Review series

Ventilator modes and settings during non-invasive

Thorax: first published as 10.1136/thx.2010.142661 on 14 October 2010. Downloaded from http://thorax.bmj.com/ on May 23, 2020 by guest. Protected by copyright.
ventilation: effects on respiratory events and
implications for their identification
Claudio Rabec,1 Daniel Rodenstein,2 Patrick Leger,3 Sylvie Rouault,4
Christophe Perrin,5 Jésus Gonzalez-Bermejo,6 on behalf of the SomnoNIV group
1
Service de Pneumologie et ABSTRACT is generally applied at night, nocturnal monitoring
Réanimation Respiratoire, Compared with invasive ventilation, non-invasive seems the best way to assess its effects. Although
Centre Hospitalier et
ventilation (NIV) has two unique characteristics: the non- nocturnal monitoring of continuous positive
Universitaire de Dijon, Dijon,
France hermetic nature of the system and the fact that the airway pressure (CPAP) has been codified in the
2
Service de Pneumologie, ventilator-lung assembly cannot be considered as treatment of patients with obstructive sleep
Cliniques Universitaires Saint a single-compartment model because of the presence of apnoea syndrome,6 this is not the case with NIV.7 8
Luc, Université Catholique de variable resistance represented by the upper airway. Nocturnal monitoring of NIV is far more difficult
Louvain, Bruxelles, Belgium
3
Service de Pneumologie, When NIV is initiated, the ventilator settings are and unforeseen problems arise for many reasons:
Centre Hospitalier Lyon Sud, determined empirically based on a clinical evaluation and (1) sleep can induce profound ventilatory changes,
Lyon France diurnal blood gas variations. However, NIV is in particular in patients with respiratory insuffi-
4
ADEP Assistance, Paris, France predominantly applied during sleep. Consequently, to ciency; (2) these ventilatory changes are also exac-
5
Service de Pneumologie, erbated by the interactions with a preset
Centre Hospitalier de Cannes, assess overnight patientemachine ‘agreement’ and
Cannes, France efficacy of ventilation, more specific and sophisticated devicedthe ventilator can induce sleep distur-
6
Service de Pneumologie et monitoring is needed. The effectiveness of NIV might bances by itself; (3) physicians caring for these
Réanimation Respiratoire, therefore be more correctly assessed by sleep studies patients may vary greatly in their methods of
Hôpital de la Pitié-Salpétriere than by daytime assessment. The most available and assessing the effects of NIV from a single blood gas
and UPMC ER10, Paris, France
simple monitoring can be done from flow and pressure measurement to full polysomnography.
Correspondence to curves from the mask or the ventilator circuit. The issue is complicated further because volume
Claudio Rabec, Service de Examination of these tracings can give useful information and pressure preset ventilators will respond differ-
Pneumologie et Réanimation to evaluate if the settings chosen by the operator were ently in certain situations and because the type of
Respiratoire, Centre Hospitalier mask used will also have different effects on
et Universitaire de Dijon, 2 Bd
the right ones for that patient. However, as NIV allows
Maréchal de Lattre de Tassigny, a large range of ventilatory parameters and settings, it is monitoring parameters.9 The semiology of
21079 Dijon, France; mandatory to have information about this to better abnormal respiratory events during sleep is there-
claudio.rabec@chu-dijon.fr understand patienteventilator interaction. Ventilatory fore completely different depending on the tech-
modality, mode of triggering, pressurisation slope, use or nology used. Pressure and flow dynamics also differ
Received 14 May 2010
not of positive end expiratory pressure and type of markedly among the different type of ventilators
Accepted 10 August 2010
Published Online First exhalation as well as ventilator performances may available for home care.
14 October 2010 all have physiological consequences. Leaks and upper This article will deal with the equipment avail-
airway resistance variations may, in turn, modify able for NIV, in particular the ventilator types,
these patterns. This article discusses the equipment modes and settings. Its goal is to give the back-
available for NIV, analyses the effect of different ground necessary to understand their impact on
ventilator modes and settings and of exhalation and nocturnal monitoring of NIV.
connecting circuits on ventilatory traces and gives the
background necessary to understand their impact on
nocturnal monitoring of NIV. ISSUES OF PARTICULAR IMPORTANCE DURING
NIV: LEAKS, UPPER AIRWAY RESISTANCE, TYPE
OF EXHALATION PORT AND NIV VENTILATORS
Compared with invasive ventilation, NIV has two
Since the first studies in the early 1980s showing unique characteristics: the non-hermetic nature of
the usefulness of non-invasive ventilation (NIV) in the system that poses the potential risk of unin-
the management of some forms of respiratory tentional leaks and the fact that the ventilator-lung
failure,1e4 the number of patients receiving this assembly cannot be considered as a single-
treatment both in the acute setting and at home is compartment model because of the presence of
continuously increasing. This is explained by a variable resistance represented by the upper
a growing number of indications in which the airway (UA). Both situations may compromise the
effectiveness of NIV has been proven, but also by delivery of an effective tidal volume. As a conse-
major technological advances that led to the avail- quence, increasing the delivered volume or the
ability of high-performance portable ventilators as delivered inspiratory pressure during NIV does not
well as to the development of technical support necessarily result in increased effective ventilation
infrastructure.5 reaching the lungs.10 When monitoring NIV using
When starting NIV, it seems desirable to verify flow or pressure tracings, these two peculiarities
whether this particular form of treatment is effec- will unavoidably influence the features of the
tive and has no untoward effects. Since home NIV respiratory traces.

