Download Mask Proportional Assist vs Pressure Support

Mask Proportional Assist vs Pressure
Support Ventilation in Patients in
Clinically Stable Condition With Chronic
Ventilatory Failure*
Roberto Porta, MD; Lorenzo Appendini, MD; Michele Vitacca, MD;
Luca Bianchi, MD; Claudio F. Donner, MD, FCCP; Roberta Poggi, MD; and
Nicolino Ambrosino, MD, FCCP
Objective: To compare the short-term physiologic effects of mask pressure support ventilation
(PSV) and proportional assist ventilation (PAV) in patients in clinically stable condition with
chronic ventilatory failure (CVF).
Design: Randomized, controlled physiologic study.
Setting: Lung function units of two pulmonary rehabilitation centers.
Patients: Eighteen patients with CVF caused by COPD (11 patients) and restrictive chest wall
diseases (RCWDs) [7 patients].
Methods: Assessment of breathing pattern and minute ventilation (V̇E), respiratory muscles and
lung mechanics, and patient/ventilator interaction during both unassisted and assisted ventilation. After baseline assessment during spontaneous breathing (SB), mask PSV and PAV were
randomly applied at the patient’s comfort, with the addition of the same level of continuous
positive airway pressure (2 cm H2O or 4 cm H2O in all patients), for 30 min each, with a 20-min
interval of SB between periods of assisted ventilation.
Results: A longer time was spent to set PAV than PSV (663 ⴞ 179 s and 246 ⴞ 58 s, respectively;
p < 0.001). Mean airway opening pressure (Pao) computed over a period of 1 min, but not peak
Pao, was significantly lower with PAV than with PSV (151 ⴞ 45 cm H2O/s/min and 207 ⴞ 73 cm
H2O/s/min, respectively; p < 0.002). Tidal volume (VT) exhibited a greater variability with PAV
than with PSV (variation coefficient, 16.3% ⴞ 10.5% vs 11.6% ⴞ 7.7%, respectively; p < 0.05).
Compared with SB, both modalities resulted in a significant increase in VT (by 40% and 36% with
PAV and PSV, respectively, on average) and V̇E (by 37% and 35%) with unchanged breathing
frequency and duty cycle. Both modalities significantly reduced esophageal (by 39% and 51%)
and diaphragmatic (by 42% and 63%) pressure-time products, respectively. Ineffective efforts
were observed with neither modes of assistance in any patient.
Conclusions: In resting, awake patients in clinically stable condition with CVF caused by either
COPD or RCWD, noninvasive application of PAV, set at the patient’s comfort, was not superior
to PSV either in increasing VT and V̇E or in unloading the inspiratory muscles. We failed to find
any difference in patient/ventilator interaction between ventilatory modes.
(CHEST 2002; 122:479 – 488)
Key words: breathing pattern; COPD; hypercapnia; noninvasive mechanical ventilation; respiratory failure; respiratory
muscles; restrictive chest wall disease
Abbreviations: CPAP ⫽ continuous positive airway pressure; CVF ⫽ chronic ventilatory failure; El,dyn ⫽ dynamic
elastance; ⌬El,dyn ⫽ residual lung elastance; FA ⫽ flow assist; IE ⫽ ineffective efforts; IPS ⫽ inspiratory pressure
support; NPPV ⫽ noninvasive positive pressure ventilation; Pao ⫽ airway opening pressure; PAV ⫽ proportional assist
ventilation; Pdi ⫽ transdiaphragmatic pressure; Pes ⫽ esophageal pressure; PEEP ⫽ positive end-expiratory pressure;
PEEPi,dyn ⫽ dynamic intrinsic positive end-expiratory pressure; Pga ⫽ gastric pressure; PSV ⫽ pressure support
ventilation; PTPao ⫽ mean airway opening pressure computed over a period of 1 min; PTPdi ⫽ pressure-time product
of the diaphragm calculated over a period of 1 min; PTPes ⫽ pressure-time product of the inspiratory muscles calculated
over a period of 1 min; RCWD ⫽ restrictive chest wall disease; Rl ⫽ pulmonary resistance at mid inspiration;
⌬Rl ⫽ residual pulmonary resistance; SB ⫽ spontaneous breathing; Ti/Ttot ⫽ duty cycle; VA ⫽ volume assist;
V̇e ⫽ minute ventilation; Vt ⫽ tidal volume
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479
noninvasive positive pressure ventilaL ong-term
tion (NPPV) is used widely in the management
of chronic ventilatory failure (CVF) that results from
restrictive chest wall disease (RCWD) and from
COPD1; however, particularly in the latter, a strong
evidence of significant clinical benefit is lacking.2,3
Pressure support ventilation (PSV) is the most common mode of providing ventilatory assistance in the
long-term setting, with and without some level of
positive end-expiratory pressure (PEEP).4,5
Studies have shown that proportional assist ventilation (PAV)6 provides some physiologic benefit in
patients with CVF by improving the breathing pattern and arterial blood gases,7 decreasing breathlessness during exercise,8,9 and unloading the respiratory
muscles.10 The theoretic background suggests that
these effects of PAV should be associated with a
better patient/ventilator interaction because PAV
shifts the control of ventilatory assistance from the
caregiver to the patient. In fact, the pattern of
mechanical breath is proportional to the amplitude
and timing of the patient’s ventilatory demand.6
However, this theoretic advantage was never tested
in the clinical practice of NPPV. Among others, one
reason might be that a direct comparison between
PAV and PSV was never performed to understand
whether the former may improve the patient’s acceptability of long-term NPPV. This study was undertaken to investigate whether PAV will provide a
better patient/ventilator interaction than PSV, and
thus a better acceptability in patients requiring
long-term use of noninvasive ventilation.
