IIIHIIIHHHHHHH USOO51 0783OA

United States Patent (19) (11 Patent Number: 5,107,830

Younes (45) Date of Patent: Apr. 28, 1992
(54) LUNG VENTILATOR DEVICE 4,462,410 7/1984 Blais et al. ........................... 128/727
4,487,207 12/1984 Fitz ..................................... 128/728
75 Inventor: Magdy Younes, Manitoba, Canada 4,587,967 5/1986 Chu et al. .. . 28/205.8
4,617,637 10/1986 Chu et al. ............................ 364/505
73 Assignee: University of Manitoba, Winnipeg, 4,726,366 2/1988 Apple et al. .................... 28/205.8
Canada 4,823,788 4/1989 Smith et al. .................... 28/204.21
* Notice: The portion of the term of this patent 4,957,107 9/1990 Sipin ............................... 28/204.21
subsequent to Sep. 3, 2008 has been 4,971,049 li/1990 Rotariu et al. ................. 28/204.2
disclaimed. FOREIGN PATENT DOCUMENTS
(21) Appl. No.: 501,757 3306607 9/1983 Fed. Rep. of Germany .
2328452 10/1976 France .
22 Filed: Mar. 30, 1990 1541852 3/1979 United Kingdom .
2054387 2/1981 United Kingdom .
Related U.S. Application Data 2121292 12/1983 United Kingdom .
63 Continuation-in-part of Ser. No. 496,172, Mar. 20, Primary Examiner-Edgar S. Burr
1990, Pat. No. 5,044,362, which is a continuation of Assistant Examiner-Aaron J. Lewis
Ser. No. 158,752, Feb. 22, 1988, abandoned. Attorney, Agent, or Firm-Sim & McBurney
30 Foreign Application Priority Data 57 ABSTRACT
Feb. 21, 1987 GB) United Kingdom ................. 8704104 Ventilation to a patient is provided in response to pa
51) Int. Cl.............................................. A61M 16/00 tient effort. The free flow of gas from a piston, or simi
52) U.S. Cl. .......................... 128/204.18; 128/204.21; lar air source, in response to patient inhalation is de
128/204.23 tected, the instantaneous rate and volume of flow are
58) Field of Search ...................... 128/204.18, 204.21, measured and the measurements are used as control
128/204.23, 204.26 signals to a drive motor for the piston to move the
56) References Cited piston to generate a pressure which is proportional to
the sum of measured and suitably amplified rate and
U.S. PATENT DOCUMENTS volume of flow signals. Since the command signal to the
1,406,141 2/1922 Anston ........................... 128/205.18 pressure generator only changes subsequent to, and not
3,669,097 6/1972 Fitz ..................................... 128/728 in advance of, a change in flow and volume, the ventila
3,985,124 10/1976 Coleman ............................. 128/727 tor is subservient to the patient and provides a propor
4,036,221 7/1977 Hillsman et al. ... 128/204.23 tional assist to patient ongoing breathing effort during
4,121.578 10/1978 Torzala .............. ... 128/204.23 inspiration (Proportional Assist Ventilation, PAV).
4,301,810 1 1/1981 Belman ....... ... 128/200.24
4,448,192 5/1984 Stawitcke... ... 128/204.26
4,459,982 7/1984 Fry ................................. 128/204.23 16 Claims, 10 Drawing Sheets

TO PATENT
SUPPLY NV (PEEP)
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U.S. Patent Apr. 28, 1992 Sheet 1 of 10 5,107,830

9Wntion & Mold

U.S. Patent Apr. 28, 1992 Sheet 2 of 10 5,107,830

mVentilator pressure in absence of effort.

U.S. Patent Apr. 28, 1992 Sheet 3 of 10 5,107,830

Volume and flow in absence of effort

U.S. Patent Apr. 28, 1992 Sheet 4 of 10 5,107,830

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U.S. Patent Apr. 28, 1992 Sheet 9 of 10 5,107,830

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U.S. Patent Apr. 28, 1992 Sheet 10 of 10 5,107,830

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1.
that the flow and volume delivered are the same as
LUNG VENTILATOR DEVICE prescribed. The more inspiratory effort the patient
makes, the less the pressure delivered by the ventilator
REFERENCE TO RELATED APPLICATION (FIG. 2, left to right). Conversely, if the patient does
This application is a continuation-in-part of copend 5 not wish to receive the prescribed volume or flow, and
ing U.S. application Ser. No. 496,172 filed Mar. 20, 1990 fights back, the machine generates a greater pressure to
(now U.S. Pat. No. 5,044,362), which itself is a continu offset the patient's opposing effort. There results, there
ation of U.S. application Ser. No. 158,752 filed Feb. 22, fore, an antagonistic relation between patient and ma
1988 (now abandoned). chine.
10 With this volume cycled method of ventilation, the
FIELD OF INVENTION degree of loss of patient control over his own breathing
The present invention relates to a method and device varies, depending on the type of ventilation used. With
to assist in ventilating the lungs of a patient in propor continuous mandatory ventilation (CMV), loss of pa
tion to patient effort. 5
tient control is complete, as he not only cannot alter
flow and volume within each assisted breath, but also
BACKGROUND TO THE INVENTION cannot influence frequency. In the assist/control mode
Ventilators are devices which are used to force gases, (A/C), the patient can alter the frequency of the manda
usually air or oxygen-enriched air, into the lungs of tory breaths, but within each breath, the same adverse
patients who, for one reason or another, are incapable of relation between patient effort and pressure delivered
sustaining adequate ventilation entirely through their 20 applies. With the newer synchronized intermittent man
own efforts. The source of pressure may be a piston datory ventilation (SIMV), spontaneous breaths are
device, a built in blower or a high pressure line. Com permitted between the mandatory breaths. During these
mercially-available ventilators utilize various mecha breaths, the patient controls the rate and depth of
nisms to regulate the pressure applied to the patient. In breathing. However, the abnormal relation between
all cases, a breath is triggered which sets in motion a 25 effort and ventilatory output (see FIG. 1) which was
sequence of events during which pressure is applied the reason for installation of ventilatory support in the
until a volume or pressure target is reached, at which first place, continues to apply. In fact, it is worsened by
time the pressure cycle ends. Once the cycle is trig the additional load imposed on the patient by the device
gered, the ventilator proceeds in a predetermined man 30 and endotracheal tube. The patient, therefore, alternates
ner, set by adjustments of dials on the control panel of between breaths in which effort is entirely without
the unit. With available devices and methods of ventila ventilatory consequence (mandatory breaths), and
tion, the ability of the patient to modulate breathing breaths in which effort produces abnormally low venti
output through his own effort is limited or non-existent, latory return.
as is apparent from the description below. With pressure support methods of ventilatory assist
One of the principal indications for institution of 35 (PS), the delivered pressure is a predetermined function
ventilatory support is the presence of an unfavourable of time, usually an intended square wave pressure be
relation between patient effort and resulting ventilation ginning at the onset of inspiration and terminating as
(see FIG. 1). This may be because of neuromuscular flow rate declines to a specific amount. The pressure
weakness, necessitating a greater effort to produce a delivered is, therefore, independent of how much inspi
given pressure, abnormal respiratory mechanics with 40 ratory effort the patient is making during inspiration
which greater pressure output is desired to attain a (see FIG. 3). Any inspiratory effort the patient makes
given ventilation, or both. This abnormal relation im during the breath would produce greater flow and vol
pairs the patient's ability to control ventilation and ume (FIG. 3, A to C). However, since the pressure
breathing pattern in a way that insures optimal CO2 delivered is independent of effort, the ventilatory conse
removal and/or oxygenation. In addition, the high res 45
quences of patient effort are still subject to the abnormal
piratory effort results in distress and, if the effort is relation between effort and lung expansion dictated by
sufficiently high, may ultimately lead to exhaustion (i.e.
respiratory muscle fatigue). the disease (FIG. 1). Although the overall relation be
By providing positive pressure at the airway during tween effort and ventilation is improved, the ability of
inspiration, all current approaches to ventilatory sup 50 the patient to alter ventilation in response to varying
port (volume-, pressure- or time-cycled approaches) needs continues to be impaired. Furthermore, since
unload the respiratory muscles and improve the relation patient effort normally increases in a ramp fashion dur
between patient effort and achieved ventilation in a ing inspiration, while pressure delivered by the ventila
global sense. For a given inspiratory effort, the patient tor is nearly constant, the ventilator overassists early in
receives a greater volume in the assisted breath than he 55 inspiration while the assist decreases relatively as inspi
would otherwise. The price of this support, however, is ration progresses. The patient then would sense an in
a variable degree of loss of control by the patient over crease in load as inspiration is lengthened and this
his ventilation and breathing pattern. These effects are prompts the patient to breathe with short inspirations,
illustrated in FIGS. 2 to 4. resulting in a small tidal volume.
