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The effects of alterations of arterial pressure by nitroprusside and methoxamine on regional myocardial function were studied during total coronary occlusion and during partial coronary constriction in open chest dogs. Animals were instrumented with a left ventricular micromanometer, an electromagnetic flow probe around the left circumflex coronary artery, and three pairs of ultrasonic crystals in a control, marginally ischaemic, and ischaemic segments. In nine dogs, when the coronary artery was totally occluded by a hydraulic cuff placed distal to the flow probe, the end-diastolic length of all three segments was increased, and shortening was rapidly replaced by systolic expansion in the ischaemic segment. Active shortening decreased by 68% in the marginal segment, while it increased by 21% in the control segment. With nitroprusside infusion, peak systolic pressure was reduced from 15.2 to 11.3 kPa and hypokinesis of the marginal segment was improved to 45% of control value with a concomitant decrease in end-diastolic length by 5%. With methoxamine, left ventricular systolic pressure was elevated to 18.5 kPa, and marginal segment shortening further deteriorated to 19% of control with chamber enlargement. In 6 dogs, coronary flow was limited by a screw-driven metal clamp. When mean coronary flow was reduced by 50% of control value, stable hypokinesis was produced in the ischaemic segment. Nitroprusside infusion induced the same haemodynamic changes, however, shortening of the ischaemic segment deteriorated further and a passive bulge appeared. Methoxamine improved the percentage shortening from 1.2 to 6.4%. Thus, interventions with an agent which changes afterload in left ventricular ejection have multiple and complex effects on the contractile function of the ischaemic myocardium and, depending on the magnitude of coronary blood flow reduction, a potentially beneficial drug can be determined to an ischaemic myocardium.
Sasayama et al. (1980) studied this question.