Cardiac Effects of Repeated Weightlessness During Extreme Duration Swimming Compared With Spaceflight

James P. MacNamara, MD, MSCS; Katrin A. Dias, PhD; Satyam Sarma, MD; Stuart M.C. Lee, PhD; David Martin, MS, RDCS; Maks Romeijn, MD; Vlad G. Zaha, MD, PhD; Benjamin D. Levine, MD

Disclosures

Circulation. 2021;143(15):1533-1535. 

Benoît 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.[1,2] 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.

Both individuals gave permission to identify them in this report. B.L., an elite endurance swimmer who previously swam across the Atlantic Ocean, swam for 159 days (June 5, 2018–November 11, 2018), with breaks of 7 and 32 days because of unfavorable weather. He swam average of 5.8 hours (range, 1.1–9 hours) per swimming day and slept 8 hours a night, resulting in 9 to 17 h/d in the prone or supine position. He did not have a set hydration regimen. S.K. spent 340 days in space (March 27, 2015–March 1, 2016) and was prescribed exercise 6 d/wk that included a combination of cycling, treadmill, or resistive exercise. Two-dimensional and Doppler echocardiograms (Vivid Q, GE) were performed by sonographers before and by remote guidance during their respective campaigns. Left ventricular (LV) mass and LV ejection fraction were measured with the Teichholz method. Diastolic function was assessed by mitral inflow velocity and early diastolic recoil velocity. The study was approved by the institutional review board, and subjects gave informed consent. The data that support the findings of this study are available from the corresponding author on reasonable request.

LV mass declined at similar rates in both individuals. B.L.'s mass dropped by 0.72 g/wk (95% CI, −0.14 to 1.58) and S.K.'s mass dropped by 0.74 g/wk (95% CI, 0.13–1.34) when linear regression is applied (Figure). Both subjects had an initial drop in LV diastolic diameter to a lower steady state through the campaign (B.L., 5 cm to average 4.7 cm; S.K., 5.3 cm to average 4.6 cm), suggesting an initial volume loss that was greater with spaceflight, although biological variability cannot be excluded. LV ejection fraction and markers of diastolic function did not consistently change in either individual throughout their campaign.

Figure.

Individual data for Benoît Lecomte and Scott Kelly across their respective campaigns.
The overall trend of left ventricular (LV) mass was similar between Benoît Lecomte (▴) and Scott Kelly (○) (Top, left). Linear regression equation and best-fit line are shown. LV diastolic diameter shows an initial decline in Scott Kelly (Top, right). Ejection fraction (Bottom, left), diastolic recoil (e', Bottom, middle), and E/A ratio (Bottom, right) show no consistent trend. BL indicates Benoît Lecomte; LV, left ventricular; and SL, Scott Kelly.

Both individuals lost LV mass over the duration of their campaigns despite substantial amounts of exercise. Full-time bed rest studies, which serve as analogs to weightlessness, have shown reductions in plasma volume, cardiac mass, and orthostatic tolerance.[2] B.L.'s awake time out of the water and ad libitum hydration were likely insufficient to completely preserve his plasma volume. Daily lower-body negative pressure (partially reinstating hydrostatic gradients) does not preserve plasma volume.[3] We anticipated that a long duration of swimming exercise would have been enough of a stimulus to result in increases in LV mass and volume. Swim training of 1 to 3 hours daily at high intensity is associated with increased LV size and mass.[4] It is surprising that low-intensity, long-duration swimming was insufficient to overcome the effects of repeated exposure to a weightlessness analog, leading to a cardiac adaptation similar to that in prolonged spaceflight. This study was limited by real-world issues, including delays between initial scans, interruptions of B.L.'s swim (for his safety), lack of body mass and blood volume measures, and reliance on the Teichholz method to estimate mass, limited the study. Both subjects meet the geometric assumptions of the Teichholz method, and S.K.'s results, including the initial decline in LV mass, are consistent with prior studies of bed rest and spaceflight.[2]

These individuals and their extraordinary feats provide unique insight into the effects of extreme duration swimming and spaceflight. Extended loss of gravitational hydrostatic gradients through weightlessness or prone positioning in water immersion without proper countermeasures resulted in loss of cardiac mass. Consistent, low-intensity exercise may be insufficient to prevent cardiac atrophy during extreme-duration swimming, though further study is needed.

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