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.

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Abstract
The effects of dopamine (DP), dobutamine (DB), and isoproterenol (ISO) were examined in 38 conscious dogs after coronary artery occlusion on measurements of overall left ventricular (LV) function (i.e., LV pressure, dP/dt [rate of pressure change], 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. Regional myocardial blood flow was measured 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 .mu.g/kg per min, and ISO, 0.03 .mu.g/kg per min, i.v. 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. Significantly different (P < 0.01) effects occurred 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 .mu.g/kg per min, i.v., increased LV dP/dt 3- to 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. 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. .beta.-Adrenergic agents do not always elicit a coronary steal or deleterious effects on ischemic myocardial function, despite coronary dilation in the normal zone. When inotropic stimulation is coupled with tachycardia, blood flow falls and contractile function deteriorates in the ischemic myocardium.