Evaluation of an impedance threshold device in patients receiving active compression-decompression cardiopulmonary resuscitation for out of hospital cardiac arrest

Article (PDF Available)inResuscitation 61(3):265-71 · July 2004with24 Reads
DOI: 10.1016/j.resuscitation.2004.01.032 · Source: PubMed
Abstract
The purpose of this multicentre clinical randomized controlled blinded prospective trial was to determine whether an inspiratory impedance threshold device (ITD), when used in combination with active compression-decompression (ACD) cardiopulmonary resuscitation (CPR), would improve survival rates in patients with out-of-hospital cardiac arrest. Patients were randomized to receive either a sham (n = 200) or an active impedance threshold device (n = 200) during advanced cardiac life support performed with active compression-decompression cardiopulmonary resuscitation. The primary endpoint of this study was 24 h survival. The 24 h survival rates were 44/200 (22%) with the sham valve and 64/200 (32%) with the active valve (P = 0.02). The number of patients who had a return of spontaneous circulation (ROSC), intensive care unit (ICU) admission, and hospital discharge rates was 77 (39%), 57 (29%), and 8 (4%) in the sham valve group versus 96 (48%) (P = 0.05), 79 (40%) (P = 0.02), and 10 (5%) (P = 0.6) in the active valve group. Six out of ten survivors in the active valve group and 1/8 survivors in the sham group had normal neurological function at hospital discharge (P = 0.1). The use of an impedance valve in patients receiving active compression-decompression cardiopulmonary resuscitation for out-of-hospital cardiac arrest significantly improved 24 h survival rates.

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Resuscitation 61 (2004) 265–271
Evaluation of an impedance threshold device in patients receiving active
compression–decompression cardiopulmonary resuscitation
for out of hospital cardiac arrest
Patrick Plaisance
a,
, Keith G. Lurie
b
, Eric Vicaut
c
, Dominique Martin
d
,
Pierre-Yves Gueugniaud
e
, Jean-Luc Petit
f
, Didier Payen
a
a
Department of Anesthesiology and Critical Care, Lariboisière University Hospital, 2 Rue Ambroise Paré, 75010 Paris, France
b
Department of Emergency Medicine, Hennepin County Medical Center, University of Minnesota,
and Advanced Circulatory Systems, Incorporated, Eden Prairie, MN, USA
c
Clinical Research Unit, Lariboisière University Hospital, 2 Rue Ambroise Paré, 75010 Paris, France
d
Emergency Department-SMUR, Vernon, France
e
Department of Anesthesiology and Critical Care, Lyon, France
f
Emergency Department-SMUR, Thionville, France
Received 20 November 2003; received in revised form 12 January 2004; accepted 19 January 2004
Abstract
Aims: The purpose of this multicentre clinical randomized controlled blinded prospective trial was to determine whether an inspiratory
impedance threshold device (ITD), when used in combination with active compression–decompression (ACD) cardiopulmonary resuscitation
(CPR), would improve survival rates in patients with out-of-hospital cardiac arrest. Methods and results: Patients were randomized to receive
either a sham (n = 200) or an active impedance threshold device (n = 200) during advanced cardiac life support performed with active
compression–decompression cardiopulmonary resuscitation. The primary endpoint of this study was 24 h survival. The 24h survival rates
were 44/200 (22%) with the sham valve and 64/200 (32%) with the active valve (P = 0.02). The number of patients who had a return of
spontaneous circulation (ROSC), intensive care unit (ICU) admission, and hospital discharge rates was 77 (39%), 57 (29%), and 8 (4%) in the
sham valve group versus 96 (48%) (P = 0.05), 79(40%)(P = 0.02), and 10 (5%) (P = 0.6) in the active valve group. Six out of ten survivors
in the active valve group and 1/8 survivors in the sham group had normal neurological function at hospital discharge (P = 0.1). Conclusion:
The use of an impedance valve in patients receiving active compression–decompression cardiopulmonary resuscitation for out-of-hospital
cardiac arrest significantly improved 24 h survival rates.
