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Copyright: Aerospace Medical Association
Aviation, Space, and Environmental Medicine x Vol. 85, No. 7 x July 2014 687
RESEARCH ARTICLE
R EHNBERG L, A SHCROFT A, B AERS JH, C AMPOS F, C ARDOSO RB,
V
ELHO R, G EHRKE RD, D IAS MKP, B APTISTA RR, R USSOMANO T. Three
methods of manual external chest compressions during microgravity
simulation. Aviat Space Environ Med 2014; 85:687 – 93.
Introduction: Cardiopulmonary resuscitation (CPR) in microgravity is
challenging. There are three single-person CPR techniques that can be
performed in microgravity: the Evetts-Russomano (ER), Handstand (HS),
and Reverse Bear Hug (RBH). All three methods have been evaluated in
parabolic fl ights, but only the ER method has been shown to be effective
in prolonged microgravity simulation. All three methods of CPR have yet
to be evaluated using the current 2010 guidelines. Methods: There were
23 male subjects who were recruited to perform simulated terrestrial
CPR (+1 G
z ) and the three microgravity CPR methods for four sets of
external chest compressions (ECC). To simulate microgravity, the sub-
jects used a body suspension device (BSD) and trolley system. True
depth (D
T ), ECC rate, and oxygen consumption ( .
V O
2 ) were measured.
Results: The mean ( 6 SD) D
T for the ER (37.4 6 1.5 mm) and RBH meth-
ods (23.9 6 1.4 mm) were signifi cantly lower than +1 G
z CPR. However,
both methods attained an ECC rate that met the guidelines (105.6 6 0.8;
101.3 6 1.5 compressions/min). The HS method achieved a superior D
T
(49.3 6 1.2 mm), but a poor ECC rate (91.9 6 2.2 compressions/min).
.
V O
2 for ER and HS was higher than +1 Gz; however, the RBH was not.
Conclusion: All three methods have merit in performing ECC in simu-
lated microgravity; the ER and RBH have adequate ECC rates, and the HS
method has adequate D
T . However, all methods failed to meet all criteria
for the 2010 guidelines. Further research to evaluate the most effective
method of CPR in microgravity is needed.
Keywords: Evetts-Russomano , Handstand , Reverse Bear Hug , cardiopul-
monary resuscitation .
B ASIC LIFE SUPPORT (BLS), of which external chest
compressions (ECC) form the main initial part, is es-
sential to maintain organ perfusion until a defi brillator
is available and advanced life support (ALS) can be ap-
plied. The absence of gravitation effects during space-
fl ight makes performing effective cardiopulmonary
resuscitation (CPR) additionally challenging. Several
studies with parabolic fl ights and ground simulations
have shown that performing CPR in microgravity may
be challenging to provide effectively ( 7 , 10 , 17 ). In addi-
tion to the conventional method of performing CPR
during spacefl ight using restraint equipment, other
methods have been developed as single-person meth-
ods of performing CPR: the Hand Stand (HS), Reverse
Bear Hug (RBH), and Evetts-Russomano (ER) methods.
The advantage of these techniques over restrained
methods is that BLS can be initiated immediately after a
cardiac arrest, thus minimizing organ damage and po-
tentially increasing chances of survival [International
Liaison Committee on Resuscitation (ILCOR)] ( 12 , 15 ).
These single-person methods have numerous benefi ts
over performing conventional CPR in microgravity, yet
each also has individual strengths and limitations. The
HS and RBH have been studied in parabolic fl ights ( 7 )
using mannequin and swine models ( 8 ), and using the
1998 and 2000 American Heart Association guidelines,
respectively. Preliminary comparison of the three meth-
ods has been carried out in simulated microgravity using
the 2010 ILCOR guidelines ( 10 ).
The HS method has been shown in parabolic fl ights
( 7 , 8 ) and ground-based simulations of microgravity ( 10 )
to be the most effective of the three methods, as well as
the least fatiguing. A limitation of the HS method is that
it is rescuer height dependent and, hence, there is a
height limitation on who can use this method ( 7 ). In ad-
dition, the force generated by the legs against a surface
in order to compress the chest might damage the cap-
sule wall or cause excess vibrations.
