Effect of Gravitational Gradients on Cardiac Filling and Performance

Background: Gravity affects every aspect of cardiac performance. When gravitational gradients are at their greatest on Earth (i.e., during upright posture), orthostatic intolerance may ensue and is a common clinical problem that appears to be exacerbated by the adaptation to spaceflight. We sought to elucidate the alterations in cardiac performance during preload reduction with progressive upright tilt that are relevant both for space exploration and the upright posture, particularly the preload dependence of various parameters of cardiovascular performance. Methods: This was a prospective observational study with tilt-induced hydrostatic stress. Echocardiographic images were recorded at four different tilt angles in 13 astronauts, to mimic varying degrees of gravitational stress: 0 degrees (supine, simulating microgravity of space), 22 degrees head-up tilt (0.38 G, simulating Martian gravity), 41 degrees (0.66 G, simulating approximate G load of a planetary lander), and 80 degrees (1 G, effectively full Earth gravity). These images were then analyzed offline to assess the effects of preload reduction on anatomical and functional parameters. Results: Although three-dimensional end-diastolic, end-systolic, and stroke volumes were significantly reduced during tilting, ejection fractions showed no significant change. Mitral annular e' and a' velocities were reduced with increasing gravitational load (P < .001 and P = .001), although s' was not altered. Global longitudinal strain (GLS; from -19.8% +/- 2.2% to -14.7% +/- 1.5%) and global circumferential strain (GCS; from -29.2% +/- 2.5% to -26.0% +/- 1.8%) were reduced significantly with increasing gravitational stress (both P < .001), while the change in strain rates were less certain: GLSR (P = .049); GCSR (P = .55). Endsystolic elastance was not consistently changed (P = .53), while markers of cardiac afterload rose significantly (effective arterial elastance, P < .001; systemic vascular resistance, P < .001). Conclusions: Preload modification with gravitational loading alters most hemodynamic and echocardiographic parameters including e' velocity, GLS, and GCS. However, end-systolic elastance and strain rate appear to be more load-independent measures to examine alterations in the cardiovascular function during postural and preload changes, including microgravity.