Ventilator

Waveforms
Graphical Presentation
of Ventilatory Data

PCIRC 50
CMH2O
40
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VE07100
Table of Contents
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
PRESSURE-TIME CURVES . . . . . . . . . . . . . . . . . . . 3
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Identifying Breath Types . . . . . . . . . . . . . . . . . . . . 4
Ventilator-Initiated Mandatory Breaths . . . . . . . . . . . . 5
Patient-Initiated Mandatory Breaths . . . . . . . . . . . . . . 5
Spontaneous Breaths . . . . . . . . . . . . . . . . . . . . . . . . . 6
Pressure Support Ventilation . . . . . . . . . . . . . . . . . . . . 6
Pressure Control Ventilation . . . . . . . . . . . . . . . . . . . . 7
Pressure Control With Active Exhalation Valve . . . . . . 7
BiLevel Ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Airway Pressure Release Ventilation (APRV). . . . . . . . . 8
Assessing Plateau Pressure . . . . . . . . . . . . . . . . . . 9
Assessing the Work to Trigger a Breath . . . . . . . . . . . 9
Evaluating Respiratory Events . . . . . . . . . . . . . . . . . . 10
Adjusting Peak Flow Rate . . . . . . . . . . . . . . . . . . . . . 10
Measuring Static Mechanics . . . . . . . . . . . . . . . . . . . 11
Assessing Rise Time . . . . . . . . . . . . . . . . . . . . . . . . . 12
Setting Rise Time . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Assessing Auto-PEEP Maneuver . . . . . . . . . . . . . . . . 13
FLOW-TIME CURVES . . . . . . . . . . . . . . . . . . . . . . . 15
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Verifying Flow Waveform Shapes . . . . . . . . . . . . . 16
Detecting the Type of Breathing . . . . . . . . . . . . 17
Determining the Presence of Auto-PEEP . . . . . . 18
Missed Inspiratory Efforts Due to Auto-PEEP . 19
Evaluating Bronchodilator Response . . . . . . . . . 20
Evaluating Inspiratory Time Setting
in Pressure Control . . . . . . . . . . . . . . . . . . . . . . . . 20
Evaluating Leak Rates With Flow Triggering . . . 21
Assessing Air Leaks and Adjusting Expiratory
Sensitivity in Pressure Support . . . . . . . . . . . . . . 22
BiLevel Ventilation . . . . . . . . . . . . . . . . . . . . . . . . . 23
APRV in BiLevel Mode . . . . . . . . . . . . . . . . . . . . . . . 23

i
VOLUME-TIME CURVES . . . . . . . . . . . . . . . . . . . . 24 Assisted Breaths . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 BiLevel Ventilation Without Spontaneous Breathing . 47
Detecting Air Trapping or Leaks . . . . . . . . . . . . . 25 BiLevel/APRV Ventilation With Spontaneous Breathing . 47
BiLevel Ventilation . . . . . . . . . . . . . . . . . . . . . . . . . 25 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
APRV in BiLevel Mode . . . . . . . . . . . . . . . . . . . . . . . 26 Assessing the Work to Trigger a Breath . . . . . . . . . . 49
COMBINED CURVES . . . . . . . . . . . . . . . . . . . . . . . 27 Assessing Compliance . . . . . . . . . . . . . . . . . . . . . . . 50
Pressure and Volume-Time Curves . . . . . . . . . . . . . 28
Assessing Decreased Compliance . . . . . . . . . . . . . . . 50
Assist Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Assessing Resistance. . . . . . . . . . . . . . . . . . . . . . . . . 51
SIMV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Detecting Lung Overdistention . . . . . . . . . . . . . . . . . 51
SPONT (CPAP). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Determining the Effects of Flow Pattern
Pressure Support… . . . . . . . . . . . . . . . . . . . . . . . . . . 31 on the P-V Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
Pressure Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Adjusting Inspiratory Flow . . . . . . . . . . . . . . . . . . . . 53
BiLevel Ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Detecting Air Leaks or Air Trapping . . . . . . . . . . . . . 53
APRV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 FLOW-VOLUME LOOP . . . . . . . . . . . . . . . . . . . . . . . 55
Volume and Flow-Time Curves . . . . . . . . . . . . . . . . 34 Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
Assist Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Evaluating the Effect of Bronchodilators . . . . . . . . . . 56
SIMV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
SPONT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Pressure Support Ventilation . . . . . . . . . . . . . . . . . . . 36
Pressure Control Ventilation . . . . . . . . . . . . . . . . . . . 37
BiLevel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
APRV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Pressure and Flow-Time Curves . . . . . . . . . . . . . . 39
Assist Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
SIMV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
SPONT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Pressure Support. . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Pressure Control Ventilation . . . . . . . . . . . . . . . . . . . 42
BiLevel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
APRV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
PRESSURE-VOLUME LOOP . . . . . . . . . . . . . . . . . . 44
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Inspiratory Area . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Breath Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Mandatory Breaths . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Spontaneous Breaths . . . . . . . . . . . . . . . . . . . . . . . . 46

ii iii
INTRODUCTION Waveforms can help the clinician evaluate the
effects of pressure, flow and volume on the
This pocket guide will help you identify different following four aspects of ventilatory support:
ventilatory waveform patterns and show you how • Oxygenation and ventilation
to use them when making ventilator adjustments.
• Lung damage secondary to mechanical
Graphically displayed waveforms can help you
ventilation (barotraumas/volutrauma)
better understand the patient-ventilator
relationship and the patient’s response to the • Patient rest and/or reconditioning

many types of ventilatory support. • Patient comfort

Waveforms are graphical representations of data Waveform analysis can also help the clinician
collected by the ventilator either integrated with detect circuit and airway leaks, estimate imposed
changes in time (as in Pressure-Time, Flow-Time or ventilatory work, and aid in assessing the efficacy
Volume-Time curves) or with one another (as in of bronchodilator therapy.
Pressure-Volume or Flow-Volume loops). In this workbook, all waveforms depicted are
Waveforms offer the user a “window” into what color-coded to represent the different types of
is happening to the patient in real-time in the breaths or breath phases represented by the
form of pictures. The digital values generated and waveforms displayed.
displayed by the ventilator generally lag by at least – GREEN represents a mandatory inspiration.
one breath and in some cases 4 to 8 breaths. – RED represents a spontaneous inspiration.

– YELLOW represents exhalation.