The RBH method was shown in parabolic fl ight to be
a potentially feasible method of CPR in microgravity as
it produced adequate ECC depth and only a slightly
lower than adequate rate of compression using the
2000 guidelines ( 7 ). The short duration of the parabolas
meant the effectiveness of the RBH over a longer duration
could not be assessed. It was shown that the RBH was
the most physically taxing and provided the smallest
depth of compression of the 3 methods ( 10 ) over 3 sets of
30 compressions. Other authors have also mentioned
the lack of ease of performing mouth-to-mouth ventila-
tions with the RBH and HS methods ( 7 ).
The ER and RBH methods are not limited in terms of
rescuer height and are independent of equipment and
capsule parameters, unlike the HS. The ER method has
From the John Ernsting Aerospace Physiology Laboratory, Micro-
gravity Centre, PUCRS, Porto Alegre, Brazil, and the Centre of Human
and Aerospace Physiological Sciences, School of Biomedical Sciences,
Kings College London, London, UK.
This manuscript was received for review in September 2013 . It was
accepted for publication in March 2014 .
Address correspondence and reprint requests to: Lucas Rehnberg,
39 Brook Square, London SE18 4NB, UK; lukirehnberg@gmail.com .
Reprint & Copyright © by the Aerospace Medical Association,
Alexandria, VA.
DOI: 10.3357/ASEM.3854.2014
Three Methods of Manual External Chest
Compressions During Microgravity Simulation
Lucas Rehnberg , Alexandra Ashcroft , Justin H. Baers ,
Fabio Campos , Ricardo B. Cardoso , Rochelle Velho ,
Rodrigo D. Gehrke , Mariana K. P. Dias ,
Rafael R. Baptista , and Thais Russomano
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688 Aviation, Space, and Environmental Medicine x Vol. 85, No. 7 x July 2014
COMPARISON OF CPR POSITIONS — REHNBERG ET AL.
been evaluated in parabolic fl ight ( 5 ) and ground-based
simulation ( 17 ) and found to be an effective method of
CPR in microgravity, in accordance with the 2005 Amer-
ican Heart Association guidelines. Despite providing
adequate depth and rate of compression, it was shown
that women were not able to perform effective ECC us-
ing the ER method in microgravity simulation ( 11 ). The
ER method was shown to be effective over 3 mins, in
terms of depth and ECC rate, with the 2005 guidelines.
However, when using the 2010 ILCOR guidelines, the
ER method, as well as the HS and RBH, performed be-
low the recommended depth of compression ( 10 ).
The aim of this study was to compare the effi cacy of
the three single-person microgravity CPR methods —
HS, ER, and RBH. The three methods were assessed us-
ing the most recently updated ILCOR guidelines, which
state that the chest should be ‘ depressed at least 5 cm ’
and the ECC rate should be at a rate of ‘ at least 100/
min ’ . This study compares the depth and ECC rate of
the three microgravity methods in simulated micrograv-
ity against each other and +1 G
z CPR over a period of
1.5 min.
This study builds upon previous work by measuring
more subjects over a longer period of time and measur-
ing oxygen consumption ( V
o 2 ) to allow more accurate
evaluation of physical exertion while performing each
CPR method. Recorded in this study were heart rate
(HR), perceived exertion, skin folds, and anthropomet-
ric measurements to estimate muscle mass. Accurate es-
timates of subject muscle mass will allow for corrections
of
V
o 2 for each subject.
METHODS
Subjects
Participating in this study were 23 male subjects at
the John Ernsting Aerospace Physiology Laboratory,
Microgravity Centre, Pontifícia Universidade do Rio
Grande do Sul (PUCRS), Brazil. The age of the subjects
ranged from 18 to 27, with a mean of 22 6 3.1 (mean 6
SD) yr of age, with an average height and weight of
180.8 6 7.1 cm and 75.6 6 12.5 kg. The study protocol
was approved by the PUCRS Ethics and Research
Committees. Subjects were recruited on a voluntary ba-
sis after receiving informed consent. Subjects with mus-
cle, bone, or cardiovascular disease that could hinder
their performance during CPR were excluded.
