Importance of heart rate in determining the effects of sympathomimetic amines on regional myocardial function and blood flow in conscious dogs with acute myocardial ischemia.

We examined the effects of dopamine (DP), dobutamine (DB), and isoproterenol (ISO) in 38 conscious dogs after coronary artery occlusion on measurements of overall left ventricular (LV) function (i.e., LV pressure, dP/dt, mean arterial pressure, and heart rate), while regional myocardial function was assessed, using an ultrasonic gauge to measure segment length (SL) in normal and ischemic areas of myocardium. We measured regional myocardial blood flow, using the radioactive microsphere technique. Coronary occlusion resulted in graded reductions of blood flow and function from the normal to severely ischemic zones. DP and DB, 4.0 pg/kg per min, and ISO, 0.03 fig/kg per rain, iv, increased LV dP/dt similarly; i.e., by 20%, and increased blood flow and systolic SL shortening in the normal zone. ISO increased heart rate (29.9 ± 4.6%) and decreased SL shortening and blood flow (32.9 ± 6.1%) in the severely ischemic zone. In contrast, we observed significantly different (P < 0.01) effects with these doses of DP and DB, which caused no increases in heart rate or decreases in blood flow or contractile function in the ischemic zone. DP and DB, 10 pg/kg per min, iv, increased LV dP/dt 3to 4-fold more than with the lower dose, but still failed to elicit reductions in contractile function or blood flow in the severely ischemic zone when heart rate did not rise. In contrast, when the same doses of DP and DB increased heart rate in another group of dogs, blood flow and contractile function fell in the ischemic zone. Thus, /J-adrenergic agents do not always elicit a "coronary steal" or deleterious effects on ischemic myocardial function, despite coronary dilation in the normal zone. However, when inotropic stimulation is coupled with tachycardia, blood flow falls and contractile function deteriorates in the ischemic myocardium. Circ Res 45: 793-803, 1979