1 2
PRESSURE-TIME CURVES Applications

The pressure-time curve can provide the clinician with

PCIRC 50
CMH2O A B C the following information:
40
PPEAK
30 • Breath type delivered to the patient
20

10
Baseline
• Work required to trigger the breath
Mean Airway Pressure (PMEAN)
0
• Breath timing (inspiration vs exhalation)
-10
1 2 3 4 5 6S

• Pressure waveform shape

Figure 1. Typical Pressure-Time Curve • Adequacy of inspiration
Pressure is defined as “force per unit area.” • Adequacy of inspiratory plateau
Commonly measured at or near the circuit wye,
pressure for mechanical ventilation applications is • Adequacy of inspiratory flow
typically expressed in cm H2O and abbreviated as • Results and adequacy of a static mechanics
PAW (Airway Pressure). maneuver
Figure 1 shows a graphic representation of pressure • Adequacy of the Rise Time setting
changes over time. The horizontal axis represents
time; the vertical axis represents pressure. Identifying Breath Types
Inspiration is shown as a rise in pressure (A to B in The five different breath types listed below can be
the figure). Peak inspiratory pressure (PPEAK) appears identified by viewing the pressure-time curve, as
as the highest point of the curve. Exhalation begins shown on the following pages.
at the end of inspiration and continues until the next
inspiration (B to C in the figure). 1. Ventilator-initiated mandatory breaths

Beginning pressure is referred to as the baseline, 2. Patient-initiated mandatory breaths

which appears above zero when PEEP/CPAP is 3. Spontaneous breaths
applied. Average (mean) pressure is calculated from
the area under the curve (shaded area) and may be 4. Pressure support breaths
displayed on the ventilator as PMEAN or MAP. 5. Pressure control breaths
Several applications for the pressure-time curve are
described below.

3 4
1. Ventilator-Initiated Mandatory Breaths 3. Spontaneous Breaths
PCIRC 50 PCIRC 50
CMH2O CMH2O
40 40

30 30

20 20 B
A A
10 10

0 0

-10 -10
1 2 3 4 5 6S 1 2 3 4 5 6S

Figure 2. A Ventilator-Initiated Mandatory Breath (VIM) Figure 4. Spontaneous Breath

With no flow-triggering applied, a pressure rise with- Spontaneous breaths (without Pressure Support) are
out a pressure deflection below baseline (A) indicates represented by comparatively smaller changes in
a ventilator-initiated breath (Figure 2). pressure as the patient breathes above and below the
baseline (Figure 4). Pressure below the baseline repre-
2. Patient-Initiated Mandatory Breaths
sents inspiration (A) and pressure above the baseline
PCIRC 50 represents exhalation (B).
CMH2O
40

30 4. Pressure Support Ventilation

20 PCIRC 50
A CMH2O
10 40
Plateau
0 30

-10 20
1 2 3 4 5 6S
10
Figure 3. A Patient-Initiated Mandatory Breath (PIM)
0

A pressure deflection below baseline (A) just before a -10

1 2 3 4 5 6S
rise in pressure indicates a patient’s inspiratory effort
Figure 5. Pressure Support
resulting in a delivered breath (Figure 3).
Breaths that rise to a plateau and display varying
NOTE: Flow-triggering almost completely eliminates
inspiratory times indicate pressure supported breaths
the work imposed on the patient to trigger a breath
(Figure 5).
from the ventilator.

5 6
5. Pressure Control Ventilation BiLevel Ventilation
PCIRC 50 PCIRC 50
CMH2O CMH2O
40 40
A C
Plateau
30 30

20 20
B
10 10

0 0

-10 -10
1 2 3 4 5 6S 1 2 3 4 5 6S

Figure 6. Pressure Control Figure 8. BiLevel Ventilation With Spontaneous Breathing

at PEEPH and PEEPL
Figure 6 shows breaths that rise to a plateau and
display constant inspiratory times, indicating pressure Figure 8 shows BiLevel™ ventilation with spontaneous
controlled breaths. breathing occurring at both PEEPH (A) and PEEPL (B).
Note, also, that the BiLevel mode synchronizes the
Pressure Control With Active Exhalation Valve transition from PEEPH to PEEPL with the patient’s own
PCIRC 50 spontaneous exhalation (C).
CMH2O
40
A A
30
Airway Pressure Release Ventilation (APRV)
20 PCIRC 50 A
CMH2O
10 40

0 30
B
-10 20
1 2 3 4 5 6S
10
Figure 7. Pressure Control With Spontaneous Breathing
0
at Peak Pressure
-10
1 2 3 4 5 6S
Figure 7 shows pressure control ventilation with
Figure 9. Airway Pressure Release Ventilation (APRV)
spontaneous breathing occurring at peak pressure Using BiLevel Mode
during the plateau period (A). This pattern is
commonly seen in ventilators that employ an active Figure 9 depicts Airway Pressure Release Ventilation
expiratory valve. (APRV) showing the characteristic long inspiratory
time (TIMEH) (A) and short “release” time (TIMEL) (B).
Note that all spontaneous breathing occurs at PEEPH.

7 8
Assessing Plateau Pressure trigger sensitivity setting on the ventilator or a slow
response time by the ventilator itself.
PCIRC 50
CMH2O
40
A
Evaluating Respiratory Events
30
PCIRC 50
20 CMH2O A B
40 C
10
30 D
0
20
-10
1 2 3 4 5 6S 10

0
Figure 10. Plateau Pressure
-10
1 2 3 4 5 6S
Figure 10 shows that during pressure control or
Figure 12. Respiratory Time Calculations
pressure support ventilation, failure to attain a
plateau pressure (A) could indicate a leak or inability Figure 12 shows several respiratory events. A to B
to meet the patient’s flow demand. indicates the inspiratory time; B to C indicates the
expiratory time.
NOTE: In some cases the ventilator may not be able
to accelerate the flow delivery quickly enough to sus- If the pressure during exhalation does not return to
tain the patient’s flow requirement. baseline before the next inspiration is delivered (D),
the expiratory time may not be adequate.
Assessing the Work to Trigger a Breath
Adjusting Peak Flow Rate
PCIRC 50
CMH2O DTOT PCIRC 50
40
CMH2O B
30
40

20 30
Pressure
A
10 PT 20

0 10
Time
-10
1 2 0

-10
1 2 3 4 5 6S

Figure 11. Work to Trigger Figure 13. Peak Flow Adjustment

In Figure 11, the depth of the pressure deflection Figure 13 shows that during volume ventilation, the
below the baseline (PT) and the time the pressure rate of rise in pressure is related to the peak flow
remains below the baseline (DTOT ) indicates the setting. A lag or delay (A) in achieving the peak
patient’s effort to trigger a breath. pressure could indicate an inadequate flow setting.
Larger trigger pressures (PT) and/or longer trigger A very fast rise to pressure (B), often accompanied
delay times (DT ) may also indicate an inadequate by an increased peak pressure, could indicate an
inappropriately high flow setting.