Materials and Methods
The investigative protocol was approved by the Ethics Committee of the S. Maugeri Foundation IRCCS. The study was
conducted according to the declaration of Helsinki. Patients gave
their informed consent to participate in the study.
Patients
The study was conducted in the Lung Function Units of
Gussago and Veruno, S. Maugeri Foundation, from March 1999
to March 2000. Eighteen patients (11 patients with COPD and 7
patients with RCWD caused by kyphoscoliosis) with CVF were
recruited for this study. Diagnosis of COPD was made according
From the Fondazione Salvatore Maugeri IRCCS, Pulmonary
Departments, Scientific Institutes of Gussago (Drs. Porta, Vitacca, Bianchi, and Ambrosino) and Veruno (Drs. Appendini and
Donner), Ospedale Maggiore di Borgo Trento, Azienda Ospedaliera di Verona (Dr. Poggi), Italy.
This study was partially supported by Respironics Inc., Murrysville, PA.
Manuscript received July 12, 2001; revision accepted February
20, 2002.
Correspondence to: Nicolino Ambrosino, MD, FCCP, Fondazione
S. Maugeri, Lung Function Unit, Istituto Scientifico di Gussago,
I-25064 Gussago (BS), Italy; e-mail: [email protected]
to the American Thoracic Society.11 The diagnosis of CVF was
based on the clinical records showing values of Paco2 persistently
⬎ 45 mm Hg during room air spontaneous breathing (SB) in the
months if not years preceding the study. All patients were in
stable clinical condition, as assessed by an arterial pH ⬎ 7.35 and
were free from exacerbations in the preceding 4 weeks. Patients
with other chronic organ failure, cancer, or inability to cooperate
were also excluded from the study. All of the patients were
receiving drug treatment according to the prescriptions of their
primary physicians. In particular, patients with COPD were
receiving regular treatment with inhaled bronchodilators, avoiding either systemic or inhaled steroids apart from exacerbations.
At the time of the study, 16 of the 18 patients were receiving
long-term oxygen therapy. Six patients with COPD and two
patients with RCWD had been receiving long-term home NPPV
by nasal mask for 16 to 36 months, with volume-cycled ventilators
in assisted mode for two patients, and with pressure-cycled
ventilators in PSV mode for six patients. Indications for long-term
NPPV for these patients were chronic hypercapnia and nocturnal
hypoventilation for all patients. The mean use of NPPV was
approximately 7 h per night. Six of 18 patients were admitted to
the hospital for indication of domiciliary NPPV. Four of these six
patients were discharged with home NPPV. The other patients
underwent respiratory rehabilitation programs. The patients’ characteristics, according to their diagnoses, are shown in Table 1.
Measurements
Lung function and arterial blood gas levels were assessed 1 to
3 days before the study. Routine static and dynamic lung volumes
were measured with constant volume body plethysmographs
(CAD-NET system 1085; Medical Graphics; St. Paul, MN, and
V̇max Series/6200 Autobox DL; SensorMedics; Yorba Linda, CA)
with the patients in the seated position according to standard
procedure.12 The predicted values of Quanjer13 were used.
Arterial blood was sampled at the radial artery with the patients
in a semirecumbent position and breathing room air. Pao2,
Paco2, and pH were measured with automated analyzers (model
840; Ciba Corning; Medfield, MA, and ABL 620; Radiometer;
Copenhagen, Denmark).
For the experimental procedure of this study, flow was measured using a heated pneumotachograph (Fleisch n°1; Fleisch;
Lausanne, Switzerland) connected to a flow transducer (HewlettPackard 47304A; Hewlett-Packard; Cuppertino, CA) inserted
between the nasal mask and the plateau valve of the ventilator
Table 1—Demographic, Anthropometric, and
Functional Characteristics of Patients*
Characteristics
Age, yr
Male/female sex, No.
