EFFECTS OF 12 DAYS EXPOSURE TO SIMULATED MICROGRAVITY ON CENTRAL CIRCULATORY HEMODYNAMICS IN THE RHESUS-MONKEY

Central circulatory hemodynamic responses were measured before and dur ing the initial 9 days of a 12-day 10 degrees head-down tilt (HDT) in 4 night-sized juvenile rhesus monkeys who were surgically instrumented with a variety of intrathoracic catheters and blood flow sensors to a ssess the effects of simulated microgravity on central circulatory hem odynamics. Each subject underwent measurements of aortic and left vent ricular pressures, and aortic flow before and during HDT as well as du ring a passive head-up postural test before and after HDT. Heart rate, stroke volume, cardiac output, and left ventricular end-diastolic pre ssure were measured, and dP/dt and left ventricular elastance was calc ulated from hemodynamic measurements. The postural test consisted of 5 min of supine baseline control followed by 5 minutes of 90 degrees up right tilt (HUT). Heart rate, stroke volume, cardiac output, and left ventricular end-diastolic pressure showed no consistent alterations du ring HDT, Left ventricular elastance was reduced in all animals throug hout HDT, indicating that cardiac compliance was increased. IIDT did n ot consistently alter left ventricular +dP/dt, indicating no change in cardiac contractility. Heart rate during the post-HDT HUT postural te st was elevated compared to pre-HDT while post-HDT cardiac output was decreased by 52% as a result of a 54% reduction in stroke volume throu ghout HUT. Results from this study using an instrumented rhesus monkey suggest that exposure to microgravity may increase ventricular compli ance without alterating cardiac contractility. Our project supported t he notion that an invasively-instrumented animal-model should be viabl e for use in spaceflight cardiovascular experiments to assess potentia l changes in myocardial function and cardiac compliance. (C) 1998 Else vier Science Ltd. All rights reserved.