9 10
During pressure ventilation, this variation in rise to Assessing Rise Time
pressure may indicate a need to adjust the ventilator’s PCIRC 50
rise time setting. CMH2O
40
A B C
30
Measuring Static Mechanics
20

ml cmH2O
10
C 35 cmH2O R 18 L/s PPL 28 cmH 2O
PCIRC 50 0
CMH2O
40 -10
C 1 2 3 4 5 6s
A
30
B
Fig 15. Using the Pressure-Time Curve to Assess Rise to Pressure
20

10 The rise to target pressure in pressure ventilation

0 often varies among patients due to differences in
-10
1 2 3 4 5 6s lung impedance and/or patient demand. These
variables may result in a suboptimal pressure
Figure 14. Static Measurements waveform during breath delivery.
Figure 14 illustrates a stable static pressure plateau Many clinicians believe the ideal waveform for patients
measurement that differentiates the pressure caused receiving pressure ventilation is roughly square in shape
by flow through the breathing circuit and the pressures (Figure 15, B) with a rapid rise to target pressure so
required to inflate the lungs. The pressure-time curve that the target pressure is reached early in the inspira-
can be used to verify the stability of the plateau tory phase and remains at the target pressure for the
when calculating static compliance and resistance. duration of the inspiratory time. This delivery pattern
(A) represents the peak pressure. may help satisfy the patient’s flow demand while
contributing to a higher mean airway pressure.
(B) represents the static pressure, or pressure in the
lungs for the delivered volume. If compliance or flow demand is uncharacteristically
high, the rise to pressure may be too slow. The result
(C) represents an unstable pressure plateau, possibly is target pressure is achieved late in the inspiratory
due to a leak or the patient’s inspiratory effort. Using phase, causing a decreased mean airway pressure (A).
this plateau pressure to calculate compliance or Patient comfort and synchrony can also be influenced
resistance may result in inaccurate respiratory if the rise time is too slow.
mechanics values.
A rise time that is too fast could result in delivered
pressure exceeding the set target pressure and poten-
tially exposing the patient to higher-than-acceptable
pressures (C). “Overshoot” in pressure ventilation is
commonly seen with low compliance and/or high
resistance.

11 12
Setting Rise Time Figure 17 depicts a successful expiratory pause
PCIRC 50
maneuver for a determination of Auto-PEEP, or
CMH2O
40 Intrinsic PEEP (PEEPI ). An expiratory pause allows
30
A B C
pressure in the lungs to equilibrate with pressure in
20 the circuit, which is measured as Total PEEP (PEEPTOT).
10 An algorithm then subtracts the set PEEP, and the
0 difference is considered Auto-PEEP.
-10
1 2 3 4 5 6s
A successful expiratory pause maneuver requires
Fig 16. Using the Pressure-Time Curve to Set Rise Time % sufficient pause time for full equilibration between
An adjustable Rise Time setting allows the clinician the lungs and circuit. (A) in the figure represents
to tailor breath delivery in pressure ventilation to the point of equilibration and also represents the
more closely meet the patient’s demand and clinical minimum adequate time for the expiratory pause.
conditions. A shorter pause time would not allow complete
pressure equilibration, resulting in a potential
If the patient’s demand is excessive or compliance is underreporting of the PEEPTOT and therefore an
very high, resulting in a slow rise to pressure (Figure underestimation of the patient’s Auto-PEEP.
16, A), increasing the flow output with the Rise Time
setting may result in a more ideal “square” pressure Observing the pressure-time curve during the
waveform (B). Auto-PEEP maneuver allows the clinician to assess
the quality of the maneuver and the accuracy of the
If the patient’s compliance is very low or the resist- reported PEEPI value.
ance is high, the rapid rise to pressure may produce
an undesirable pressure overshoot (C). A slower rise
time may reduce or eliminate the overshoot (B).

Assessing Auto-PEEP Maneuver

PEEPI 6.1 cmH2O PEEPTOT 16.1 cmH2O

PCIRC 50
CMH2O A
40

30

20
Total PEEP
10 Set PEEP

-10
1 2 3 4 5 6s

Figure 17. Assessing the Auto-PEEP Maneuver

13 14
FLOW-TIME CURVES Applications

The flow-time curve can be used to detect:

INSP 120 A B
C
80
D • Waveform shape
40
.
V
0 • Type of breathing
L
min 40
• Presence of Auto-PEEP (Intrinsic PEEP)
80
E
EXP 120
1 2 3 4 5 6S
• Patient’s response to bronchodilators
Figure 18. Typical Flow-Time Curve • Adequacy of inspiratory time in pressure
control ventilation
Flow is defined as a volume of gas moved or
displaced in a specific time period; it is usually • Presence and rate of continuous air leaks
measured in liters per minute (L/min). Figure 18
shows flow (vertical axis) versus time (horizontal axis). Verifying Flow Waveform Shapes
NOTE: Flow shown above the zero flow line is INSP 120
inspiratory flow and flow shown below the zero 80 SQUARE DESCENDING RAMP SINE DECELERATING

flow line is expiratory flow. 40

.
V
Inspiratory time is measured from the beginning of L
0
min 40
inspiration to the beginning of exhalation (A to B).
80
Expiratory time is measured from the beginning of
EXP 120
exhalation to the beginning of the next inspiration 1 2 3 4 5 6S

(B to C). Figure 19. Flow Patterns

The peak inspiratory flow is the highest flow rate Inspiratory flow patterns can vary based on the flow
achieved during inspiration (D). The expiratory peak waveform setting or the set breath type as illustrated
flow rate is the highest flow rate achieved during in Figure 19.
exhalation (E).
In volume control ventilation, the ventilator can be
NOTE: Some ventilators do not measure flow at the set to deliver flow in:
wye. Instead, inspiratory flow is measured at the gas
supply flow sensor; expiratory flow is measured at • A square wave pattern, where the peak flow
the exhalation flow sensor. rate is set and the flow is constant through the
inspiratory phase. Square flow waveforms can
result in higher peak pressures.

• A descending ramp flow wave, where the set

peak flow is delivered at the beginning of the

15 16
 breath and decreases in a linear fashion until the The decelerating flow waveform characteristic of
volume is delivered. Descending flow waveforms pressure ventilation may actually display a flow of
can produce lower peak pressures but can zero at the end of inspiration, in Pressure Control, if
increase the inspiratory time significantly. the inspiratory time is set long enough.