Weight, kg
FEV1, % predicted
FVC, % predicted
FEV1/FVC, %
Residual volume, % predicted
Inspiratory capacity, % predicted
Total lung capacity, % predicted
pH
Pao2, mm Hg
Paco2, mm Hg
COPD
(n ⫽ 11)
RCWD
(n ⫽ 7)
66 ⫾ 10
9/2
76 ⫾ 17
26 ⫾ 6
47 ⫾ 13
45 ⫾ 11
189 ⫾ 43
42 ⫾ 22
104 ⫾ 15
7.40 ⫾ 0.04
54 ⫾ 11
58 ⫾ 8
55 ⫾ 15
6/1
58 ⫾ 11
31 ⫾ 9
35 ⫾ 8
75 ⫾ 15
82 ⫾ 17
35 ⫾ 9
53 ⫾ 11
7.38 ⫾ 0.02
53 ⫾ 10
57 ⫾ 5
*Data are presented as mean ⫾ SD unless otherwise indicated.
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Clinical Investigations
circuit.14 Volume was obtained by numerical integration of the
flow signal. Airway opening pressure (Pao) was measured with a
differential pressure transducer (Honeywell 143PCO3D; Honeywell; Freeport, IL) connected to one port of the nasal mask.
Changes in pleural and abdominal pressures were estimated from
changes in esophageal pressure (Pes) and gastric pressure (Pga),
respectively (Transducer Motorola X2010 ⫾ 100 cm H2O; Colligo, Elekton; Agliano Terme, Italy) using the balloon-catheter
technique, with esophageal and gastric balloon catheters, as
previously described.10 Transpulmonary pressure and transdiaphragmatic pressure (Pdi) were obtained by subtraction of Pes
from Pao and Pga, respectively.
Data Analysis
All signals were digitized by an analog-to-digital converter with
12-bit resolution, connected to a personal computer, at a sampling frequency of 100 Hz. Subsequent analyses of breathing
pattern and pulmonary mechanics were performed using an
edition of the software package (ANADAT 5.2; RHT-Infodat;
Montreal, PQ, Canada) interfaced with the respiratory monitoring system used in the present study.
Using the Abreath facility of ANADAT, the mean value of each
physiologic variable was computed and used subsequently for
statistical analysis. Tidal volume (Vt), respiratory frequency,
minute ventilation (V̇e), total cycle duration, inspiratory time,
expiratory time, and duty cycle (Ti/Ttot) were calculated from
the flow signal as average values from 5 min of continuous
recording of flow and volume. Breathing pattern variability was also
computed on flow tracing and expressed as coefficient of variation
(SD/mean ⫻ 100). Dynamic intrinsic PEEP (PEEPi,dyn) was
measured as the negative deflection in Pes from the onset of the
inspiratory effort to the start of the inspiratory flow. In the presence
of expiratory muscle activity, the value of PEEPi,dyn was reduced by
the decrease in Pga measured in the same time interval.15 Changes
in the magnitude of the inspiratory muscles and diaphragm effort
were estimated from changes in Pes and Pdi swings, respectively, as
well as from changes in the pressure-time product of the inspiratory
muscles calculated over a period of 1 min (PTPes) and pressure-time
product of the diaphragm calculated over a period of 1 min
(PTPdi).15 The latter measurements were expressed also as pressure
developed per liter of ventilation (PTPes/V̇e and PTPdi/V̇e).
Transpulmonary pressure was used to calculate dynamic elastance
(El,dyn) according to the Mead and Whittenberger technique,16
and pulmonary resistance at midinspiration (Rl) according to the
Neergaard-Wirtz elastic subtraction technique.16
The maximal positive level of Pao signal (peak Pao) was
calculated as average values from 10 consecutive respiratory acts
in which breathing pattern and mechanics were calculated. In
addition, the average area subtended by Pao of the same tidal
swings, from the onset of inspiratory effort to the inspiratory flow
tracing inversion (from inspiration to expiration) was computed
and calculated over a period of 1 min, multiplying it for
respiratory rate (mean Pao computed over a period of 1 min
[PTPao]) [Fig 1].
Patients’ inspiratory efforts that were unable to trigger a new
ventilator cycle despite a negative deflection in Pes were termed
ineffective efforts (IE).17 The mean number of IE per minute,
recorded over 5 min, was expressed as percentage of the
respiratory rate (IE per minute/respiratory frequency [beats per
minute] ⫻ 100).