Equipment and Materials
To simulate microgravity for the three positions, two
methods of simulation were used: a body suspension
device (BSD) and a trolley system ( Fig. 1 ). The BSD was
used to perform the ER ( Fig. 1B ) and RBH ( Fig. 1C )
methods. The BSD consists of a pyramidal frame with
steel bars 6 3 3 cm in thickness and has a height of 200 cm
with a base of 300 cm 3 226 cm. The subjects were fully
suspended from the BSD via a steel crossbar 120.5 cm
in length and with a diameter of 27.5 mm. The steel
crossbar hung from the BSD with reinforced steel wir-
ing. Attached to the steel wiring of the crossbar was a
static nylon rope with carabineers attached to each end.
The carabineers were clipped to the hip region of the
body harness worn by the subjects, with a safety carabi-
neer attached to the back of the subject.
The HS method ( Fig. 1A ) was performed on the trol-
ley system. This system consisted of a padded trolley
contained within fi xed tracks. The trolley measured 88 cm
in length and 64 cm in width, standing at 87 cm high.
The trolley aimed to only provide support to the back
while the subjects placed their hands and feet on fi xed
walls to perform the HS method. The whole system was
contained between two fi xed walls set at 194 cm apart,
the standing height in the European Space Agency Co-
lumbus Orbital Facility ( 14 ).
A standard CPR mannequin (Resusci Anne Skill Re-
porter, Laerdal Medical Ltd., Orpington, UK) was modi-
fi ed to include a linear displacement transducer capable
of measuring ECC depth and rate. The mannequin’s
chest steel spring depressed 1 mm for each 1 kg of ap-
plied weight. Real-time feedback of each ECC was pro-
vided to the subjects via an LED display. The LED
display consisted of a series of colored lights that indi-
cated depth of ECC (red: 0-39 mm, yellow: 40-49 mm,
green: 50-60 mm), in accordance with the new 2010
ILCOR guidelines. The depth of compression was ana-
lyzed in two ways: max depth and true depth (D
T ), as
conducted in previous work ( 18 ). The display also pro-
vided audio cues with an electric metronome set to a
rate of 100 compressions/min. A 6-s interval between
each set of ECC represented the time required to per-
form two mouth-to-mouth ventilations; however, these
were not carried out in this experiment.
An Aerosport V
o 2000 analyzer (MedGraphics, St. Paul,
MN) recorded minute ventilation ( V
E ) and
V
o 2 , with the
latter being calculated and recorded directly by a com-
puterized ergospirometric system (Aerograph 4.3, Aero-
Sport Inc., Ann Arbor, MI). All V
o 2 measurements were
normalized to ml · kg
2 1 · min
2 1 using the estimates of
muscle mass discussed below. An Onyx 9500 fi ngertip
pulse oximeter (Nonin Medical Inc., Plymouth, MN)
measured HR and the Borg Scale was used to measure
the rate of perceived exertion (RPE) ( 2 ).
Skinfold thickness was measured using Lange skin-
fold calipers (Beta Technology Incorporated, Cambridge
Scientifi c Instruments, Witchford, Ely, UK). To get an ac-
curate estimation of bone mass, bone diameter calipers
were used (Tommy 3 small bone caliper, Rosscraft In-
novations Incorporated, Vancouver, BC, Canada). A metal
tape was used to measure the circumference of the fore-
arm and abdomen.
A DataQ acquisition device with eight analog and
six digital channels, 10 bits of measurement accuracy,
rates up to 14,400 samples/s, and USB interface was
used (DATAQ Instruments Inc., Akron, OH). The de-
vice supported a full-scale range of 6 10 V and a reso-
lution of 6 19.5 mV. WinDaq data acquisition software
allowed for the conversion of volts to the necessary
units used. One input channel was used during data
collection, measuring changes in depth of the manne-
quin chest.
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Aviation, Space, and Environmental Medicine x Vol. 85, No. 7 x July 2014 689
COMPARISON OF CPR POSITIONS — REHNBERG ET AL.