• A sine waveform, where the inspiratory flow Spontaneous Breaths

gradually increases and then decreases back to A spontaneous breath without pressure support will
zero. This method of delivering flow may cause result in a sine-like inspiratory flow pattern often dis-
patient discomfort. playing a lower peak flow rate.

• A decelerating flow waveform, where the flow A pressure support breath will be represented by a
is highest at the beginning of inspiration but decelerating flow waveform which does not return
decelerates exponentially over the course of to zero at the end of inspiration.
inspiration due to the effects of lung impedance.
Decelerating flow is generated in pressure venti- Determining the Presence of Auto-PEEP
lation modalities, such as pressure control or
INSP 120
pressure support. 80

40
.
Detecting the Type of Breathing V
0
L
min 40
Mandatory Breaths Spontaneous Breaths
C
80 A B
INSP 120
DESCENDING DECELERATING- DECELERATING-
SQUARE PRESSURE CONTROL SINE
80 RAMP PRESSURE SUPPORT EXP 120
1 2 3 4 5 6S
40
.
V
0
Figure 21. Auto-PEEP
L
min 40
Auto-PEEP, or Intrinsic PEEP (PEEPI ) refers to the pres-
80
ence of positive pressure in the lungs at the end of
EXP 120
1 2 3 4 5 6S exhalation (air trapping). Auto-PEEP is most often the
Figure 20. Flow-Time Curves Indicating Breath Types result of insufficient expiratory time.

Figure 20 shows five typical flow-time curves for Auto-PEEP (Figure 21) is indicated by an expiratory
different types of breaths. flow that does not return to zero before the next
inspiration begins (A).
Mandatory Breaths
The square and descending ramp flow patterns are A higher end-expiratory flow generally corresponds
characteristic of volume control mandatory breaths to a higher level of Auto-PEEP (B).
with the volume, flow rate and flow waveform set by A lower end-expiratory flow generally corresponds
the clinician. to a lower level of Auto-PEEP (C).

17 18
NOTE: The flow-time waveform can indicate the Evaluating Bronchodilator Response
presence and relative levels of Auto-PEEP but should
INSP 120
not be used to predict an actual Auto-PEEP value. 80
PRE-BRONCHODILATOR POST-BRONCHODILATOR

40
Missed Inspiratory Efforts Due to Auto-PEEP .
V
0
L
min
INSP 120 40
A
80 A
80
B B
40 EXP 120
. 1 2 3 4 5 6S
V
0
L
min 40 Figure 23. Bronchodilator Response
80

EXP 120
A A
Figure 23 shows flow-time curves before and after
1 2 3 4 5 6S
the use of a bronchodilator. Compare the peak
Figure 22. Missed Inspiratory Efforts expiratory flow rates (A) and the time to reach zero
flow (B). The post-bronchodilator curve shows an
Patients who require longer expiratory times are increased peak expiratory flow rate and a reduced
often unable to trigger a breath if the inspiratory time to reach zero flow, potentially indicating
times are too long resulting in auto-PEEP. improvement following bronchodilator therapy.
Figure 22 illustrates the presence of patient inspirato- This improvement in expiratory air flow may also be
ry efforts that did not trigger a breath. This occurs seen after the patient is suctioned.
when the patient has not been able to finish exhaling
when an inspiratory effort is made (A). Evaluating Inspiratory Time Setting in
Pressure Control
To trigger a breath, the patient must inspire through
the Auto-PEEP and meet the set trigger threshold to INSP 120
80
trigger the ventilator. Patients with weak inspiratory A B C
40
efforts are often unable to trigger breaths when sig- V
.
0
nificant Auto-PEEP is present. L
min 40

80

EXP 120
1 2 3 4 5 6S

Figure 24. Inspiratory Time Adjustment

Figure 24 shows the effect of inspiratory time in

pressure control on flow delivery to the patient.
Shorter inspiratory times may terminate inspiration
before the inspiratory flow reaches zero (A).

19 20
Increasing the inspiratory time so the inspiratory flow Assessing Air Leaks and Adjusting Expiratory
reaches zero before transitioning into exhalation (B) Sensitivity in Pressure Support
can result in the delivery of larger tidal volumes B
INSP 120
without increasing the pressure. 80
C
40 A
Further increasing the inspiratory time beyond the V
.
zero flow point will generally not deliver any addi- L
0
min 40
tional tidal volume but results in a pressure plateau
80
(C), which may be desirable in some cases.
EXP 120
1 2 3 4 5 6S

Evaluating Leak Rates With Flow Triggering

Figure 26. Setting Expiratory Sensitivity (ESENS)
INSP 120
80 Figure 26 displays how leaks can affect the inspiratory
B
40 time of pressure support breaths. Typically, pressure
.
V
0 support breaths cycle into exhalation when the inspi-
L
min 40 ratory flow decelerates to a termination threshold.
A
80 With some ventilators this breath termination criteria
EXP 120
1 2 3 4 5 6S
(or expiratory sensitivity) is fixed at a value typically
expressed as a percent of the peak flow delivered for
Figure 25. Leak Rate that breath (10%, 25%). Other ventilators allow the
Figure 25 shows a flow-time curve for a patient with clinician to vary the breath termination criteria to
flow triggering and a continuous air leak (e.g., compensate for the effects of leaks or variations in
uncuffed ET tube, bronchopleural fistula). When the lung impedance on inspiratory time.
flow trigger sensitivity is set higher than the leak Air leaks can often prevent the flow rate from decel-
rate, the flow-time curve can display the leak. erating to the set termination threshold (A), resulting
The leak allows some of the ventilator’s base flow to in a long inspiratory time (B). Adjusting the expiratory
escape the circuit during the expiratory phase, as sensitivity level to a higher percentage of peak flow
shown on the flow-time curve (B). (C) permits the breath to terminate earlier, decreasing
the patient’s inspiratory time and helping to restore
The distance between the zero flow baseline (A) and patient-ventilator synchrony.
the flow curve (B) represents the actual leak rate in
L/min.