Figure 1. Representative tracing of Pao delivered by PAV (upper panel) and PSV (lower panel). The
maximal positive level of the Pao signal (peak Pao) was calculated as average values from 10 consecutive
respiratory acts in which breathing pattern and mechanics were also calculated. The average area
subtended by Pao of the same 10 tidal swings (magnified dashed areas in squares) was computed and
calculated over a period of 1 min multiplying it for respiratory rate (PTPao).
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481
Ventilatory Settings
A commercial nasal mask of adequate size for each patient’s
nose was used (Respironics; Murrysville, PA). In each patient,
both modalities were delivered using the same portable ventilator
able to compensate for leaks and to operate in the continuous
positive airway pressure (CPAP) mode, PSV mode, and PAV
mode (BiPAP Vision; Respironics). The ventilator circuit was
equipped with the Sanders NRV-2 plateau valve (Respironics) to
prevent CO2 rebreathing.14 Before starting the protocol, PAV
and PSV settings were separately assessed as follows.
PAV: The ventilator delivers pressure according to the motion
equation, generating a pressure that is proportional to patient’s
spontaneous effort.6 A portion of the total mechanical workload,
ie, elastance and resistance, is taken over according to a level of
assistance, which has been decided by the caregiver and can
specifically unload the resistive burden (flow assist [FA]) and the
elastic burden (volume assist [VA]). VA and FA were set separately; VA and FA were set initially at the minimum value of 2 cm
H2O/L/s and 1 cm H2O/L/s, respectively, in all patients. Then,
leaving FA unchanged, VA was increased slowly by steps of 2 cm
H2O/L until the patient indicated that breathing was uncomfortable. Each step lasted the time necessary for the ventilatory
setting to be described as comfortable or uncomfortable. Then,
that level of assist was decreased by 2 cm H2O/L. This level was
considered as the maximum tolerated. To set FA, a similar
stepwise approach was used, by keeping VA at 2 cm H2O/L and
slowly increasing FA from 1 cm H2O/L/s by small steps of 1 cm
H2O/L/s until the patient noted being uncomfortable with that
level of assistance. Then, that level of assistance was decreased by
1 cm H2O/L/s. This level was considered to be the maximum
tolerated. The setting of PAV that was applied to patients
corresponded to 80% of the maximal individual tolerated values
of VA and FA.10 Those values of FA and VA were used to
evaluate the residual pulmonary resistance (⌬Rl) and residual
lung elastance (⌬El,dyn) faced by respiratory muscle contraction
to generate inspiratory flow and volume, being subtracted from
Rl (⌬Rl ⫽ Rl ⫺ FA) and El,dyn (⌬El,dyn ⫽ El,dyn ⫺ VA),
respectively. No data were available about the residual resistance
and elastance of the chest wall.
PSV: The level of inspiratory pressure support (IPS) was
increased slowly by steps of 2 cm H2O, starting from 2 cm H2O,
until the patients indicated that breathing was uncomfortable.
Hence, that level of IPS was decreased by 2 cm H2O, and the
resultant level was applied. A default level of 2 cm H2O of CPAP
was added to both modalities in each individual patient.
Experimental Procedure
The whole procedure was performed under continuous monitoring of arterial oxygen saturation by pulse oximetry (Oxicap
Monitor; Ohmeda; Louisville, CO). The patients were evaluated
in the morning (patients with COPD were evaluated at least 2 h
after inhalation of their bronchodilating medications) and were
free to choose the most comfortable position. All patients
adopted a semirecumbent position.
The procedure to evaluate respiratory muscles and mechanics
has been extensively detailed elsewhere.4,5,10,15 Briefly, in all
patients, after the application of topical anesthesia (xylocaine
spray 10%), two balloon-tipped catheters were consecutively
inserted through the nose into the middle third of the esophagus
and into the stomach and thereafter inflated to 0.5 mL and 1 mL
for the esophageal and gastric catheters, respectively. Then, the
nasal mask was applied and connected to the pneumotachograph.
The occlusion test18 was finally performed to check the proper
functioning of the esophageal balloon. A pneumatic shutter was
inserted in line and proximally to the pneumotachograph only to
perform this maneuver, and then removed. The occlusion test
was satisfactory in every instance.
Initially, all patients were not connected to the ventilator and
breathed room air through the nose mask for approximately 20
min. All of the patients were instructed to breathe through the
nose mask and to keep their mouths closed during the experimental procedure to prevent leaks. Subsequently, PAV and PSV
were applied in random order for 30 min, and the trials were
separated by 20-min periods of SB through the mask. All of the
measurements were obtained at an inspiratory oxygen fraction
of 0.21.
All of the physiologic signals were recorded in the last 5 min of
each unsupported and supported breathing period. In two patients with RCWD, the signal from the gastric balloon was
unsatisfactory. Hence, complete data of respiratory muscles and
mechanics were available in 16 of 18 patients.
