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January 19, 2005
Elizabeth Vall Reports
New CPR method improves results 500%
Giving mouth-to-mouth, is never anyone's favorite part of
CPR, but, now two University of Arizona physicians say it's
a dangerous waste of time.
In an article printed in Wednesday's journal of the
American Medical Association, heart experts at UA, say
national standards should no longer require mouth-to-mouth
ventilation for adults suffering cardiac arrest.
New studies show fast and forceful chest compressions are
more valuable. Doctors say it moves oxygenated blood to the
brain and heart, sustaining the body for up to ten minutes.
Dr. Arthur Sanders from the Sarver Heart Center, says "We
improved survival from 13% for people receiving
ventilation and compression, to 80% survival rate where
they were getting continuous chest compressions."
Tucson firefighters and paramedics are trained to give
consistent compression CPR. Yet, every year, the American
Red Cross trains 20,000 citizens on the old method.
It's a more complicated combination of 15 compressions for
every two mouth to mouth breaths.
Richard White from the Southern Arizona Red Cross says,
"They are learing tried and true tested techniques
internationaly respected that have been saving lives for
more than 50 years."
The Red Cross and the American Heart Association will be
reviewing this new research to see if national CPR
standards should be revised. The Red Cross says it's
possible changes could be made by 2006.
----------------------
Circulation. 2002 Feb 5;105(5):645-9.
Importance of continuous chest compressions during
cardiopulmonary resuscitation: improved outcome during a
simulated single lay-rescuer scenario.
Kern KB, Hilwig RW, Berg RA, Sanders AB, Ewy GA.
University of Arizona Sarver Heart Center, Section of
Cardiology, 85724, USA. [EMAIL PROTECTED]
BACKGROUND: Interruptions to chest compression-generated
blood flow during cardiopulmonary resuscitation (CPR) are
detrimental. Data show that such interruptions for
mouth-to-mouth ventilation require a period of "rebuilding"
of coronary perfusion pressure to obtain the level achieved
before the interruption. Whether such hemodynamic
compromise from pausing to ventilate is enough to affect
outcome is unknown.
METHODS AND RESULTS: Thirty swine (weight 35 +/- 2 kg)
underwent 3 minutes of untreated ventricular fibrillation
before 12 minutes of basic life support CPR. Animals were
randomized to receive either standard airway (A), breathing
(B), and compression (C) CPR with expired-gas ventilation
in a 15:2 compression-to-ventilation ratio or continuous
chest compression CPR. Those randomized to the standard
15:2 group had no chest compressions for a period of 16
seconds each time the 2 ventilations were delivered.
Defibrillation was attempted at 15 minutes of cardiac
arrest. All resuscitated animals were supported in an
intensive care environment for 1 hour, then in a
maintenance facility for 24 hours. The primary end point of
neurologically normal 24-hour survival was significantly
better in the experimental group receiving continuous chest
compression CPR (12 of 15 versus 2 of 15; P<0.0001).
CONCLUSIONS: Mouth-to-mouth ventilation performed by single
layperson rescuers produces substantial interruptions in
chest compression-supported circulation. Continuous chest
compression CPR produces greater neurologically normal
24-hour survival than standard ABC CPR when performed in a
clinically realistic fashion. Any technique that minimizes
lengthy interruptions of chest compressions during the
first 10 to 15 minutes of basic life support should be
given serious consideration in future efforts to improve
outcome results from cardiac arrest.
Resuscitation. 2004 Aug;62(2):219-27.
Continuous intratracheal insufflation of oxygen improves
the efficacy of mechanical chest compression-active
decompression CPR.
Steen S, Liao Q, Pierre L, Paskevicius A, Sjoberg T.
Department of Cardiothoracic Surgery, Heart-Lung Division,
University Hospital of Lund, SE-221 85 Lund, Sweden.
[EMAIL PROTECTED]
The aim of the present study was to compare the efficacy of
intratracheal continuous insufflation of oxygen (CIO) with
intermittent positive pressure ventilation (IPPV) regarding
gas exchange and haemodynamics during mechanical chest
compression-active decompression cardiopulmonary
resuscitation (mCPR) provided by the LUCAS device.
Ventricular fibrillation (VF) was induced electrically and
ventilation was discontinued in 16 pigs, mean body weight
23 kg (range 22-27 kg). They were randomized into two
groups (CIO versus IPPV). After 8 min of VF, mCPR was
started and run for 30 min in normothermia, after which
defibrillation was attempted during on-going mCPR. Return
of spontaneous circulation was obtained in eight of eight
CIO pigs and in four of eight IPPV pigs. Arterial oxygen
tension (P < 0.05) and coronary perfusion pressure (P <
0.01) were significantly higher in the CIO pigs. Arterial
CO(2)-tension was subnormal in both groups and
significantly (P < 0.05) lower in the IPPV-pigs (around 4.5
versus 3.0 kPa). The intratracheal pressure differed
significantly (P < 0.001) between the two groups. It was
negative in each decompression phase in the IPPV pigs in
spite of 6 mmHg of PEEP. The CIO pigs had a positive
intratracheal pressure during the whole cycle of mCPR, with
a minimum pressure of 8 mmHg during each decompression
phase. To conclude, mCPR combined with CIO gave adequate
ventilation and significantly better oxygenation and
coronary perfusion pressure than mCPR combined with IPPV.