170 Thorax 2011;66:170e178. doi:10.1136/thx.2010.142661

 Review series

Influence of unintentional leaks will have similar features whatever the type of positive pressure
Unintentional leaks are very common in NIV.11 12 Leakage may device used for ventilation.

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be absent or minimal when the patient is awake but may
worsen during sleep as a result of the loss of voluntary control
VENTILATOR MODES AND SETTINGS: WHAT’S IN A NAME?
and decreased muscle tone. Leaks can take place at the mouth or
When NIV was introduced there were a very limited number of
between the skin and the mask, and air could also be deposited
modalities and types of ventilators with very few possible
in the oropharyngeal reservoir and even flow into the digestive
settings. We now have more than 30 brands offering numerous
tract (‘internal’ leaks).13
options for settings.22 Moreover, ventilators are not submitted
Leaks can impair the quality of both ventilation and sleep.
to stringent medical regulations. This leaves manufacturers free
They can largely affect ventilator triggering, pressurisation,
to give different names to the same ventilator modalities and
volume delivered, rate of inspiratory pressuring and cycling
settings and even to ‘create’ new modalities that frequently
function and induce sleep fragmentation.
correspond only to small modifications of a known class. This
explains the wide variety of existing terminology describing NIV
Influence of the upper airway modalities.
During NIV a variable resistance constituted by the UA is
interposed between the ventilator and the lungs. The UA may Influence of the ventilator mode
change its resistance to airflow, compromising the delivery of an Theoretically, NIV could be delivered using all the modalities
effective tidal volume to the lungs. Intermittent obstruction of used for invasive ventilation but, in real life, most ventilators
the UA is common during NIV and may be related to two used for NIV deliver either volume or pressure-targeted venti-
mechanisms. The first corresponds to obstructive events at the lation. Because the place of other anecdotal modalities proposed
oropharyngeal level because of UA collapse as a result of by some NIV devices such as synchronised intermittent
insufficient expiratory airway pressure. This mechanism may be mandatory ventilation or other ‘hybrid’ modalities is not yet
present in patients with an unstable UA.14 Another mechanism clear, this paper will mainly focus on the former two modalities.
corresponds to episodes of intermittent obstruction at
the glottic level reflecting cyclic glottic closure induced by
Volume-targeted ventilation (VTV)
hyperventilation, a type of ‘ventilation resistance’ reaction.15e18
In this modality the ventilator delivers a fixed volume during
a given time and will generate whatever pressure is necessary to
Influence of type of exhalation device and connecting circuits achieve this, regardless of the patient contribution to ventila-
Whereas ICU ventilators classically use a double circuit with an tion. Pressure in the airways (Paw) is not constant and results
integrated expiratory valve, two different types of circuits can be from the interaction between ventilator settings, compliance
used to provide NIV. The first uses a similar assembly to those and resistance of the respiratory system and spontaneous
used in ICU devices and includes either single or double tubing inspiratory efforts (figure 2). It should be emphasised that any
in which inspiration and expiration are separated and a true additional inspiratory effort will not lead to changes in delivered
expiratory valve is present so that carbon dioxide (CO2) volumes or flows but only result in a decrease in Paw. As each
rebreathing is not a significant problem (figure 1A). A second breath is delivered with the same predetermined flow-time
type does not have a true exhalation valve and often uses profile and as the area under the flow-time curve defines the
a single-limb circuit with a risk of rebreathing. To avoid volume, the advantage of this modality is the strict delivery, in
rebreathing, this system includes a calibrated leak (called an the absence of leaks, of the preset volume, whatever the values