At the end of each period of assistance, 10 patients (5 patients
with each diagnosis) of the 18 patients scored their comfort
sensation under NPPV by means of an arbitrary scale ranging
from 0 (worst comfort) to 10 (best comfort). At the end of the two
ventilation sessions, the patients indicated their preferred mode
of ventilation. The time spent by the operator to set the two
modalities was recorded by an attending nurse.
Statistical Analysis
Results are shown as mean ⫾ 1 SD. Differences between
treatments and within treatment were evaluated by analysis of
variance for repeated measures. Because of the lack of differences between values assessed during the two SB periods,
baseline values were considered as the mean levels of the two
measurements. Differences between paired groups of data were
evaluated with post hoc paired t test with Bonferroni adjustment
and were applied as requested by analysis of variance interaction.
A p value ⬍ 0.05 was considered significant.
Results
All patients accepted NPPV well throughout the
procedure. No patients reported side effects or
refused the procedure with either modality. Because
no significant difference between PAV and PSV was
found according to the diagnosis, when not specified,
data from patients with different diagnoses are reported together.
Setting of the Ventilator
The individual and mean levels of assistance with
the two modalities and patients’ elastance and resistance are shown in Table 2. With PAV, no “runaway” phenomenon6 was observed in any patient.
The run-away phenomenon was defined by the
continuation of positive pressure after the end of the
patients’ inspiratory effort into the patients’ neural
expiration. In this condition, the pressure delivered
by the ventilator exceeds the patients’ elastic recoil
opposing force, generating inspiratory flow and volume well beyond the termination of the inspiratory
muscle effort, until total respiratory elastance
increases above VA value at total lung capacity.6
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Clinical Investigations
Table 2—Individual and Mean Levels of Assistance and Lung Mechanics During Spontaneous and
Supported Ventilation
PSV
Patient No.
1
2
3
4
5
6
7
8
9
10
11
Average ⫾ SD
12
13
14
15
16
17
18
Average ⫾ SD
Average ⫾ SD
Group
COPD*
COPD
COPD
COPD
COPD*
COPD*
COPD*
COPD
COPD*
COPD*
COPD*
COPD
RCWD*
RCWD
RCWD*
RCWD
RCWD*
RCWD*
RCWD*
RCWD
Total
PAV
IPS cm
H2O
VA cm
H2O/L
FA cm
H2O/L/s
El,dyn cm
H2O/L
Rl cm
H2O/L/s
⌬El,dyn
cm H2O/L
⌬Rl cm
H2O/L/s
16
6
14
13
8
10
14
10
16
17
10
12.2 ⫾ 3.6
11
11
10
11
14
10
16
11.9 ⫾ 2.3
12 ⫾ 3
16
10.4
4.8
10.4
7.2
3
8
10
11
16
6
12.2 ⫾ 5.6
24.8
29.6
12
22.4
22.4
22
28
27.8 ⫾ 8†
14.7 ⫾ 8
4
4
2.4
5.6
4.8
9
10
4
3
4
1
4.7 ⫾ 2.7
4
8
4
8.8
11.2
7
5
6.9 ⫾ 2.7
5.5 ⫾ 2.8
10.5
29.1
4.5
62
3.1
7.2
10.1
17.2
13.2
16.7
6.9
16.4 ⫾ 16.8
7.1
8.8
22.2
17.7
6.9
22.7
41.7
18.2 ⫾ 12.4
17.1 ⫾ 14.9
14.6
19.9
12
31
2.7
15.3
16.7
23
16.3
12.4
4.8
15.3 ⫾ 7.9
2.8
6.4
6.4
5.3
8.5
13.5
13.6
8.1 ⫾ 4.1
12.5 ⫾ 7.5
⫺ 5.5
18.7
⫺ 0.3
51.6
⫺ 4.1
4.2
2.1
7.2
2.2
0.7
0.9
7.1 ⫾ 16.1
⫺ 17.7
⫺ 20.8
10.3
⫺ 4.7
⫺ 15.5
0.7
13.7
⫺ 4.9 ⫾ 13.8
2.4 ⫾ 16
10.6
15.9
9.6
25.4
⫺ 2.1
6.3
6.7
19
13.3
8.4
3.8
10.6 ⫾ 7.6
⫺ 1.2
⫺ 1.6
2.4
⫺ 3.5
⫺ 2.7
6.5
8.6
1.2 ⫾ 4.7
7⫾8
*Receiving home ventilation.
†p ⬍ 0.001 between COPD and RCWD patients.
Table 2 also shows patients discharged with home
mechanical ventilation. There was no significant
difference in levels of VA and FA between patients
who required home ventilatory assistance and patients
who did not.