------------------------------------
Resuscitation. 2004 Apr;61(1):75-82.
Reducing ventilation frequency combined with an inspiratory
impedance device improves CPR efficiency in swine model of
cardiac arrest.
Yannopoulos D, Sigurdsson G, McKnite S, Benditt D, Lurie
KG.
Department of Medicine, University of Minnesota (DY),
Minneapolis, MN 55455, USA.
BACKGROUND: The basic premise that frequent ventilations
during cardiopulmonary resuscitation (CPR) are a necessity
for tissue oxygenation has recently been challenged. An
inspiratory impedance threshold device (ITD) recently has
also been shown to increase CPR efficiency, principally by
augmenting circulation with little impact on ventilation.
The optimal compression to ventilation (C/V) is not known
for this new device. The purpose of this study was to
compare the currently recommended C/V ratio of 5:1 with a
10:1 ratio, +/- the ITD, to optimize circulation and
oxygenation during CPR.
METHODS: Thirty-two adult pigs weighing 26-31 kg were
randomized to CPR with varying C/V ratios +/- the ITD as
follows: A = 5:1, B = 5:1+ITD, C = 10:1, D = 10:1+ITD.
After 6 min of untreated ventricular fibrillation (VF),
closed-chest standard CPR was performed with an automatic
piston device that does not impede passive chest wall
recoil, at a continuous compression rate of 100 min(-1).
Synchronous breaths were given every 5 or 10 compressions
during the decompression phase depending on the group. CPR
was performed for 6 min and physiological variables were
measured throughout the experimental protocol.
RESULTS: A reduction in the frequency of ventilation from
5:1 to 10:1 resulted in significantly improved arterial and
coronary perfusion pressure in a pig model of cardiac
arrest. Addition of an ITD resulted in further increases in
arterial and coronary perfusion pressures with both 5:1 and
10:1 C/V ratios, without compromising oxygenation.
CONCLUSION: CPR efficiency can be optimized by changing the
compression: ventilation ratio from 5:1 to 10:1 and with
concurrent use of the inspiratory threshold device.
-----------------------------------------------
Resuscitation. 1993 Dec;26(3):251-60.
Cardiopulmonary resuscitation without intermittent positive
pressure ventilation.
Okamoto K, Kishi H, Choi H, Morioka T.
Division of Intensive and Critical Care Medicine, Kumamoto
University School of Medicine, Japan.
The purpose of this study was to examine whether tracheal
insufflation of oxygen (TRIO) could be used as a substitute
for intermittent positive pressure ventilation (IPPV)
during cardiopulmonary resuscitation (CPR) in dogs with
orotracheal intubation. Twenty-seven anesthetized,
paralyzed and intubated dogs were used. The tip of the
insufflation catheter was placed 1 cm distal to the top of
the endotracheal tube. The effects of TRIO at a flow rate
of 10 l/min with or without a continuous positive airway
pressure (CPAP) of 5 cmH2O during external cardiac
compressions were compared with those managed under the
standard CPR. During CPR, TRIO without CPAP maintained
adequate gas exchange. Peak airway pressures in the TRIO
groups were significantly lower than that in the standard
CPR group. No significant differences were observed in
arterial, pulmonary artery and diastolic right atrial
pressures during CPR among the three groups. However, the
coronary perfusion pressures in the TRIO group with CPAP
always tended to be low during CPR. The present study
suggests that TRIO without CPAP should be a promising
substitute for IPPV during CPR when IPPV is not feasible.
Anesthesiology. 1999 Apr;90(4):1078-83.
Cardiopulmonary resuscitation: effect of CPAP on gas
exchange during chest compressions.
Hevesi ZG, Thrush DN, Downs JB, Smith RA.
Department of Anesthesiology, University of South Florida
College of Medicine, Tampa 33642-4799, USA.
BACKGROUND: Conventional cardiopulmonary resuscitation
(CPR) includes 80-100/min precordial compressions with
intermittent positive pressure ventilation (IPPV) after
every fifth compression. To prevent gastric insufflation,
chest compressions are held during IPPV if the patient is
not intubated. Elimination of IPPV would simplify CPR and
might offer physiologic advantages, but compression-induced
ventilation without IPPV has been shown to result in
hypercapnia. The authors hypothesized that application of
continuous positive airway pressure (CPAP) might increase
CO2 elimination during chest compressions.