With volume cycled ventilation (FIG. 2), the ventila 60 With airway pressure release ventilation (APRV),
tor delivers pressure in whatever amount and time pat pressure at the airway alternates between high and low
tern is necessary to cause a predetermined flow pattern levels with a time sequence that is independent of pa
and tidal volume to be achieved during the assisted tient effort (see FIG. 4). The periodic cycling of pres
breath. The operator of the ventilator (i.e. physician or sure insures a minimum ventilation. The patient can also
therapist), and not the patient, determines the flow and 65 obtain spontaneous breaths independent of the pro
volume to be delivered. If the patient makes an inspira grammed cycles. During these, he receives no assist
tory effort during the inhalation phase, the ventilator (i.e., similar to SIMV). The relation between effort and
simply decreases the pressure it provides in such a way ventilatory consequences during these breaths contin
 5,107,830 4
3
ues to be poor, as dictated by disease, limiting the pa the piston type which expands in proportion to the
tient's ability to alter flow and ventilation in response to exhaled air.
varying needs. In fact, since with APRV, operating U.S. Pat. No. 3,669,097 describes a device for increas
lung volume is increased, the relation between effort ing the capacity and strength of lungs. An expansible
and ventilatory consequences is further compromised 5 bellows chamber is connected to a conduit having a
on account of the well established adverse effect of mouthpiece. A selectively-adjustable valve is present in
increased lung volume on neuromechanical coupling the conduit for constricting the passage from the
(i.e. pressure generation for a given muscle activation). mouthpiece to the inlet to the bellows chamber, so that
I am aware of specific prior art proposals to effect a force in excess of the normal pressure developed by
modifications to commercially-available pressure-pow O the lungs is required to expand the bellows.
ered ventilators to allow the pressure produced to vary U.S. Pat. No. 4,301,810 describes a ventilatory muscle
with electrical activity recorded from a respiratory training apparatus comprising a reservoir and a mouth
nerve, as described in Remmers et al, "Servo Respirator piece and also having a simple valving system to vent
Constructed from a Positive-Pressure Ventilator', J. stale air from the reservoir during exhalation and let
Appl. Physiol. 41: 252 to 255, 1976. To the extent that 15 fresh air into the reservoir during inhalation. The air
activity in inspiratory nerves reflects effort these modi flow through the mouthpiece is monitored to ensure the
fications would permit a ventilator to deliver pressure in intended manner of use of the apparatus is maintained.
proportion to effort, as is intended in the present inven U.S. Pat. No. 4,462,410 describes a recording spirom
tion. These modifications, which were developed for eter for performing a breath test which uses a movable
animal use where inspiratory nerves are accessible and 20 pusher plate which is moved in response to the breath
can be recorded from, implicity require direct measure ing of the patient and a recording medium which ena
ment of inspiratory muscle or nerve activity which is bles a record to be made of the volume of air expelled
not practical in humans requiring ventilatory support. by a patient as a function of time.
In contrast, the present invention permits the delivery 25 U.S. Pat. No. 4,487,207 describes a lung exercise
of pressure in proportion to patient effort without the device which has a mouthpiece through which a patient
inhales. A conduit connects the conduit to an air inlet
need for direct recording of activity, and through the and a valve is located in the conduit, normally biased to
use of algorithms that permit the inference of degree of a closed position. Upon inhaling, the valve is opened
effort from easily measurable variables, such as flow and the amount of air inhaled is monitored.
and volume. Poon et al, "A Device to Provide Respira 30 U.S. Pat. No. 4,459,982 by Fry discloses a lung venti
tory Mechanical Unloading", IEEE Trans. Biomed. lator device which comprises a chamber means which
Eng. 33: 361 to 365, 1986, described a modification to a delivers respiratory gases to a patient. This patent de
commercially-available volume ventilator which per scribes, as one embodiment, the provision of a flow rate
mits the ventilator to deliver pressure in proportion to directly controlled by the patient's instantaneous de
inspired flow. Although this device was developed 35 mand under spontaneous breathing of the patient, and
originally to simulate and amplify the effect of helium in hence, at first sight, may be considered relevant to the
reducing respiratory resistance during experimental present invention. However, the operation of the device
studies on exercising humans, it can theoretically be and its control require movement of the piston in the
used in patients to provide partial ventilatory support. chamber to maintain the pressure at the patient's airway
As such, the device would develop pressure with a time constant and equal to a reference pressure determined
pattern that resembles inspiratory flow, which is highest by the operator. Since the device operates to maintain
early in inspiration and declines later. Since this pattern airway pressure constant throughout inspiration, it is
is poorly (or even negatively) correlated with patient apparent that it does not deliver pressure in proportion
inspiratory effort, which rises continuously during the to patient effort (which varies throughout inspiration),
inspiratory phase, this pattern of support is quite unlike 45 as is the case of the present method (PAV).
the method of the present invention, where pressure is British Patent No. 1,541,852 describes a piston, driven
intended to be a function of inspiratory effort through by a motor which alters the pressure in the piston ac
out inspiration, as described in more detail below. Not cording to the power supplied to motor. This system is
only is the method completely different (simple resistive designed to deliver pressure according to a predeter
unloading vs assist in proportion to effort) but the appa 50 mined pressure-time profile (as in pressure cycled venti
ratus described herein is much more suitable for our lators) or to force a given volume of gas into the patient
method (Proportional Assist Ventilation, PAV) than a (as in volume cycled methods). In the method of the
modification of existing volume ventilators which are present invention, neither pressure nor flow and volume
designed to regulate flow and not pressure. The design is/are predetermined. Rather, the patient determines his
of the invention permits unlimited flow and there is no 55 own flow pattern and tidal volume through his own
delay between onset of inspiratory effort and onset of effort, while the ventilator delivers pressure in a manner
flow from ventilator to patient, since no triggering is that parallels the patient's ongoing effort (which is obvi
required before the gas delivery system is communi ously not predetermined).
cated to the patient. The system of the invention also Other representative prior art employing motor
can effect proportional assist through positive pressure 60 driven pistons that deliver predetermined pressure vs
at the airway or negative pressure at body surface, time or volume vs time patterns know to the applicant
whereas a modified positive pressure, gas powered ven are Hillman, U.S. Pat. No. 4,036,221, Chu, U.S. Pat. No.
tilator can serve the former function only. 4,617,637 and Apple, U.S. Pat. No. 4,726,366.
Some known prior art patents describe a variety of Stawitcke (U.S. Pat. No. 4,448,192, U.K. Patent No.
breathing devices. U.S. Pat. No. 3,985,124 describes a 65 2,121,292) describes a system with motor driven pistons
spirometer for measurement of the rate and volume of and extremely complex controls, the intent of which are
expiratory breathing to create a graphic record of the to reduce conflict between patient and ventilator. A
same. This device possesses an expansible chamber of control system continuously computes an ideal pres
 5, 107,830 6
5
sure-volume trajectory designed to cause the ventilator mum ventilation in the absence of effort. Any influence
to deliver gas in the amount (tidal volume) and flow rate the patient may have on his breathing is subject to the
specified by the physician while allowing for different limitation imposed by disease (the slope of the line with
degrees of patient effort. Although this system permits SIMV, PS and APRV is similar to the of the disease
the patient to over-ride physician-specified delivery line). With PAV, in contrast, the relation between effort
pattern within an inhalation or over a brief period, the and ventilatory consequences is normalized at different
control system readjusts the terms in the equation, so levels of effort.
that physician-determined criteria are ultimately met. In accordance with one aspect of the present inven
The principle of control is, therefore, similar to volume tion, there is provided a novel type of lung ventilator
cycled methods, in that an increase in patient effort is 10 apparatus which delivers PAV with a relatively simple
met with a decrease in machine assist to return ventila electrically-powered operation. In the present inven
tor output to what the physician prescribes. The main tion, the gas pressure at the patient's airway is deter
difference from other volume-cycled methods is in the mined by the action of an electric motor acting on a
freedom the patient is accorded to transiently over-ride piston reciprocating in a chamber in relation to any
the prescriptions by the physician. This principle of 15 desired command input. The electrical motor moves the
operation is diametrically opposite to that executed by piston with a force proportional to the magnitude of the
the method of the present invention. Thus, in the Sta power applied to the motor. On-going rate of flow (V)
witcke system, the physician sets targets for volume, and volume of flow (V) of gas moving from chamber to
flow and timing and the machine alters the various patient are monitored with electronic circuitry that
parameters in the control system equation in such a way 20 permits the independent adjustment of the gain or am
as to meet physician requirements. By contrast, in the plifications of each of the two signals. The electronic
present invention, the proportionalities between pres circuitry provides electric power to the motor in pro
sure, on one hand, and volume and flow on the other portion to the sum of the suitably amplified V and V
hand are the parameters that are predetermined, while signals. As is described in more detail below, this proce
the patient is left entirely free to select volume, amount 25 dure causes the device to provide pressure in propora
and pattern of flow, and timing of each breath. tion to patient effort.
