Reverse-thrust ventilation in hypercapnic patients with acute respiratory distress syndrome. Acute physiological effects.

Abstract

Techniques of tracheal gas insufflation (TGI) have been shown to enhance CO(2) clearance efficiency in mechanically ventilated patients with acute respiratory distress syndrome (ARDS). Clinical studies have explored the effects of such techniques only at moderate intratracheal gas flow rates, with TGI superimposed to mechanical ventilation in a continuous fashion, or synchronized to the expiratory phase of the duty cycle. We examined the effects of intratracheal pulmonary ventilation (ITPV), delivering the entire tidal volume (VT) in the proximity of the tracheal carina, with all the gas flow supplied continuously through a reverse-thrust catheter (RTC). A potential limitation in the application of TGI is dynamic hyperinflation. Therefore, in a subgroup of patients, we also evaluated the effects of ITPV on end-expiratory lung volume (EELV) by respiratory inductive plethysmography (RIP). Eleven patients with ARDS under volume-cycled mechanical ventilation were subsequently switched to ITPV at the same baseline respiratory rate, I:E ratio, and VT. At the same minute volume, Pa(CO(2)) decreased from 70 +/- 12.3 to 59 +/- 9.5 mm Hg, with a percent reduction of 15 +/- 4% (range from 10 to 20%). The CO(2) decrease was greater in patients with higher baseline Pa(CO(2)) levels (DeltaPa(CO(2)) = 0.29 x Pa(CO(2)) - 9.48, r = 0.95). During transition from mechanical ventilation to ITPV, tracheal positive end-expiratory pressure (PEEP(tr)) decreased with a correspondent decrease in EELV. Both were restored by increasing the PEEP at the ventilator by 3.6 +/- 2.0 cm H(2)O. These data suggest that in patients with ARDS ITPV effectively reduces dead space ventilation and the employment of the RTC may limit or avoid dynamic hyperinflation.

Full text

Page 1

Am J Respir Crit Care Med
Internet address: www.atsjournals.org
Vol 162. pp 363–368, 2000
Reverse-Thrust Ventilation in Hypercapnic Patients
with Acute Respiratory Distress Syndrome
Acute Physiological Effects
NICOLA ROSSI, GUIDO MUSCH, FABIO SANGALLI, MURIEL VERWEIJ, NICOLO PATRONITI, ROBERTO FUMAGALLI,
and ANTONIO PESENTI
Department of Anesthesia and Intensive Care, Ospedale San Gerardo Nuovo dei Tintori, University of Milan, Monza, Milan, Italy
Techniques of tracheal gas insufflation (TGI) have been shown to
enhance CO
2
clearance efficiency in mechanically ventilated pa-
tients with acute respiratory distress syndrome (ARDS). Clinical
studies have explored the effects of such techniques only at mod-
erate intratracheal gas flow rates, with TGI superimposed to me-
chanical ventilation in a continuous fashion, or synchronized to
the expiratory phase of the duty cycle. We examined the effects of
intratracheal pulmonary ventilation (ITPV), delivering the entire
tidal volume (V
T
) in the proximity of the tracheal carina, with all
the gas flow supplied continuously through a reverse-thrust cathe-
ter (RTC). A potential limitation in the application of TGI is dy-
namic hyperinflation. Therefore, in a subgroup of patients, we also
evaluated the effects of ITPV on end-expiratory lung volume
(EELV) by respiratory inductive plethysmography (RIP). Eleven pa-
tients with ARDS under volume-cycled mechanical ventilation
were subsequently switched to ITPV at the same baseline respira-
tory rate, I:E ratio, and V
T
. At the same minute volume, Pa
creased from 70
?
12.3 to 59
?
9.5 mm Hg, with a percent reduc-
tion of 15
?
4% (range from 10 to 20%). The CO
greater in patients with higher baseline Pa
0.29
?
Pa
CO2

?
9.48, r
?
0.95). During transition from mechanical
ventilation to ITPV, tracheal positive end-expiratory pressure
(PEEP
tr
) decreased with a correspondent decrease in EELV. Both
were restored by increasing the PEEP at the ventilator by 3.6
cm H
2
O. These data suggest that in patients with ARDS ITPV effec-
tively reduces dead space ventilation and the employment of the
RTC may limit or avoid dynamic hyperinflation.