Protocol
The subjects were required to be available for a single
session lasting 90 min, which included carrying out all
measurements and performing all four CPR methods
(+1 G
z , ER, RBH, and HS). Before the BLS protocol
began, anthropometric measurements (height, weight,
arm, and leg length), skin folds, circumference, and
bone diameters were measured to allow calculation of
muscle mass. This enabled V
o 2 to be corrected. To assess
subcutaneous fat, skinfold measurements were taken at
the following sites: tricep, subscapular, chest, midaxil-
lary, abdomen, suprailiac, and medial calf. All measure-
ments were repeated three times, all on the right hand
side of the body, and the median of the three measure-
ments from each anatomical site was adopted as the
value ( 22 ). From the skinfold thickness, body density
was calculated using the predictive equations proposed
by Petroski ( 16 ), which was corrected for our young
population of Brazilian students, incorporating circum-
ference of the forearm and abdomen, as well as bone di-
ameter of the wrist and knee.
Subjects were then familiarized with the equipment
and all the techniques, +1 G
z CPR and all three micro-
gravity CPR techniques. Subjects were given full ex-
planations of the techniques and had to demonstrate
that they had mastered all BLS methods, in accordance
with ILCOR guidelines, before the beginning of each
protocol.
Subjects rested for 5 min prior to the beginning of the
BLS protocol to record baseline values for HR and V
o 2 .
Fig. 1. The three microgravity CPR methods: A) Handstand; B) Evetts-Russomano; and C) Reverse Bear Hug.
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690 Aviation, Space, and Environmental Medicine x Vol. 85, No. 7 x July 2014
COMPARISON OF CPR POSITIONS — REHNBERG ET AL.
All subjects began the BLS protocol by performing +1 G
z
CPR. They were required to carry out four sets of ECC
over a period of 1.5 min in accordance with ILCOR
guidelines, 30:2 ratio, allowing 6 s for ventilations, which
were not performed in this protocol. At the end of per-
forming four sets of ECC, the subjects were then given a
minimum of 10 min rest; this was done between each
BLS protocol. After performing +1 G
z CPR, the order of
performing the three microgravity CPR methods was
randomized.
The ECC rate and depth were measured throughout
the experiment. The exhaled gases were sampled con-
tinuously and analyzed every three breaths automati-
cally. HR was recorded before (resting HR) and
immediately after the completion of each protocol. The
rate of perceived exertion, using the Borg scale, was
also taken at the end of each protocol. The Aerosport V
o 2000 analyzer used its own software and was auto-
calibrated prior to each protocol. The mannequin was
calibrated between each CPR method using inputs of 0
and 60 mm.
Data Analysis
A one-way ANOVA test, assuming equal variance,
was performed to determine the differences in ECC rate,
depth, and physiological variables between all methods
(GraphPad Prism v6.0a). A 95% confi dence interval cal-
culation around the mean was used. A Tukey ’ s post hoc
test was carried out if the data were found to be signifi -
cantly different at P ⱕ 0.05.
RESULTS
All the physiological measurements collected from
the subjects and mannequin are summarized in Table I .
The results show that regular terrestrial +1 G
z CPR per-
formed by all subjects met the ILCOR criteria in terms of
D
T (Mean, 55.6 6 0.6 mm) and ECC rate (Mean, 103.4 6
0.4 compression/min) over the four sets. Among the
three microgravity CPR methods the D
T and ECC rate
varied signifi cantly ( Fig. 2 and Fig. 3 ) .
The HS method proved to be the best method to con-
sistently achieve a D
T close to that stated by the ILCOR
guidelines ( Fig. 2 ). The D
T fell into the 40-50 mm range
[F(3, 88) 5 128.3; P 5 0.0026; 49.3 6 1.2 mm] rather than
the minimum 50-mm range. The HS method had a sig-
nifi cantly lower ECC rate (92 6 2.2 compression/min)
compared to +1 G
z , ER, and RBH. Despite having a
lower D
T [F(3, 88) 5 128.3; P , 0.0001] compared to
+1 G
z ( Fig. 2 ), the ER and RBH both had ECC rates above
100 compressions/min (105.6 6 0.9; 101.3 6 1.5, respec-
tively) ( Fig. 3 ). The worst performance in terms of D
T
was the RBH, achieving a D
T of ECC lower than the
ILCOR guidelines in the fi rst set of ECC [32.5 6 1.8 mm,
F(3, 88) 5 128.3; P , 0.0001] and this continued to de-
cline over the four sets. Even when looking at the maxi-
mum depth achieved by the RBH, it still fell short of the
ILCOR guidelines ( Fig. 2 ).
