Short-term Exposure to Microgravity and the Associated Risk of Sudden Cardiac Arrest: Implications for Commercial Spaceflight

Abstract

The likelihood of trained astronauts developing a life threatening cardiac event during spaceflight is relatively rare, whilst the incidence in untrained individuals is unknown. Space tourists who live a sedentary lifestyle have reduced cardiovascular function, but the associated danger of sudden cardiac arrest (SCA) during a suborbital spaceflight (SOSF) is unclear. Risk during SOSF was examined by reviewing several microgravity studies and methods of determining poor cardiovascular condition. Accurately assessing cardiovascular function and improving baroreceptor sensitivity through exercise is suggested to reduce the incidence of SCA during future SOSFs. Future studies will benefit from past participants sharing medical history; allowing creation of risk profiles and suitable guidelines.

Full text

SHORT-TERM EXPOSURE TO MICROGRAVITY AND THE ASSOCIATED RISK OF
SUDDEN CARDIAC ARREST: IMPLICATIONS FOR COMMERCIAL SPACEFLIGHT
Kevin J. C. Laing (1), Thais Russamano (1)
(1) Centre of Human & Aerospace Physiological Sciences, King's College London,
Guys Campus, London, SE1 1UL, UK. kevin.laing@kcl.ac.uk
ABSTRACT
The likelihood of trained astronauts developing a life
threatening cardiac event during spaceflight is relatively
rare, whilst the incidence in untrained individuals is
unknown. Space tourists who live a sedentary lifestyle
have reduced cardiovascular function, but the associated
danger of sudden cardiac arrest (SCA) during a
suborbital spaceflight (SOSF) is unclear. Risk during
SOSF was examined by reviewing several microgravity
studies and methods of determining poor cardiovascular
condition. Accurately assessing cardiovascular function
and improving baroreceptor sensitivity through exercise
is suggested to reduce the incidence of SCA during
future SOSFs. Future studies will benefit from past
participants sharing medical history; allowing creation
of risk profiles and suitable guidelines.
1. INTRODUCTION
The likelihood of a life threatening cardiac event
occurring in space is relatively low, due to the current
rigorous selection procedures in place for national
agency astronauts [1]. However, the commercialisation
of spaceflight is expected to rapidly increase the number
of humans travelling into space. With a rise in numbers
comes the increased possibility that an individual will
experience sudden cardiac arrest (SCA) during
spaceflight. Carrying out pre-flight tests on potential
commercial spaceflight participants (CSFPs) will allow
the risk of an individual to be assessed and reduce the
chances of compromising a costly mission. Selecting the
most suitable methods to evaluate cardiovascular risks
remains essential to making key decisions regarding
fitness to participate. Current medical standards have
been developed for use by national space agencies and
with the long-term adaptations of microgravity in mind.
Many of these stringent guidelines are not applicable to
commercial spaceflight, as they are based on the long-
term deleterious effects of microgravity [2]. A
suborbital spaceflight (SOSF), expected to last 5-6
minutes, does not elicit the same adaptations seen
during a long-term mission and this must be considered
when determining the risk of an individual.
It is widely believed that Virgin Galactic will be the first
commercial operator to carry humans into space. The
future of SOSF and its success as a commercial venture
is heavily dependent on participant safety. Due to the
limited evacuation and treatment capabilities during
spaceflight, an emphasis is currently placed on
prevention through adequate screening [3]. The
expected profile of participants and unique
physiological stresses experienced during brief exposure
to microgravity has been examined in this review.
Parabolic flight data provides a useful insight to the
physiological responses of the cardiovascular system as
does the initial measurements made during space
missions. The majority of astronauts in the past 50 years
of manned spaceflight have been of very good physical
health. However, when the data collected during the
initial stages of a long-term mission is analysed in
conjunction with various terrestrial based analogues, it
can prove useful in predicting the physiological
responses of the general population. Commercial
spaceflight has opened up the possibility of
experiencing microgravity to individuals who may have
previously been excluded by national agencies based on
their health. The typical CSFP is predicted to be older
(due to the large cost involved) and less physically fit
than their career astronaut counterparts [4]. It is
therefore reasonable to assume that in the future
potential CSFP will be exposed to a greater risk of
developing SCA due to their predicted inferior
cardiovascular fitness profiles.
Disruption to the normal ability of the heart to contract
rhythmically can lead to SCA. The two types of serious
SCA, ventricular fibrillation (VF) and asystole, will
become medical emergencies if left to persist. These
types of arrhythmias have been shown to occur in
microgravity [1]; however, dysrhythmia leading to
severe circulation problems or SCA has yet to be
recorded during spaceflight.
This review aims to investigate how the risk of SCA,
due to short-term microgravity exposure during a SOSF,
can be best managed and reduced. Suitable pre-flight
screening methods for CSFP exclusion and possible
methods to attenuate risk have been explored.
2. DYSRHYTHMIAS
Cardiac rhythm disturbances in microgravity remain
relatively rare with most due to cardiovascular disease;
however, it remains unclear as to whether or not
microgravity alone or the exacerbation of a pre-existing
undiagnosed condition is the cause [5]. It is known that
several factors can contribute such as dehydration,
documented in the literature as Apollo 15 Space
Syndrome [6] and electrolyte imbalances. In particular
reference to asystole, bradycardia and high pCO2 has
been shown to be aggravating factors [7]. One of the