METHODS: After appropriate instrumentation and measurement
of baseline data, ventricular fibrillation was induced in
18 pigs. Conventional CPR was performed as a control
(CPR(C)) for 5 min. Pauses were then discontinued, and
animals were assigned randomly to receive alternate trials
of uninterrupted chest compressions at a rate of 80/min
without IPPV, either at atmospheric airway pressure
(CPR(ATM)) or with CPAP (CPR(CPAP)). CPAP was adjusted to
produce a minute ventilation of 75% of the animal's
baseline ventilation. Data were summarized as mean +/- SD
and compared with Student t test for paired observations.
RESULTS: During CPR without IPPV, CPAP decreased PaCO2
(55+/-28 vs. 100+/-16 mmHg) and increased SaO2 (0.86+/-0.19
vs. 0.50+/-0.18%; P < 0.001). CPAP also increased
arteriovenous oxygen content difference (10.7+/-3.1 vs.
5.5+/-2.3 ml/dl blood) and CO2 elimination (120+/-20 vs.
12+/-20 ml/min; P < 0.01). Differences between CPR(CPAP)
and CPR(ATM) in aortic blood pressure, cardiac output, and
stroke volume were not significant.
CONCLUSIONS: Mechanical ventilation may not be necessary
during CPR as long as CPAP is applied. Discontinuation of
IPPV will simplify CPR and may offer physiologic advantage.
Anesth Analg. 2001 Apr;92(4):967-74.
The effects of positive end-expiratory pressure during
active compression decompression cardiopulmonary
resuscitation with the inspiratory threshold valve.
Voelckel WG, Lurie KG, Zielinski T, McKnite S, Plaisance P,
Wenzel V, Lindner KH.
Cardiac Arrhythmia Center, Cardiovascular Division,
Department of Medicine, University of Minnesota,
Minneapolis 55455, USA.
The use of an inspiratory impedance threshold valve (ITV)
during active compression-decompression (ACD)
cardiopulmonary resuscitation (CPR) improves perfusion
pressures, and vital organ blood flow. We evaluated the
effects of positive end-expiratory pressure (PEEP) on gas
exchange, and coronary perfusion pressure gradients during
ACD + ITV CPR in a porcine cardiac arrest model. All
animals received pure oxygen intermittent positive pressure
ventilation (IPPV) at a 5:1 compression-ventilation ratio
during ACD + ITV CPR. After 8 min, pigs were randomized to
further IPPV alone (n = 8), or IPPV with increasing levels
of PEEP (n = 8) of 2.5, 5.0, 7.5, and 10 cm H(2)O for 4
consecutive min each, respectively. Mean +/- SEM arterial
oxygen partial pressure decreased in the IPPV group from
150 +/- 30 at baseline after 8 min of CPR to 110 +/- 25
torr at 24 min, but increased in the PEEP group from 115
+/- 15 to 170 +/- 25 torr with increasing levels of PEEP (P
<0.02 for comparisons within groups). Mean +/- SEM
diastolic aortic minus diastolic left ventricular pressure
gradient was significantly (P < 0.001) higher after the
administration of PEEP (24 +/- 0 vs 17 +/- 1 mm Hg with 5
cm H(2)O of PEEP, and 26 +/- 0 vs 17 +/- 1 mm Hg with 10 cm
H(2)O of PEEP), whereas the diastolic aortic minus right
atrial pressure gradient (coronary perfusion pressure) was
comparable between groups. Furthermore, systolic aortic
pressures were significantly (P < 0.05) higher with 10 cm
H(2)O of PEEP when compared with IPPV alone (68 +/- 0 vs 59
+/- 2 mm Hg). In conclusion, when CPR was performed with
devices designed to improve venous return to the chest,
increasing PEEP levels improved oxygenation. Moreover, PEEP
significantly increased the diastolic aortic minus left
ventricular gradient and did not affect the decompression
phase aortic minus right atrial pressure gradient. These
data suggest that PEEP reduces alveolar collapse during ACD
+ ITV CPR, thus leading to an increase in indirect
myocardial compression.
IMPLICATIONS: Inspiratory impedance during active
compression-decompression cardiopulmonary resuscitation
improves perfusion pressures, and vital organ blood flow
during cardiac arrest. Increasing levels of positive
end-expiratory pressure during performance of active
compression-decompression cardiopulmonary resuscitation
with an inspiratory impedance valve improves oxygenation,
and increases the diastolic aortic-left ventricular
pressure gradient and systolic arterial blood pressure.
-----------------------------------
(Circulation. 2002;105:124.)
© 2002 American Heart Association, Inc.