Patients with RCWD tolerated higher levels of VA
under PAV than patients with COPD. Peak Pao was
not significantly different between ventilator modes
(11 ⫾ 4 cm H2O and 12 ⫾ 3 cm H2O with PAV and
PSV, respectively). By contrast, PTPao was lower
with PAV (151 ⫾ 45 cm H2O/s/min) than PSV
(207 ⫾ 73 cm H2O/s/min; p ⬍ 0.002). During PAV,
⌬Rl and ⌬El,dyn workloads showed a different
behavior in patients with COPD and patients with
RCWD (Table 2). In fact, in patients with RCWD,
VA was above the El,dyn, whereas ⌬Rl fell within
normal limits.19 By contrast, in patients with COPD,
⌬El,dyn fell within normal limits, and, on average,
⌬Rl was mildly increased (Table 2). The caregivers
spent more time to set PAV than PSV (663 ⫾ 179 s
and 246 ⫾ 58 s, respectively; p ⬍ 0.001).
Breathing Pattern and Lung Mechanics
Figure 2 shows a polygraphic tracing from a
representative patient during the different conditions of the study protocol. Ineffective efforts were
not observed with either modes of assistance in any
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patient. In the breathing pattern, Vt showed a greater
variability under PAV than under PSV (coefficient of
variation, 16.3% ⫾ 10.5% and 11.6% ⫾ 7.7%, respectively; p ⬍ 0.01).
Mean values of breathing pattern and lung mechanics during SB and assisted ventilation are shown
in Table 3. On average, both PAV and PSV improved
Vt (by 40% and 36%, respectively), V̇e (by 37% and
35%, respectively), and mean inspiratory flow (by
33% and 51%, respectively) without changes in
respiratory frequency and Ti/Ttot. Neither mode
modified El,dyn or Rl compared with baseline.
Inspiratory Muscles
Table 4 shows that on average, the magnitude of
the patients’ inspiratory muscle effort was significantly
reduced by both PAV and PSV, which resulted in lower
Pes (⫺ 33% and ⫺ 51%), Pdi (⫺ 34% and ⫺ 52%),
PTPes (⫺ 39% and ⫺ 51%), PTPdi (⫺ 42% and
⫺ 63%), PTPes/V̇e (⫺ 53% and ⫺ 63%), and PTPdi/
V̇e (⫺ 56% and ⫺ 72%, respectively; Fig 3). Although
there was no significant difference, PSV unloaded the
inspiratory muscles slightly more than PAV.
Comfort
Comfort score was available in 10 patients (5
patients with COPD and 5 patients with RCWD).
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483
Figure 2. Polygraphic recording during different periods of protocol study. From top to bottom: flow,
Pao, Pes, Pdi, and Vt.
No patients rated low scores of comfort under either
modality of ventilation, indicating good adaptation to
NPPV (7.0 ⫾ 1.3 and 7.4 ⫾ 1.6 for PAV and PSV,
respectively). Moreover, no significant difference
was found between scores during PAV and PSV.
There was no clear preference about a specific
ventilatory mode because six patients preferred PSV
and four patients preferred PAV.
Discussion
The results of this study show that in resting,
awake patients in clinically stable condition with
Table 3—Breathing Pattern and Lung Mechanics
During Spontaneous and Supported Ventilation*
Variables
Respiratory frequency,
beats/min
Vt, mL
Ve, L/min
Ti/Ttot
Vt/Ti, L/s
PEEPi,dyn, cm H2O
El,dyn, cm H2O/L
Rl, cm H2O/L/s
SB
PAV
PSV
20.8 ⫾ 5.7
20.8 ⫾ 5
19.6 ⫾ 5.4
452 ⫾ 168
8.7 ⫾ 1.6
0.36 ⫾ 0.05
0.40 ⫾ 0.1
1.6 ⫾ 1.2
16.7 ⫾ 12.3
12.6 ⫾ 6.6
579 ⫾ 190†
11.5 ⫾ 2.9†
0.34 ⫾ 0.04‡
0.52 ⫾ 0.17†
1.5 ⫾ 1.1
12.2 ⫾ 7.4
12.1 ⫾ 5.7
621 ⫾ 182†
11.5 ⫾ 2.4†
0.32 ⫾ 0.05‡
0.59 ⫾ 0.17†
1.3 ⫾ 1.3
12.3 ⫾ 6.2
11.7 ⫾ 4.5
*Data are presented as mean ⫾ SD. Vt/Ti ⫽ mean inspiratory flow
†p ⬍ 0.001 vs SB.
‡p ⬍ 0.02 vs SB.