reasons for the occurrence of VF, among many, is poor
cardiac muscle conductance, such as that seen in post-
myocardial infarction patients.
The potential for an astronaut to experience cardiac
arrhythmia remains present and there have been several
such cases documented, where the occurrence has not
been life threatening [3]. During the MIR space station
programme, the first ventricular tachyarrhythmia was
logged whilst the ECG of a crew member was recorded
(Fig. 1). This incident occurred two months into a four-
month mission with the astronaut remaining
asymptomatic throughout. This observation was made
during a particularly stressful segment of the mission
programme, suggesting factors other than microgravity
were contributing to the overall effect [8]. Whilst this
data is worth noting it remains insignificant in relation
to SOSF where such a stressful situation would only
arise in an emergency.
Figure 1. Record of a non-sustained 14-beat ventricular
tachycardia (VT) from a MIR crewmember (n = 1) [8]
3. TERRESTRIAL ANALOGUES
One of the most significant cardiovascular adaptations
during microgravity exposure is the head-ward fluid
shift and this phenomenon can be observed during
parabolic flights and the resultant physiological
responses recorded. A typical parabolic flight involves
repeated parabolas that simulate a normal, hyper- and
microgravity environment during progressive phases
[11]. Several studies have demonstrated a significant
decrease in heart rate (HR) and increase in mean arterial
pressure (MAP), in both humans and animals due to the
transition from hyper- to microgravity and vice versa
(Fig. 2) [9-11].
It is worth noting that no parabolic flight profile is the
same as different aircrafts and varying heights are used
during research to obtain data. Unique gravity profiles
are created during each parabolic flight and this
complicates analysis due to the requirement of specialist
analysis techniques [12]. However, these studies remain
significant in relation to commercial spaceflight as they
mimic the profile of hyper-gravity followed by
microgravity experienced during SOSF. Parabolic flight
also allows responses to be measured on simulated re-
entry without the various adaptations to long-term
microgravity.
Entering microgravity, whilst standing, causes blood
from the lower extremities to move towards the central
compartment. The sudden redistribution of body fluid
pressure gradients is similar to an extreme posture
change from head-down tilt to upright and requires
specific responses to maintain adequate blood flow.
Cardiovascular function adapts quickly and readily to
during spaceflight and there is little, if any persistent
change in MAP [11]. However, the arterial
baroreceptors located in the transverse aortic arch and
the carotid sinuses of the left and right internal carotid
arteries are at their most active during exercise and G-
level transition.
Figure 2. HR (bpm) responses to different G-levels
during parabolic flight [11]
The changes in HR and MAP during parabolic flight
suggest the largest cardiac risk to a potential CSFP
would be during entry and re-entry. A Bainbridge-like
reflex seems to play a significant part in microgravity
induced tachycardia [14]. Reference [12] compiled data
from several parabolic flight campaigns to show that the
following changes in HR were observed at different
gravitational transitions, +13 bpm at 1 g  1.8 g; -23
bpm at 1.8 g  0 g; and +19 bpm at 0 g  1.6 g (Tab.
1).
Gravitational
transition (g)
Change in heart
rate (bpm)
n = 36
Stroke volume
(100% at 1 g)
n = 18
1  1.8
+ 13
- 16
1.8  0
- 23
+ 20
0  1.6
+ 19
- 2
Table 1. Cardiovascular data at different gravity
transitions. Control HR at 1 g was 83 bpm.
Data compiled for various parabolic
flight campaigns [12]
Controlling this and minimising it could be key to
preventing a CSFP entering a potential life-threatening
ventricular tachycardia or asystole. During prolonged
exposure to microgravity cardiac events have occurred,
however, various plausible explanations can be offered.