© 2004 Elsevier Ireland Ltd. All rights reserved.
Keywords: Cardiac arrest; Active compression–decompression; Cardiopulmonary resuscitation (CPR); Impedance threshold device; Outcome
Resumo
Objectivo: O objectivo deste estudo cl
´
ınico multic
ˆ
entrico prospectivo aleatorizado cegamente controlado foi determinar se um aparelho
do limiar de imped
ˆ
ancia inspiratório (IDT), quando utilizado em combinação com a reanimação cardio-pulmonar (CPR) com Compressão–
Descompressão activa (ACD), melhora a taxa de sobreviv
ˆ
encia nas v
´
ıtimas de paragem card
´
ıaca extra-hospitalar. Método e resultados: Os
doentes foram aleatorizados para receber quer uma simulação (n = 200) ou um aparelho do limiar de imped
ˆ
ancia activo (n = 200) durante o
suporte avançado devida realizado comreanimação cardio-pulmonar com compressão-descompressão activa.O primeiro alvo desteestudo foi
a sobreviv
ˆ
encia às 24 horas. A taxa desobreviv
ˆ
encia às 24 horas foi 44/200 (22%)com aválvula simuladae 64/200 (32%) com a válvula activa
(P = 0.02). O número de doentes que recuperou a circulação espont
ˆ
anea (ROSC), as admissões na Unidade de Cuidados Intensivos (ICU),
e as taxas de alta hospitalar foram 77 (39%), 57 (29%), e 8 (4%) no grupo de simulação versus 96 (48%) (P = 0.05), 79 (40%) (P = 0.02),
e 10 (5%) (P = 0.6) no grupo com válvula activa. Seis de dez sobreviventes no grupo válvula activa e 1/8 sobreviventes no grupo simulação
tinham uma função neurológica normal na altura da alta hospitalar (P = 0.1). Conclusão: A utilização de uma válvula de imped
ˆ
ancia nos
Corresponding author. Tel.: +33-1-49-95-61-61; fax: +33-1-49-95-61-60.
E-mail address: plaisance@claranet.fr (P. Plaisance).
0300-9572/$ – see front matter © 2004 Elsevier Ireland Ltd. All rights reserved.
doi:10.1016/j.resuscitation.2004.01.032
266 P. Plaisance et al. /Resuscitation 61 (2004) 265–271
doentes que recebem reanimação cardio-pulmonar com compressão-descompressão activa para a paragem card
´
ıaca extra-hospitalar melhora
de forma significativa a taxa de sobreviv
ˆ
encia às 24 horas.
© 2004 Elsevier Ireland Ltd. All rights reserved.
Palavras chave: Paragem card
´
ıaca; Compresão-descompressão activa; Reanimação cardio-pulmonar (CPR); Válvula do limiar de imped
ˆ
ancia; Prognóstico
Resumen
Objetivo: El propósito de este estudio cl
´
ınico prospectivo, randomizado, multicéntrico ciego controlado fue determinar si un dispositivo de
umbral de impedancia inspiratoria (ITD), usado en combinación con reanimación cardiopulmonar (CPR) con compresión y descompresión
activa (ACD), mejorar
´
ıa las tasas de sobrevida en pacientes con paro cardiorespiratorio extrahospitalario(OHCA). Métodos y Resultados: Los
pacientes fueron randomizados para recibir un dispositivo simulado(n = 200) o un dispositivo de impedancia activa (n = 200) durante el
soporte vital avanzado realizado con CPR de ACD. La meta primaria de este estudio fue la sobrevida a las 24 horas. Las tasas de sobrevida a
las 24 hrs fueron 44/200 (22%) con la válvula simulada y 64/200 (32%) con la válvula activa (P = 0.02). El número de pacientes que tuvieron
retorno a circulación espontánea (ROSC), admisión a unidad de cuidados intensivos (ICU), y tasas de alta hospitalaria fue de 77 (39%), 57
(29%), y 8 (4%) en el grupo simulado versus 96 (48%) (P = 0.05), 79 (40%) (P = 0.02), y 10 (5%) (P = 0.6) en el grupo de la válvula
activa. 6 de 10 sobrevivientes en el grupo de válvula activay1de8delossobrevivientes del grupo simulado ten
´
ıan función neurológica
normal al alta hospitalaria (P = 0.1). Conclusión: El usode una válvula de impedancia en pacientes que reciben CPR ACD para paro cardiaco
extrahospitalario mejoró significativamente las tasas de sobrevida a las 24 horas.
