Cardiac Effects of Repeated Weightlessness During Extreme Duration Swimming Compared With Spaceflight
James P. MacNamara, Katrin A. Dias, Satyam Sarma, Stuart M. C. Lee, David Martin, Maks Romeijn, Vlad G. Zaha, Benjamin D. Levine
Abstract
enot Lecomte (B.L.) swam 2821 kilometers over 159 days, and Scott Kelly (S.K.) spent 340 days in space. 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. Water immersion and supine bed rest, ground-based models for spaceflight, initially increase central blood volume as a result of reversed hydrostatic gradients but over time lead to diuresis partially through atrial natriuretic peptide stimulation and antidiuretic hormone inhibition. 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. 2 Without countermeasures, extended spaceflight results in cardiac atrophy and orthostatic intolerance. In this study, we compare the cardiac effects of extreme-duration swimming and spaceflight to determine whether low-intensity, long-duration exercise counteracts the effects of repeated weightlessness.