CO2
de-
2
decrease was
levels (
?
CO2
Pa
CO2

?
?
2.0
The recent recognition of the potential for ventilator-induced
lung injury (V ILI) in patients with acute respiratory distress
syndrome (A RDS) has focused attention on ventilatory strat-
egies aimed at reducing airway pressure and tidal volumes
(V
T
) (1–3).
A lmost invariably, however, low V
hypercapnia (4), which, in turn, carries many side effects, such
as pulmonary hypertension and increased intracranial pres-
sure in patients with head trauma. Reducing the potential for
barotrauma and V ILI remains, however, of major importance
in the management of patients with A RDS.
Tracheal gas insufflation (TGI) improves CO
by its effect of bypassing or washing out the anatomical dead
space. The addition of TGI techniques to mechanical ventila-
tion allows the use of very small V
an appropriate respiratory rate, may reach the target of avoid-
ing dangerous ventilatory settings while maintaining normocap-
T
ventilation results in
2
elimination
T
that, when combined with
nia. TGI techniques have been developed and investigated
both in animal (5–8) and human studies (9–15).
Intratracheal pulmonary ventilation (ITPV ) is a TGI tech-
nique that delivers the entire V
T
carina, while the ventilator acts as a shutter that directs the flow
alternatively in and out of the respiratory system (16–18).
To our knowledge, the implementation of continuous TGI
on patients with acute respiratory failure has been tested as an
adjunctive support to mechanical ventilation at relatively low
flow rates only (up to 6 L/min) (13, 15, 19). ITPV alone or as an
adjunct to mechanical ventilation has been applied on patients
with acute lung injury, although clinical experience is still lim-
ited (20, 21). A continuous flow of fresh gas as the only source
of ventilation has never been evaluated systematically on se-
dated and paralyzed patients with A RDS. The present study
evaluates the CO
2
clearance efficiency of continuous ITPV de-
livering the entire V
T
through the intratracheal catheter and
compares it with volume-cycled mechanical ventilation.
A possible major problem with ITPV in adult patients is re-
lated to the high gas flow that is needed at the carina to pro-
vide the entire V
T
: high flows, when delivered through stan-
dard catheters, commonly create an obstacle to exhalation,
resulting in hyperinflation and incomplete expiration.
A special intratracheal reverse-thrust catheter (RTC) has
been designed to direct the gas flow out of the lung. Such a cath-
eter has been proven to help the expiratory phase, avoiding
dynamic hyperinflation through a Venturi effect (17, 22). To ver-
ify this additional benefit, we used an RTC in the present study,
and in a subgroup of patients we also measured end-expiratory
lung volume (EELV ) by respiratory inductive plethysmog-
raphy (RIP).
in the proximity of the tracheal
METHODS
Eleven patients meeting the A RDS criteria (23) with a Murray score
of 2.5
?
0.3 at the time of the study were enrolled. A ll patients were
under stable conditions without significant variations in hemody-
namic status or body temperature. In nine patients a thermodilution
Swan-Ganz catheter was in place before enrollment, and all had an in-
traarterial catheter and a central vascular line for clinical monitoring.
Patients’ demographics and clinical data are shown in Table 1. No in-
vasive procedures were performed for at least an hour before data
collection to limit baseline metabolic rate and CO
variations.
The experimental protocol was reviewed and approved by the
Ethical Committee of the S. Gerardo Hospital and informed consent
was obtained from the family member closest in kinship.
2
elimination (
CO2
)
Instrumentation and Monitoring
Each patient had undergone nasotracheal or orotracheal intubation
with a standard endotracheal tube (ETT) (Mallinckrodt Laboratories
Ltd., A thlone, Ireland) (range 7.5–9.0 mm i.d.). During the study pe-
riod all patients received a continuous intravenous infusion of fenta-
nyl and propofol at approximately 1
tively. A dequate muscle relaxation was achieved by administration of
pancuronium bromide in boluses. Patients were connected to a Servo
?
g/kg/h and 2 mg/kg/h, respec-
V·
(
Supported by departmental funds.