© 2004 Elsevier Ireland Ltd. All rights reserved.
Palabras clave: Paro card
´
ıaco; Compresión-descompresión activa; Reanimación cardiopulmonar (RCP); Válvula de umbral de impedancia; Resultado
1. Introduction
Most people die from cardiac arrest, despite receiving car-
diopulmonary resuscitation (CPR). There are many reasons
for the high mortality rates, notably the time to the start of
CPR, and the inherent inefficiency of CPR itself. While time
to initiation of CPR is dependant upon the overall efficiency
of the emergency medical services, standard manual CPR is
intrinsically inefficient as the chest compression promotes
forward flow but there is little to promote the return of blood
back into the heart. Indeed, standard CPR only provides
10–20% of normal blood flow to the heart and 20–30% of
normal blood flow to the brain [1–3]. Many have tried to de-
velop new CPR techniques [4–9]. Most efforts have focused
on means to increase systemic pressures directly during the
compression phase of CPR. Two approaches have been re-
cently discovered to increase venous return to the heart dur-
ing the decompression phase, thereby priming the pump for
each subsequent compression phase.
The first one is active compression–decompression (ACD)
CPR [10–13]. It is performed with a hand-held suction de-
vice fixed on the anterior chest wall. During the compression
phase, the chest is compressed and blood is forced out of
the heart to perfuse the vital organs. When actively pulling
up with the device, a vacuum is created within the thorax,
drawing more blood back into the heart. This technique is
known to improve haemodynamics [3,10] and, in some stud-
ies, survival rates in patients in cardiac arrest compared with
patients receiving standard CPR [12,13]. A second approach
involves use of the impedance threshold device (ITD) dur-
ing CPR [14–16]. It is a small (35ml) valve with a sili-
cone diaphragm that can be attached to the tracheal tube or
face mask or laryngeal mask airway [16,17] (see Fig. 1).
When not actively ventilating the patient, the diaphragm is
designed to selectively impede inspiratory airflow into the
patient when the intrathoracic pressure is less than 0atm,
thereby increasing the degree of negative intrathoracic pres-
sure with each chest decompression when compared with
CPR alone. This creates and maintains a vacuum within the
chest to improve venous return back to the heart further.
During active ventilation by the rescuer, neither inspiratory
Fig. 1. Figure demonstrates the use of ITD with a resuscitator bag. The
ITD was colored blue to prevent the user from knowing if it was active
or sham.
P. Plaisance et al. /Resuscitation 61 (2004) 265–271 267
nor expiratory gas exchange to the patient is impeded by the
ITD. Similarly, with chest compression, there is no resis-
tance to the movement of air out of the chest [15]. Should
the patient gasp spontaneously, or begin to breathe on his
or her own, then ventilation is possible through a side port
safety check valve set to open at 21cm H
2
O. The ITD is
removed after there is a return of spontaneous circulation
(ROSC).
Recently, animal studies have demonstrated that ITD im-
proves vital organ blood flow and survival rate during stan-
dard and, to a greater degree, during ACD CPR because
of the enhancement of the decrease in intrathoracic pres-
sure with active chest wall decompression [1,18–20]. Based
upon the improvement in blood pressure and coronary per-
fusion pressure in patients treated with the combination of
the ACD CPR and ITD [14], this device combination was
given a Class IIb recommendation in the International CPR
Guidelines 2000 (“acceptable alternative” to standard CPR)
[21,22].
