Exercise tolerance and compensatory sympathetic tone during beta-blockade after myocardial infarction

Using chronically instrumented dogs with a healed anterior wall myocardial infarction, we have assessed the effect of acute beta-adrenergic blockade on exercise tolerance, left ventricular (LV) performance, and myocardial VO2. Mongrel dogs were instrumented with Doppler flow probes around the left circumflex coronary artery, LV pressure cells, epicardial ECG electrodes, and left atrial and coronary sinus catheters. Myocardial infarction was produced by ligation of the left anterior descending coronary artery at time of instrumentation (approximately 20% LV mass). After a 1-month recovery period, dogs were subjected to two identical submaximal graded exercise tests on a treadmill on separate days, once without (control) and once with (propranolol, 1 mg X kg-1, i.v.) acute beta-adrenergic blockade. Heart rate, LV pressure, dP/dt max, left circumflex blood flow velocity, and myocardial VO2 index were measured at each stage of the graded exercise test. All variables were significantly reduced from control during beta-block in exercise. beta-Adrenergic blockade in the presence of a 1-month-old anterior wall myocardial infarction did not compromise exercise capacity during submaximal exercise.