Correspondence and requests for reprints should be addressed to Nicola Rossi,
M.D., Department of Anesthesia and Intensive Care, Ospedale Nuovo San Ger-
ardo dei Tintori, Via Donizetti 106, 20052 Monza, Milan, Italy. E-mail: rossin@
planet.it
Received in original form August 5, 1999 and in revised form December 17, 1999)

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364
AMERICAN JOURNAL OF RESPIRATORY AND CRITICAL CARE MEDICINE VOL 1622000
ventilator 900 C (Siemens Elema, Solna, Sweden) and mechanically
ventilated in the volume control mode (V CV ) with a V
ml/kg and a respiratory rate (RR) of 15
expiratory (I:E) ratio was set at 1:1 and the inspired oxygen fraction
(F
IO
2
) was set at 1 throughout the study.
Under bronchoscopic guidance the ETT was advanced to a level 1
to 2 cm above the carina. A 7-fr-i.d. RTC, providing a backward flow
through an annular orifice, was slid inside the ETT through an adap-
tor. The RTC was meant to lie within the distal end of the ETT to
avoid any damage to the tracheal mucosa (17). The correct position of
the catheter was previously determined by inserting the RTC inside
an ETT of the same size in use for the patient.
Pressure at the airway opening (Pao) was measured at the proxi-
mal end of the ETT. Peak inspiratory pressure (PIP
end-expiratory pressure (PEEP
tr
) were measured in the main airways
via a small catheter (1.5 mm i.d.) provided with side holes, inserted
through the ETT.
A n esophageal balloon catheter (Smartcath, Bicore Monitoring
Systems, Irvine, CA ) was positioned in the distal third of the esopha-
gus and attached to a pressure transducer to record esophageal pres-
sure (Pes). Pao, tracheal pressure, and Pes were measured with cali-
brated pressure transducers (Bentley Trantec Inc., A rmstrong, CA )
and simultaneously recorded on an eight-channel thermal recorder
(TA 5000 recorder; Gould Instruments, Cleveland, OH). Flow was
measured by a heated pneumotachograph (Fleisch #2) mounted at the
Y -piece, connected to a differential pressure transducer (V alidyne
MP 45; V alidyne Co., Northridge, CA ) and calibrated with oxygen.
V olume was computed by digital integration of the flow signal (
low
). The flow, Pes, Pao, and tracheal pressures signals were pro-
cessed via an analog-to-digital converter (80 samples/s per channel)
by an IBM portable PC for storage and later analysis.
In the last six patients studied, bands for respiratory inductive
plethysmography (RIP) (Respitrace Plus; NIMS, Inc., Miami Beach,
FL) were placed around the chest and the abdomen to detect changes
of EELV while switching from one mode of ventilation to another, as
well as to help ensure a constant V
corrent-coupled mode; signals were recorded on paper and processed
by a PC for later analysis.
A mixing chamber (5 L) attached to the expiratory port of the ven-
tilator allowed continuous sampling of mixed expired CO
tion (Datex Normocap, Instrumentarium Corp., Helsinki, Finland).
A t the beginning of each study the CO
gas mixture of known CO
2
concentration (5.3%). The efficiency of
the mixing chamber was individually tested in each patient by com-
paring mixed expired CO
2
concentration with the data obtained from
a standard expiratory gas collection procedure.
A rterial and mixed venous blood samples were analyzed at 37
(Radiometer A BL, Copenhagen, Denmark) and corrected for body
temperature. Right-to-left shunt (
lated arterial and mixed venous oxygen contents.
T
of 8.2
?
1.9
?
5 bpm. The inspiratory to
tr
) and positive
see be-
T
(24). RIP was used in the direct
2
concentra-
2
analyzer was calibrated with a
?
C
S
/
T
) was computed from calcu-
Q
.
Q
.
