Functional Cardiac Adaptation to 60 Days Strict Head-Down-Tilt Bedrest

Abstract

Background: Reduced physical activity increases the risk for heart failure. Myocardial strain measurements capture subtle abnormalities in myocardial function. We tested the hypothesis that bedrest deconditioning produces cardiac dysfunction in healthy persons. Methods: In the AGBRESA study, which assessed artificial gravity through centrifugation as potential countermeasure for space travel, 24 healthy persons (8 women) were submitted to 60 days strict -6°-head-down-tilt bedrest. 8 subjects each were included in a control group or groups subjected to continuous artificial gravity training on a short-arm centrifuge [30 minutes/day] or intermittent centrifugation [6x5 min/day]. We assessed cardiac morphology, function, strain and hemodynamics by cardiac-MRI (baseline, end of bedrest, recovery) and echocardiography (baseline, end of bedrest). Before and after bedrest, we assessed orthostatic heart rate responses as measure of cardiovascular deconditioning. Results: We conducted a pooled analysis because cardiovascular responses to bedrest did not differ between groups. Supine heart rate (baseline: 64±9.6bpm; bedrest: 72.3±10.5bpm; recovery: 69.6±10.5bpm, p<0.0001) and diastolic blood-pressure (69.6±7.3mmHg; bedrest: 78.5±6.9mmHg; recovery: 70.3±6.3mmHg, p<0.0001) increased with bedrest. With head-up tilt, heart rate increased 22.8±10.5bpm before and 45.9±213bpm at the end of bedrest (p<0.0001) consistent with cardiovascular deconditioning. Cardiac-output decreased from 6.6±0.9l/min to 6±1l/min at end of bedrest (recovery: 6.8±1.2l/min, p=0.0096). Left ventricular ejection fraction and mass-index did not change. Echocardiographic global longitudinal strain (baseline: -19.90±2.1%; bedrest: -18.1±2.1%, p<0.0001) decreased, whereas global circumferential strain in MRI tended to increase (baseline: -18.6±1.7%; bedrest: -19.1±1.6%, p=0.0843). After four days of recovery MRI measurements had returned to baseline. Conclusion: In healthy persons, cardiovascular deconditioning through 60 days head-down-tilt bedrest does not lead to cardiac atrophy or sustained reductions in cardiac performance. The transient nature of cardiac strain changes suggests functional rather than structural changes in the heart.