intentional leak) either at the mask level or in the circuit (figure of compliance (C) and resistance (R). A major disadvantage of
1B). Single circuit pressure-targeted ventilators provided with VTV is precisely that delivery of this fixed ventilatory assistance
a calibrated leak (called bilevel ventilators) are most commonly does not allow account to be taken of patients’ varying
used for NIV nowadays. These devices cycle between a higher requirements. Another inconvenience is that, if there is a leak,
inspiratory positive airway pressure (IPAP) and a lower expira- there will be no increase in flow rate to compensate for it and
tory positive airway pressure (EPAP) that can be independently the generated pressure will be lower, so that the effectively
adjusted. With these devices, a minimum mandatory EPAP level delivered volume will be reduced in proportion.
of 4 cm H2O is needed to ensure an effective washout of CO2.20
Interestingly, a recent study showed that the exhalation port Pressure-targeted ventilation (PTV)
position influences CO2 rebreathing with a more efficient CO2 In this modality the ventilator is set to deliver airflow by
washout when the leak is positioned within the mask.21 generating a predefined positive pressure in the airways for
When monitoring NIV using polygraphy, the mode of exha- a given time. Airflow is therefore adjusted in order to establish
lation used and the position of the flow sensor (in relation to the and maintain a constant Paw. Constant analysis of the flow rate
expiratory device) both have a major influence on features of and airway pressure determines the flow variations necessary to
respiratory traces. Typical traces of these different assemblies maintain a flat or ‘square wave’ pressure. Flow is brisk at the
can be seen in figure 1A and B. beginning of inspiration when the gradient between the circuit
pressure and the pressure target is large. As this gradient narrows
Influence of the ventilator: ICU vs home devices the flow decelerates until driving pressure no longer exists and
Both ICU and home ventilators can be used to deliver NIV. The flow ceases (figure 2).
main technical characteristic differentiating them is that, for the Thus, for a given patient, the volume delivered is not fixed and
former, the driving pressure is supplied by compressed gas, will depend on the interaction between the preset pressure,
whereas the latter devices incorporate their own pressure source. patient inspiratory effort, the physical characteristics of the
Nevertheless, as the type of ventilatory support that they respiratory system (C and R) and inspiratory time. An impor-
provide is similar, this difference has little influence on respira- tant advantage of PTV is the ability to compensate for mild to
tory traces. Therefore, when monitoring NIV, respiratory traces moderate leaks.

Thorax 2011;66:170e178. doi:10.1136/thx.2010.142661 171

 Review series

Figure 1 Type of circuits used in

non-invasive ventilation (NIV) with (A)

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an expiratory valve and (B) intentional
leak, and typical traces obtained with
these different assemblies. Note that
when a circuit with an expiratory valve
is used, the expiratory valve may be
interposed in the circuit (single circuit)
(A) or included in the ventilator (double
circuit) (B); when a single circuit with
intentional leak is used, the leak may be
interposed in the circuit (C) or
incorporated at the mask (D). Flow
traces can be influenced by the type of
circuit used, but also by the position of
the pneumotachograph with regard to
the expiratory device. In the case of
a double circuit provided by an
expiratory valve, the expiratory slope
will reflect the expired volume only
when the flow sensor is interposed
between the mask and the exhalation
device (trace 2 in A), but not when it is
placed distally to the expiratory valve
(trace 1 in A). On the other hand, when
using a single circuit with intentional
leak, the expiratory slope does not
reflect the expired tidal volume and may
be absent (B). In this case, the position
of the pneumotachograph with regard to
the leak device does not significantly
change the expiratory slope. Modified
from Perrin et al19 with permission.