The RPE increased in the ER, HS, and RBH [F(3, 88)
5 93.23; P , 0.0001] compared to terrestrial +1 G
z CPR
(8.8 6 0.34). RBH was perceived as the hardest (18.2 6 0.4),
the ER method slightly less so (16.3 6 0.4), and the HS
perceived as the least fatiguing (13.7 6 0.5). A similar
trend was seen with subject HR; however, the ER
method did show a higher mean HR (150.6 6 3.9 bpm)
than the RBH (145.7 6 4.02 bpm), despite having a lower
Borg value ( Table I ). Combining the results of the HS D
T
and perceived exertion shows the HS to be effi cient at
providing D
T while also being the least fatiguing tech-
nique. Conversely, when combining the same results for
the RBH, the inadequate D
T and high RPE, with in-
creased aerobic demand ( Table I ), shows that the RBH is
TABLE I. SUMMARY OF PHYSIOLOGICAL VALUES AND VALUES
OBTAINED FROM MANNEQUIN.
Physiological Variables Values
+1 G
z
Borg Scale (6 – 20) 8.8 6 0.34
Heart Rate (bpm) 111.1 6 2.88
Oxygen Consumption;
.
V O
2 , ml · kg
2 1 · min
2 1 (baseline)
25.01 6 1.64 (7.05)
Minute Ventilation;
L · min
2 1 (BTPS; baseline)
26.34 6 1.07 (10.41)
Evetts-Russomano
Borg Scale (6 – 20) 16.26 6 0.40 *
Heart Rate (bpm) 150.6 6 3.87 *
Oxygen Consumption;
.
V O
2 , ml · kg
2 1 · min
2 1 (baseline)
35.85 6 1.01 * (7.05)
Minute Ventilation;
L · min
2 1 (BTPS; baseline)
58.38 6 2.90 * (10.41)
Reverse Bear Hug
Borg Scale (6 – 20) 18.17 6 0.39 *
Heart Rate (bpm) 145.7 6 4.03 *
Oxygen Consumption;
.
V O
2 , ml · kg
2 1 · min
2 1 (baseline)
25.45 6 0.90 (7.05)
Minute Ventilation;
L · min
2 1 (BTPS; baseline)
48.24 6 2.32 * (10.41)
Handstand
Borg Scale (6 – 20) 13.74 6 0.53 *
Heart Rate (bpm) 123.3 6 3.96 *
Oxygen Consumption;
.
V O
2 , ml · kg
2 1 · min
2 1 (baseline)
27.47 6 1.04 * (7.05)
Minute Ventilation;
L · min
2 1 (BTPS; baseline)
39.89 6 2.01 * (10.41)
* Signifi cantly different from +1 G
z at P , 0.001, one-way ANOVA.
Fig. 2. Mean true depth (D
T ) of external chest compressions ( 6 SEM)
over 1.5 min for terrestrial and microgravity CPR using the three meth-
ods. Dashed line represents the greater than 50 mm of depth set by the
ILCOR 2010 guidelines; N 5 23. *Signifi cantly different from +1 G
z at
P , 0.001, one-way ANOVA; **signifi cantly different from +1 G
z , ER,
HS, and RBH at P , 0.0001, one-way ANOVA.
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Aviation, Space, and Environmental Medicine x Vol. 85, No. 7 x July 2014 691
COMPARISON OF CPR POSITIONS — REHNBERG ET AL.
the hardest and least effi cient CPR method in simulated
microgravity.
All methods of CPR, +1 G
z , ER, HS, and RBH had a
signifi cantly increased V
o 2 compared to the baseline
value [F(4,110) 5 147.3; P , 0.0001]. The V
o 2 for
the HS and ER were additionally signifi cantly higher
[F(4,110) 5 147.3; P , 0.0001] than the +1 G
z CPR
value, which is to be expected in microgravity and
correlates with the increase in HR and Borg values
( Table I ). However, the mean V
o 2 for +1 G
z CPR and
the RBH were similar and not signifi cantly different
[F(4,110) 5 147.3; P 5 0.9964], but the RBH did show
a larger variation ( Table I ).
