Regional myocardial blood flow in lambs with concentric right ventricular hypertrophy.

Chronically hypertrophied right ventricles function normally under high systolic pressures, whereas normal right ventricles fail when they are acutely subjected to similar pressures. This phenomenon may be partly due to adaptation of the coronary circulation as well as to hypertrophy. Knowledge of the magnitude and distribution of coronary blood flow and the degree of coronary vascular reserve are important in understanding the function of hypertrophied myocardium. We studied these variables in 16 awake, 5-12-week-old, tranquilized lambs; 9 of the lambs had had their main pulmonary artery banded at 2 days of age. Pressures, cardiac output, and coronary blood flow (radioactive microsphere method) were measured at rest and during two stress states—isoproterenol and dextran infusion. Ventricular function was similar in control and banded lambs. We found significant right ventricular hypertension and both right ventricular and septal hypertrophy in the banded lambs. Total coronary blood flow per gram was slightly higher in banded lambs in all states, and right ventricular flow per gram was significantly elevated at rest and during isoproterenol infusion. Right ventricular coronary resistance per gram was significantly lower in banded lambs in all states. These changes demonstrate that hypertrophied right ventricular tissue is not ischemic at rest, since there is vascular reserve. The changes also raise the question of increased vascularity in hypertrophied myocardium. Right ventricular oxygen supply per unit oxygen demand was increased in banded lambs at rest and during isoproterenol infusion. This finding suggests that hypertrophied myocardium has inefficient oxygen utilization, low oxygen extraction, or both.