CVF caused by either COPD or RCWD, noninvasive
application of PAV, set at the patient’s comfort, was
not superior to PSV either in increasing Vt and V̇e
and or in unloading the inspiratory muscles. The only
differences between PSV and PAV were that the latter
achieved those physiologic benefits at a lower level of
PTPao but required more time to set the ventilator. We
failed to find any difference in patient/ventilator interaction between ventilatory modes.
This study confirms and extends previous observations,4,5,10 but it provides the first comparison in
the NPPV setting between PAV, a recently proposed
mode of mechanical ventilation, and PSV, which still
remains the most widely used mode of ventilatory
assistance for both short-term and long-term patients.1 In patients with COPD and in patients with
Table 4 —Inspiratory Effort During Spontaneous and
Supported Ventilation*
Variables
Pes, cm H2O
Pdi, cm H2O
PTPes, cm H2O/min
PTPdi, cm H2O/min
PTPes, cm H2O/s/L V̇e
PTPdi/V̇e, cm H2O/s/L
SB
PAV
PSV
12.6 ⫾ 4.7
14.3 ⫾ 4.9
218 ⫾ 83
249 ⫾ 106
27 ⫾ 14
30 ⫾ 15
8.3 ⫾ 4.5†
9.7 ⫾ 4.9†
126 ⫾ 81†
143 ⫾ 75†
12 ⫾ 8†
13 ⫾ 7†
6.8 ⫾ 4.4†
7.6 ⫾ 5.5†
106 ⫾ 82†
97 ⫾ 76†
10 ⫾ 8†
8 ⫾ 8†
*Data are presented as mean ⫾ SD.
§p⬍0.001 vs SB.
†p ⬍ 0.001 vs SB.
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Figure 3. Individual and mean changes in PTPes/V̇e (upper panel) and PTPdi/V̇e (lower panel)
corrected per liter of ventilation during SB and assisted ventilation with the two modes studied. Values
are given according to diagnosis (full circles and continuous line ⫽ COPD; open squares and dashed
lines ⫽ RCWD). Individual and mean values of SB represent the actual baseline measurement before
each assisted period.
RCWD, the magnitude of improvement in V̇e and
Vt (Table 3), as well as the amount of the reduction
of the patients’ inspiratory effort (Table 4), were not
significantly different between PAV and PSV, although PSV determined a slightly greater reduction
of patients’ effort, on average. Similar changes in
breathing pattern7 and respiratory mechanics10 were
www.chestjournal.org
obtained in our previous studies with PAV. In this
connection, it should be mentioned that all of the
patients in this study and in the previous studies7,10
were receiving low levels of CPAP, which contributed to the reduction of the patients’ inspiratory
effort.4,15
A direct comparison between PAV and PSV was
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485
performed in intubated patients with acute respiratory failure.20 –22 The settings (ie, ICUs), the patients
(ie, in acute condition), and the method of delivering
mechanical ventilation (ie, through the endotracheal
tubes) are too different from the conditions of our
study to make any comparison. However, it should
be mentioned that in those studies,20 –22 PAV allowed
a greater variability of Vt than PSV in the face of an
increased ventilatory demand. Also in our study,
patients receiving PAV showed a greater variability
of Vt than those receiving PSV. However, we failed
to find any significant clinical benefit from this
difference in our long-term patients. In fact, none of
the two modes (ie, PAV or PSV) prevailed in the
patients’ choice of a greater comfort. This might be
caused by the fact that, among others, both modalities were set at the patients’ comfort since the
beginning of the procedure, whereas in the acute
settings, PAV was set after measuring the patients’
respiratory mechanics using the occlusion technique.20 –22 This procedure, which might allow a
better physiologic tailoring of the level of assistance
following the theoretic background of PAV, can be
applied without discomfort in ventilator-dependent
patients, but it is difficult to use in awake patients
reacting to the airway occlusion. Furthermore,
NPPV needs patients’ cooperation, which cannot be
obtained in condition of discomfort. Therefore, regardless the mode of mechanical assistance, the
ventilator must be set at the patient’s comfort to
ensure cooperation.
In comparing different modalities of mechanical
ventilation, a major problem is to avoid the risk to
match up to “apples and oranges.”23 In our study,
this risk was reduced by a few issues. First, the same
ventilator was used to deliver the mechanical assistance. Second, the two modes were applied according to a random order with a similar period (30) of
spontaneous, unsupported breathing before each
mode. Third, the patients were in stable clinical
condition; therefore, it is unlikely that changes in the
patient’s state might affect the results of the physiologic measurements. Finally, both PSV and PAV
were set with the same procedure, namely at the
patient’s comfort.
Having obtained similar physiologic results, PAV
was associated with a lower mean Pao, as assessed by
PTPao, than PSV (151 ⫾ 45 cm H2O/s/min vs
207 ⫾ 73 cm H2O/s/min). This may be relevant in
view of studies24 showing that NPPV can significantly reduce cardiac output in patients with COPD.