A comparative analysis of the two modalities and of corre- reached, the breath converts to a flow-cycled modality by
sponding flow and pressure patterns is summarised in table 1. prolonging the inspiratory time), but most of them progressively
adjust the pressure level during several cycles to provide a tidal
Volume-targeted pressure ventilation volume as close as possible to the target volume set by the
A limitation of pressure ventilation is that it cannot guarantee clinician.
a tidal volume delivered to the patient. Volume targeting is Whether this feature improves the effectiveness of ventilation
a feature available in some new ventilators that could allow this is not yet clear.23 24 Different algorithms used to provide
limitation to be overcome. This hybrid modality combines volume-targeted pressure are shown in figure 3.
features of pressure and volume ventilation. The ventilator
estimates the delivered tidal volume and adjusts its parameters Recent ventilator modes: matching physiology and practice?
to ensure a predetermined target tidal volume. Some ventilators Conventional ventilation delivers fixed pressure or volumes
adjust a target volume within each cycle (each breath starts as rates. Some recent modes use a more physiological delivery of
a pressure-limited breath and if the set tidal volume is not assistance following the spontaneous inspiratory signal coming

172 Thorax 2011;66:170e178. doi:10.1136/thx.2010.142661

 Review series

Table 1 Comparison between pressure and volume-targeted

ventilators

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Volume-targeted Pressure-targeted
Pressure curve pattern

Flow curve pattern

Type of ventilatory Fixed volume in spite of Fixed pressure. Tidal

assistance delivered changing resistance volume may vary with
(R) and compliance (C) changes in C and R
Controlled variable Maintains a constant Maintains a constant
inspiratory preset flow inspiratory preset
pressure
Breath-to-breath Not possible: ventilator Possible: flow and
adjustments delivers a fixed assistance volume can be varied
in a breath-to-breath
basis
Possibility to guarantee Yes (if no leaks) No
a fixed delivered tidal
volume
Peak airway pressure Not limited* Limited (useful in
patients at risk of
barotrauma
or gastric distension
Leak compensation Poor, leaks may Good for mild to
significantly reduce moderate leaks
delivered volume and
induce hypoventilation
*Except if pressure reaches predetermined high pressure security limit.

from the patient. Hence, they are supposed to improve patient

ventilator synchronism.
One of these modes, proportional assisted ventilation (PAV),
relies on the inspiratory flow of the spontaneously breathing
pattern and adds a fixed proportional assistance. If the patient’s
flow increases the assistance increases proportionally. The degree
of assistance is determined by the practitioner based on esti-
mations of the patient’s resistance and compliance.25 Even if
PAV proves to work according to theoretical assumptions, clin-
ical studies using this mode both in invasive ventilation and NIV
failed to show any clear advantages of this mode over
others.26e29
The other mode, called neurally-adjusted ventilatory assist
(NAVA), relies on the EMG of the diaphragm recorded from
oesophageal electrodes. Ventilatory assistance is therefore not
influenced by factors such as airway resistance, elastance,
intrinsic positive end expiratory pressure (PEEP) or air leaks.30
This modality, available only in critical care ventilators, was
tested only in patients under invasive ventilation. As there are
very few data on NAVA application, its clinical impact cannot be
assessed at present.