It will be apparent from this discussion, that prior art The difference between proportional assist ventila
patient ventilation systems provide ventilatory support tion and other modalities is that the patient has total
to a patient in accordance with parameters which are control over his breathing pattern. In this nodality, the
determined mainly by a physician and not by the pa 30 apparatus works on the principle of positive feedback,
tient. Generally, the prior art has required some target namely the more volume and flow the patient takes, the
flow rate, target pressure, target volume, and/or target more pressure the machine provides. The adjustable
frequency or inspiratory or expiratory time. Such re parameter is not a target pressure or a target volume,
quirements give rise to the various problems discussed but a degree of assist (or proportionality) to the patient's
above. 35 breathing pattern. Thus, if the patient's respiratory sys
SUMMARY OF INVENTION
tem stiffness (elastance) is such that it requires 40
cmH2O per liter of inflation, the machine may be ad
The present invention is directed to a quite different justed to provide a specified amount of pressure/unit of
approach to ventilatory support from those of the prior inhaled air. If in this case, the proportionality is set to 20
art described above. In the present invention, the pres 40 cmH2O/L, the patient has to do half the elastic work
sure delivered by the ventilator increases in direct pro and the machine does the other half, and so on. There is
portion to patient effort, and the proportionality applies no requirement that the patient take in a certain volume
from breath to breath, as well as continuously through or reach a certain pressure. As soon as the patient de
out each inspiration. In effect, patient effort is amplified, creases his own effort, air stops flowing in and the ma
thereby normalizing the relation between effort and 45 chine stops pumping.
ventilation, while leaving the patient entirely in control Accordingly, in one aspect of the present invention,
of all aspects of his breathing. In the present invention, there is provided a method for providing breathing
the pressure delivered by the ventilator to the patient assistance in proportion to patient ongoing inspiratory
reflects the amplitude as well as the time pattern of effort, which comprises providing a free flow of gas
patient effort (see FIG. 5). There is no target flow rate, 50 from a gas delivery system to a patient in response to a
no target pressure, no target volume, and no target pressure gradient generated by patient inspiratory ef
frequency or inspiratory or expiratory time, as there is fort; determining the rate and volume of flow of the gas
in the prior art. These parameters are determined solely to the patient; independently amplifying signals corre
by the pattern of patient effort and the ventilator simply sponding to the determined rate and volume of flow;
amplifies the ventilatory consequences. The net effect 55 and providing a pressure assist to the gas in proportion
of this approach is that the abnormal relation between to the sum of the determined and amplified rate and
effort and ventilatory consequences is restored towards volume of flow.
normal, returning the patient's ability to alter ventila The present invention also provides apparatus for
tion and flow pattern, as dictated by his own varying delivering proportional assist ventilation to a patient,
needs, including the optimization of respiratory sensa 60 comprising means for delivering a free flow of gas to
tion. The ventilation procedure of the present invention the patient in response to patient inhalatory effort;
may be termed proportional assist ventilation (PAV). means operatively connected to the gas delivery means
In the left-hand graph in FIG. 6, there is provided a for generating pressure in the free flow of gas in re
summary of the relation between patient effort and sponse to an electrical command signal; detection
pressure delivered by the ventilator with different ven means for detecting the instantaneous volume and rate
tilatory methods, while the right-hand graph shows the of gas flow to the patient and for generating a separate
relation between patient effort and ventilatory conse electrical signal corresponding in magnitude to each of
quences. With SIMV, PS, and APRV, there is a mini the detected values; means for selectively applying
 5, 107,830
7 8
amplification to each of the electrical signals; and means dysfunction is common in the weaning period and may
for generating the electrical command signal to the be due, in part, to protracted inactivity of the respira
pressure generating means in proportion to the sum of tory centres produced by machine settings that promote
the amplified electrical signals corresponding in magni apnea with other modalities of ventilatory support. The
tude to the instantaneous rate and volume of flow. lesser likelihood of central and peripheral muscle dys
There are several advantages to this type of ventila function facilitates weaning.
tion, in relation to the prior art ventilation devices and An improved efficiency of negative pressure ventila
modalities. tion is achieved. The ability of negative pressure de
The first advantage is one of comfort, in that the vices to reduce intrathoracic pressure is limited, making
relation between machine and patient is not only syn O them quite ineffective in patients with severe mechani
chronous, but entirely harmonious. There is no fighting cal abnormalities (e.g. chronic obstructive pulmonary
between patient and machine, and, in fact, the ventilator disease (COPD) or pulmonary fibrosis). Furthermore,
becomes an extension of the patient's own muscles. the lack of harmony between pump pressure and upper
Since with PAV pressure is only delivered if patient airway dilator muscle activity promotes airway col
effort is present, and the pressure is proportional to 15 lapse. If the negative pressure applied to a body surface
patient effort throughout inspiration, the patient must is harmonized with patient's own inspiratory effort,
contribute a fraction of the pressure used to expand the upper airway narrowing is decreased and the combined
patient's chest. The ventilator needs to deliver less pres pump and patient generated pressure may be adequate
sure for a given tidal volume, as compared with other for ventilation. These two factors may make it possible
devices. With other forms of ventilation, patient contri to ventilate patients with severe lung disease during
bution cannot be relied upon and often patient contribu sleep using the present invention. At present, cuirass
tion is antagonistic. The required peak airway pressure ventilation in these patients (e.g. COPD) is feasible only
is higher with such devices than in the present inven during wakefulness.
tion. In preliminary studies, with the ventilation proce An essential and distinguishing feature of the proce
dure of the invention, it was found that peak airway 25 dure of the present invention is that the pressure gener
pressure with PAV was only one-third to one-half that ated by the ventilator follows the exchange of gas from
with SIMV at the same level of ventilation. Since the machine to patient as opposed to leading or causing it,
required peak pressure is lower in the present invention, as in conventional devices. Flow and volume must first
it is possible to ventilate some patients through a nose or be altered before the ventilator alters its pressure out
face mask and avoid intubation altogether. Intubation, 30 put.
directly or indirectly, is one of the main causes of mor Although the ventilating apparatus of the invention
bidity and mortality in ventilated patients. was designed specifically for the purpose of delivering
Intrathoracic pressure remains negative during inspi this proportional assist node, the design principles used
ration, as in normal subjects, thereby reducing the he to accomplish this end also make it possible to use the
modynamic complications of artificial ventilation. The 35 machine, with very minor electronic additions, to de
greater comfort, lack of fighting and possible avoidance liver pressure in proportion to any desired input. This
of intubation greatly reduces the need for sedation or extreme versatility permits adaptations to fit future
paralysis of patients, both of which impair the patient's needs with minimal modification.
defenses.