Based upon these promising results, the present study was
designed to determine if the combination of ACD CPR with
the ITD would result in improved short-term survival rates
when compared with ACD CPR alone.
2. Material and methods
This prospective multi-site clinical randomized blinded
trial complies with the Declaration of Helsinki. It was
approved by the Consultative Council for the Protection
of Persons Volunteering for Biomedical Research of Lari-
boisière Saint-Louis University, Paris. This ethical commit-
tee waived the requirement for informed consent since it
was not possible under the clinical circumstances and the
committee felt that the study was justified based upon a
previous haemodynamic study with the same devices [14].
The study was performed in northern Paris metro areas,
Vernon, Gisors, Lyon, and Thionville. A two-tiered re-
sponse system is responsible for answering all emergency
calls in these EMS systems [13]. Patient enrollment began
in September 1999 and ended in August 2000. In these
study sites, the first-tier response is made by fire rescue per-
sonnel. They were equipped with basic life support (BLS)
equipment and automatic external defibrillators. They all
practiced basic CPR systematically with the ACD as the
primary initial resuscitation technique. The second-tier re-
sponse provided advanced care. This was a physician-based
and led response team. They provided advanced airway
support, advanced life support (ALS) with pharmacologi-
cal therapy and manual defibrillation. As usually practiced,
BLS, and ALS were both performed on the scene until the
patients were successfully resuscitated or if the physician
decided to stop the manoeuvers except for specific cases i.e.
profound hypothermia. Resuscitated patients were directly
transported by the medical team to an intensive care unit
(ICU).
Patients older than 18 years of age with an out-of-hospital
cardiac arrest were enrolled in the study. Adhering to stan-
dard operating clinical protocols, no patient received CPR
from the advanced life support team who had presumed ir-
reversible death, known terminal illness, traumatic injuries,
those who had made “do not resuscitate” orders, those in
whom the known time from cardiac arrest to start of ba-
sic CPR exceeded 30 min and those in whom a spontaneous
palpable carotid or femoral pulse was restored before the
arrival of the medical team. Patients meeting these criteria
were excluded from the study.
The personnel had been previously trained in ACD CPR
and, for the purposes of this study, no additional ACD
CPR training was provided. There was no specific train-
ing associated with the use of the ITD, with the exception
that co-investigators were instructed to attach the ITD to
the tracheal tube and remove it as soon as there was a
return of spontaneous circulation. During BLS, ventilation
was performed using manual bag/valve/mask ventilation
with 100% oxygen and a ventilation/compression ratio
of 2:15.
Upon the arrival of the ALS team, patients were ran-
domized to receive either ACD CPR with an active ITD or
ACD CPR with a sham valve. The active impedance valves
(CPRx LLC, Minneapolis, Minnesota) were designed to
prevent airflow into the patient during chest wall decom-
pression. If the negative pressure within the thorax reached
a value of more than 21cm H
2
O, the valve would open
allowing inflow of oxygen. Specifically for this study, sham
valves were created by the manufacturer (CPRx LLC, Min-
neapolis, Minnesota) by removing the silicone diaphragm
and by occluding the diaphragm venting ports. As such, the
sham valves functioned as hollow conduits. Furthermore, all
the valves were colored dark blue so that the rescuers could
not determine whether or not the silicone diaphragm was
present. All valves were labeled with a serial number and a
code. Each site received at least one box of 10 valves. Half
of them were sham valves and the other one-half were active
valves. The randomization code was held by an independent
statistician (E.V.) and this remained unknown to all investi-
gators until after study enrollment was completed. Investi-
gators were instructed to use the valves sequentially for each
patient in cardiac arrest who required on-going CPR with
ALS.