Influence of ventilator cycling: who controls the ventilator and

Figure 2 Flow and mask pressure during (A) spontaneous breathing,
who controls ventilation?
(B) volume-targeted ventilation, (C) pressure-targeted ventilation. 1, Appropriate settings are decisive to obtain optimal patient-
controlled cycle; 2, assisted cycle. In (B) and (C), dashed lines represent ventilator synchrony.8 When delivering mechanical ventilation
spontaneous breathing (SB) traces from (A). Paw, pressure in the there are two ventilatory pumps acting together: the ventilator
airways; PEEP, positive end expiratory pressure. on the one hand and the patient’s own respiratory muscles on the
other. These two pumps may work in harmony but, in fact, they
can interact in any number of ways, many of which will create
problems rather than solving them. Hence, patient-ventilator

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 Review series

Figure 3 Different algorithms of

volume-targeted pressure. Pressure,

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volume and flow recording traces using
(A) an algorithm that progressively
adjusts the pressure support level over
several cycles between a preset range
of pressure to achieve a target tidal
volume (Vt) and (B) an algorithm that
adjust a target volume intracycle. Note
that, in this case, each breath starts as
a pressure-limited breath and if the set
Vt is not reached, the breath converts to
a flow-cycled mode by prolonging the
inspiratory time. In both cases the
ventilator is able to estimate the cycle to
cycle Vt effectively delivered to the
patient and to adjust the ventilatory cycle to ensure a target Vt. Press, inspiratory pressure. Reproduced with permission of A Cuvelier.

asynchrony is quite common in patients during NIV.31 32 ventilator is triggered by the patient. The pressure is maintained
Asynchronies may occur at two levels: during inspiratory trig- as long as a minimal preset inspiratory flow is occurring. End of
gering in situations in which there is a mismatch between inspiratory assistance (eg, cycling from inspiration to expiration)
patient inspiratory effort and ventilator triggering (ie, ineffective occurs when inspiratory flow reaches a predetermined
inspiratory effort, double triggering or autotriggering); or during percentage of peak inspiratory flow. In this mode, a targeted
cycling from inspiration to expiration when ventilator cycling inspiratory pressure, inspiratory trigger sensitivity and
does not coincide with the end of patient effort (ie, premature or a percentage threshold of peak flow for cycling to expiration (see
delayed cycling).32 below) must be selected. In some ventilators these can all be set
Thus, the most logical approach to explain how a ventilator by the clinician whereas in others only the inspiratory pressure
acts is to analyse the different phases of a typical positive can be set. Because each cycle is terminated by a flow criterion
pressure ventilatory cycle (figure 4). rather than by volume or time, the patient retains control of
We will start this section by reviewing the triggering and cycle length as well as its depth and flow profile This mode is
cycling modes which refers, respectively, to how inspiration and also called ‘pressure support ventilation’ (PSV) (figure 5A).
expiration start and stop.
Assist mode (A)
In this mode the patient controls the onset of inspiration but the
How is the onset of ventilator inspiration determined? inspiratory length is regulated by the operator. The clinician
The patient can control the initiation (eg, triggering) and the must select a targeted volume or pressure, an inspiratory:expi-
end of inspiration (eg, cycling into expiration) or, on the ratory (I:E) ratio or an inspiratory time and an inspiratory trigger
contrary, neither of them, and the ventilator controls the initi- sensitivity (figure 5B).
ation and the end of inspiration.
Assist-control mode (A/C)
Spontaneous mode (S) This mode operates as an assist one but also allows selecting
In this mode, only available in PTV, the patient controls the a back-up respiratory rate (RR). If the patient’s spontaneous
beginning and end of inspiration. Inspiration starts when the frequency is lower than the preset ventilator back-up RR, the
system moves to control mode. Therefore, this mode allows the
patient to trigger the ventilator but also grants a minimum
back-up rate. In this mode the clinician must select the same
settings as in assist mode but also add a back-up rate.
Control mode (C)
In the control mode there is a preset automatic cycle based on
time. The ventilator controls the beginning and end of inspira-
tion and thus the RR. One therefore expects the ventilator to
capture and inhibit the patient’s respiratory centre and for the
patient to follow the settings imposed by the ventilator. In this
mode the clinician must select a targeted volume or pressure,
a fixed RR and an I:E ratio or inspiratory and expiratory dura-
tions. With this mode, the entire work of breathing is supposed
to be performed by the ventilator. In some ventilators this mode
is also called ‘Timed (T) mode’ and is rarely used (figure 5C).
A particular combination of these modes is available in some
NIV ventilators. This mode, called S/T, is basically a PSV that
provides a back-up rate. In this particular mode, cycling from
inspiration to expiration is flow-limited in patient-triggered
Figure 4 The ventilatory cycle as seen from a mask pressure-time cycles and switches to time-limited when the patient’s sponta-
tracing. I to E, inspiratory to expiratory; PEEP, positive end expiratory neous RR falls below the back-up RR, and also when the
pressure. inspiratory time exceeds a predetermined maximal length during