21 22
BiLevel Ventilation VOLUME-TIME CURVES
INSP 120
80 A Volume is defined as a quantity of gas in liters.
40 B C
Figure 29 shows a typical volume-time curve. The
.
V
0 upslope (A) indicates inspiratory volume while the
L
min 40 downslope (B) indicates expiratory volume.
80
D
Inspiratory time (I Time) is measured from the begin-
EXP 120
1 2 3 4 5 6S
ning of inspiration to the beginning of exhalation.
Expiratory time (E Time) is measured from the begin-
Figure 27. BiLevel Ventilation With Spontaneous Breathing ning of exhalation to the beginning of inspiration.
Figure 27 shows inspiratory and expiratory flow dur- VT 500 I Time
mL
ing BiLevel ventilation. The high inspiratory flows 400
E Time

indicate the beginning of the mandatory breath (A) 300

with the lower inspiratory flows indicating sponta- 200 A B

neous inspirations during both TIMEH (B) and TIMEL 100

(C). The high peak expiratory flow represents the 0 C

mandatory breath exhalation (D). 100

1 2 3 4 5 6S

APRV in BiLevel Mode Figure 29. Typical Volume-Time Curve

INSP 120 A
In Figure 29, the patient has exhaled fully after 1.7
80 seconds and again after 3.3 seconds. Because of the
.
40 significant time between the end of exhalation and
V
L
0 the beginning of the next inspiration, increasing the
min 40
C respiratory rate in this example would probably not
80
cause air trapping.
EXP 120 B
1 2 3 4 5 6S
Applications
Figure 28. APRV in BiLevel Mode With Spontaneous Breathing
The volume-time curves may be used to detect:
Figure 28 shows inspiratory and expiratory flow
during APRV with its characteristically long TIMEH (A) • Air trapping
and short “release time” (B). The high inspiratory • Leaks in the patient circuit
flows represent the beginning of the mandatory
breaths, and the lower inspiratory flows represent the
spontaneous breathing during the TIMEH. Also note
the presence of Auto-PEEP (C), which is also
characteristic of APRV.

23 24
Detecting Air Trapping or Leaks APRV in BiLevel Mode

VT 500 VT 500
mL mL
400 400

300 300

200 200
A
100 100

0 0

100 100
1 2 3 4 5 6S 1 2 3 4 5 6S

Figure 30. Air Trapping or Leaks Figure 32. APRV Using BiLevel Ventilation

Figure 30 shows exhalations that do not return to Figure 32 shows APRV using BiLevel ventilation with
zero (A). Volume in and volume out are not always spontaneous breathing at PEEPH.
equal. Air leaks or air trapping often result in an expi-
ratory volume that is lower than the inspired volume.
The plateau displayed during exhalation (A) is the
expired volume until the start of the next inspiration.

BiLevel Ventilation

VT 500
mL
400
B B
300

200
A
A
100
C
0
100
1 2 3 4 5 6S

Figure 31. BiLevel Ventilation

Figure 31 shows flow delivery during the mandatory

breaths (A) and spontaneous breathing during at
PEEPH (B) as well as at PEEPL (C).

25 26
COMBINED CURVES Pressure and Volume-Time Curves

Figures 33-39 compare pressure and volume over time.

Many conditions can be identified by viewing two
curves simultaneously. The curve examples that Assist Control
follow show combined pressure, flow and volume- PCIRC 50

time curves in these five ventilatory modalities: cmH 2 O 40

30
• Assist Control (A/C or CMV) 20
A
• Synchronized Intermittent Mandatory Ventilation 10

0
(SIMV)
-10
1 2 3 4 5 6S
• Spontaneous (SPONT or CPAP) VT 500
mL
400
• Pressure Support 300

200
• Pressure Control
100

• BiLevel 0
100
• APRV 1 2 3 4 5 6S

Figure 33. Assist Control

Figure 33 shows volume and pressure during A/C

ventilation. Note that volume and pressure rise
simultaneously. A patient-initiated breath is indicated
by a slight negative deflection in pressure (A).

27 28
SIMV SPONT (CPAP)
PCIRC 50 PCIRC 50
cmH 2 O 40 cmH 2 O 40

30 30
B
20 20
A B A
10 10

0 0
-10 -10
1 2 3 4 5 6S 1 2 3 4 5 6S
VT 500 VT 500
mL mL
400 400

300 300

200 200

100 100

0 0
100 100
1 2 3 4 5 6S 1 2 3 4 5 6S

Figure 34. SIMV Figure 35. SPONT

Figure 34 shows volume and pressure changes during Figure 35 shows volume and pressure changes during
SIMV. The small pressure fluctuations and concurrent SPONT breathing. The small fluctuations in pressure
volume changes indicate spontaneous breathing (A). and volumes (A and B) indicate spontaneous breath-
Negative pressure deflections just before a pressure ing. The variability in volume from breath to breath is
rise indicate a patient-initiated breath (B). characteristic of spontaneous breathing.

NOTE: If flow triggering is active on the ventilator,

there may be little negative pressure deflection
during inspiration.

29 30
Pressure Support Figure 37 shows volume and pressure during pressure
PCIRC 50 control ventilation. The characteristic plateau on the
cmH 2 O 40
volume curve (A) appears as the target pressure is
30 maintained for the set inspiratory time. This is
20 because inspiratory flow has reached zero at this
10
point in the inspiratory time.
0
-10 BiLevel Ventilation
1 2 3 4 5 6S
VT 500 PCIRC 50
mL cmH 2 O 40
400
A A
300 30
A
200 20
B
100 10
0 0
100 -10
1 2 3 4 5 6S 1 2 3 4 5 6S
VT 500
Figure 36. Pressure Support mL
400 A

300
Figure 36 shows volume and pressure during pressure
200
support. Note the changes in volume (A) that corre- B
100
spond to changes in the patient’s inspiratory time 0
and inspiratory effort. 100
1 2 3 4 5 6S

Pressure Control
Figure 38. BiLevel Ventilation With Spontaneous Breathing
PCIRC 50
cmH 2 O 40
Figure 38 shows pressure and volume during BiLevel
30 ventilation, with spontaneous breathing at both
20 PEEPH (A) and PEEPL (B).
10

0
-10
1 2 3 4 5 6S
VT 500
mL
400
A A
300

200

100

0
100
1 2 3 4 5 6S

Figure 37. Pressure Control

31 32
APRV Volume and Flow-Time Curves
50
PCIRC
cmH 2 O 40
Figures 40-46 compare volume and flow over time.

30 Assist Control
20
VT 1000
10 mL
800
A B
0 600
-10
400
1 2 3 4 5 6S
VT 500 200
mL
400 0

300 -200
1 2 3 4 5 6S
200 NSP 120
80
100
. 40
0
V
100 0
L
1 2 3 4 5 6S min 40

80
Figure 39. APRV EXP 120
1 2 3 4 5 6S

Figure 39 depicts volume and pressure with APRV,

with spontaneous breaths occurring at PEEPH. Figure 40. Assist Control

Figure 40 shows volume and flow during A/C ventila-

tion with a square wave. Inspired volume corresponds
to inspiratory flow (A); exhaled volume corresponds
to expiratory flow (B).