It could be extrapolated that the lower mean Pao
with PAV was likely to determine less changes in the
patients’ hemodynamics than PSV. However the lower
mean Pao with PAV was associated with a trend to less
respiratory muscle unloading than with PSV.
PAV required a longer time to be set. There are
two possible explanations for this finding. First, PAV
is not common in the clinical practice, whereas PSV
is the most widely used mode of NPPV. Conceivably,
the caregivers are far more familiar with PSV than
with PAV. In these circumstances, the time to set
PAV will be remarkably reduced by practice. Second, with PSV only Pao must be regulated whereas
with PAV two independent variables (ie, VA and FA)
must be coordinated. Although the manufacturers
are trying to simplify this issue, there is little doubt
that at present the setting of PAV requires more time
and awareness than PSV. Time might not be so
relevant for long-term patients in stable condition,
but it may be crucial for patients needing mechanical
ventilation to treat acute respiratory failure.
In line with previous studies,25 both PAV and PSV
unloaded the patients’ respiratory muscles without
causing changes in lung mechanics (Tables 3, 4). The
low levels of PEEPi,dyn in our patients with COPD
are not surprising in view of their stable conditions.
The most interesting aspect of Table 2 is the
comparison between the levels of assistance (VA and
FA) and patients’ lung mechanics (lung resistance
and elastance). Apparently, FA underassisted the
resistive component in patients with COPD, whereas
VA overassisted the elastic component in patients
with RCWD. The first of these results is in line with
our previous study.10 Similar to those patients, most
of the patients with COPD in this study also did not
tolerate levels of FA ⬎ 5 cm H2O/L/s. The present
study was not tailored to address directly this finding,
but it can be hypothesized that patients with COPD,
by means of dyspnea perception, chose a low level of
FA to avoid excessive increase in Vt, thus protecting
themselves from the development of dangerous levels of dynamic hyperinflation and intrinsic PEEP. In
fact, dynamic hyperinflation is known to increase
dyspnea perception in patients with COPD.26 Moreover, the fact that FA algorithm is linear, and Rl
during inspiration is not,6 may add to the above
hypothesis. It may be that early during inspiration,
Rl was underassisted because of its high value; on
the contrary, with increasing the inspiratory volume
and airway caliber, FA might have been better
matched to a decreasing Rl. In a recent study,
Younes and colleagues27 suggested a new method for
measuring resistance to set PAV in intubated patients with acute respiratory failure. Whether that
method may allow a better tailoring of FA also with
NPPV remains to be established in the clinical
setting.
Finally, the present study does not provide experimental data explaining the apparent VA overassistance of the patients with RCWD. In fact we could
only measure El,dyn, whereas in patients with
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RCWD the major mechanical abnormality is well
known to be related to stiff chest wall. Chest wall
elastance could not be measured in this study, but it
can be assumed to be greater than the level of VA
overassistance indicated by ⌬El,dyn. This hypothesis is indirectly supported by the fact that the
run-away phenomenon was not observed when VA
overwhelmed lung elastance. All together, the above
data on chest wall and lung mechanics suggest that
the measurement of lung mechanics using the
esophageal catheter system can be not only uncomfortable for the patients, but also of little help to set
PAV for clinical purposes when chest wall mechanics
abnormalities are present. The regulation at patient’s
comfort still seems the most acceptable way to set
any mode of NPPV in the common clinical situation.5
Limitations of the Study
PAV has been designed to assist the respiratory
muscles in coping with changes of the ventilatory
demand.6 This particular feature of PAV should
distinguish it from other modes of ventilatory assistance. This study failed to show any advantage of
PAV over PSV to claim as relevant for clinical
purposes, apart from the lower mean Pao. However
we documented that PAV behaved as well as PSV
under resting conditions in very sick patients in
stable clinical condition.
Our study was performed in awake patients,
whereas home NPPV is usually prescribed at night.
Therefore, the correct ventilator setting theoretically
should be tested during a formal sleep study. Nevertheless, we think that the lack of studies of comparison of PAV and PSV would warrant a daytime
investigation, in particular when one takes into account the invasive techniques needed to measure
patient’s respiratory muscle function. Furthermore,
daytime mechanical ventilation in awake patients was
reported to be as equally effective in reversing
chronic hypercapnia as nocturnal mechanical ventilation.28
In conclusion, in resting, awake patients in clinically stable condition with CVF, noninvasive application of PAV, set at patient’s comfort, was not
superior to PSV either in increasing Vt and V̇e, in
unloading the inspiratory muscles, or in patient/
ventilator interaction.
ACKNOWLEDGMENT: We want to thank Dr. Andrea Rossi
for useful talks, comments, and suggestions.
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