Low-intensity exercise may fail to preserve heart health in long-term space travelers
Sustained, low-intensity exercise may not be enough to preserve left ventricular mass and diastolic diameter in long-term space travelers, according to a research letter published in Circulation.
Researchers compared the echocardiograms of former NASA astronaut Scott Kelly, who spent 340 days aboard the International Space Station, with those of Benoît Lecomte, who previously swam across the Atlantic Ocean, to determine whether sustained, low-intensity exercise in relative weightlessness was enough to preserve heart function.
“What do extreme duration swimming and extreme duration spaceflight have in common, and how are they different? Both are associated with removal of gravitational loading of the musculoskeletal system and the absence of weight-bearing activities,” James P. MacNamara, MD, MSCS, cardiology research fellow at the University of Texas South-western Medical Center in Dallas, and colleagues wrote. “During spaceflight, the loss of a gravitational gradient results in a similar short-term rise in preload, followed by a compensatory decrease in blood volume and a long-term reduction in preload. Without countermeasures, extended spaceflight results in cardiac atrophy and orthostatic intolerance.”
For this analysis, researchers used data from echocardiograms performed before and during Lecomte’s and Kelly’s respective feats to identify changes in LV and diastolic function.
Lecomte swam for 159 days for an average of 5.8 hours per day and slept 8 hours a night, resulting in 9 to 17 hours per day in the prone or supine position.
Kelly spent 340 days in space and was prescribed exercise 6 days per week that included a combination of cycling, treadmill and resistive exercise.
“The heart is used to pumping ‘uphill’ against gravity and both the presence of weightlessness and the confinement of the space station leads to an overall reduced workload on the heart which causes physiological (not pathological) atrophy,” Benjamin D. Levine, MD, FACC, FAHA, FACSM, professor of medicine and cardiology and distinguished professorship in exercise science at UTSouthwestern Medical Center, told Healio. “This change is rapidly reversible with increased activity on return to earth. From other data that we have collected using MRI, the astronauts who are most fit before flight lose the most mass, and those that begin relatively unfit can actually induce physiological hypertrophy.”
Researchers reported that LV mass declined at a similar rate for both individuals (0.72 g per week for Lecomte; 95% CI, 0.14 to 1.58; vs. 0.74 g per week for Kelly; 95% CI, 0.13-1.34).
Both Lecomte and Kelly experienced an initial drop in LV diastolic diameter to a lower steady state throughout their respective feats, with Lecomte’s decreasing from 5 cm to average 4.7 cm and Kelly’s dropping from 5.3 cm to average 4.6 cm.
According to the report, these findings suggest spaceflight is associated with greater initial LV volume loss; however, biological variability cannot be excluded.
“I was surprised with Mr. Lecomte’s result, because based on previous work we have done with bedrest simulations, we thought he would be doing enough exercise to counteract the microgravity effect,” Levine told Healio. “However, it turns out that his swimming was very low intensity, and the gravitational effect won out.”
Markers for LV ejection fraction and diastolic function did not consistently change in either individual throughout their campaign, researchers reported.
"Physiological atrophy does not mean 'unhealthy',' Levine told Healio. "The rise and fall in cardiac mass with activity is an appropriate physiological response to load, and up to 75% of the heart’s muscle mass is plastic, responsive to changes in physical activity: from a maximal atrophy of approximately 25% in [patients with spinal cord injury], to a 50% hypertrophy in elite endurance athletes."
As for exercise training that could be helpful to those who will be in space long-term, "I am a big fan of rowing (Hastings JL, et al. J Appl Physiol. 2012;doi:10.1152/japplphysiol.00019.2012) which is superb endurance and strength exercise, and NASA is likely to use a special flywheel that allows rowing for endurance, but can be locked down to allow strength training as well," Levine said in an interview.
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