Mechanism of spaceflight-induced changes in left ventricular mass.

Decrements in left ventricular (LV) mass observed after microgravity exposure have been previously postulated to be a central component of spaceflight-induced cardiovascular deconditioning. In this study, echocardiographic measurements of LV mass in astronauts demonstrated a comparative 9.1% reduction in postflight LV mass that returned to preflight values by the third day of recovery. A ground-based study in normal subjects determined that these pre- to postflight LV mass changes could be reproduced by simple dehydration. Reductions in LV mass observed immediately after spaceflight may be secondary to simple physiologic fluid exchanges.