Basic Science Reports
Use of an Inspiratory Impedance Valve Improves
Neurologically Intact Survival in a Porcine Model of
Ventricular Fibrillation
Keith G. Lurie, MD; Todd Zielinski, MS; Scott McKnite, BS;
Tom Aufderheide, MD; Wolfgang Voelckel, MD
>From the Cardiac Arrhythmia Center, University of
Minnesota, Minneapolis (K.G.L., T.Z., S.M., W.V.); the
Department of Emergency Medicine, Medical College of
Wisconsin, Milwaukee (T.A.); and the Department of
Anesthesiology, Leopold-Franzens-University, Innsbruck,
Austria (W.V.).
Correspondence to Keith G. Lurie, MD, Department of
Medicine, University of Minnesota, MMC 508, AHC, 420
Delaware St SE, Minneapolis, MN 55455. E-mail
[EMAIL PROTECTED]
Abstract
Background— This study evaluated the potential for an
inspiratory impedance threshold valve (ITV) to improve
24-hour survival and neurological function in a pig model
of cardiac arrest.
Methods and Results— Using a randomized, prospective, and
blinded design, we compared the effects of a sham versus
active ITV on 24-hour survival and neurological function.
After 6 minutes of ventricular fibrillation (VF), followed
by 6 minutes of cardiopulmonary resuscitation (CPR) with
either a sham or an active valve, anesthetized pigs
received 3 sequential 200-J shocks. If VF persisted, they
received epinephrine (0.045 mg/kg), 90 seconds of CPR, and
3 more 200-J shocks. A total of 11 of 20 pigs (55%) in the
sham versus 17 of 20 (85%) in the active valve group
survived for 24 hours (P<0.05). Neurological scores were
significantly higher with the active valve; the cerebral
performance score (1=normal, 5=brain death) was 2.2±0.2
with the sham ITV versus 1.4±0.2 with the active valve
(P<0.05). A total of 1 of 11 in the sham versus 12 of 17 in
the active valve group had completely normal neurological
function (P<0.05). Peak end-tidal CO2 (PETCO2) values were
significantly higher with the active valve (20.4±1.0) than
the sham (16.8±1.5) (P<0.05). PETCO2 >18 mm Hg correlated
with increased survival (P<0.05).
Conclusions— Use of a functional ITV during standard CPR
significantly improved 24-hour survival rates and
neurological recovery. PETCO2 and systolic blood pressure
were also significantly higher in the active valve group.
These data support further evaluation of ITV during
standard CPR.
Key Words: cardiac arrest • fibrillation •
cardiopulmonary resuscitation • valves • survival •
arrhythmia • brain
Introduction
Survival rates remain poor for most patients who suffer
from a cardiac arrest. Studies on the mechanism of blood
flow during cardiopulmonary resuscitation (CPR) have
recently focused on the importance of the decompression
phase of CPR.1–4 During the decompression phase of
standard CPR, a small vacuum is created within the chest
relative to the rest of the body every time the chest wall
recoils back to its resting position.5 This draws venous
blood back into the right heart. In addition, during the
decompression phase of standard CPR, air is drawn into the
lungs. We previously described the use of an impedance
threshold valve (ITV) to prevent the inflow of respiratory
gases during the active chest wall recoil phase, or
decompression phase, of standard CPR.4,5 The ITV is a small
(35-mL) disposable plastic valve that is attached to the
endotracheal tube or a face mask. It works by allowing the
rescuer to freely ventilate the patient but impeding
inspiratory airflow during the decompression phase of CPR
when the patient is not being actively ventilated. This
creates a small vacuum within the chest to further enhance
venous return.
We recently demonstrated in a porcine model that use of the
ITV resulted in a nearly 2-fold increase in blood flow to
the brain and the heart after 6 minutes of ventricular
fibrillation and 6 minutes of standard CPR.6 Although use
of the ITV during standard CPR has been reported previously
in 2 studies involving >30 animals, to date there have been
no definitive data in support of a survival benefit from
the use of the ITV with standard CPR.4,6 Thus, the purpose
of this investigation was to test the hypothesis that the
ITV would improve neurological function and 24-hour
survival in an established animal model of cardiac arrest
during performance of standard CPR.
Methods
Preparatory Phase
The study was approved by the Committee of Animal
Experimentation at the University of Minnesota. Anesthesia
was used in all surgical interventions to avoid all
unnecessary suffering. This study was performed according
to Utstein-style guidelines7 on 40 female farm pigs
weighing 28 to 33 kg. Each animal received 7 mL (100 mg/mL)
of ketamine HCl (Ketaset, Fort Dodge Animal Health) IM for
initial sedation. Intravenous access with an 18-gauge
angiocatheter (Jelco Ethicon, Inc) was rapidly obtained
through a lateral ear vein. Propofol anesthesia (PropoFlo,
Abbott Laboratories) (2.3 mg/kg) was initially delivered as
an intravenous bolus. While the animals were spontaneously
breathing but heavily sedated, they were intubated with a
7.0F endotracheal tube (Medline Industries Inc). The
animals were then given an additional 30 mg of propofol and
were maintained on a propofol infusion of 160 µg • kg-1
• min-1 until just after induction of ventricular
fibrillation.
