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ArWatch Staff, and Dr Peter Norsk

Clinical Articles, Featured

Long-duration space flight may pose heart problems

A study1 from the US National Aeronautics and Space Administration (NASA) has found some unexpected changes in haemodynamics following prolonged weightlessness. They observed that, while ambulatory heart rate does not change, systemic blood pressure decreases during prolonged space flight.

Other changes included a shift of blood and fluid from the lower to the upper body (caused by weightlessness) which was much higher and the blood pressure much lower than previously thought.

The researchers led by NASA and Universities Space Research Association (USRA) physician and space scientist, Dr Peter Norsk, measured stroke volume and cardiac output and monitored the blood pressure in eight astronauts aged between 45–53 years during a 24 hour period. Recordings were taken before, during and after 3–6 months of spaceflight. A venous blood sample was also collected to analyse the concentration of norephinephrine, reflecting the sympathetic nervous activity of the cardiovascular system.

Dr Peter Norsk (NASA and Universities Space Research Association)

Dr Peter Norsk (NASA and Universities Space Research Association)

The team found that the shift of blood and fluid from the lower to the upper body caused by weightlessness was much higher than previously thought. The blood volume burden to the heart was also more than expected despite the heart rate remaining the same. At the same time, blood pressure was considerably reduced by 10 mmHg, which corresponds to changes seen with antihypertensive medication. This effect was induced by a 39% decrease in peripheral vascular resistance by dilatation of arterial resistance vessels.

Dr Norsk said: “The discovery is important because during long duration missions, the blood volume burden to the heart could constitute a health problem during future long duration missions. We know that some astronauts experience vision problems some months into spaceflight and this may in fact be caused by the augmented fluid and blood volume shift to the upper body.”

“Although the blood volume burden to the heart is higher than expected, blood pressure is lower because the blood vessels are more relaxed (dilated). This is actually good for the body and the blood vessels. Thus, there are one or more spaceflight factors leading to these changes, which haven’t been identified yet. In the future, the spaceflight factors that induce the fluid shifts and relaxation of the blood vessels should be identified,” Dr Norsk continued.

BJC Arrhythmia Watch interviewed Dr Norsk to explore these findings further, asking:

(BJC) How soon into weightlessness are these shifts measurable?

Screen shot 2015-01-29 at 15.58.12(PN) The fluid shifts in space occur immediately within seconds of weightlessness. We have known this for several decades and the fluid shifts have been characterised pretty well during shorter flights of 1–2 weeks of duration. What we didn’t know was how the fluid shifts adapt to long duration flights for months. We found in this study that the fluid and blood shift to the heart is even larger than within the initial week of flight, because earlier2 we measured the same variables with same methodology and found that stroke volume and cardiac output only increased half as much as what we see now after three months of flight.

(BJC) could this volume shift lead to pulmonary oedema?

(PN) Before the advent of manned spaceflight, it was speculated that the fluid shift could lead to pulmonary oedema. However, nothing of the sort has ever been observed in space, where until now 536 people have flown. Previous studies looking at this issue showed that pulmonary oedema did not occur and it is thought that pulmonary oedema is unlikely to occur.3

(BJC) Have any pathologic changes been observed with this phenomenon, e.g. presence of atrial fibrilllation, cardiac failure etc..

(PN) Cardiac arrhythmia is of concern for spaceflight, because there have been a few events of this kind during some of the previous flights. Two space research studies concluded that there is no increased arrhythmia because of the spaceflight itself. These studies indicated that spontaneous arrhythmia may occur on earth as well as in space, nor was there any arrhythmia occurring specifically in space. Although it is not expected that there will be any arrhythmia occurring in space that is a health danger, there is an automated external defibrillator (AED) and cardiac drugs to treat dangerous arrhythmia that could occur on the International Space Station (ISS). This has never needed to be done.

(BJC) Have you conducted Holter ECG monitoring in astronauts?

(PN) Although I have not conducted Holter monitoring in space, there are several published studies for both short and long duration space flight. The data indicates as above that there is no increased cardiac arrhythmias associated with space flight. An arrhythmia did occur in an Apollo astronaut who went on to have a heart attack about a 1.5 years later but there was no direct indication it was related to this occurrence. There has been another publication showing a short ‘run’ of ventricular tachycardia in a completely asymptomatic astronaut who did not know it was occurring that was seen on a Holter monitoring which caused no health problem.4

(BJC) Are there measures which could be adopted, e.g. pressure suits to avoid such chronic shifts and volume expansion?

(PN) One could use lower body negative pressure to counteract the fluid shift as well as bracelets (venous congestion). The Russians use limp bracelets for this purpose as a countermeasure against the fluid shifts in space. NASA has initiated a large study to understand the effects of lower body negative pressure and later bracelets upon the fluid shifts and the effects on intracranial pressure. So the answer to your question is yes, lower body negative (not positive) pressure do have an effect on the fluid shifts as well as limb venous congestion, and it is being used routinely by the Russian space program. However, we need more documentation as to the efficiency of these countermeasures.

(BJC) Are there any ‘good’ effects associated with this phenomenon?

(PN) We do not know yet whether the cardiovascular changes that occur in space crews on the ISS are just a normal resetting of the cardiovascular system or if these changes may have unintended consequences that could be unhealthy or even healthy for the space traveller either during or after the space mission. Current research studies are trying to determine if there is. We know that the vessels in the part of the body below the heart decrease in filling, volume and diameter and that the opposite occurs above the heart. The latter may be the cause for some vision acuity problems, which many astronauts experience some time into their missions on the Space Station.

BJC Arrhythmia Watch would like to thank:

Dr Peter Norsk
NASA Element Scientist – Human Research Program, Johnson Space Center, Houston, Texas, USA

Reference

1. Norsk P, Asmar A, Damgaard M, Christensen NJ. Fluid shifts, vasodilatation and ambulatory blood pressure reduction during long duration spaceflight. J Physiol 2015. http://dx.doi.org/10.1113/jphysiol.2014.284869

2. Norsk P, Damgaard M, Petersen L, Gybel M, Pump B, Gabrielsen A, Christensen NJ. Vasorelaxation in space. Hypertension 2006;47:69–73. http://dx.doi.org/10.1161/01.HYP.0000194332.98674.57

3. Prisk GK. Microgravity and the respiratory system. Eur Respir J 2014;43:1459–71. http://dx.doi.org/10.1183/09031936.00001414

Published on: January 29, 2015

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