Effects of orthostatic and anti-orthostatic stress on patent and stenotic coronary arteries in swine.

Head-up tilt (HUT) followed by head-down tilt (HDT) has been used to simulate the acute phase of adaptation to microgravity. This study evaluates the effects of HUT and HDT on the coronary circulation before and during coronary stenosis. Seven pigs were placed in the prone position and exposed to the following orientations for 20 min each: 1) 0 degrees horizontal (HZ); 2) +70 degrees HUT; and 3) -15 degrees HDT. The swine were then placed in the HZ position for 20 min while hemodynamics returned to baseline. The diameter of the left anterior descending (LAD) coronary artery was reduced to a point slightly less than that which produced a decrease in blood flow and the protocol was repeated. Heart rate (HR), contractility (dP/dt), mean arterial blood pressure (MABP), left ventricular pressure (LVP), coronary sinus pressure (CSP), left-ventricular end-diastolic pressure (LVEDP), coronary blood flow (CBF), coronary vascular resistance (CVR), myocardial oxygen consumption (MVO2) and coronary sinus lactate were determined after 20 min in each position. The transition from HUT to HDT elicited a significant (p < 0.05) increase in MABP, LVP, CSP, LVEDP, MVO2, and CBF and a significant decrease in CVR. During HDT, MVO2 increased 20 +/- 10% from Hz and 68 +/- 15% from HUT. There was a comparable increase in coronary artery flow. The hemodynamic responses were not significantly altered during pneumatic coronary occlusion. However, coronary sinus lactate was significantly elevated in the HDT positions. This study demonstrates a substantial increase in CBF during HUT to HDT with and without coronary stenosis.(ABSTRACT TRUNCATED AT 250 WORDS)