The animals were positioned in the supine position. Femoral
artery cannulation was performed under aseptic conditions,
and arterial blood pressures were monitored and recorded as
previously described.2,6 Continuous ECG monitoring was
recorded with a lead II ECG. Data were digitized, recorded,
and analyzed as previously described.2,3,6 Intrathoracic
pressures were measured with a micromanometer-tipped
catheter positioned 2 cm below the tip of the endotracheal
tube. End tidal CO2 (ETCO2) (CO2 SMO Plus Respiratory
Profile Monitor, Novametrix Medical Systems), arterial
pressures, and intrathoracic pressures were recorded
continuously during both the preparatory phase and the
experimental protocol. Animals received 400 mL of normal
saline before the induction of ventricular fibrillation.
Temperature was recorded with a rectal thermometer and
maintained between 37.5°C and 39.5°C with either a fan
and a cooling blanket or a Bair Hugger, Temperature
Management System (Augustine Medical, model 505), as
needed.
Animals were positioned on a rigid cradle for standard CPR
with an automated CPR device. A circular compression pad
with a diameter of 6.4 cm was attached to a pneumatically
driven compression-piston device (CPR Controller, Ambu
International) and positioned over the lower third of the
sternum.
During the preparatory phase, the animals were ventilated
with room air by use of a positive-pressure ventilator
(Harvard Apparatus Co, model 607) at an average rate of 16
breaths per minute and a tidal volume of 20 mL/kg. The rate
was adjusted on the basis of analysis of arterial blood
gases every 30 minutes (IL Synthesis, model 20,
Instrumentation Laboratory).
Protocol
Ventricular fibrillation was induced in the anesthetized
animals with a 3-second, 60-Hz, 140- to 160-V AC shock
applied across the thorax with 2 half-circle stainless
steel surgical needles as electrodes. CPR was performed
continuously at a rate of 80 compressions per minute, with
a compression-decompression duty cycle of 50%. Compressions
were performed to a depth of 25% of the anteroposterior
diameter of the thorax with a circular compression pad.
During the decompression phase, the compression pad was
elevated at a rate of 7.5 in/s to allow for the natural
recoil of the anterior chest wall. After ventricular
fibrillation was induced, the intravenous saline and
propofol infusions were immediately discontinued, and the
animal was disconnected from the mechanical ventilator.
After 6 minutes of untreated cardiac arrest, standard
closed-chest CPR was delivered continuously with an
automated pneumatic piston device as described above.
Thirty seconds before initiation of CPR, either a sham or
an active ITV was attached to the endotracheal tube. The
ITVs used in this study have been described previously in
detail.5,6 Assignment of each valve was made according to a
computer-generated randomization list. Researchers were
blinded to the kind of valve used until after the pigs were
euthanized 24 hours after resuscitation.
During CPR, ventilation was delivered during the
decompression phase. Animals were ventilated during CPR
with 100% O2 with a demand valve resuscitator (Life Support
Products, Inc, model L063-05RM) through either a sham
(nonfunctional) or active functional ITV (ResQ-Valve, CPRx
LLC) at a compression-to-ventilation ratio of 5:1. It was
not possible, when looking at the blue impedance valves, to
determine whether or not there was a silicone diaphragm
within the valve. In addition, the silicone diaphragm
venting ports were occluded during the manufacturing of the
sham valves, such that they functioned as a hollow conduit
for respiratory gas exchange. As such, half of the ITVs
were made as sham valves and the other half were active.
Figure 1 depicts the function of the ITV during the chest
compression and decompression phases of CPR.2,5,6
Figure 1. Schematic of respiratory gas flow through ITV.
After 12 minutes of ventricular fibrillation and a total of
6 minutes of CPR, the impedance valve was removed, and each
animal was immediately defibrillated with up to 3
sequential 200-J transthoracic monophasic shocks (Lifepak
6, Physio-Control). Animals that were successfully
resuscitated were reconnected to the automatic ventilator.
Animals that remained in cardiac arrest received a single
dose of intravenous epinephrine (0.045 mg/kg) and an
additional 90 seconds of CPR with the previously assigned
sham or active ITV. Each animal that remained in cardiac
arrest then received up to 3 additional sequential 200-J
transthoracic shocks. All animals with successful
restoration of spontaneous circulation were treated with
intravenous fluids. Dopamine, at a concentration of 1.6
mg/mL, was administered to maintain the systolic blood
pressure at >70 mm Hg, as needed. Mechanical ventilation
with supplemental oxygen (10 L/min) was continued
throughout the immediate postresuscitation period. The
endotracheal tube was removed once the animal was able to
breathe independently, as judged by measurement of peak
inspiratory flow rates and maintenance of adequate
ventilation as the mechanical ventilation rate was
progressively reduced. Each animal was transferred to a
heated holding area until it woke up and was able to move
around and drink water independently. The survivors were
held in an observation area for 24 hours before undergoing
further assessment. They then underwent euthanasia and
autopsy 24 hours after resuscitation.