BRIEF DESCRIPTION OF DRAWINGS
With PAV, the ventilator is simply an extension of 40
the patient's own muscles and is, hence, subject to all FIG. 1 is a graphical representation of patient effort
the intrinsic control mechanisms that adjust ventilation and respiratory flow and volume;
to varying needs and protect against various injuries. FIG. 2 is a graphical representative of volume cycled
Such control mechanisms can potentially be of consid ventilation (VCV) according to one prior art proce
erable benefit. For example, the respiratory system is 45 dure;
endowed with powerful reflexes to protect the lungs FIG. 3 is a graphical representation of pressure sup
from overdistension. Overdistension would reflexly port ventilation (PSV), according to another prior art
cause inhibition of inspiratory effort and recruitment of procedure;
expiratory effort. Since the ventilator ceases its pressure FIG. 4 is a graphical representative of airway pres
when inspiratory effort is terminated, overdistension 50 sure release ventilation (APRV) according to a further
would reflexly cause the ventilator to cycle off. The risk prior art procedure;
of barotrauma is reduced. Furthermore, a patient's net FIG. 5 is a graphical representative of proportional
abolic needs may vary greatly from time to time as a assist ventilation (PAV) according to the present inven
result of movement, shivering or temperature changes. tion;
Normally, the respiratory control changes inspiratory 55 FIG. 6 is a graphical representative comparing pro
effort so that ventilation matches metabolic demands. portional assist ventilation (PAV) in accordance with
With PAV, such adjustments in inspiratory effort the present invention with prior art procedures;
would be met by concordant changes in machine assist. FIG. 7 shows a schematic of a device for providing
This effect tends to control arterial blood gas tensions proportional assist ventilation and a graphical represen
within narrow limits. Furthermore, the patient would tative of ventilator gain and patient elastance;
not fluctuate from periods of overassist to underassist, FIG. 8 is a schematic representation of a lung ventila
and hence distress, as his metabolic demands change. tor device provided in accordance with one embodi
With the present invention, there is less likelihood of ment of the invention;
inspiratory muscle disuse because a minimum level of FIG. 9 is a schematic representation of the electronic
inspiratory muscle activity must be present with the 65 circuitry used with the apparatus of FIG. 8;
PAV modality. If PAV is used throughout the illness, FIG, 10 is a schematic representation of the various
there would be no period in which the central control command signals for the electronic circuitry of FIG. 9;
mechanisms are inactive (apnea). Central respiratory and
 5, 107,830 10
FIG. 11 is a schematic representative of a lung venti During supported ventilation, it follows that the ins
lator device for effecting proportional assist ventilation taneous relation between the opposing forces is deter
in accordance with another embodiment of the inven mined by the relationship:
tion.
Pinus + Pent= V-Es- VRs (1)
GENERAL DESCRIPTION OF INVENTION
Or
In the present invention, patient effort is detected and
responded to. Inspiratory effort can be defined as the Pinus= V-Es-- VRs-Pent (2)
degree of inspiratory muscle activation (Ei) relative to
the maximal possible activation (Ena). Inspiratory ac O
tivity results in generation of inspiratory pressure Since V,V and Pent can be continuously measured and
(Pnui) according to the relationship: Es and Rs can be measured or estimated, Pnus can be
computed continuously, by any convenient computing
(i.e. Emax)Prmusi=fEi/Enax device, from the measured values, and the ventilator
5 device made to change pressure in proportion to the
where f is the function that governs the conversion of instantaneous Pnus.
activity to pressure. This function is primarily influ Pent=A-Pinus
enced by muscle strength, lung volume (V) and inspira
tory flow (V) (see Younes et al, A model for the relation where A is the proportionality between Pent and Pnus in
between respiratory neural and mechanical outputs. 20 cmH2O/cmH2O.
I.Theory, J.Appl. Physiol 51:963-978, 1981, incorpo A simpler and more versatile approach is to design
rated herein by reference). Since maximal inspiratory the ventilator such that it delivers pressure according to
pressure is not affected by volume in the resting tidal the following equation:
volume range and flow at resting levels is associated
with very small velocities of muscle shortening relative 25 Pet=K-- K2V (3)
to the maximal possible velocity (Younes et al, supra),
the funcction that governs the relation between effort where K1 is a gain factor applied to the volume signal
and Pinus is primarily affected by muscle strength under (in cmH2O/L) and K2 is a gain factor applied to the
resting levels of ventilation. This is very frequently flow signal (in cmH2O/L/sec). Although constants are
abnormal in patients requiring ventilatory support be 30 used in this analysis, non-linear functions can be used
cause of primary neuromuscular disease, secondary to where appropriate.
nutritional or metabolic derangements, or as a conse If A is the desired proportionality with Pinus and Pinus
quence of hyperinflation as in the case in patients with is given by equation (2) above, it follows that:
severe asthma or chronic obstructure pulmonary dis
ease (COPD). Under these conditions, a given effort 35 Penr=A. W. Es-- A.V.Rs-A-Pent
results in less than normal pressure and hence less ex
pansion. so that
At any instant during the breath, the pressure applied
to the respiratory system (Pappi) is dissipated against the Pen(1 +A)= AE V--ARV
elastic and resistive elements of the respiratory system, 40
since inertial losses are negligible at resting levels of
ventilation. Thus:
Pappl= Pel--Pres
45 The latter equation has the same form as equation (3)
where Pei is a function of lung volume above passive above, where K1 is a fraction (ie A/(1 +A)) of respira
Functional Residual Capacity (FRC) (Pei=fv), as dic tory elastance and K2 is the same fraction of respiratory
tated by the pressure-volume relation of the respiratory resistance. Accordingly, Pent can be made proportional
system, and Presis a function of flow, as dictated by the to Pmus without actually having to calculate Pnus ac
pressure-flow relation of the respiratory system. Al SO cording to equation (2). Hence, if it is desired to pro
though both functions may be complex and non-linear, duce a Pent which is three times as much as Pinus (A=3),
linear functions are used in this discussion for conve K1 is set to be 0.75 Es and K2 is set to be 0.75 Rs. It
nience. Thus: follows that either equation (2) or equation (3) can be
used to control Pent. However, the use of equation (3)
Poppi= V-Es--VRs 55 is advantageous, for the following reasons.
With equation (2), Pent is used to derive the com
where V is the volume above ERC, E is the elastance mand signal to the pressure generating mechanism,
of the respiratory system in the linear range (in while being, itself, the controlled variable. Particularly
cmH2O/L), V is the flow rate and Rs is respiratory in view of the inevitable lag between command signal
system (including ventilating apparatus) resistance (in 60 and Pent, this situation promotes oscillations which
cmH2O/L/sec). require elaborate filtration to avoid.
During spontaneous breathing, the only source of With equation (3), the use of individually adjustable
Poppl is pressure generated by the respiratory muscles gain factors for flow and volume permits unequal un
(Pn). In patients attached to a ventilator, applied pres loading of the resistive and elastic properties, which
sure is the sum of patient generated pressure and ma 65 may have advantages in particular clinical situations. In
chine generated pressure (Pen). The latter may take the addition, in many cases measurement of Es and Rs are
form of positive pressure at the airway or negative difficult to obtain or are unreliable, but with the use of
pressure at a body surface (e.g. by tank or cuirass). equation (3), the gain factors for V and V can be sepa
 5,107,830
11 12
rately adjusted to patient comfort, thereby obviating the pressure in the system (i.e. Pa) to increase accord
the necessity of having to know Es and Rs. An example ing to the volume signal and the chosen gain factor.
of a system which operates in accordance with this Assuming the gain factor is set at 0.5 units of press
principle is shown in FIG. 11 and described below. ure/unit of volume, the ventilator will increase Pa by 2
An essential and distinguishing feature of propor units of pressure in response to the initial four units of
tional assist ventilation (PAV) according to the inven volume, so that the total pressure (Pappi=Pnus --Pa.)
tion, is that the pressure generated by the ventilator then has increased to six units, thereby causing two
follows the exchange of gas from machine to patient, as more units of volume to be transferred. This increased
opposed to leading or causing it. Flow and volume must volume, in turn, is sensed and, according to the gain
first be altered before the ventilator alters its pressure O factor; causes Pent to rise an additional one unit, which
output. Any device which delivers PAV to a patient, itself results in the transfer of one more unit of volume,
therefore, must permit free flow of air from the device and so on.
to the patient in response to changes in pressure at the It can be seen that, because the gain factor is less than
patient's end and this requirement is most readily exem the elastance of the patient, the magnitude of the steps
plified by breathing from an easily-movable bellows. 15 decreases progressively and, accordingly, there is no
Since, at any instant, the elastic and resistive pressure tendency to "run-away'. In fact, if Pinus remains con
losses (right side of equation 1) are balanced by the sum stant, the volume rises to an asymptote. This effect can
of Pinus and Pent, any increase in Pinus causes a change in be seen graphically in FIG. 7, the volume asymptote is
total applied pressure and a corresponding change in twice the value produced by the patient in the absence
flow and volume. The device then responds by chang 20 of assist.
ing its pressure, thereby causing a greater change in With a larger step change in Pinus, for example the
flow and volume.