ITPV System and Calculations
During ITPV , a continuous flow of humidified oxygen was supplied to
the RTC through a flowmeter. Gas flow was delivered through two in-
series Conchatherm III temperature-controlled humidifiers (Hudson
RCI, Temecula, CA ). Pressure proximal to humidifiers and tempera-
ture of the two heating columns were continuously monitored. The
ventilator was set in the pressure control mode at 0 cm H
PEEP level (i.e., no flow was delivered). The inspiratory valve re-
mained closed at all times, while the expiratory valve of the ventilator
acted as a shutter. When it was closed all flow delivered through the
RTC entered the lungs; when it opened, the expiratory V
continuous intratracheal flow from the RTC were exhaled (18). The
expiratory valve opening and closure times depended on the RR and
I:E ratio, which were checked on the PC recordings of the airway pres-
sures and gas flow tracings. Because during ITPV all continuous flow
was delivered directly to the catheter and inspiratory flow from the
ventilator equaled zero, the actual V
cle (V
T
,
ITPV
) was proportional to the expiratory valve closure time.
A pproximating for V
insp

?
V
exp
in the respiratory system, and given an
I:E ratio of 1:1, the effective inspiratory V
from the total expiratory flow (
exp
V˙T, ITPV
V˙exp
I (×
∫
V·
2
O above the
T
plus the
T
delivered at each respiratory cy-
T
(V
T
,
ITPV
) was obtained
) and computed as follows:
CO2
was calculated as the product of the measured mixed expired
(F
CO2
) and total minute volume. Physiologic dead space fraction
/V
T
) was calculated from the Enghoff modification of the Bohr
equation (25) as follows:
V˙CO2
760
×([
–=
CO
(V
2
D
During baseline V CV , static respiratory compliance (Cpl,rs) and total
inspiratory resistance of the respiratory system (Raw,rs) were as-
sessed by an occlusion maneuver. A ppropriate corrections were made
for compressible volumes (approximately 0.7 ml/cm H
auto-PEEP (PEEP
i
) was measured during the end-expiratory occlu-
sion. Cpl,rs was obtained by dividing the expired volume by the differ-
ence between end-inspiratory elastic recoil pressure and end-expira-
tory occlusion airway pressure (PEEP
from the pressure tracing measured at the airway opening during an
end-inspiratory occlusion, as previously described (26).
During both V CV and ITPV , the difference between end-inspira-
tory and end-expiratory esophageal pressure (
an indirect index of inspiratory V
T
the airway opening and MA P
tr
were calculated by time averaging the
signals over five respiratory cycles.
2
O) (26). Static
?
PEEP
i
). Raw,rs was computed
?
Pes) was measured as
. Mean airway pressure (MA P) at
Experimental Protocol
A fter patient preparation, we allowed 60 min of equilibration before
collecting the first baseline data set. Measurements were taken before
and after the RTC insertion (without flow) to detect changes on respi-
V·
IE)
+
⁄
1 2
⁄
V˙exp
∫
==
VD VT
⁄
1
)
PaCO2
RR
×()⁄] V
⁄
T
TABLE 1
PATIENT POPULATION*
Patient
No. Age (yr)
Sex
M/F ()DiagnosisPEEP (cm H
2
O)
Pa
(mm Hg
O2
/F
IO2
)
Cpl,rs
ml/cm H(
2
O) MV (L/min)
1
2
3
4
5
6
7
8
9
19
70
72
23
43
34
24
69
42
32
65
M
M
M
M
M
M
F
M
M
M
M
Polytrauma
Legionella pneumonia
Abdominal sepsis
Polytrauma
Septic shock
Bacterial pneumonia
Abdominal sepsis
Bacterial pneumonia
Bacterial pneumonia
Hematologic malignancy
Septic shock
12167
111
190
158
141
200
161
121
130
170
195
60
35
61
36
45
32
25
60
34
71
50
7.2
10.4
7.4
12.6
11.9
8.8
6.2
8.9
9.1
8.8
13.4
8
10
14
15
10
12
10
10
10
10
10
11
Mean
?
SD45
?
2110/111.0
?
2.0 185
?
4546.3
?
159.5
?
2.3
Definition of abbreviations
oxygen ratio; PEEP
* Patients’ clinical data at the entry in the study protocol.
: Cpl,rs
?
respiratory system compliance; MV
positive end-expiratory pressure at the airway opening.
?
minute volume; Pa
O2
/F
I
O2

?
arterial Po
2
to inspired fraction of
?

