Transmural coronary vasodilator reserve and flow distribution in unanesthetized calves sojourning at 3500 m

Regional myocardial blood flow (MBF; 15-micron-diam radionuclide-labeled microspheres) was studied in six unanesthetized calves sojourning at 3500 m (PB = 500 mm Hg) for 53 +/- 2 days. These high-altitude (HA)-exposed calves were studied during chronic hypoxemia (PaO2 = 48 +/- 1 mm Hg), maximal coronary vasodilation, and during acute normoxemia (PaO2 = 91 +/- 1 mm Hg). Nine calves born and raised at sea level (SL) were also studied at matched PaO2 during chronic normoxemia, maximal coronary vasodilation, and acute hypoxemia to serve as control. Marked pulmonary hypertension and right ventricular (RV) hypertrophy were present in HA calves. Left ventricular (LV) MBF of HA calves during chronic hypoxemia (1.05 +/- 0.11 ml X min-1 X g-1) was similar to that of normoxemic SL calves (1.11 +/- 0.06 ml X min-1 X g-1) but MBF in their hypertrophied RV (1.65 +/- 0.21 ml X min-1 X g-1) exceeded that in normoxemic SL calves (0.47 +/- 0.06 ml X min-1 X g-1). More interesting was the finding that RV and LV MBF of HA calves did not change between chronic hypoxemia and acute normoxemia. By contrast, acute hypoxemia of a similar degree caused a dramatic increase in RV as well as LV MBF of SL calves. Minimal LV coronary vascular resistance was similar in the two groups of calves. This meant that functional cross-sectional area of LV coronary vascular bed was not altered in response to sojourn at HA. Minimal RV coronary vascular resistance of HA calves was also not different from that of SL calves. This means that functional cross-sectional area of the RV coronary vascular bed in HA calves increased proportionately with the increase in their RV mass.