A system that operates according to this sequence second step in FIG. 7, the volume exchanged under the
displays positive feedback. On its own, such a system is patient's own power is larger, and so is the response of
inherently unstable and tends to "run-away', since, as 25 the ventilator but, again, there is no tendency to run
some air leaves the system, it generates pressure which away, and the volume exchanged in this second step
causes more air to leave, causing the system to generate bears the same proportionality to Pinus as the smaller
even more pressure, and so on. When attached to a first step.
patient, however, the positive feedback inherent to the Since the elastic recoil at any instant is sustained
ventilator is counteracted by the negative feedback 30 jointly by patient and ventilator, if the patient decreases
provided by the mechanical properties of the patient, his contribution, as signified by the arrow in FIG. 7, the
namely elastance and resistance. pressure applied to the respiratory system, namely
FIG. 7 schematically illustrates how the interaction Pnus--Pent, is no longer sufficient to sustain the sys
between a PAV system and a patient tends to eliminate tem's elastic recoil. As in the case of spontaneous
the potential for "run-away" and causes the system to 35 breathing, when Pinus decreases at the end of inspiration,
simply amplify the ventilatory consequences of patient a positive gradient develops from alveoli to airway and
effort. The upper portion of FIG. 7 illustrates a simple exhalation begins.
design which satisfies the criteria of a PAV delivery The extent of amplification of the elastic component
system. The patient is attached to a freely-moving pis of Pinus is a function of the gain on the ventilator volume
ton. Movement of air from the piston to patient is sensed signal (K1) relative to the patients own elastance (E).
by a flowmeter, resulting in flow and volume signals. Since the total elastic pressure (V.E) is balanced in
The latter signals as measurements of the instantaneous part by the elastic component of Pius (Pnus (el)) and in
rate and instantaneous volume of flow of ga are used to part by the elastic assist of the ventilator (V.K), it
activate a motor which applies force to the back of the follows that:
piston in proportion to the flow and volume signals. 45
Separate gain controls determine the proportionalities
between flow and volume on the one hand and the force Pinus (el) = V Es - Pent (el)
exerted, namely resulting pressure, on the other hand.
Such gain volumes are analogous to the terms K1 and = V. E - V. K.
K2 in equation (3). 50 set V (Es - K)
To facilitate description of the interaction between
the patient and a PAV system, the hypothetical case The ratio of total elastic pressure applied, namely
will be considered where all changes (Pinus and Pent) V.E, to that developed by the patient (Pinus (el)),
take place in discrete sequential steps, illustrated by the which is the elastic amplification factor (F(el)), there
lower three graphs in FIG. 7, and all generated pressure 55 fore, is given by the equation:
is applied against the elastance of the respiratory system
(i.e. the resistance is zero). Patient elastance is expressed Fel)- V.E/ME-K)=E/IE-K)
in arbitrary units and is assigned a value of one (i.e. 1
unit pressure/unit volume). Hence, if K1 is Es, as in the example of FIG. 7, then
Before the onset of inspiration, i.e. Pnus=zero, the the amplification factor is 2. For K1-0.75 E, the am
airway pressure and volume are stable, reflecting the plification factor is 4, and so on. Where K equals or
level of positive expiratory pressure (zero in the illus exceeds Es, amplification would be infinite and a run
trated case). Assume the patient makes a step change in away situation would result. However, since E in
Pinus of 4 units. According to patient elastance, four creases progressively as long as volume increases, a
units of volume move from ventilator to patient. The 65 run-away situation produced by the gain factor acciden
airway pressure is still zero, as seen by the interval tally being higher than Es near functional residual ca
between the first pair of vertical lines. The transfer of pacity, would soon be aborted as lung volume increases
gas (volume and flow), however, is sensed and causes and Es increases.
 5,107,830 14
13
The example shown in FIG. 7 is intended as an over consequences, it is necessary to provide support to both
simplification to explain the principles involved in the flow and volume, according to equation (3) above. In
present invention. For example, the resistance is never this way, Pnus is amplified regardless of how it is parti
zero and part of Pnus is expended against the resistance tioned between elastic and resistive components.
elements of the system (patient plus equipment). How The illustration shown in FIG. 7 also should not be
ever, similar analysis and consideration can be made to interpreted as implying that the control should proceed
the resistance component of Pnus, i.e. Pnu(res). At any in a sequential manner as illustrated. Analog control
instant, the total pressure used to offset the flow-related circuitry can function just as well. In both cases (digital
pressure gradient is given by V. R, where R is the total and analog) the command signal to the pressure genera
resistance for airflow from ventilator to chest wall and 10 tor changes only subsequent to, and not in advance of,
includes equipment resistance. This pressure is provided a change in flow and volume thereby rendering the
in part by the ventilator (V.K2) and in part by the resis ventilator subservient to the patient and not the oppo
tance component of Pinus (Pinus(res)). Using a similar site.
analysis as that for elastic pressure, it can be shown that
the amplification of the resistance component of Pnus 15 DESCRIPTION OF PREFERRED
(F(res)) is a function of the flow gain on the ventilator EMBODIMENTS
(K2) relative to the total resistance (R), according to the For the delivery of PAV, motor driven bellows is the
relationship: preferred basic design, since the relation between con
Fres)= RMR-K2) 20
mand signal and pressure output is more direct, as op
posed to gas powered ventilators where pressure can be
Because volume rises gradually during inspiration varied only by controlling flow. The relation between
whereas flow peaks early on and declines later in inspi flow and system pressure is extremely complex. Deli
ration, the partition of Pnus between elastic and resis cate servo control would be essential and this would
tance components varies greatly with inspiratory time. 25 tend to cause oscillations in the PAV nodality, unless
Pinus (res) contributes the largest fraction of Pnus early severe filtering is applied which considerably dampens
in inspiration with this fraction declining to nearly zero the response, as described above.
by the end of inspiration. The correlation between in Two embodiments, utilizing motor driven bellows,
stantaneous flow, and hence Pnus (res), and instanta are described below. The first, more complex one
neous effort, i.e. total Pinus, is quite poor and, in fact, (FIGS. 8 to 10), is recommended where the relation
negative later in inspiration. An assist related to flow 30 between power supplied to motor and pressure output is
only, therefore, would amplify effort early in inspira not intrinsically linear. This may arise because the prop
tion, where effort is small, while leaving the muscles erties of the mechanical components (bellows and mo
essentially unsupported late in inspiration, where effort tor) are such that a significant amount of the force gen
is greatest. erated by motor is dissipated in overcoming the inertia
Similarly, because Pnu(el) constitutes a different 35 and resistance of the piston/motor. Since acceleration
fraction of total Pinus at different times of inspiration, and flow vary greatly during inspiration, inertial and
volume is not a perfect correlate of effort, although, resistive force losses also would be highly variable,
because both Pnus and volume rise during inspiration, resulting in an unreliable intrinsic relation between
the correlation is better than in the case of flow. None force and pressure. Alternatively, nonlinearities be
theless, assist proportional to volume only would pro tween applied power and pressure output may arise if
vide Pinus amplification primarily late in inspiration, but the motor operates in its non-linear range. In both cases,
would leave the early part of inspiration essentially these non-linearities must be compensated for through
unsupported. This result would not be particularly ad servo-feedback where the power supplied to the motor
verse if resistance is normal since, in this case, the rela is related to an error signal (difference between actual
tion between effort and ventilatory consequences early 45 and desired pressure), as opposed to being related to the
in inspiration would be normal and not requiring sup primary signal itself (sum of flow and volume in the case
port. However, since R is abnormal in all ventilated of PAV).
patients, at least because of the added resistance of the The second embodiment (FIG. 11) is suitable where
endotracheal tube and apparatus resistance, the failure the relation between power delivered to motor and
of Pinus amplification early in inspiration would leave 50 pressure in the bellows is linear in the range of motion
the patient with a sense of loading which may lead to required for ventilation. This is achieved by mechanical
respiratory distress. The patient's own resistance also design that minimizes inertial and resistive losses during
may be high, although the increase in inspiratory air movement of piston or bellows and by insuring that the
way resistance even with very severe obstructive dis 55 motor operates in a linear range (i.e. fixed relation be
eases is only modest and the main load is still primarily tween power supply and force output) through the
elastic, as dictated by dynamic hyperinflation. entire clinically-useful volume range of piston or bel
Another consideration regarding the use of support in
proportion to either flow or volume alone is the fact lows. The use of such mechanical design obviates the
that amplification of one component of Pnus namely need for using pressure feedback and error signal to
Pn(el) or Pnu(res), inevitably results in an increase in control the motor. As indicated earlier, the use of pres
the contribution of the other, unsupported, component. sure feedback in the PAV modality destabilizes the
Thus assist in proportion to flow will initially increase device (because the desired pressure is not itself a fixed
flow. Lung volume, and hence elastic component of reference or function but is highly labile and influenced
Pinus, rises at a faster rate. The contribution of Pnus by the pressure in the system). Eliminating the pressure
(res), and hence the assist, must subsequently decline as 65 feedback from the command signal, therefore, results in
a result. a much more stable system. Filtering can be minimal
It follows from this discussion that, in order to nor with enhanced responsiveness of the device. The opera
malize the relation between effort and its ventilatory tion of such device then would be essentially errorless.
