Influence of norepinephrine on myocardial oxygen consumption under controlled hemodynamic conditions

Studies were performed to ascertain the effect of norepinephrine on the contractility and myocardial oxygen consumption of the isolated, supported heart preparation with aortic pressure, heart rate and stroke volume held constant. Graded infusions of norepinephrine were given into the left main coronary artery and the fall of left ventricular end-diastolic pressure used as a measure of the increase in contractility obtained. Low doses of norepinephrine caused pronounced falls of left ventricular end-diastolic pressure with no change, a slight rise or a slight fall in myocardial oxygen consumption. Thereafter, increases in the infusion rate of norepinephrine had relatively little influence on left ventricular end-diastolic pressure but produced a marked increase in myocardial oxygen consumption. These data suggest that the concept of the so-called oxygen-wasting effect of norepinephrine may be attributable to either (1) the use of a dose which exceeds that required for a near maximal inotropic effect or (2) at low doses, an inadequate consideration of the influence of induced hemodynamic alterations (rise of heart rate and aortic pressure) on myocardial oxygen consumption.