Survival rates, complication rates, and neurological status
were evaluated 24 hours after resuscitation. Neurological
function was evaluated 24 hours after resuscitation by 2
investigators (S.M., T.Z.) who remained blinded to the
device that was used. Evidence of pulmonary congestion, as
judged by blood gas analysis, was assessed 1 to 3 hours
after resuscitation. Pulmonary congestion was also assessed
by observing for pink frothy exudate in the endotracheal
tube during and after CPR and at autopsy. Neurological
function was assessed quantitatively, as described by
Bircher, Safar, Vaagenes, and colleagues.8,9 The Swine
Neurologic Deficit Score was used to evaluate level of
consciousness, respiratory pattern, cranial nerve function,
motor and sensory function, and behavior evaluation,
including ability to drink, chew, stand, and walk.8 The
Cerebral Performance Score was also used.9 It is a
neurological assessment based on a 5-point evaluation of
the level of consciousness.
Statistical Analysis
Hemodynamic and perfusion parameters were analyzed by ANOVA
(a value of P<0.05 was considered statistically
significant). The 2 test was used for survival rate
analysis. A priori, the sample size was calculated on the
basis of expected differences in 24-hour survival between
groups. All data are expressed as mean±SEM.
Results
Survival Outcomes
The main study end points were 24-hour survival and
neurological function. Twenty animals received standard CPR
plus a sham valve, and 20 received standard CPR plus an
active valve. Twenty-four-hour survival was 55% in the sham
valve group and 85% in the active valve group (P<0.05).
Neurological function was significantly improved in the
24-hour survivors that received treatment with the active
valve. The cerebral performance score, based on a 5-point
scoring system (1=normal, 5=brain death),9 was 2.2±0.2 for
animals treated with the sham valve versus 1.4±0.2 for
those treated with the active valve (P<0.002). With the
Swine Neurologic Deficit Score,8 there were similar
statistically significant improvements in the active valve
group (Table 1, Figure 2). The neurological deficits in
survivors treated with the sham valve were striking. The
animals often appeared docile and disoriented, frequently
walking into the wall of the cage without apparently
realizing that it was there. Only 1 of 11 survivors had a
completely normal neurological score. By contrast, 12 of 17
survivors treated with the active valve had normal
neurological function (P<0.05). Improved neurological
function was observed in many of the different categories
in animals treated with the active valve (Tables 1 and 2).
Table 1. Twenty-Four Hour Survival and Neurological
Assessment Score
Sham Valve (n=20)
Active Valve (n=20)
24-hour survival, n (%) 11 (55)* 17 (85)*
Neurological assessment
Consciousness 25.0 ±6.2 10.6 ±4.4*
Respiratory pattern 10.8 ±8.5* 0.0 ±0.0*
Painful stimulus 13.3 ±4.1 4.7 ±2.1
Muscle tone 16.7 ±5.6 5.9 ±2.7
Standing 5.0 ±2.6 1.2 ±1.2
Walking 13.3 ±3.3 5.3 ±2.1*
Restraint 30.8 ±5.3* 12.9 ±4.8*
Total deficit score
16.4 ±3.3
5.8±1.8
*P<0.05.
P<0.02.
Figure 2. Pittsburgh neurological deficit score for all
animals receiving standard CPR with either sham (n=11 of 20
survivors 24 hours after resuscitation) or active (n=17 of
20 survivors 24 hours after resuscitation) valve and
subgroup of animals that were resuscitated without (w/o)
epinephrine. All values are mean±SEM. *P<0.03.
Table 2. Twenty-Four Hour Survival and Neurological
Assessment Score by Level of Care (BLS or ALS) Received
Sham Valve (n=20)
Active Valve (n=20)
BLS
ALS
BLS
ALS
24-hour survival, n (%) 6 (30) 5 (25) 12 (60) 5 (25)
Neurological assessment
Consciousness 25.0 ±9.2 25.0 ±9.2* 13.9 ±5.5 0.0 ±0.0*
Respiratory pattern 5.0 ±5.0 16.7 ±16.7* 0.0 ±0.0* 0.0
±0.0
Painful stimulus 15.8 ±7.8 10.8 ±3.3* 6.6 ±2.6 0.0
±0.0*
Muscle tone 18.3 ±8.9 12.5 ±6.0 6.0 ±2.8 6.3 ±6.3
Standing 6.7 ±4.2 3.3 ±3.3* 1.5 ±1.5 0.0 ±0.0*
Walking 13.3 ±5.6 13.3 ±4.2 6.2 ±2.7 2.5 ±2.5
Restraint 30.0 ±8.2 32.0 ±7.5* 15.4 ±6.0 5.0 ±5.0*
Total deficit score
16.3 ±3.4*
16.3 ±3.6
7.0 ±2.2*
2.0 ±1.0
*P<0.05.