Once intubated, the valves were inserted between the
tracheal tube and the inspiratory circuit and they continued
to receive ACD CPR for a minimum of 30min by the ALS
team and the first response team. Chest compressions were
performed at a frequency of 100min
1
with the aid of a
metronome. Ventilation was performed asynchronously by
means of an automatic pressure cycle portable ventilator
(Airox, Bio MS, Pau, France) at a rate of 10min
1
, a tidal
volume of 8ml/kg of body weight and an inspired oxygen
fraction of 100%. This is part of the routine care for patients
undergoing CPR in France. The teams rotated performance
of ACD CPR every 3–5min to prevent fatigue [13].Low
268 P. Plaisance et al. /Resuscitation 61 (2004) 265–271
dose adrenaline (epinephrine) (1mg) was administered
every 3–5min according to international guidelines [21,22].
After successful resuscitation the valve was removed from
the circuit.
We followed the Utstein Template Guidelines for data
collection [23]. Data collection forms were completed for
each patient. There were no deviations from the routine care
provided to patients in the study sites, with the exception
that either a sham or active valve was used.
The primary endpoint of this study was 24h survival. Sec-
ondary endpoints included the rate of return of spontaneous
circulation, intensive care unit admission rates, hospital dis-
charge rates, neurological scores at the time of hospital dis-
charge, as determined by the cerebral performance score
(Glasgow–Pittsburgh cerebral performance category (CPC)
neurological scoring systems) [23] and the complication and
adverse event rates. Data were also analyzed based upon the
initial rhythm that was observed and whether or not the ar-
rests were witnessed.
Data were analyzed on an intention to treat basis. Sample
size was determined on the following bases: we estimated
that the percentage of patients surviving after 24 h in the
sham valve group will be close to 20% and we calculated
that n = 200 patients per group will allow a 80% power to
detect a 12% difference in the percentage of patients sur-
viving after 24h corresponding to an OR = 1.9 using a
chi-square test with a two-sided significance level fixed at
5%. Chi-square test and 95% confidence intervals (CI) were
used for analysis of the primary and secondary endpoints.
When conditions of validity of chi-square or for asymptotic
method for calculation of CI were not fulfilled, Fisher’s ex-
act probability test and exact CI were used. All tests were
made using StatXact (from cytel Software Cambridge MA,
USA). A P value of <0.05 was considered statistically sig-
nificant.
3. Results
A total of 618 emergency calls for cardiac arrests were
recorded during the study period. Of these, 218 patients
were excluded from enrollment and did not receive ALS.
BLS was not provided in 57 patients because of irre-
versible death and in three patients with do-not-resuscitate
orders. ALS was not provided in 56 patients with a ter-
minal illness and in the 69 patients with a known time
from collapse to initiation of basic CPR greater than
30min. A total of 33 patients recovered following BLS
and early defibrillation, before the arrival of the medical
team. The remaining 400 patients were randomized in
this clinical trial: 200 received a sham valve and 200 re-
ceived an active valve. The clinical characteristics of the
patients, the essential time intervals during the resuscita-
tion effort, and the dose of adrenaline used during CPR
are shown in Table 1. The groups were similar in all cate-
gories.
Table 1
Patient characteristics
Variable ACD CPR
+ sham ITD
(n = 200)
ACD CPR
+ active ITD
(n = 200)
Age ± S.E.M (years) 60 ± 1.3 58 ± 1.3
Gender (%)
Male 66.5 66.5
Female 33.5 33.5
Arrest witnessed (%) 75 74
Bystander CPR (%) 10 9.5
Response intervals (min ± S.E.M)
call to BLS arrival 8.8 ± 0.5 8.4 ± 0.4
call to MICU arrival 18.5 ± 0.7 17.4 ± 0.7
Initial rhythm (%)
V-fib/pulseless V-tach 23 26
PEA 2.5 5
Asystole 74.5 69
Arrest site (%)
Home 64 69
Public place 12.5 11.5
Street 14.5 12.5
Work place 2.5 1
Medical environment 6.5 6
CPR duration
a
(min ± S.E.M) 27.0 ± 1.03 29.1 ± 1.39
Total Adr administered
a
(mg ± S.E.M)
9.2 ± 0.4 10.0 ± 0.5
ACD, active compression–decompression; CPR, cardiopulmonary resus-
citation; ITD, impedance threshold device; Adr, adrenaline (epinephrine);
BLS, basic life support; ALS, advanced life support; PEA, pulseless elec-
trical activity; V-fib, ventricular fibrillation; V-tach, ventricular tachycar-
dia; SEM, standard error of the mean.