Page 3

Rossi, Musch, Sangalli, et al.: Reverse-thrust Ventilation in ARDS
365
ratory mechanics and/or gas exchange. Measurements obtained after
the insertion of the RTC were considered as the baseline (B
after, ITPV was started after switching to the pressure control mode
as described, while the intratracheal flow was progressively increased
to reach the same V
T
as during baseline V CV .
To verify baseline reproducibility and stability, each patient was
returned to baseline V CV (B
1
). Each experimental period lasted 30 to
40 min, after which time data were collected.
PEEP
tr
was meant to be kept constant during ITPV . F
1 and was kept constant throughout the protocol, as well as RR and
the I:E ratio. When available, RIP signals allowed continuous moni-
toring of inspired V
T
and EELV . A nalysis of recorded data was per-
formed when full stability of V
T
and PEEP
perimental stage, mixed venous and arterial blood samples were
obtained and hemodynamic parameters were collected.
0
). There-
IO
2
was set at
tr
was achieved. A t each ex-
Statistical Analysis
Two-way analysis of variance (A NOV A ) for repeated measurements
was used to evaluate differences for each variable among B
and B
1
data sets. Relationships between selected parameters were
evaluated by simple regression analysis. Regression equations, when
reported, are expressed with the same unit of measurement used in ta-
bles or in the text. Data are expressed as means
ues lower than 0.05 were considered significant.
0
, ITPV ,
?
SD. Probability val-
RESULTS
Raw,rs before and after the insertion of the RTC (without
flow) increased from 14.3
?
8.6 to 17.4
spectively (p
?
0.05). PEEP, Cpl,rs, and gas exchange were
not affected by the presence of the RTC (data not reported).
Baseline PEEP
i
during V CV was 0.43
cm H
2
O). The intratracheal flow rate averaged 20.1
min (range from 14.5 to 25.8 L/min). The working pressure,
developed in the gas delivery system, ranged from 350 to 820
mm Hg (540
?
190 mm Hg). Temperature of the delivered gas
was steadily maintained at about 37
were detected during any experimental stage.
?
6.5 cm H
2
O/L/s, re-
?
0.61 (range 0.2–2.0
?
3.8 L/
?
C. No adverse events
Gas Exchange
Stability in V T between V CV and ITPV stages was achieved
(8.2 ? 1.9 and 8.9 ? 1.8 ml/kg, respectively, p ? 0.37), and I:E ra-
tio was constant at 1:1. No changes in ?Pes were observed be-
tween experimental periods (Table 2). While V T was constant,
V D/V T significantly decreased during ITPV (Table 2) due to a de-
crease in V D of 65 ? 45 ml (p ? 0.01). Consequently, alveolar ven-
tilation (
VCV to 4.54 ? 0.94 L/min during ITPV, p ? 0.005). A rterial PCO2
(PaCO2) consistently decreased with an average percentage reduc-
tion relative to baseline of 15 ? 4% (range 10 to 20%); accord-
ingly, arterial pH improved (Table 2). Individual PaCO2 changes
are shown in Figure 1. A s expected, the greatest CO2 decrease
was observed in patients with the highest baseline PaCO2 levels
(?PaCO2 ? 0.29 ? PaCO2 ?9.48; r ? 0.95, p ? 0.001). A fter return
to B1, PaCO2 was slightly but significantly lower than B0 (Table 2).
CO2 was significantly higher during ITPV and then returned to
its baseline value during B1 (Table 2). Changes in arterial PO2
(PaO2) did not reach the level of significance, although s/
significantly different among the three stages (Table 2).
A) increased by 26 ? 12% (from 3.52 ? 0.82 L/min at
T was
Lung Volume and PEEP
When RIP was available, the measured V T was constant
throughout the protocol. During transition from V CV to
ITPV , as intratracheal catheter flow was started and then pro-
gressively increased to match baseline V T, dynamic PEEPtr
decreased. A ccordingly, EELV progressively declined (Table
3). Baseline values were restored by increasing the external
PEEP at the ventilator by an average of 3.6 ? 2.0 cm H2O
(Figure 2).