The V
E of the three microgravity CPR methods, as
well as the +1 G
z CPR, increased from the baseline V
E . In
a pattern comparable to that of the HR, the V
E was the
highest with the ER method (58.4 6 2.9 L · min
2 1 ) and
lowest with the RBH (48.2 6 2.3 L · min
2 1 ), despite an
opposite pattern seen with the Borg values. The HS had
the lowest V
E out of the three microgravity CPR meth-
ods (39.9 6 2.01 L · min
2 1 ), but all were signifi cantly
higher than +1 G
z CPR [F(4,110) 5 86.64; P , 0.0001]
( Table I ).
DISCUSSION
This study builds on the preliminary work done
evaluating these three micrgravity CPR methods ( 10 )
by gathering more subjects, assessing changes in
V
o 2 ,
and evaluating each method according to the new 2010
ILCOR and European Resuscitation Council guidelines
( 12 ). This study has also built on previous microgravity
CPR studies and has now measured more accurately the
depth of ECC by using the D
T . This gives us the true
depth during each ECC by taking into account the recoil
as well as the depth of compression. This was developed
as there is often a trend for subjects to not fully decom-
press the chest as much as 46% of the time ( 1 ), especially
when tiring. This would, therefore, give a false record of
the depth of ECC ( 17 ). In addition, this study evaluated
these three microgravity CPR methods using the 2010
ILCOR guidelines, as well as evaluating them over a
prolonged period of time, as opposed to the short dura-
tion microgravity exposure seen in parabolic fl ights
( 5 , 7 , 8 ).
The minimum requirements for CPR, outlined in
the ILCOR guidelines, are needed to provide suffi -
cient hemodynamics from the time of cardiac arrest
until ALS can be applied or if there is spontaneous
return of circulation. Despite the increase to a mini-
mum depth of 50 mm, all subjects could perform CPR
to the ILCOR guidelines, with both depth and rate of
compression. There was also only a small increase
from baseline in RPE (8.8 6 0.3), V
o 2 , and V
E (10.41 to
26.34 L · min
2 1 ) compared to the three microgravity
CPR methods.
Out of the three microgravity CPR methods, the HS
appeared overall to be the most effective CPR method.
When combining the compression data with V
o 2 and
RPE, it could be argued that the HS was the most effi cient
technique. The HS provided the greatest D
T [F(3, 110) 5
128.3; P 5 0.0026; 49.3 6 1.2 mm] out of the three meth-
ods while also being perceived as the least fatiguing,
with a V
o 2 similar to that of +1 G
z CPR. However, the
HS method did have a lower than required rate of com-
pression (92 6 2.2 compression/min), which some stud-
ies have suggested can be detrimental to the blood fl ow
generated in CPR, potentially resulting in poorer sur-
vival outcomes ( 9 , 15 ).
Subjects were not able to achieve the new ECC depth
of 50 mm using the ER method, but a satisfactory rate of
compression was achieved (105.6 6 0.8 compression/
min). Looking at maximum depth over the four sets, it
suggests that subjects managed to keep the depth be-
tween 40-50 mm (46.5 6 1.8 mm), which could be seen
as acceptable with some clinical benefi t, although not
ideal. Yet, if you look at the D
T ( Fig. 2 ), it clearly shows
that the depth (37.4 6 1.5 mm) does not meet the ILCOR
criteria and this suggests poor compression of the chest
as the subjects grew fatigued during CPR performance
( 17 ). Despite these compressions potentially having a
clinical effect, it highlights the importance of accurately
measuring the true depth of ECC.
The ER also had a higher V
o 2 and RPE compared to
+1 G
z CPR. These data support previous fi ndings show-
ing the ER method is highly fatiguing ( 17 ); however, our
study found the ER method was unable to meet the
guidelines. The authors also highlighted the advantages
the ER method has over the HS method, in that ER
method performance is not affected by rescuer height or
capsule diameter/width.