33 34
SIMV Figure 42 shows volume and flow during SPONT. The
VT 1000 sine-like flow waveform and reduced volume are
mL
800 MANDATORY SPONTANEOUS characteristic of spontaneous breathing. Inspiration
600 (A) and exhalation (B) are plotted simultaneously for
400
both volume and flow.
200

0 Pressure Support Ventilation

-200
VT 1000
1 2 3 4 5 6S
NSP 120 mL
800
B
80
600

. 40 400
V A
0 200
L
min 40 0
80 -200
1 2 3 4 5 6S
EXP 120 NSP 120
1 2 3 4 5 6S
C
80

Figure 41. SIMV .

V
40

0
L
Figure 41 shows volume and flow during SIMV with min 40

a square wave. The figure also shows the differences 80

in volume and flow waveform between mandatory EXP 120

1 2 3 4 5 6S
and spontaneous breaths.
Figure 43. Pressure Support Ventilation
SPONT
VT 1000 Figure 43 shows volume and flow during pressure
mL
800 support ventilation. The slope of the volume curve
600
A B
may be very steep during the early part of inspiration
400
(A). As flow decreases in a decelerating pattern (C)
200
the slope of the inspiratory volume curve decreases
0
(B).
-200
1 2 3 4 5 6S
NSP 120
80

. 40
V
0
L
min 40

80

EXP 120
11 2 3 4 5 6S

Figure 42. SPONT

35 36
Pressure Control Ventilation BiLevel
VT 1000 VT 1000
mL A mL
800 800
A
600 600

400 400

200 200

0 0

-200 -200
1 2 3 4 5 6S 1 2 3 4 5 6S
NSP 120 NSP 120
B B
80 80

. 40 . 40
V V
0 0
L L
min 40 min 40

80 80

EXP 120 EXP 120

1 2 3 4 5 6S 1 2 3 4 5 6S

Figure 44. Pressure Control Ventilation Figure 45. BiLevel Ventilation

Figure 44 shows volume and flow during pressure Figure 45 shows volume and flow in BiLevel
control ventilation. Delivered volume (A) increases as ventilation.
inspiratory time (B) increases.
APRV
VT 1000
mL
800

600

400

200

0
-200
1 2 3 4 5 6S
NSP 120
80

. 40
V
0
L
min 40

80

EXP 120
1 2 3 4 5 6S

Figure 46. Airway Pressure Release Ventilation

Figure 46 shows volume and flow in APRV.

37 38
Pressure and Flow-Time Curves SIMV
P CIRC 50
Figures 47-53 compare pressure and flow over time. cmH 2 O A
40

Assist Control 30

20
P CIRC 50
B
cmH 2 O 10
40
0
30
-10
20 1 2 3 4 5 6S
NSP 120
10 A
80
B
0
. 40
-10 V
1 2 3 4 5 6S 0
NSP 120 L
min 40
80
80
. 40
V EXP 120
0 1 2 3 4 5 6S
L
min 40

80
Figure 48. SIMV
EXP 120
1 2 3 4 5 6S Figure 48 shows pressure and flow in a square flow
wave mandatory breath (A) and a non-pressure
Figure 47. Assist Control
supported, spontaneous breath (B).
Figure 47 shows pressure and flow with a square
SPONT
flow pattern (A) and a descending ramp flow pattern
P CIRC 50
(B). Note the characteristic lower peak pressure and cmH 2 O
40
longer inspiration of a descending ramp flow pattern. 30

20

10

0
-10
1 2 3 4 5 6S
NSP 120

80

. 40
V
0
L
min 40

80

EXP 120
1 2 3 4 5 6S

Figure 49. SPONT

39 40
Figure 49 shows pressure and flow during SPONT Pressure Control Ventilation
ventilation. Inspiration and exhalation are plotted P CIRC 50
A
simultaneously for both pressure and flow. cmH 2 O
40

30
Pressure Support 20

P CIRC 50 10
cmH 2 O
40 0
B
30 -10
1 2 3 4 5 6S
20 NSP 120
A
10 80

0 40 BD
. C
V
-10 0
1 2 3 4 5 6S L
NSP 120 min 40
C
80 80

. 40 EXP 120
V D 1 2 3 4 5 6S
0
L
min 40 Figure 51. Pressure Control Ventilation
80

EXP 120 Figure 51 shows pressure and flow during pressure

1 2 3 4 5 6S
control ventilation. The two waveforms can be used
Figure 50. Pressure Support together to adjust inspiratory pressure and inspiratory
time. As inspiratory time is increased and plateau
Figure 50 shows pressure and flow during pressure
pressure is sustained (A), note the reduction in
support ventilation. The negative deflection in the
inspiratory flow rate (B). A short inspiratory time
pressure tracing at the beginning of inspiration (A)
may result in an inspiratory flow that does not reach
indicates patient-initiated breaths. Pressure increases
zero (C). Increasing the inspiratory time can allow
to the target pressure support level above PEEP (B).
flow to reach zero (D), resulting in a larger delivered
The decelerating flow waveform represents the high
tidal volume.
initial flow rate (C) that decreases as the target
pressure is reached. The pressure support breath ter-
minates when the inspiratory flow decreases to
a set level or percentage of the peak flow for that
breath (D).

41 42
BiLevel PRESSURE-VOLUME LOOP
P CIRC 50
cmH 2 O
40
Introduction
30

20 Graphical “loops” are the result of two of the three

10 ventilator variables (pressure, flow and volume) plot-
0
ted against one another as opposed to the scalar
-10
1 2 3 4 5 6S curves that plot one variable against time. In this
NSP 120

80
booklet, pressure-volume loops, or P-V loops, are
40
plotted with pressure on the horizontal axis and vol-
.
V
0
ume on the vertical axis. The P-V loop is composed of
L
min 40
two segments representing the inspiratory and expi-
80 ratory phases of ventilation.
EXP 120
VT 1000
1 2 3 4 5 6S Insp Area
mL 900
0.000
800
Figure 52. BiLevel Ventilation 700
600

Figure 52 shows pressure and flow during BiLevel cm

500
400
3.0 H2O

ventilation. 300
200
A B
100
APRV 0
-100 PCIRC
P CIRC 50 -30 -20 -10 0 10 20 30 40 50 60 cmH2O
cmH 2 O
40

30
Figure 54. Pressure-Volume Loop Axes
20 In a P-V loop, inspiration is drawn first, starting at the
10 point where the two axes intersect (A). Properly posi-
0
tioned, the volume axis should intersect the pressure
-10
1 2 3 4 5 6S axis at a point representing the patient’s baseline, or
NSP 120