 5,107,830 16
15
Referring to FIGS. 8 to 10 the drawings, a lung venti The motor 16 is of the type which possesses a moving
lating apparatus 10 comprises a plurality of components part, such as a shaft or coil, to exert a force in a forward
housed in housing 12. The components located in the (positive) or backward (negative) direction in propor
housing 12 comprise a rolling seal piston head 14 of low tion to the power applied to the motor 16. The maxi
inertia and low resistance, a drive motor 16 for the 5 mum force requirement of the motor is determined by
piston 14, an electrical power supply 18 for the unit 10, the area (A) of the piston head and the maximum pres
and air blower 20 and electronic controls 22. sure range (Pmax) for which the unit 10 is built. The
The piston head 14 is mounted for reciprocal move equation to calculate the required force is:
ment in the housing 12 to provide a chamber 24 of
variable volume depending on the position of the piston
head 14. The cross-sectional area of the piston head 14
as well as the maximum distance of reciprocation deter
F(kg) = nam, m)
mine the range of ventilation for which the unit 10 may Hence, for example, for a piston of area 200 cm2 and
be used. maximum pressure range 50 cm H2O, the motor 16
For best patient response, the piston head 14 should 15 should be able to generate up to 10 kg of force.
move with little inertia and frictional resistance, so that A velocity transducer 57 (FIG. 9) is mounted to the
the initial movement of the piston follows free flow of core of the motor 16 to generate a signal 58 propor
gas to the patient upon initial patient inhalation effort. tional to the rate of motion of the piston head 14. This
For this purpose, a light plastic material may be en signal is used for a variety of purposes, as described
ployed as the material of construction, or, as illustrated, 20 below with respect to FIG. 9. The motor 16 is powered
an aluminum sheel 26 with a foamed polymer reinforce by a power line 60 from the electronic module 22 (see
ment, such as, Styrofoam. Frictional resistance to piston also FIG. 9).
movement may be decreased by using high quality bear While we found the above described mechanical
ings and low resistance seals, such as a rolling seal 28 design suitable, clearly any other motor/bellows design
between the piston head 14 and the interior chamber 25 or configuration that satisfied the low inertia/low resis
wall 30.
The chamber 24 is provided with an inlet/outlet tance requirement of this application would be suitable.
opening 32 which is arranged to be joined by a flexible The electronic module 22 functions to provide the
air pipe 34 to the airway of a patient. A one-way valve electrical signal drive input 60 to the motor 16 to cause
36 communicates with the pipe 34, to allow ambient gas, the motor 16 to generate a drive force to the piston 14
usually air, to enter the piston chamber 24 through the of a pattern and magnitude that reproduces, as closely
inlet/outlet opening 32 during the exhalatory phase of as possible, the desired pattern and magnitude of pres
the respiratory cycle, as described in more detail below. sure to be delivered to the patient from the chamber 24.
The chamber 24 is illustrated as possessing an addi It is preferred to employ a fast-responding motor, so
tional gas inlet 38 controlled by a solenoid valve 40 to 35 that a change in drive input results in an almost instanta
permit the optional introduction of selected gas mix neous change in drive force, thereby permitting almost
tures, if desired, to the chamber 24. The solenoid valve unlimited patterns of pressure applications in response
40 is opened during expiration as the piston head 14 to patient inspiratory effort.
reciprocates to its baseline position. Alternatively, the A large variety of electronic components may be
oxygen content of the inspired gas may be enriched by 40 used in the electronic circuitry in conjunction with the
admitting a continous flow of oxygen into the chamber pressure generating device (i.e. the combination of
24 through an optional gas inlet 42. motor 16, piston head 14 and power supply 18) to pro
The piston head 14 is mounted on a piston rod 44 for duce a corresponding variety of pressure patterns, as
reciprocation of the piston head 14 for the purpose of described further below.
alternatively decreasing and increasing the volume of 45 The operation of the ventilating device 10 now is
the cylinder 24. The piston rod 46 is mounted in sliding described in conjunction with the electronic circuitry
relation to a suitable bearing 45. A position sensing shown schematically in FIG. 9 and the command sig
device 46, in the form of a potentiometer, is mounted on nals shown schematically in FIG. 10.
the piston rod 44 to provide an electrical signal corre The motor 16 responds to the instantaneous differ
sponding to the position of the piston head 14. 50 ence, after suitable amplification, between a desired
A pressure transducer 48 is operatively connected to output, being the command signal, and the actual out
the piston chamber 24 to determine the pressure in the put. As seen in FIG. 9, the desired output is inputted by
chamber 24 which, in turn, determines the force output line 70 to a summing amplifier 72 to which also is fed a
required by motor 16, as explained in more detail below. feedback signal (corresponding to the actual state of
The piston chamber 24 also is connected to a three 55 affairs) by line 74.
way solenoid valve 50. The solenoid valve 50 is con To produce. a signal that is proportional to the in
nected via tube 51 to an expiratory valve 52 connected spired volume, the inspired volume signal is connected
by flexible tubing 54 to the patient airway. During the to the command signal. The pressure in the piston
inspiratory phase of operation of the piston 14, the sole chamber 24, as measured by the pressure transducer 48,
noid valve 50 connects the chamber 24 to the expiratory 60 is used as feedback. If the measured pressure in the
valve 52, shutting it off. During the expiratory phase of chamber 24 is different from the one desired as deter
operation of the piston 14, the solenoid valve 50 con mined by the summing amplifier 72, then an error signal
nects the expiratory valve 52 to a variable pressure in is generated in line 76. The error signal, after suitable
order to set the level of positive end-expiratory pressure amplification by amplifiers 78, 80, then controls the
(PEEP). The variable pressure is produced by control 65 output of a power amplifier82 which provides the con
ling the resistance of a needle valve 55 mounted into the trol signal in line 60 to the motor 16.
pipe 56 connecting the solenoid valve 50 to the blower When the pressure in the piston chamber 24 is mea
20. sured by the pressure transducer 48 as just described,
 5,107,830 18
17
the ventilator unit 10 can deliver pressure from the resulting signal 89 is used as a command signal in line 70
piston chamber 24 in proportion to inspired volume (as for the ventilator unit 10 to produce pressure in propor
just described) or inspired flow rate (with the flow rate tion to inspired flow (i.e. resistive assist). A gain control
used as the command signal). For the PAV operation, 90 permits the selection of the magnitude of the assist by
the ventilator unit delivers pressure from the piston in suitable amplification of the electrical signal corre
response to a combination of the instantaneous inspired sponding in magnitude to the detected instantaneous
volume and inspired flow rate, and in accordance with inspired flow rate. Clearly, other conventional methods
the desired proportional assist. of measuring the rate of flow from chamber to patient
This basis design also can be used to implement other would be equally suitable.
methods of assist as per prior art methods. For example, 10 (b) Inspired volume: The signal related to inspired
if a sine wave or a ramp voltage is used as the command flow (line 89) may be integrated in integrator 92 to
signal, the ventilator unit produces pressure in the cor provide a signal corresponding to inspired volume and
responding manner. As a result of this versatility, it is hence represents, at any given time, the instantaneous
possible to employ the unit 10 as a high frequency venti gas flow volume. When the resulting signal is routed to
lator alone or in combination with volume and flow 15 the summing amplifier 72 by line 70, the ventilator unit
assist. 10 develops pressure in proportion to inspired volume.
If inspired volume is used as a feedback, the unit also The magnitude of the assist obtained again may be con
delivers pressure in a pattern which causes inspired trolled by a gain device 96 by suitable amplification of
volume to follow the pattern of the command signal, the electrical signal corresponding in magnitude to the
thereby functioning as a volume ventilator. 20
detected instantaneous inspired flow volume.