During long-term exposure to microgravity, the
electrolyte balance in the body fluctuates and changes in
blood concentration of calcium and potassium are
believed to have an effect on cardiac rhythm disturbance
[5]. In addition, stressful mission tasks such as
extravehicular activity (EVA) and the inadvertent
pressures placed on an astronaut to achieve results have
been shown to induce ventricular tachycardia [8].
Nonetheless, these situations have occurred during
orbital spaceflight and it seems reasonable to infer such
adaptation during only 5-6 minutes of sub-orbital
spaceflight would not be long enough to induce such
changes.
The medical standards required of CSFPs are therefore
lower, as these individuals as essentially passive
participants without any operational objectives or
responsibilities [4]. It is for this reason that during
future SOSFs the largest risk to an individual is
suggested to occur during gravitational force transitions.
The ability of CSFPs to cope with the associated
physiological changes, during this time, will play a large
role in determining the risk of an individual to SCA.
4. DETERMINING RISK
When assessing the suitability of an individual to be
part of a commercial spaceflight appropriate testing
methods need to be employed. Current ESA astronaut
selection procedure dictates an individual has to pass a
JAR-FCL 3 Class 2 medical examination. This is a
relatively efficient protocol to detect most health related
exclusion factors of pilots and hence potential
astronauts. With regards to assessing cardiovascular
function, a 12-lead resting ECG can be taken and, in
relation to potential risk in microgravity, much can be
gained through the use of a simple questionnaire [5].
The answers to an initial set of questions can then
provide the basis of further directed examination. A
possible screening procedure using a simple scoring test
could be used to determine the risk of SCA with an
emphasis placed on the medical history of the
participant and, in particular, previous myocardial
infarction (MI), heart failure, abnormal heart rhythms
and SCA or unexplained fainting episodes. A participant
heart ejection fraction of less than 40% is also indicative
of increased SCA risk [17].
Individuals deemed to be at particular risk of SCA could
be placed on suitable exercise programmes with the goal
of reducing their risk during spaceflight. Intensive
exercise used as a training tool in this way has been
shown effective in reducing the deviations seen to
physiological measurements in spaceflight and the
susceptibility of an individual to sudden cardiac death
[15, 16]. It is believed trained individuals are better
placed to cope with the unique stresses and therefore the
risk of pre-trained individuals developing SCA is
reduced.
The baroreceptor reflex remains an important
homeostatic control mechanism of the body and it is
essential in responding to pressure changes.
Baroreceptors remain crucial to maintaining normal
functioning BP in all conditions and their impairment
may play an adverse role in SCA [17-19]. Measuring
baroreflex sensitivity may be useful in defining the
cardiovascular risk of an individual during spaceflight
and potentially serve as an excluding factor if the values
measured deviate too far from the norm. Currently there
are several laboratory techniques used to determine
baroreceptor sensitivity (Tab. 2) and numerous studies
have demonstrated their prognostic value in various
cardiovascular diseases [20, 21].
 Carotid sinus massage
 Electrical stimulation of carotid sinus nerves
 Anaesthesia of carotid sinus nerves and vagi
 Occlusion of common carotid artery
 Valsalva manoeuvre
 Head-up tilting
 Lower-body negative pressure application
 Intravenous bolus injection of vasoactive agents
with no (or limited) direct effect on the heart
 Intravenous stepwise infusion of vasoactive agents
 Assessment of reflex changes in muscle
sympathetic nerve activity induced by BP changes
following vasoactive drug infusion
 Neck chamber technique
Table 2. Lab based assessments methods for
determining baroreflex sensitivity in human [22]
The baroreceptor reflex works antagonistically with the
Bainbridge reflex to control HR [14]. When either of
these mechanisms fails to respond adequately to
pressure changes, problems indicative of cardiovascular
disease are displayed. During microgravity insufficient
control of BP in this environment can have detrimental
effects and potentially lead to aborting a mission. It is
therefore important each CSFP is adequately examined
and the correct risk determined to prevent an
unexpected medical emergency, such as SCA, occurring
during SOSF.
5. CONCLUSION
One of the main cardiovascular responses seen during
SOSF is the head-ward fluid shift and the associated
mechanisms this elicits, such as the changes in stroke
volume (SV) and HR. Various parabolic flight data
shows that the reduction to HR when entering
microgravity standing is far greater than when in a
supine position [10]. Data also suggests that HR can fall
much faster than it can increase, suggesting possible
attenuation by the parasympathetic efferents during
SOSF [23]. Position matters when measuring the
cardiovascular responses during gravitational transition.
This knowledge can be used to place CSFPs in a
suitable supine position during entry and re-entry to
minimise the stresses experienced and reduce the
possibility of SCA.

The baroreceptor reflex is initiated due to the variable
body fluid hydrostatic pressure gradients during SOSF.
Brief exposure to microgravity is thought to induce
interaction between sympathetic and parasympathetic
efferent activities, leading to the specific characteristic
of the HR changes during brief microgravity exposure.
Problems relating to baroreflex sensitivity could lead to
inappropriate adjustments in HR during spaceflight and
cardiac arrhythmia due to other mechanisms attempting
to bring HR under control. Increasing the baroreceptor
sensitivity of potential CSFPs with reduced reflex
response could prove useful is reducing the risk of SCA
in these individuals.
Regular bouts of intense exercise has been shown to be
the most effective way at reducing the incidence of SCA
on Earth during periods of elevated HR [24]. It seems
logical to assume the same would be applicable in
microgravity; however, it remains important to suitably
assess the cardiovascular condition of an individual
prior to ensure any exercise regime would be of benefit.
6. FUTURE STUDIES
Future studies into the cardiovascular risks associated
with spaceflight will benefit greatly from the
willingness of participants to share their medical and
family history. When performance in space can be
linked to previous medical history the correct
procedures and best practices can be followed to
provide a safer SOSF.
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