a
In those patients not achieving return of spontaneous circulation.
The rates of ROSC, ICU admission, 24h survival, hospi-
tal discharge and neurological scores at hospital discharge
are shown for all patients in Table 2. There was a statistically
significant increase in 24 h survival in patients treated with
the active ITD. In addition, we observed a significant in-
crease in ICU admission rates. All patients discharged from
the hospital had a CPC score of either 1 or 2. Normal neuro-
logical function (CPC = 1) was observed in 6/10 survivors
treated with the active valve and 1/8 survivors treated with
a sham valve (P = NS).
A total of 46 patients in the active valve group and 52
patients in the sham valve group had an initial rhythm of
ventricular fibrillation. The 24 h survival rates were 54% in
the active valve group and 40% in the sham valve group
(NS). Only five patients survived with ventricular fibrillation
in each group to hospital discharge. Neurological function
was normal in 4/5 patients in the active valve group and 1/5
patients in the control group (P = NS).
As shown in Table 3, the frequency rate of rib fractures,
sternal fractures, and pulmonary edema were compared be-
tween groups. There were no adverse events associated with
the use of the active valve.
P. Plaisance et al. /Resuscitation 61 (2004) 265–271 269
Table 2
Clinical outcomes
Outcome All patients:
sham valve
(n = 200)
All patients:
active valve
(n = 200)
All patients
OR (95% CI)
Witnessed
patients: sham
valve (n = 148)
Witnessed
patients: active
valve (n = 150)
Witnessed patients:
OR (95% CI)
ROSC, n (%) 77 (38.5) 96 (48) 1.48 [0.99–2.19] 72 (48.7) 84 (56) 1.34 [0.85–2.19]
P = 0.056 P = 0.2
ICU admission, n (%) 57 (28.5) 79 (39.5) 1.64 [1.08–2.49] 54 (36.5) 72 (48) 1.60 [1.01–2.55]
P = 0.02 P = 0.04
>24h survival n (%) 44 (22) 64 (32) 1.67 [1.07–2.6] 41 (27.7) 58 (38.7) 1.64 [1.01–2.68]
P = 0.02 P = 0.045
Hospital discharge n (%) 8 (4) 10 (5) 1.26 [0.49–3.27] 8 (5.4) 10 (6.7) 1.25 [0.48–3.26]
P = 0.63 P = 0.65
Neuro function:
CPC = 1 1 6 10.5 [0.7–550.2] 1 6 15 [0.7–824]
P = 0.11 P = 0.10
CPC = 274 7 4
Neuro, neurological; CPC, Cerebral performance category (CPC of 1 is normal cerebral function); ROSC, return of spontaneous circulation; CI, confidence
interval; OR, odds ratio; ICU, intensive care unit.
Table 3
Complications
Complications, n (%) ACD CPR
+ sham valve
(n = 200)
ACD CPR
+ active valve
(n = 200)
P value
Rib Fractures 60 (30) 78 (39) 0.06
Sternal Fractures 12 (6) 5 (2.5) 0.13
Pulmonary edema 14 (7) 8 (4) 0.27
ACD, active compression–decompression; CPR, cardiopulmonary resus-
citation.