The results show the RBH to be the worst technique in
terms of both ECC depth and RPE (18.2 6 0.4). Interest-
ingly, the RBH had the highest Borg scale value, but it
had an almost identical
V
o 2 to terrestrial +1 G
z CPR. It
has been noted as a simple method to deploy with mini-
mal training and it has been suggested that the RBH
could be used as an alternative method to the HS if res-
cuers are too short to perform the HS ( 7 ). Nonetheless,
the results from this study clearly show that, even from
the fi rst minute of CPR, the RBH method could not achieve
Fig. 3. Mean rate of external chest compression ( 6 SEM) over 1.5
min for terrestrial and microgravity CPR using the three methods.
Dashed line represents the lower limit of 100 compressions/min set by
the ILCOR 2010 guidelines; N 5 23. *Signifi cantly different from +1 G
z ,
ER, and RBH at P , 0.0001, one-way ANOVA.
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COMPARISON OF CPR POSITIONS — REHNBERG ET AL.
the required depth of compression (32.5 6 1.8 mm)
needed and this further declined over the remaining pe-
riod of ECC.
Combining this data with the RPE values highlights
that the RBH was ineffective and highly fatiguing. As-
tronauts must have an adequate aerobic reserve to act in
an emergency to respond to a crewmember who is in
distress ( 13 ). Despite an adequate ECC rate, the RBH is
simply not able to produce adequate force for effective
ECC and is too strenuous to perform for any period of
time. The inability to directly visualize the front of the
patient may be a factor in the poor performance of the
RBH method. Overall the RBH is not a practical CPR
method to be used in microgravity as it would not pro-
vide suffi cient blood fl ow and perfusion of vital organs
until ALS could be deployed.
The similarity between the V
o 2 of +1 G
z CPR and the
RBH was unexpected, especially as the HR and Borg
values for the RBH were the highest of the three micro-
gravity CPR methods. Studies have shown that regular
+1 G
z CPR is primarily aerobic exercise that is reason-
ably well tolerated in healthy untrained individuals
( 23 ). The RBH, however, as with all the microgravity
methods, lacks the ability to accelerate the weight of the
rescuer’s body to aid in the force of the ECC and solely
relies upon the strength generated by the upper limbs,
mainly fl exion of the biceps, to generate force.
It could be suggested that the RBH may predomi-
nantly be a resistance exercise and therefore be anaero-
bic, which could potentially explain the lower than
expected V
o 2 as anaerobic metabolism is used as the
main source of fuel for the muscle rather than oxygen
consumption ( 21 ). Studies have shown that continuous
V
o 2 measurements are the gold-standard for aerobic ex-
ercise, but intense muscle contractions can ‘ pinch off ’
blood fl ow and, therefore, affect the accuracy of V
o 2
measurements ( 20 ). The extent of anaerobic exercise in
the RBH could be measured in future studies by mea-
suring blood lactate as well as V
o 2 ( 19 ).
The limitation of the Resusci Annie mannequin has
been noted in previous studies. Despite our modifi ca-
tions, these mannequins are designed for regular +1 G
z
CPR training. The mannequin may not accurately refl ect
upper limb morphology, as this is an important factor in
adopting some of these CPR techniques ( 17 ). The man-
nequin also does not fully replicate the changes seen in
chest mechanics ( 3 , 4 ) or other physiological effects of
microgravity. This is also true of the BSD in simulating
microgravity.
Another limitation of the study was the assumption
that CPR in microgravity would conform to the ideal
30:2 ratio stated in the guidelines. As only rate and
depth of ECC were evaluated, the subjects did not have
to leave their CPR position to perform ventilations to
then reposition themselves to re-start CPR. This re-
peated repositioning between compressions and venti-
lations, in a real cardiac arrest, could potentially affect
the rate and quality of ECC given to the patient. The
study also made the assumption that CPR began imme-
diately after a cardiac arrest took place in microgravity
aboard the International Space Station (ISS). However,
aboard the ISS, spacecraft, or during an EVA, there could
potentially be delays by the other crewmembers reach-
ing the patient. Specifi c to this study, the harness for the
BSD could potentially restrict movement and hinder re-
positioning in future studies evaluating ventilation dur-
ing microgravity CPR. It was out of the scope of this
study to evaluate these multiple factors, as this study
primarily focused on evaluating these three CPR meth-
ods as to how effective they are, so that research can
progress into developing more effi cient CPR/BLS pro-
tocols. However, these are important facts to remember
as they are likely scenarios that can happen and will all
affect the effectiveness of CPR in microgravity and the
patients ’ survival.