In the electronic circuitry 22, the summing amplifier (c) Ramp generator: This mode of operation permits
72 also receives feedback from the velocity transducer the ventilator unit 10 to function independent of patient
57 by line 58. This additional feedback tends to prevent effort and provides a controlled ventilation. This func
rapid changes in pressure which otherwise may trigger tion can be activated by the operator by throwing
oscillations, as previously discussed with reference to 25 switch 98 to bring the function generator 100 into the
equation (2). The output of the velocity transducer 57 circuit. Alternatively, provision may be made for the
reflects the motion of the piston as air moves into the ramp generator to be routed automatically to the sun
subject as well as motion related to compression or ming amplifier 72 in the event of the failure of the pa
decompression of gas in the system. The former compo tient to breathe spontaneously for a specified period of
nent is a relatively slow event whereas the latter com 30 time (not shown). M
ponent incorporates high frequency components, which (d) D.C. output: An adjustable DC output provided
are desirable as feedback to prevent oscillations. For by an offset amplifier 101 also is routed to the summing
this reason, the signal in line 58 is passed through a high amplifier 72, to result in the generation of continuous
pass filter 84. The filter output in line 106 is used as a pressure.
velocity feedback in three arms. 35
The electronic circuitry 22 also includes cycling con
One arm of the net velocity signal in line 106 is passed trols (not shown) to take into account that air is ex
through a bidirectional peak clipping circuit 108, which changed between patient and ventilator unit 10 only
clips the signal above and below an adjustable level. during the inspiration phase of the respiratory cycle and
The signal so produced in line 110 is amplified and it is necessary to reset various controls in the "off"
passed to the summing amplifier 72. phase in preparation for the next cycle. These controls
A second arm of the net velocity signal in line 106 is also effect closure of the expiratory valve 52 during the
passed through a window clipping circuit 112, which pumping phase.
passes only that part of the signal above or below an During inspiration, air moves from the piston cham
adjustable level and is designed to abort massive oscilla ber 24 to the patient via tube 34. During expiration, air
tions in the event of failure of other feedback. The level 45
above or below which net velocity signal is passed is escapes by the expiratory valve 52 while the one-way
adjusted to be above the signal level associated with valve 36 prevents expired air from re-entering the
response to the control signal. chamber 24.
The third arm, in line 114, containing the entire net At the commencement of inspiration, the piston head
velocity signal, is passed through an adjustable attenua 50 14 begins to move forward in the chamber 24, either as
tor with the resulting signal passing by line 118 to the a result of the patient pulling in (assist mode) or as a
summing amplifier 72. With the two clipping circuits result of the piston pushing (in response to a ramp corn
108 and 112 in place, the gain on the main velocity mand in the controlled mode). This forward motion
signal in line 114 need only be small. then generates a flow signal which, when it exceeds a
Generally, switches permit the operator to select the 55 predetermined level, causes the expiratory valve 52 to
function or combination of functions to be channeled to close to ensure that pressure is conveyed to the patient.
the summing amplifier 72 and variable gain controls This result is achieved by summing the flow signal in
permit selection of the magnitude of the assist. The line 89 with a DC offset voltage in line 102 from offset
various functions available for the illustrated embodi amplifier 103 and passing the summed signal in line 104
ment are shown schematically in FIG. 10 and will now through a zero crossing detection circuit.
be described: The level of the offset voltage 102 may be varied and
(a) Inspired flow: When the high frequency compo usually is kept slightly above zero. Once the flow ex
nents of the output of the velocity transducer 57 in line ceeds this minimal level, a square voltage output is
58 are filtered out, the remaining signal agrees very well generated which activates valve 50, thereby connecting
with flow measured independently at the airway 34 and 65 the chamber 24 to the valve 52 and closing the latter.
hence represents, at any given time, the instantaneous Flow then continues into the patient as long as patient
gas flow rate. Accordingly, the velocity flow signal in effort plus pressure output by the ventilator unit 10
line 58 is passed through a low pass filter 88 and the exceed the elastic recoil of the respiratory system.
 5,107,830
19 20
When patient effort declines at the end of the patient's position of the piston 201 is also monitored using a
spontaneous inspiration, inspiratory flow ceases, potentiometer 211 or other appropriate device, as de
As the flow signal crosses the offset level on its return scribed above with respect to FIGS. 8 to 10.
to zero at the end of the inspiratory cycle, the valve 50 The instantaneous flow signal (i.e., an electrical signal
is inactivated, so releasing the pressure in the expiratory indicative of the instantaneous rate of flow of gas
valve 52 and allowing the patient to exhale. through the inhalation conduit 210) is conditioned
At the same time, a negative voltage is gated to the through an externally adjustable gain control 212. This
summing amplifier 72, which causes the piston head 14 is the equivalent of K2 in equation 3 above. The amplifi
to pull back allowing air to enter the chamber 24 cation may be constant or variable to allow for non-lin
through the one-way valve 36, if air is the desired gas. 10 ear behaviour of the pressure-flow relation of patient
If desired, the solenoid valve 40 may be opened at the and/or external tubing. The instantaneous flow signal is
same time to permit pressurized gas of a specific compo also integrated using an integrator 213 to provide a
sition to be admitted. The piston head 14 retracts until signal corresponding to instantaneous inhaled volume
the piston position, as indicated by the output of the 214 (i.e., an electrical signal indicative of the instanta
potentiometer 46, reaches a preset level. 15 neous volume of flow of gas through the inhalation
When this condition is reached, the negative voltage conduit 210). The latter is also conditioned through an
to the summing amplifier 72 is interrupted and the sole externally adjustable gain control 215. This is the equiv
noid valve 40 is closed. The active command signal is alent of K1 in equation 3 above and again may be con
the output of the offset amplifier. The pressure in the stant or variable to allow for non-linear elastic proper
piston chamber 24 remains at this level throughout the ties of the respiratory system. The amplified flow and
remainder of the expiratory phase and until the begin volume signals 216, 217 are summed using a summing
ning of the next inspiratory phase. The output of the amplifier 218, to generate a composite output signal.
offset amplifier is normally set just below its PEEP The summing amplifier may also receive other inputs
level, so that the patient need only exert minimal effort 219 to permit the unit to be operated in a non-propor
to get the piston head 14 moving at the beginning of 25 tonal assist mode. These additional inputs are described
inspiration. below.
The electronic circuitry 22 may include safety fea A switch mechanism 220 channels the output of the
tures powered by battery in the event of power failure. summing amplifier 218 to the motor 205 during the
For example, an alarm may be made to sound in the inhalation phase and the later part of the exhalation
event that there is a power failure, a failure of pressure 30 phase. During the early part of the exhalation, the
to cycle for a preset period, an excess pressure or a large switch 220 channels instead a constant negative voltage
inspired volume. 221 to return the piston 201 to a preset location, as
A variety of gauges, read-outs and recorders may be judged by the potentiometer signal 211. In either case,
provided to display, store or compilate a variety of the command signal 222 is first suitably amplified using
parameters, including breath by breath or moving aver 35 a power amplifier 223 and power supply 224.
ages, of tidal volume, ventilation frequency, inspiratory An exhalation tube 225 and exhalation valve 226
duration, duty cycle (inspiratory duration/cycle dura insure gas flow from patient to the ambient atmosphere
tion) and respiratory system compliance. during the exhalation phase. Any of a variety of con
Turning now to FIG. 11, there is illustrated therein mercially available exhalation valves can be used. The
another preferred embodiment of the invention, which 40 exhalation valve 226 is opened or closed according to its
is a much simplified version of the embodiment of FIG. valve control mechanism 228.
8, and currently is the best mode known to the applicant A differential pressure transducer 227 measures the
for carrying the invention (PAV) into effect. A low pressure gradient between a point upstream and a point
inertia, low resistance rolling seal piston 201 freely downstream from the one-way valve 204 that directs
moves within a chamber 202. The chamber 202 receives flow on the inhalation line 210. The downstream point
gas through a one-way valve with an appropriate open is preferably as close as possible to the patient to en
ing pressure level 203 during the exhalation phase hance the sensitivity of the transducer 227 to flow,
while, during the inhalation phase, gas moves from the while the upstream point may be conveniently located
chamber 202 to patient 208 through another one-way at the chamber 202. The inhalation phase is defined as
valve 204. The piston 201 is coupled to the coil of a 50 any time during which the pressure upstream is higher
linear drive motor 205, which itself moves freely within than the pressure at the point nearer the patient. The
a fixed magnet 206. The coil 205 pushes or pulls on the exhalation phase is when pressure on the patient side of
piston 201 in proportion to the magnitude of the drive the valve 204 is higher than in the chamber 202. The
potential supplied to the coil 205 via a cable 207. output of the differential transducer 227 is channeled to
Clearly, any other type of bellows-motor combination 55 the exhalation valve control mechanism 228 and to the
would be suitable, provided that the requirement is switching mechanism 220 for the motor drive 205. At
satisfied that there is permitted a free flow of gas to the transition from inhalation to exhalation, as defined by
patient under a gradient generated by the patient. This transducer 227 output, the integrator 213 is reset to
requirement can be met either through the inherent return the volume signal 214 to zero, in preparation for
mechanical properties (i.e. low inertia, low resistance) 60 the next cycle.
