Minimal impairment of myocardial blood flow responses to exercise in the remodeled left ventricle early after myocardial infarction, despite significant hemodynamic and neurohumoral alterations.

OBJECTIVES Previous studies have demonstrated a decreased flow reserve in the surviving hypertrophied left ventricle (LV) early after myocardial infarction. We hypothesized that exacerbation of hemodynamic abnormalities and neurohumoral activation during exercise could exhaust coronary flow reserve and thereby impair myocardial O(2) supply. Consequently, we studied hemodynamic, neurohumoral and regional myocardial perfusion and metabolic responses to exercise in pigs with LV hypertrophic remodeling 3 weeks after a myocardial infarction produced by permanent left circumflex coronary artery ligation. METHODS Chronically instrumented pigs were exercised on a treadmill up to 85% of maximum heart rate. Pigs with a myocardial infarction (MI) had a lower cardiac output (21%), stroke volume (28%), LVdP/dt(max) (18%), systemic (22%) and pulmonary (20%) vascular conductance, and increased left atrial (225%) and pulmonary artery (75%) pressures, compared to normal pigs. In MI, the exercise-induced increases in cardiac pump function, and systemic and pulmonary vasodilation were blunted compared to normals. Consequently, perfusion of visceral organs became impaired during strenuous exercise, but cerebral and skeletal muscle blood flows were maintained. Exercise-induced increases in norepinephrine and endothelin levels were exacerbated and, while relative sympathetic drive was maintained, cardiac responsiveness to norepinephrine was blunted. Despite lower capillary densities in the hypertrophied non-infarcted LV and relative subendocardial hypoperfusion during strenuous exercise, which necessitated a slight increase in O(2) extraction, there was no metabolic evidence of overt myocardial ischemia during strenuous exercise as indicated by the arterio-coronary venous pH difference. CONCLUSIONS LV dysfunction and neurohumoral activation were present in pigs with a 3-week-old infarction, particularly during exercise. However, although myocardial perfusion and O(2) supply were slightly impaired, myocardial ischemia did not occur even during exercise up to 85% of maximum heart rate, suggesting that perfusion abnormalities do not contribute to LV dysfunction early after infarction.