The trolley used as a simulation for the HS method,
like all simulations, has its advantages and disadvan-
tages. The trolley system allowed for an easy adoption
of the correct position. No biomechanics were specifi -
cally done, but it could be said that the trolley may have
supported core muscles (such as the erector spinae and
abdominal muscles). This could make the HS method
easier to perform than in actual microgravity. The trol-
ley had plastic wheels fi xed within wooden tram braces.
Despite the addition of a lubricant, the friction and the
momentum created by the weight of the trolley could
have affected the performance of the HS, and could ex-
plain the low ECC rate.
A limitation of the methods was the lack of counter-
balance. The order in which the subjects performed the
microgravity CPR methods were randomized, but this
was not counterbalanced so an equal proportion of sub-
jects began with each CPR method. Also, the population
of subjects may not be accurately representative of an
astronaut population in terms of age, fi tness, and the
lack of any female subjects. Yet with the potential for
increased commercial suborbital fl ights, a broad spec-
trum of subjects should be evaluated ( 6 ). In this study,
V
o 2 was used, in conjunction with RPE, as a quantitative
measure of the aerobic demand of each of the CPR meth-
ods. However, future studies to evaluate these CPR
methods may benefi t from pre-testing
V
o 2max and pre-
senting data as a percentage of
V
o 2max ; this would allow
greater insight into individual aerobic demands during
each CPR method.
The next step in evaluating these microgravity CPR
methods would be to carry out a parabolic fl ight cam-
paign to: validate the data seen in ground-based simula-
tions of microgravity; test the effi cacy of each of the CPR
methods with the new ILCOR guidelines; and include
female subjects in these evaluations. This would allow
validation and further evaluation of these three micro-
gravity CPR methods while also providing data regard-
ing the similarity of the simulated microgravity provided
by the BSD and the microgravity achieved during a par-
abolic fl ight.
All three methods have merits in performing ECC
in microgravity; the ER and RBH have shown ade-
quate ECC rate, and the HS method showed adequate
D
T . However, as none of the three methods were able to
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Copyright: Aerospace Medical Association
Aviation, Space, and Environmental Medicine x Vol. 85, No. 7 x July 2014 693
COMPARISON OF CPR POSITIONS — REHNBERG ET AL.
meet all the criteria from the ILCOR guidelines, none of
the three methods can be considered an effective form of
CPR in these microgravity simulations. Further research
needs to look at more accurate astronaut populations or
those more likely to take commercial suborbital fl ights,
and to evaluate the most effective method of CPR in mi-
crogravity during parabolic fl ight.
ACKNOWLEDGMENTS
We would like to thank King ’ s College London, the Colt Foundation,
the John Ernsting Aerospace Physiology Lab/MicroG Centre, PUCRS,
Valquiria Gomes, and Isadora Serpa for their support in undertaking
this study. We would also like to thank Professor Valter P. Albuquerque
for providing statistical expertise in this study.
Authors and affi liations: Lucas Rehnberg, B.Sc., M.Sc., Alexandra
Ashcroft, B.Sc., and Justin H. Baers, B.Sc., M.Sc., Centre of Human
and Aerospace Physiological Sciences, Kings College London,
London, UK; Fabio Campos , Ricardo B. Cardoso, Rodrigo D. Gehrke,
Mariana K. P. Dias, Rafael R. Baptista, M.Sc., Ph.D., and Prof. Thais
Russomano, M.D., Ph.D., John Ernsting Aerospace Physiology
Laboratory, Microgravity Centre, PUCRS, Porto Alegre, Brazil; and
Rochelle Velho, B.Sc., M.B.B.S., NHS, University Hospitals Coventry
and Warwickshire NHS Trust, West Midlands Deanery, UK.
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