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 Review series

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Figure 5 Different modes of inspiratory and expiratory triggering. (A) Spontaneous mode: the patient controls the beginning and end of inspiration.
Inspiration starts when the ventilator is triggered by the patient and cycling from inspiration into expiration occurs when the inspiratory flow reaches
a predetermined percentage of peak inspiratory flow. This mode is also called ‘pressure support’ and, in some ventilators, ‘spontaneous (S) mode’. (B)
Assisted mode: the patient controls the onset of inspiration but the end of inspiration is time triggered. When a backup respiratory rate (RR) is preset,
this mode is called assist/control. In this last mode, if the patient’s RR is lower than the preset ventilator backup RR, the system moves to control
mode. (C) Control mode: there is a preset automatic cycle based on time. The ventilator controls the beginning and end of inspiration and thus the RR.
In some ventilators this mode is also called ‘timed (T) mode’. Paw, pressure in the airways.

S cycles (see below). A patient-triggered cycle can be seen in EPAP level below the trigger threshold (when using pressure
curves of ventilation as a negative inspiratory deflection in triggering), both situations being able to lead to autotriggering
pressure and flow curves (see trace 2 in figures 2B and C). (figure 6).
The newer technologies (microprocessors, servo valves and
Type of trigger fast blowers) have substantially improved trigger responses.
As described above, in the A and A/C modes the ventilator has Moreover, adjustable inspiratory trigger is an option presently
to recognise the patient’s inspiratory effort. This is called trig- available in most home ventilators. Some of these also propose
gering function. Classically, NIV devices have two types of automated complex trigger algorithms (‘flow waveform method
triggers. The first, called ‘pressure-based’ trigger, present in old of triggering’) in which the flow-time waveform is used to
ventilators, is based on a drop in proximal airway pressure and trigger the ventilator. The respective advantages of these
requires a closed circuit. The second, called ‘flow-based’ trigger, sophisticated trigger systems have not been assessed in rigorous
present in almost all recent ventilators, is based on detection of studies. It must be emphasised that some adjustable trigger
an inspiratory flow in the presence of continuous flow washing devices are graduated by arbitrary units which makes them
out the circuit during expiration. Asynchrony during inspiratory difficult to use.
triggering is quite common during sleep in patients during NIV
and may compromise ventilatory efficacy and sleep quality. It is Pressurisation rate
mainly influenced by the delay duration (that can vary between As correct pressurisation is essential to decrease inspiratory
different ventilators33), the trigger sensitivity and the amount of effort and improve synchronisation, during this phase inspira-
inspiratory effort (which depends itself on respiratory drive and tory flow should be sufficient to match inspiratory demand.36
muscle strength).31 Circumstances influencing the rate of pressurisation are the level
Ventilators that use flow triggering generally have the shorter of ventilatory support, the amount of time required to reach the
trigger delays.34 35 Leaks may greatly affect trigger function, pressure target (pressurisation slope, also called ‘rise time’), the
either by preventing the detection of patient inspiratory effort compliance and resistance of the respiratory system and the
(leading to ineffective inspiratory effort) or by mimicking an patient inspiratory effort. Studies comparing different ventila-
‘inspiratory flow’ (when using flow triggering) or dragging the tors also emphasise the influence of the type of device on

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 Review series

Figure 6 Example of autotriggering.