80
PEEP, pressure. Figure 54 represents the axes of a P-V
40
plot with the volume axis positioned at a PEEP of 3
.
V
0
cm H2O (B). The baseline setting for the plot is dis-
L
min 40
played to the left of the plot grid.
80

EXP 120 Inspiratory Area

1 2 3 4 5 6S

The box to the right of the plot grid displays a

Figure 53. Airway Pressure Release Ventilation
numerical value that represents the calculation of the
Figure 53 shows pressure and flow over time with APRV. area of the loop to the left of the volume axis. With

43 44
the baseline set correctly, the inspiratory area gives Spontaneous Breaths
an approximation of the work imposed by the venti- VT 1000
Insp Area
mL 900
lator. The higher the number, the greater the work 800
0.116

imposed by the ventilator. This calculated value may 700

600
change from breath to breath. 500
3.0
cm
H2O
400 B
300 A
Breath Types 200
100
Pressure-Volume loops are plotted differently for 0
PCIRC
-100
mandatory and spontaneous breaths. -30 -20 -10 0 10 20 30 40 50 60 cmH2O

Mandatory Breaths Figure 56. Spontaneous Breath

VT 1000
mL 900 Insp Area Figure 56 shows a pressure-triggered, spontaneous
0.000
800
700
breath. Since the P-V loop plots inspiration first (A)
600
C
and then exhalation (B), and the inspiratory pressure
500
3.0
cm
H2O
400
B during a spontaneous breath is usually less than PEEP,
300 the spontaneous P-V loop is drawn in a clockwise
200
100
A B A
direction. The value displayed in the “Insp Area” box
0
-100 PCIRC indicates the degree of work imposed on the patient.
-30 -20 -10 0 10 20 30 40 50 60 cmH2O

Assisted Breaths
Figure 55. Mandatory Breath VT 1000
mL 900
The loop in Figure 55 represents a mandatory breath. 800
700
It is plotted in a counterclockwise direction, starting 600
at PEEP, with inspiration (A) being drawn first, then 500 Insp Area
cm 400
exhalation (B). Since mandatory breaths normally 3.0 H2O

300
0.017

result in the delivery of pressures greater than PEEP, 200

100
the loop is drawn to the right of the volume axis, in 0
A PCIRC
the positive pressure area of the grid. The end of -100
-30 -20 -10 0 10 20 30 40 50 60 cmH2O

inspiration (C) reflects both the peak inspiratory pres-

sure and the delivered volume for that breath. After Figure 57. Assisted Breath
inspiration is finished, the expiratory portion of the
Assisted breaths, as shown in Figure 57, begin plotting
loop (B) reflects pressure and volume changes as the
clockwise due to the patient’s initial inspiratory effort
patient exhales, with the volume returning to zero
(A). When the ventilator begins to deliver flow to the
and the pressure returning back to PEEP.
patient, the pressure becomes positive and the plot
direction shifts to counterclockwise. Note the charac-
teristic “trigger tail” of the assisted breath P-V loop.

45 46
 VT 1000
BiLevel Ventilation Without Spontaneous Breathing mL 900
Insp Area
0.00
VT 1000 800
Insp Area
mL 900 700
0.00
800 600
700 500
600 3.0
cm
H2O
400
500 300
cm 400 200
3.0 H2O
A
300 100
200 0
-100 PCIRC
100
-30 -20 -10 0 10 20 30 40 50 60 cmH2O
0
PCIRC
-100
-30 -20 -10 0 10 20 30 40 50 60 cmH2O Figure 60. BiLevel /APRV With Spontaneous Breathing

It is also common to see a P-V loop that represents

Figure 58. BiLevel Ventilation
only the spontaneous breath taken at PEEPH. This
Figure 58 shows BiLevel ventilation without sponta- appears as a small loop beginning and ending in the
neous breathing at PEEPH, resulting in a typical upper right area of the P-V axes (Figure 60), since the
mandatory breath P-V loop. breath actually begins and ends at a pressure greater
than the PEEPL.
BiLevel/APRV Ventilation With Spontaneous Breathing
VT 1000
Insp Area
Applications
mL 900
0.00
800
700 Pressure-Volume loops may be used to detect the
600
500
following:
cm 400
• Inspiratory area calculations
3.0 H2O

300
200
100
• Work to trigger a breath
0
-100 PCIRC
-30 -20 -10 0 10 20 30 40 50 60 cmH2O
• Changes in compliance and resistance
Figure 59. BiLevel /APRV With Spontaneous Breathing • Lung overdistention
In most cases the P-V loop will represent one • Adjustments to pressure support
complete breath cycle (inspiration and exhalation).
• Inflection points
Since BiLevel ventilation and APRV allow sponta-
neous breathing during TIMEH, the P-V loop for a • Adequacy of peak flow rates
patient breathing spontaneously with BiLevel (or
APRV) will likely show the mandatory inspiration and
a short expiratory phase at PEEPH (Figure 59) that is
displayed just before a spontaneous inspiratory
effort is made.

47 48
Assessing the Work to Trigger a Breath Assessing Compliance
VT 1000 VT 1000
Insp Area Insp Area
mL 900 mL 900
0.017 0.000
800 800 Slope
B
700 700
600 600
500 500 A
3.0
cm
H2O
400 3.0
cm
H2O
400
300 300
200 200
100 100
0 0
-100 PCIRC -100 PCIRC
-30 -20 -10 0 10 20 30 40 50 60 cmH2O -30 -20 -10 0 10 20 30 40 50 60 cmH2O

Figure 61. Assessing Work to Trigger Figure 63. Compliance Changes

Figure 61 shows a P-V loop for a pressure-triggered, Figure 63 shows a typical pressure-volume loop for a
mandatory breath. If the ventilator is set correctly (i.e., mandatory breath. The slope (or steepness) of the
the inspiratory flow meets the patient’s demands), then loop reflects the relationship between volume and
the inspiratory area calculation is an estimate of the pressure, or compliance. A change in the slope of the
work to trigger a breath. The larger the “trigger tail,” P-V loop indicates a change in compliance. A shift in
with its higher inspiratory area value, the more work the slope toward the pressure axis (A) indicates a
patient is doing to trigger the breath from the ventilator. decrease in compliance, while a shift toward the vol-
VT 1000
ume axis (B) indicates an increase in compliance.
Insp Area
mL 900
0.049
800 Assessing Decreased Compliance
700
600 VT 1000
Insp Area
500 mL 900
0.00
cm 400 800 Slope
3.0 H2O

300 700
200 600
100 500
0 3.0
cm
H2O
400
-100 PCIRC 300 A
-30 -20 -10 0 10 20 30 40 50 60 cmH2O
200
100
Figure 62. Assessing Work to Trigger 0
-100 PCIRC
-30 -20 -10 0 10 20 30 40 50 60 cmH2O
Figure 62 shows a pressure-triggered, mandatory
breath with a pressure sensitivity that is too high, and Figure 64. Compliance Changes – Decreased Compliance
therefore imposes more work on the patient to trigger
a breath. The higher inspiratory area value and the Figure 64 shows a shift in slope of the P-V loop
larger “trigger-tail” (A) suggest that the pressure sen- toward the pressure axis (A). This graphically illus-
sitivity is too high. Optimizing the pressure sensitivity trates an increase in the pressure required to deliver
will decrease the work to trigger, resulting in a lower the same volume, hence a decrease in compliance.
inspiratory area value and a smaller “trigger-tail.”

