Coronary blood flow reserve during +Gz stress and treadmill exercise in miniature swine

The purpose of this study was to compare the coronary blood flow reserve (CBFR) that exists during maximal +Gz stress to the CBFR during maximal exercise stress. Maximal exercise stress was defined as an exercise intensity greater than or equal to that necessary to produce maximal levels of O2 consumption (VO2max). Coronary blood flows (CBF) were determined with the use of the microsphere technique in chronically instrumented conscious miniature swine during +Gz stress and exercise stress at 70 and 100% of maximal tolerance (for each stress) before and after maximal coronary vasodilation with 1-2 mg/kg dipyridamole. CBFR was measured as the amount of blood flow increase produced by maximal coronary vasodilation. During exercise at VO2max, dipyridamole produced 20-30% increases in CBF, whereas it induced no coronary vasodilation or changes in CBF during +Gz stress. Dipyridamole also produced decreases in the animals' tolerance to +Gz in that all five animals could maintain a steady state for 60 s at 7 +Gz before dipyridamole, whereas only two of these animals could maintain a steady state for 60 s at 7 +Gz after dipyridamole. These results confirm that CBFR exists during maximal exercise in normal mammals. However, this dose of dipyridamole produced no coronary vasodilation during either level of +Gz stress.