Autotriggering is defined as the

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occurrence of at least three consecutive
pressurisations at a ventilator frequency
of >40/min not synchronised with
patient respiration.43

pressurisation, in particular in situations of high inspiratory emphasised that, even if recent ventilators have a good capa-
demand.33 bility to compensate mild to moderate leaks, greater leaks may
A faster rise time has been shown to unload respiratory compromise the ability of the device to attain a desired level of
muscles more completely.36 As the slope becomes flatter, the inspiratory pressure.
machine delivers lower flow rates and the patient’s work of
breathing increases.36 On the other hand, it must be emphasised Cycling from inspiration to expiration
that, if slow pressurisation could increase inspiratory work, an Switching from inspiration to expiration can be time-cycled or
excessive peak flow could also have adverse effects as it may flow-cycled. In time-cycled modes, ventilators use time criteria
increase the sensation of dyspnoea,37 induce double triggering32 chosen by the clinician (figure 4B). In flow-cycled modes, cycling
and lead to high peak mask pressure, favouring leaks. occurs as inspiratory flow decreases to a predetermined
percentage of the peak inspiratory flow which is supposed to
Ability to sustain the inspiratory plateau indicate the end of inspiratory effort (figure 4A).
The inspiratory pressure level is one of the main determinants of The criteria used to end inspiration may have a clinically
the efficaciousness of NIV. Determination of the optimal level is relevant impact on the quality of ventilation. Ideally, cycling
the result of balancing two opposing aims: the desire to provide should coincide with the end of patient effort. However,
effective minute ventilation and the need to minimise leaks and synchronisation between the end of neural inspiration and
discomfort caused by excessive inspiratory pressure. It must be ventilator cycling into expiration is mainly determined by

Figure 7 Impact of leaks on inspiratory to expiratory (I to E) cycling during pressure support ventilation (S in bilevel devices) mode (A) without leaks
and (B) with leaks. Note that during leaks the flow increases to compensate for them, prolonging the inspiratory time (dashed lines) and switching to
time-limited if maximal inspiratory time (Timax) is available. Interestingly, Timax is adjustable in most recent non-invasive ventilation devices. Paw,
pressure in the airways.

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 Review series

respiratory mechanics, moving from a premature cycling in and back, the answer to three basic questions may be used as
restrictive diseases to a late cycling in obstructive ones.33 38 a guide to simplify terminology:

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Moreover, when flow cycling is used, leaks may also delay 1. What is the controlled variable (pressure or volume)?
switching into expiration because, in an attempt to maintain 2. What causes the start of ‘ventilator inspiration’? It can be
pressure, the flow rate is maintained above the level at which either the patient (triggering) or the machine.
cycling into expiration occurs (figure 7). Both these conditions 3. What determines the end of inspiration? It is either based on
may lead to patient-ventilator expiratory asynchrony.38 time (time-cycled) or determined by the patient (flow-cycled).40
In older ventilators cycling into expiration was fixed at 25% of Table 2 summarises the technical characteristics of NIV
peak flow, but more recent ventilators offer adjustable values. ventilators and their influence on monitoring tracings.
This may allow tailoring settings to the patient’s underlying
condition. For instance, Tassaux et al demonstrated in patients IMPACT OF VENTILATOR MODE, SETTINGS AND FUNCTION ON
with chronic obstructive pulmonary disease during invasive MONITORING DURING NIV
ventilation that increasing cycling from inspiration to expiration Using NIV, being connected to a ventilator is not synonymous to
from 10% to 70% of peak flow (this means shortening inspira- being ventilated. It is therefore essential to assess NIV efficacy. A
tion to allow a greater expiratory time) was associated with first step is to perform a clinical assessment with the patient
a marked reduction in delayed cycling and intrinsic PEEP.39 awake. However, as NIV is generally applied for several hours
Finally, additional mechanisms proposed by some ventilators during sleep, more specific and sophisticated monitoring is
control the end of inspiration to prevent undesirable prolonga- needed to assess overnight patient-machine ‘agreement’ and
tion of inspiratory time. Sudden increases in pressure (that can efficacy of ventilation.
be assumed as secondary to an active expiratory effort) produce Table 2 Summary of technical characteristics of NIV ventilators and
early cycling to expiration in almost all the devices. Another their influence on traces semiology
mechanism is to limit maximal inspiratory time. This maximal Normal semiological aspect
inspiratory time acts as a safety feature to prevent unsuitable
lengthening of inspiratory duration and is highly variable in Ventilatory modality
different ventilators. Moreover, it may be adjustable for some Volume target Squared flow
Increasing pressure
devices (figure 7). Pressure target Squared pressure
Decreasing flow
PEEP level Hybrids Variable (depends on modality of volume
targeting)
In patients with obstructive disease, intrinsic positive end expi-
ratory pressure (iPEEP) can reduce the effective trigger threshold
Inspiratory trigger function
leading to a significant delay between the onset of patient
Spontaneous Negative deflection in pressure and flow
inspiratory effort and ventilator triggering or even to ineffective curves
efforts. In these cases, providing an external PEEP may counter- Assisted Negative deflection in pressure and flow
balance iPEEP, improving patient-ventilator synchrony. PEEP is curves
an above atmospheric (positive) pressure applied during expira- Controlled e
tion. It is called EPAP in some ventilators. If PEEP is used, the Assisted/controlled Negative deflection in pressure and flow
curves (only in a patient-triggered cycle)
pressure will never come back to zero. Indeed, IPAP results from
Pressurisation
pressure support plus PEEP. As a result and with regard to the
Rise time
ventilator category (ICU or home ventilator), the PEEP setting
Fixed Inspiratory slope until reaching the
may interfere with either pressure support or IPAP levels. In fact, pressure target
Adjustable
ICU ventilators propose PEEP and pressure support settings, but
PEEP and IPAP settings are usually associated on home ventila- Ability to sustain inspiratory plateau Maintain constant pressure and/or flow
tors. Thus, the PEEP setting increases the IPAP level on ICU level
ventilators and decreases the pressure support level on home
ventilators. Providing an external PEEP during NIV has many I to E cycling
other theoretical advantages: flushing CO2 from the deadspace, Time cycled Fixed inspiratory time
preventing rebreathing within the mask, preserving the airway Flow cycled
patency in patients with unstable UA during sleep and recruiting Fixed Variable inspiratory time
alveoli. Unnecessary increases in PEEP levels must be avoided Adjustable (% of peak flow)
because inspiratory pressure must be increased in parallel if ‘Intelligent’ algorithms
inspiratory assistance is to be maintained, and this can lead to
intolerance and favour leaks. Leaks, if important, may make it PEEP level Level of expiratory pressure
impossible to maintain the set EPAP level.
Type of exhalation
Single or double limb with expiratory Expiratory slope reflects expired Vt*
NIV MODALITIES AND SETTINGS: TIME TO DEFINE valve
Single limb with calibrated leak Expiratory slope does not reflect expired
A STANDARDISED NOMENCLATURE? Vt and may be absent*
As underlined above, there is a wide variety of existing termi- Any level of positive expiratory pressure
nologies describing NIV modalities without a common nomen- is recognisedy
clature and with a lot of confusion. It even happens that the *Visual recognition of expiratory slope will depend on the position of the pneumotachograph
same acronyms correspond to different modalities and that with regard to the expiratory device. In this way the expiratory slope will reflect the expired
identical modalities are called differently. volume only when the flow sensor is interposed between the mask and the exhalation
device.
As ventilators can be categorised by the way that they deliver yAs a mandatory PEEP level is needed to impede rebreathing.
gas flow and by how they shift from inspiration to expiration NIV, non-invasive ventilation; PEEP, positive end expiration pressure; Vt, tidal volume.

Thorax 2011;66:170e178. doi:10.1136/thx.2010.142661 177

 Review series

The most available and simple monitoring can be done from 18. Delguste P, Aubert-Tulkens G, Rodenstein DO. Upper airway obstruction during
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1991;338:1295e7.
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mandatory to have accurate information about these issues to devices. Chest 1995;108:772e8.
better understand the interplay between the patient and the 21. Schettino GP, Chatmongkolchart S, Hess DR, et al. Position of exhalation port and
ventilator. Ventilatory modality, mode of triggering, pressurisa- mask design affect CO2 rebreathing during noninvasive positive pressure ventilation.
Crit Care Med 2003;31:2178e82.
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