MICROGRAVITY DECREASES HEART-RATE AND ARTERIAL-PRESSURE IN HUMANS

Spaceflight causes adaptive changes in cardiovascular physiology, such as postflight orthostatic intolerance, that can have deleterious effe cts on astronauts. in-flight cardiovascular data are difficult to obta in, and results have been inconsistent. To determine normative in-flig ht changes in Shuttle astronauts, we measured heart rate, arterial pre ssure, and cardiac rhythm disturbances for 24-h periods before, during , and after spaceflight on Shuttle astronauts performing their normal routines. We found that heart rate, diastolic pressure, variability of heart rate and diastolic pressure, and premature ventricular contract ions all were significantly reduced in flight. Systolic pressure and p remature atrial contractions also tended to be reduced in flight. Thes e data constitute the first systematic evaluation of in-flight changes in basic cardiovascular variables in Shuttle astronauts and suggest t hat a microgravity environment itself does not present a chronic stres s to the cardiovascular system.