of the bellow-motor combinations or through appropri Other inputs 219 to the summing amplifier 218 may
ate servo-control of bellows position, as in prior art. include any of a variety of functions. Two preferred
The rate of flow of gas from the chamber 202 to the functions are a constant voltage, to provide continuous
patient 208 is measured by a flow meter 209 mounted on positive airway pressure (CPAP) if desired, and an
the inhalation conduit 210 which generates an instanta 65 input designed to cancel out the effect of inhalation tube
neous flow signal. Alternatively, the rate of gas flow resistance. In the latter case, the output of the differen
can be measured using a velocity transducer that moni tial transducer 227 is channeled to the summing ampli
tors the rate of piston movement (not shown). The fier 218 after suitable amplification and rectification
 5,107,830 22
21
(positive gradients from chamber to patient only). In 1. A method for providing breathing assistance in
this fashion, the piston generates a positive pressure proportion to patient ongoing inspiratory effort, which
component that equals the pressure gradient between comprises:
chamber and patient, thereby essentially eliminating the providing a free flow of gas from a gas delivery sys
resistive effect of the tubing at all flow rates. 5 tem to a patient in response to a pressure gradient
Other inputs 219 may also include back-up functions, generated by patient inspiratory effort,
for example repetitive ramp or square wave voltage vs determining the rate and volume of flow of said gas to
time functions to insure ventilation in the event of ap said patient,
nea. Alternatively, through these other inputs, manufac independently amplifying signals corresponding to
O said determined rate and volume of flow, and
turers may wish to add other types of conventional
ventilation methods in machines that deliver PAV, so providing a pressure assist to said gas in proportion to
that these additional methods, for example, airway pres the sum of said determined and amplified rate and
sure release ventilation (APRV) or synchronized inter volume of flow.
mittent mandatory ventilation (SIMV) or pressure sup 15
2. The method of claim 1, wherein said pressure assist
port (PS), may be used jointly or alternately with PAV. is determined by the equation:
At the beginning of the exhalation phase, the piston
201 retracts under the influence of the negative voltage
221, acquiring fresh gas of a desired composition where Pent is the magnitude of the pressure assist, K1 is
through the one-way valve 203 until it reaches a preset 20 again factor applied to a variable ongoing volume sig
level. The switching mechanism 220 then disconnects nal V and K2 is a gain factor applied to a variable ongo
the negative voltage and channels the output of the ing flow-signal V.
summing amplifier 218 to the motor 205. At this point, 3. The method of claim 2 wherein non-linear func
the conditioned flow signal 216 is zero and the condi tions are employed in place of K1 and K2.
tioned volume signal 217 is also zero as a result of inte 25 4. The method of claim 1 wherein said pressure assist
grator 213 resetting. The only drive is from other inputs is determined by the equation:
219. In the event a constant voltage is applied via other
inputs 219, the motor 205 will pressurize the piston 201 Penr=A-Pinus
to a constant level in preparation for the next inhalation.
The level of constant pressure is typically set to be just 30 where Pent is the magnitude of the pressure assist, A is
below the level of desired positive end expiratory pres an independent factor which determines the proportion
sure (PEEP), as determined by the exhalation valve ally between the pressure assist and patient generated
control or other suitable PEEP device. In this way, pressure, and Pinus is the estimated instantaneous patient
flow begins from chambér to patient as soon as airway generated pressure determined by the equation:
pressure decreases below the PEEP level. Once flow 35
begins, the output of the summing amplifier changes Pinus=WE - VR-Pent
according to the sum of the conditioned flow 216 and
volume 217 signals and the constant voltage, if any, via where V is the magnitude of the variable ongoing vol
other inputs 219. This causes the pressure in the cham ume signal, V is the magnitude of the variable ongoing
ber 202 to rise. As discussed in detail above, so long as flow signal, Esis the elastance of the respiratory system
the gain factors for flow 212 and volume 215 are less of the patient and Rs is the resistance against which the
than the values of resistance and patient elastance, the respiratory system of the patient is operating.
chamber pressure is not adequate by itself to entirely 5. The method of claim 1 wherein said rate and vol
support the elastic recoil and resistive pressure losses ume of flow are determined by continuously sensing the
and the patient is, therefore, contributing to the total 45 movement of gas to the patient and generating an elec
pressure, and flow and volume remain responsive to trical signal corresponding in magnitude to each of the
patient effort. Once the patient terminates his own respi sensed rate and volume of gas, continuously separately
ratory effort, there is accordingly not enough pressure amplifying each signal to a degree required to provide
to sustain the elastic recoil of the respiratory system. 50 the pressure assist and providing a summed signal com
Pressure in the airway rises above chamber pressure and bining each of said amplified signals, and continuously
this, via the differential pressure transducer 227, causes applying said summed signal to said gas delivery sys
the opening of the exhalation valve 226, resetting of the tem, thereby to provide said pressure assist.
integrator 223 and hence loss of chamber pressure and 6. The method of claim 5 wherein said movement of
the cycle is repeated. 55
gas is sensed to generate an electrical signal correspond
Apart from its simplicity and ease of operation, this ing to the rate of flow of gas through a tube connecting
design has the distinct advantage of making it unneces said gas delivery system to the patient, and said electri
sary to utilize servo-feedback of pressure output, as, is cal signal is integrated to provide a further electrical
necessary in the embodiment of FIGS. 8 to 10. signal corresponding to the volume of flow through
said tube.
SUMMARY OF DISCLOSURE 7. The method of claim 6 wherein said gas delivery
In summary of this disclosure, the present invention system comprises bellows means and an electrical
provides a novel ventilation unit and ventilating proce motor operatively connected to said bellows means to
dure which are able to deliver air to a patient in propor generate a pressure corresponding in magnitude to the
tion to patient ongoing inspiratory effort (Proportinoal 65 magnitude of the summed signal applied to said electri
Assist Ventilation, PAV). Modifications are possible cal motor.
within the scope of this invention. 8. The method of claim 1 wherein said gas delivery
What I claim is: system develops negative pressure to assist patient's
 5, 107,830
23 24
breathing through the application of negative pressure 12. The apparatus of claim 11 wherein said bellows
to a body surface of the patient. means comprises a rolling seal piston.
9. The method of claim 1 when used in conjunction 13. The apparatus of claim 10, wherein said detection
with a method of ventilatory assist employing a prede means comprises flow rate sensing means operatively
termined relationship of pressure, flow and/or volume 5 connected to a pipe joining said bellows means to a
versus time, including continuous positive airway pres patient for generating an electrical signal indicative of
sure (CPAP), intermittent mandatory ventilation the instantaneous rate of flow of gas through said pipe
(IMV), pressure support ventilation (PSV), and airway and electrical circuit means for generating an electrical
pressure release ventilation (APRV). signal indicative of volume flow through said pipe from
10. Apparatus for delivering proportional assist venti O said electrical signal indicative of gas flow rate as said
lation to a patient, comprising: detected instantaneous value of flow volume.
means for delivering a free flow of gas to said patient 14. The apparatus of claim 13, wherein said means for
in response to patient inhalatory effort, generating an electrical command signal comprises
means operatively connected to said gas delivery 5 summing means for summing amplified electrical sig
means for generating pressure in said free flow of nals of said volume and gas flow rates, and including
gas in response to an electrical command signal, means for applying said command signal to said electri
detection means for detecting the instantaneous vol cal motor means, thereby to provide a ventilatory assist
to said patient corresponding to the magnitude of said
ume and rate of gas flow to said patient and for summed signal.
generating a separate electrical signal correspond 20 15. The apparatus of claim 10 adapted to deliver
ing in magnitude to each of said detected values, ventilatory assist in the form of negative pressure in
means for selectively applying amplification to each tended for application to a body surface.
of said electrical signals, and 16. The apparatus of claim 10, including electrical
means for generating said electrical command signal circuit means to override said means for generating said
to said pressure generating means in proportion to 25 electrical command signal to provide an alternative
the sum of said amplified electrical signals corre command signal corresponding to an operator predeter
sponding in magnitude to said instantaneous rate mined relationship of pressure, flow and/or volume
and volume of flow. versus time, including continuous positive airway pres
11. The apparatus of claim 10 wherein said gas deliv sure (CPAP), intermittent mandatory ventilation
ery means comprises bellows means and said gas pres 30 (IMV), pressure support ventilation (PSV), and airway
sure generating means comprises an electrical drive pressure release ventilation (APRV).
motor operatively connected to said bellows means. k k

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