V·
V·
·Q·Q
TABLE 2
MECHANICS AND GAS EXCHANGE DURING VOLUME-CYCLED
MECHANICAL VENTILATION AND DURING ITPV*
B0
ITPVB1
ANOVA
p-Value
PaCO2, mm Hg 70 ? 12.3 59 ? 9.569 ? 12.5
? 0.005†
? 0.025‡
NS
? 0.005†
NS
NS
? 0.005†
? 0.005†
? 0.05†
? 0.05‡
? 0.05†
NS
NS
NS
PaO2, mm Hg
pH
BE
H2CO3, mEq/L
VD, ml/kg
VD/VT
S/
201 ? 75
7.28 ? 0.05
4.3 ? 3.5
32 ? 4
5.1 ? 0.7
0.62 ? 0.14
39 ? 2
230 ? 85
7.33 ? 0.06
4.3 ? 4.4
31 ? 4
4.2 ? 0.9
0.53 ? 0.15
36 ? 10
209 ? 45
7.29 ? 0.05
4.0 ? 4.7
31.5 ? 5
5.2 ? 0.9
0.60 ? 0.13
37 ? 8
T, % (n ? 9)
CO2, Vml/min kg?1
?Pes, cm H2O
Peaktr, cm H2O
PEEPtr, cm H2O
4.4 ? 1.6
1.7 ? 0.6
32.0 ? 6.0
12.4 ? 7.0
4.6 ? 1.4
1.6 ? 1.0
33.0 ? 8.0
12.3 ? 2.5
4.3 ? 1.2
1.8 ? 0.7
32.5 ? 6.0
12.0 ? 5.5
Definition of abbreviations: ANOVA ? analysis of variance; BE ? base excess; ITPV ?
intratracheal pulmonary ventilation; NS ? not significant; ?Pes ? change in esoph-
ageal pressure; PaCO2 ? arterial PCO2; PaO2 ? arterial PO2, measured at an inspired frac-
tion of oxygen of 1; Peaktr ? peak inspiratory pressure at the carina; PEEPtr ? positive
end-expiratory pressure at the carina;
production metabolic rate; VD ? physiological dead space; VD/VT ? physiological dead
space fraction.
* All values are means ? SD. n ? 11, unless otherwise specified.
Results of ANOVA: †ITPV compared with (B0 ? B1); ‡B0 compared with B1.
S/
T ? right-to-left shunt fraction; Vco2 ? CO2
Q·Q·
V·
Q·Q·
Figure 1. PaCO2 at baseline volume-cycled mechanical ventilation (B0),
during ITPV and after return to baseline mechanical ventilation (B1) in
each patient. Horizontal lines indicate mean values (n ? 11) and boxes
indicate SD.
TABLE 3
END-EXPIRATORY LUNG VOLUME AND PEEPtr CHANGES
FROM BASELINE VOLUME-CYCLED MECHANICAL
VENTILATION (B0) to ITPV
Patient No.
?EELV (ml)
?PEEPtr (cm H2O)
6
7
8
9
?90
?200
?225
?80
?135
?320
?3.0
?4.0
?3.0
?2.0
?3.0
?7.5
10
11
Mean ? SD
?175 ? 91
?3.6 ? 2.0
Definition of abbreviations: EELV ? end-expiratory lung volume; ITPV ? intratrachial
pulmonary ventilation; PEEPtr ? positive end-expiratory pressure at the carina.

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366
AMERICAN JOURNAL OF RESPIRATORY AND CRITICAL CARE MEDICINEVOL 1622000
Hemodynamics
Patients showed no relevant temperature changes throughout
the protocol. No major hemodynamic changes were observed.
Nevertheless, cardiac output (CO) slightly decreased during
ITPV and, accordingly, there was a small but significant reduc-
tion in mixed venous saturation (Table 4). A lthough of little
clinical relevance, mean pulmonary arterial wedge pressure
() was slightly but significantly lower during ITPV (Ta-
ble 4, p ? 0.05), whereas central venous pressure (CV P) did
not significantly change at any stage.
Mean pulmonary arterial pressure (
creased during ITPV reaching the level of significance (Table
4). Regression analysis did not show any significant correla-
tion between changes in and PaCO2, pH, or PaO2.