P<0.002.
The initial return of spontaneous circulation (ROSC) rate
was 80% in the sham valve group and 95% in the active valve
group. For those animals that had ROSC after the first 3
defibrillatory shocks, there was no significant difference
in the amount of energy needed between groups. Without
epinephrine treatment (ie, with only Basic Life Support
[BLS] techniques during the resuscitation), 35% of the pigs
in the sham valve group versus 60% in the active valve
group could be resuscitated initially (P=NS). Five pigs in
the sham valve group and 2 in the active valve group died
after being successfully resuscitated, but before 24 hours.
We also analyzed 24-hour survival results in the subset of
animals that were resuscitated with DC shock alone, without
epinephrine. With BLS followed by defibrillation, 24-hour
survival in the sham controls was 30% versus 60% in the
active valve group (P=0.057). The neurological score in
this subgroup also showed a significant improvement in the
active valve group (Table 2). Similarly, when advanced life
support (ALS) measures were needed and used, use of the ITV
also resulted in a statistically significant improvement in
the neurological status of the animals 24 hours after
resuscitation.
Epinephrine therapy had no observed beneficial impact on
neurological function in the sham valve group but did
improve the likelihood of successful resuscitation in both
the sham and active valve groups. The cerebral performance
score was 2.0±0.4 in the sham valve group with BLS alone
versus 2.3±0.3 in the sham valve group that received ALS
(epinephrine and additional shocks). The total neurological
deficit scores were similar as well, with and without
epinephrine treatment, in animals treated with the sham
valve (Table 2). By contrast, pigs requiring epinephrine in
the active valve group that lived for 24 hours all had a
normal cerebral performance score.
A total of 5 animals in the sham group and 4 in the active
valve group required dopamine supportive therapy after
resuscitation, secondary to hypotension. The duration of
support varied between 1 minute and 30 minutes and was
similar between groups.
Hemodynamic Outcomes
Systolic arterial pressures (Figure 3A) as well as peak
ETCO2 (PETCO2) (Figure 4) levels were significantly higher
in the group treated with the active valve. The systolic
and diastolic pressures (Figure 3B) rose more rapidly and
remained higher in the active valve group than in controls.
With a 2 test, there was a significantly greater chance
for 24-hour survival when the diastolic blood pressure was
>21 mm Hg (80%) compared with animals with a diastolic
blood pressure of 21 mm Hg (40%) (P<0.05).
Figure 3. Standard CPR was performed with either a sham or
active ITV. Systolic pressures (A) were recorded
continuously during 6-minute study period during CPR.
Systolic pressures, plotted on y axis from 38 to 74 mm Hg,
were significantly higher during time points marked with
asterisk. Diastolic arterial pressures (B) were also
recorded continuously during study period and from 16 to 34
mm Hg. Values are mean±SEM for time period indicated on
each graph. *P 0.05.
Figure 4. End-tidal CO2 values were measured over 6-minute
study period. All values plotted from 10 to 24 mm Hg are
expressed as mean±SEM. Standard CPR was performed with
either a sham or active ITV. VF indicates ventricular
fibrillation. *P 0.05.
PETCO2 levels were significantly higher among survivors
than among the animals that died. There was a significantly
greater chance for 24-hour survival in animals with a
maximum PETCO2 level of 19 mm Hg (79%) than in animals
with a peak PETCO2 value of <18 mm Hg (45%) (P<0.05).
Changes in intrathoracic pressure were also monitored
continuously during the performance of CPR. As shown in
Figure 5, intrathoracic pressures were consistently lower
in the active valve group. There was a significantly
greater chance for 24-hour survival when intrathoracic
pressure was 1.5 mm Hg (80%) than in animals with thoracic
pressure of 1.5 mm Hg (40%) (P<0.05).
Figure 5. Intrathoracic pressures were recorded
continuously during 6-minute study period. Values represent
mean±SEM for time period indicated on each graph. Standard
CPR was performed with either a sham or active ITV. *P
0.05.
Arterial blood gas measurements suggest that pulmonary
function was not compromised by the impedance valve. As
shown in Table 3, the PO2 values after 10 minutes of
ventricular fibrillation and 4 minutes of CPR, as well as
ROSC, were not significantly different between groups. The
arterial pH, however, was significantly higher in the
active valve group than the sham group.