4. Discussion
The combination of ACD CPR with the ITD has been
shown recently to improve coronary artery perfusion pres-
sures and systemic blood pressures significantly in animals
and patients in cardiac arrest when compared with ACD
CPR alone [14,16]. Results from the present study confirm
that the haemodynamic benefit observed in previous studies
translates to a direct increase in survival rates and improved
neurological function. In the current study, the principal end-
point was 24h survival. We observed a nearly 50% increase
in 24h survival for patients treated with ACD CPR and an
active valve when compared with the sham valve. Both the
ACD CPR device and the ITD were designed to enhance
vital organ circulation by enhancing venous return during
CPR. This is the first report of any human survival data com-
paring ACD CPR plus the ITD to ACD CPR alone. While
this initial study was not designed or statistically powered to
determine if the new device combination could increase hos-
pital discharge rates or improve neurological outcome, use
of the ITD was associated with normal neurological func-
tion in 6/10 patients at hospital discharge versus 1/8 patients
in the sham valve control group.
ACD CPR has been studied and found to improve circu-
lation during cardiac arrest and, in some clinical trials, to
improve short and long-term outcomes. However, the com-
bination of ACD with the ITD optimizes the bellows action
of the thorax during CPR and increases cardiopulmonary
circulation by enhancing venous return during the decom-
pression phase and then by augmenting systemic pressure
during the compression phase. The observed benefits of the
combination of ACD CPR with the ITD demonstrate that
survival rates after cardiac arrest can be significantly im-
proved by enhancing fundamental physiological processes
during resuscitation. These findings are consistent with the
hypothesis that “priming the pump” is critical for survival
after cardiac arrest [24]. These findings are also consistent
with a recent prehospital clinical trial in Germany where
24h survival rates were found to be significantly improved
with the combination of ACD CPR and the ITD when com-
pared with standard CPR alone (37% versus 22%; P = 0.03)
[25]. The present results show that the ITD component of
this combination is beneficial. Because of its simplicity to
be taught and used, one could imagine that this device can
be sufficient to enhance venous return in cardiac arrest pa-
tients as already demonstrated in animals [20]. Nevertheless,
if the ITD is used with standard manual CPR, the rescuers
must allow the chest to fully recoil after each compression
in order to optimize the benefit of the ITD. One advantage
of the ITD + ACD combination is that the ACD component
helps to assure that the intrathoracic pressure will become
more negative (depth and duration) during the chest wall
decompression phase. In addition, correct use of the combi-
nation of ITD + ACD devices causes a more rapid decrease
in negative intrathoracic pressure than either device alone.
This contributes to a greater influx of venous blood back to
the heart with each decompression phase.
Unlike other CPR device studies, which are nearly im-
possible to perform in a blinded fashion, the current study
was performed in a prospective randomized blinded trial,
by developing a sealed disposable sham valve that looked
identical to the active valve. Furthermore, when performing
compressions and decompressions, there was no special
sound coming from the active valves that could offer the
rescuer any audible distinction from the sham valves. In
270 P. Plaisance et al. /Resuscitation 61 (2004) 265–271
addition, we intentionally did not try to retrain rescuers in
the performance of ACD CPR specially for the study be-
cause they had used this technique as standard practice for
a long time, were trained regularly, and teams were always
supervised by a physician at the scene. The actual purpose
of the study was to see what impact the impedance valve
may have in the “real world” when added to the care of pa-
tients in our EMS systems. However, in the present study, it
is important to note that the survival rates with ACD alone
(used as a standard technique) were similar to those seen in
our previous studies when used as a study technique [12,13].
Based upon previous studies [12–14], patients in the cur-
rent evaluation received at least 30min of ACD CPR after
undergoing tracheal intubation. This duration of CPR is im-
portant and in this study was sufficient to “prime the pump”
to optimize venous return. As in previous studies [12,13],
most of the patients in the current study were in asystole or
had pulseless electrical activity when the first rhythm was
recorded. This high incidence of non ventricular fibrillation
rhythms may be due, at least in part, to the long interven-
tion times for the first-tier ambulance (>8min) and for the
medical team (>17min). Nevertheless, it has been shown in
previous studies