Ppa
) marginally de-
DISCUSSION
General Comments
The main results of this study could be summarized as follows:
in a selected group of patients with A RDS, ITPV was able to
deliver the entire V T, bypassing the anatomical dead space
and providing a significant reduction of PaCO2.
Ppaw
Ppa
In spite of the use of relatively high gas flow rates, the RTC
was able to facilitate expiration, as indicated by the need for
an increased external PEEP and by a constant EELV as mea-
sured by RIP.
Gas Exchange
Our results are similar to those reported with continuous TGI
at moderate catheter flow rates (4–6 L/min) in patients with
acute respiratory failure (13, 15, 27). More recently, Belghith
and colleagues obtained an average PaCO2 reduction of ap-
proximately 20% at a catheter flow rate of 4 L/min in a group
of severely hypercapnic patients with A RDS (19).
In the current study, we enrolled moderately hypercapnic
patients with quite high metabolic rates. ITPV yielded a de-
crease in V D that accounted approximately for the estimated
apparatus dead space.
In the setting of acute lung injury, an increased alveolar dead
space fraction is expected to reduce the efficacy of intratracheal
insufflation techniques. This effect can be partially counterbal-
anced by the implementation of permissive hypercapnia with
higher alveolar CO2 concentration and a greater V D/V T. In a re-
cent experimental study on oleic acid-injured dogs, Nahum and
colleagues evaluated the CO2 clearance efficiency of expiratory
TGI at 10 L/min (28). In their experimental conditions, the au-
thors have shown how, for a given decrease in V D, PaCO2 is de-
creased by different amounts according to baseline PaCO2, V D/
V T, and V T. Such a relationship can explain the observed PaCO2
reduction in our clinical setting, in which V T was not as low as
those implemented in more severe patients with A RDS.
Nahum and colleagues tested the effect of an inverted-tip in-
sufflation catheter compared with a straight catheter with respect
to CO2 removal efficiency (8). They demonstrated that the effi-
cacy of continuous TGI is primarily related to expiratory washout
of proximal dead space. They also concluded that a minor contri-
bution to increased CO2 clearance may be related to a jet effect
created by high flow rates through the straight catheter (up to 15
L/min) with turbulent gas mixing distally to the catheter tip.
The RTC we used, with its upward thrust during expira-
tion, may be less efficient with respect to CO2 clearance, al-
though a recent experimental study conducted on spontane-
ously breathing animals did not show any difference between
the RTC and a straight catheter (29). Nevertheless, this aspect
needs further exploration.
Figure 2. Transition from base-
line volume-controlled mechan-
ical ventilation (VCV) to ITPV
for a representative patient (#9).
From top to bottom, pressure
tracing at the airway opening
(Pao), at the carina (Pawtr), and
respiratory inductive plethysmog-
raphy signal (Vol). After match-
ing baseline VT, intratracheal
gas insufflation resulted in a
reduction in PEEPtr with a re-
spective decrease in end-expi-
ratory lung volume (EELV). Both
were restored by increasing PEEP
at the airway opening (arrow).
TABLE 4
HEMODYNAMICS DURING VOLUME-CYCLED MECHANICAL
VENTILATION AND DURING ITPV*
B0
ITPVB1
ANOVA
p-Value
HR, bpm
BP, mm Hg
, mm Hg (n ? 9)
CO, L/min (n ? 9)
SVO2, % (n ? 9)
118 ? 17
80 ? 21
33 ? 2.6
10.8 ? 2.7
82.1 ? 4.6
114 ? 21
78 ? 19
31 ? 2.4
10.0 ? 2.6
79.4 ? 5.9
116 ? 23
79 ? 21
32.5 ? 3.8
10.35 ? 2.7
80 ? 5.2
NS
NS
? 0.005†
NS
? 0.05†
? 0.01‡
NS
? 0.025‡
CVP, mm Hg
PAOP, mm Hg (n ? 9)
12 ? 3
16 ? 3
11 ? 2
14 ? 3
12 ? 2.4
15 ? 3
Definition of abbreviations: ANOVA ? analysis of variance; BP ? mean systemic blood
pressure; CO ? cardiac output; CVP ? central venous pressure; HR ? heart rate; ITPV ?
intratrachial pulmonary ventilation; NS ? not significant; PAOP ? pulmonary artery oc-
clusion (wedge) pressure; ? mean pulmonary arterial pressure; SVO2 ? mixed
venous saturation.