Table 3. Arterial Blood Gas Values
Sham Valve
Active Valve
pH
Pco2
Po2
pH
Pco2
Po2
Baseline 7.45 ±0.03 32.8 ±1.2 93 ±3.2 7.46 ±0.01 34.2
±1.1
86 ±2.9
VF+10 min 7.29 ±0.04 31.3 ±1.9 153 ±24 7.33 ±0.02 31.7
±2.5
220 ±36
ROSC
7.13 ±0.03*
38.8 ±2.7
199 ±35
7.23 ±0.03*
36.05 ±2.7
198 ±31
VF indicates ventricular fibrillation.
*P<0.05.
Potential Complications
Device failure (eg, breakage of the device) was monitored
throughout the study. All valves were tested before and
after use. There was no evidence of device failure.
Animals underwent necropsy to evaluate for potential
complications, including damage to the rib cage and
evidence of lung, heart, and other vital organ damage
secondary to the CPR. There were no differences between the
animals treated with the sham versus active impedance
valve. In addition, we did not observe any evidence of
pulmonary edema, as judged by the presence of pink frothy
exudate in the endotracheal tube during or after CPR.
Discussion
The results from this pig study demonstrate that when blood
return is enhanced to the heart during the decompression
phase of standard CPR with an inspiratory impedance valve,
24-hour survival and neurological outcome are significantly
improved. By harnessing the kinetic energy associated with
the natural recoil of the chest during standard CPR, the
inspiratory impedance valve transiently prevents the inflow
of respiratory gases, thereby creating a greater vacuum in
the chest with each decompression phase recoil of the chest
wall. The results from this study demonstrate, for the
first time, that a device developed to enhance blood return
to the thorax during the relaxation, or decompression,
phase of standard CPR can improve survival rates and
neurological function. The results are consistent with
reports demonstrating that use of an inspiratory impedance
valve significantly enhances coronary perfusion pressure,
myocardial perfusion, and cerebral perfusion during
standard CPR.2,4–6
A recent study in humans10 suggests that a brief period of
"priming the pump" before the delivery of defibrillatory
shocks can significantly improve overall survival. This
stands to reason, because the fibrillating heart, in the
absence of closed-chest cardiac massage, has a high
metabolic rate and limited metabolic stores. Movement of
blood through the coronary arteries during CPR provides a
means to renew energy supplies and remove metabolic
byproducts, especially lactic acid, that are toxic to the
myocardium. The results demonstrating higher arterial blood
pH values in the active valve group provide metabolic
evidence in support of the improved perfusion with the
valve.
The natural recoil of the chest results in the development
of a -5.3±0.6 mm Hg decrease in intrathoracic pressure
with the active valve compared with -1.8±0.4 mm Hg
decrease with the sham valve. Relatively speaking, these
changes in negative intrathoracic pressure are quite small.
The negative intrathoracic pressures generated with the
ITV, however, are sufficient to enhance blood return to the
right heart during the decompression phase of CPR, which
can be circulated during the subsequent compression phase,
thereby priming the pump with each complete
compression-decompression cycle.
We observed no significant adverse effects from the use of
the inspiratory impedance valve. The blood gas data suggest
that oxygenation was adequate in the active valve group,
and no differences were observed between groups on autopsy.
Previously, we had demonstrated that the use of the valve
resulted in a decrease in minute ventilation and arterial
oxygen tension compared with standard CPR alone.2,4 In the
previous studies, however, as well as in the present study,
the PO2 measured in arterial blood was always >85 mm Hg.
Moreover, it is possible that the decrease in arterial PO2
we previously observed when the ITV was used was the result
of lower flow through the pulmonary vasculature in the
control group, resulting in a paradoxically higher PO2, as
was observed by Idris et al11 in another model of cardiac
arrest.
This study was designed to evaluate the potential impact of
the inspiratory impedance valve on 24-hour survival and
neurological function in a pig model. It is limited in that
we did not assess myocardial function 24 hours after
recovery, but there was no clinical evidence of heart
failure in either group. In addition, we used only a single
dose of epinephrine. This may have altered the outcome.
Conclusions
There was a statistically significant increase in 24-hour
survival and neurological function when the functional ITV
was used during standard CPR. The impedance valves worked
well during and after the study, without evidence of
failure. Moreover, PETCO2 appeared to be a promising and
potentially meaningful predictor of outcome in cardiac
arrest when the ITV was used. On the basis of this study,
further evaluation of the ITV during standard CPR seems
warranted.
Acknowledgments
Funding for this study was provided, in part, by National
Institutes of Health (NIH) SBIR grant 1R43-HL-65851 and by
CPRx LLC, Minneapolis, Minn.
Footnotes
Dr Keith G. Lurie is the principal investigator on an NIH
SBIR grant awarded to CPRx LLC. He is a coinventor of the
inspiratory impedance threshold valve and founded CPRx LLC
to develop this device.
Received August 7, 2001; revision received October 12,
2001; accepted October 15, 2001.
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