49 50
Assessing Resistance proceeds and the alveoli begin to exceed their vol-
VT 1000 ume capacity, their compliance begins to decrease,
Insp Area
mL 900
800
0.00 displaying an increase in pressure with little or no
700 corresponding volume increase.
600
B
500
cm 400
Determining the Effects of Flow Pattern on the P-V Loop
3.0 H2O

300
VT 1000
200 Insp Area
A mL 900
100 0.000
800
0
PCIRC 700
-100
-30 -20 -10 0 10 20 30 40 50 60 cmH2O 600
500
3.0
cm
H2O
400
Figure 65. Assessing Resistance 300
200

Figure 65 shows a P-V loop with an increased “bow” 100

0
A

to the inspiratory curve (A). An increase in the bow- -100

-30 -20 -10 0 10 20 30 40 50
PCIRC
60 cmH2O
ing of either limb of the P-V loop may indicate an
increase in resistance to flow. An increase in bowing Figure 67. Effect of Flow Waveforms
of the inspiratory curve may also indicate excessive
Figure 67 shows a P-V loop (dotted lines) during a
inspiratory flow. An increased bowing of the expira-
mandatory breath that is characteristic of a square
tory curve (B) often indicates an increase in expiratory
flow waveform often used with volume control
resistance.
ventilation. Note the uniform nature of the bowing
Detecting Lung Overdistention of the inspiratory curve as lung compliance changes
VT 1000
Insp Area
with a constant flow delivery.
mL 900
0.000
800
700
Using a decelerating flow waveform, or ramp, to
600 deliver the breath will often cause a distortion of the
500
3.0
cm
H2O
400
A inspiratory curve (A) as pressure increases rapidly
300 during the early part of inspiration when flow is
200
100 highest, and decreases as the flow decelerates. This
0
-100 PCIRC increase in inspiratory bowing resulting in a flattened
-30 -20 -10 0 10 20 30 40 50 60 cmH2O
curve at the beginning of inspiration may be
Figure 66. Lung Overdistention misinterpreted as a lower inflection point. Inflection
point assessment of the P-V loop to estimate the
Figure 66 shows a P-V loop during a mandatory critical opening pressure of the alveoli must be done
breath in which a decrease in compliance occurs using a low flow or static technique to eliminate the
toward the end of inspiration. This is represented by effect of flow on inspiratory pressure changes.
a flattening of the inspiratory curve (A) and is the
result of alveolar overdistention. As inspiration

51 52
Adjusting Inspiratory Flow Figure 69 shows a P-V loop for a mandatory breath
VT 1000 in which expiratory volume fails to return to zero.
Insp Area
mL 900
800
0.000 This can be the result of air leaks (cuff or circuit
700 leaks, chest tubes, bronchopleural fistula) or air
600
500 trapping.
A B
3.0
cm
H2O
400
300
200
100
0
-100 PCIRC
-30 -20 -10 0 10 20 30 40 50 60 cmH2O

Figure 68. Insufficient Inspiratory Flow

Figure 68 shows a P-V loop during a mandatory

breath that displays the characteristic “figure eight”
often seen if inspiratory flow is set too low to meet
the patient’s demands, or a descending ramp flow
waveform results in inadequate end-inspiratory flow.
As the patient’s demand begins to outstrip the flow
delivery of the ventilator, the pressure starts to
decrease (A) while volume continues to increase.
Exhalation becomes slightly positive at the beginning
(B) and then assumes a normal configuration as the
lungs empty.

Detecting Air Leaks or Air Trapping

VT 1000
Insp Area
mL 900
0.000
800
700
600
500
3.0
cm
H2O
400
300
200
100
0
-100 PCIRC
-30 -20 -10 0 10 20 30 40 50 60 cmH2O

Figure 69. Air Leaks or Air Trapping

53 54
FLOW-VOLUME LOOP Application

3.0
The flow-volume loop may be helpful in gauging the
2.5 EXH effects of bronchodilators on patients.
A
2.0
1.5
1.0
Evaluating the Effect of Bronchodilators
.
V 0.5
VT PRE-BRONCHODILATOR POST-BRONCHODILATOR
L 0 mL
sec 3.0 3.0
0.5 EXH C EXH
2.5 2.5
B A
1.0 2.0 2.0
1.5 1.5 1.5
D
1.0 1.0
2.0 INSP . B .
V 0.5 V 0.5
2.5 L
0
VT
mL
L
0
VT
mL
200 0 200 400 600 800 1000 1200 1400 1600 sec sec

0.5 0.5
1.0 1.0
Figure 70. Typical Flow-Volume Loop 1.5 1.5
2.0 2.0
INSP INSP
2.5 2.5
Figure 70 shows a typical flow-volume loop. The 200 0 200 400 600 800 1000 200 0 200 400 600 800 1000

peak expiratory flow rate is noted at (A); peak Figure 71. Figure 72.
inspiratory flow rate is noted at (B). Flow is plotted Pre-Bronchodilator Post-Bronchodilator
on the vertical axis and volume on the horizontal. Flow-Volume Loop Flow-Volume Loop
The lower half of the loop represents inspiration;
Figures 71 and 72 show pre- and post-bronchodilator
the upper half represents exhalation. The loop
flow-volume loops that indicate a positive response
arrangement resembles that of a pulmonary function
to the bronchodilator administration. The flow-
test, in which exhalation is plotted first followed by
volume loop before bronchodilator administration
the next inspiration.
shows a low peak expiratory flow rate (A) and a
scalloped shape near the end of exhalation (B) that
is characteristic of poor airway conductivity. The
post-bronchodilator loop shows an improved peak
expiratory flow (C) as well as improved flow toward
the end of exhalation (D).

55 56