* All values are means ? SD. n ? 11, unless otherwise specified.
Results of ANOVA: †ITPV compared with (B0 ? B1); ‡B0 compared with B1.
Ppa
Ppa

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Rossi, Musch, Sangalli, et al.: Reverse-thrust Ventilation in ARDS
367
Differences in PaCO2 between the two V CV stages, B0 and
B1, might be explained by the fact that interval times elapsed
among measurements were not long enough to achieve a com-
plete steady state.
We could not detect any significant change in PaO2, al-
though in some patients there was a slight increase in arterial
oxygenation. If MA P and mean lung volume, which are the
major determinants of arterial oxygenation, are kept constant,
PaO2 is not expected to change during TGI, as recently re-
ported in a study on oleic acid-injured dogs (30).
Changes in cardiac output were not significant, although a
slight decreasing trend could be recognized during ITPV. Expla-
nation for this nonsignificant difference is not immediately obvi-
ous. A n increase in intrathoracic (i.e., intrapulmonary) pressure
can be excluded as the cause of it, as CVP,
show either a significant decrease or a decreasing trend (Table 4).
, and all
Pressure and Volume Effects
A ccording to previous experimental observations, the turbulence
of the flow emerging from the RTC produces a jet that creates a
Venturi effect (17, 22). The pressure decrease depends on the fol-
lowing factors: (1) the ratio of the cross-sectional areas of the ETT
and the catheter; (2) the gap at the catheter tip where the catheter
flow emerges; (3) gas velocity; and (4) duration of expiration.
During constant flow ventilation, dynamic pressure measure-
ments distal to the catheter tip can be influenced by pressure fluc-
tuations related to turbulence and coexistence of inspiratory and
expiratory flows in the region of the carina (31). However, mech-
anisms of flow and pressure distribution during TGI are less
known. During continuous TGI, EELV and dynamic PEEP in-
crease in direct proportion to flow and inspiratory time, as re-
cently reported using an argon washout method (32). Nahum and
colleagues in their study conducted with an inverted-tip catheter
(8), found that tracheal pressure measurements did not accu-
rately reflect changes in lung volume and, consequently, in mean
alveolar pressure. They suggested that the inverted jet entrained
alveolar gas, accelerating expiratory flow during the early phase
of expiration with an increase in expiratory flow-resistive pres-
sure losses. Such a mechanism still remains unclear and, although
there might be some similarities with ITPV with respect to the di-
rection of flow during the expiratory phase, it is still unknown
how to apply this mechanism to the peculiar design of the RTC.
A nother recent study reported a decrease in static PEEP mea-
sured at end-expiratory occlusion during expiratory-phase TGI
with a modified ETT that produced a reverse flow (33).
In general, however, a major problem in TGI clinical ex-
periments, even when low flow rates have been used, has been
that EELV invariably increased. However, to maintain a con-
stant EELV we always had to increase external PEEP to com-
pensate for the observed decrease in PEEPtr. Therefore, the
RTC may be a useful tool to prevent hyperinflation during tra-
cheal insufflation at high flow rates, as it was in our case.
Nevertheless, further investigations of these aspects are re-
quired, including setups that may allow static mean alveolar
pressure measurements.
CONCLUSIONS
In the present study, ITPV effectively reduced dead space
ventilation in patients with A RDS. Our preliminary data sug-
gest a potential benefit from the use of the RTC. Some techni-
cal concerns may arise in the long-term clinical application of
the technique. Particularly, humidification of the delivered
flow at such flow rates may be troublesome. Further techno-
logical improvement is required to apply the technique with
Ppaw
Ppa
appropriate safety devices and more studies are needed to test
its efficacy at different ventilator settings.
Acknowledgment: The authors would like to thank Dr. T. Kolobow for his
advice and technical support in providing the reverse-thrust catheter. They
also thank the nursing staff for its helpful cooperation without which this
study could not have been realized.
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