Effect of local ischaemia on the myocardial oxygen balance and its response to heart rate elevation.

The capacity of ischaemic myocardium to respond to the inotropic stimulus of tachycardia was investigated in open-chest anaesthetized dogs following ligation of a branch of the anterior descending coronary artery. In two areas of the left ventricular surface (ischaemic and non-ischaemic), local coronary blood flow was measured by thermistors and isometric contractile force was recorded with strain gauge arches. NADH redox state was measured simultaneously in both regions using a two-channel surface fluorometer. It was found that ligation was followed by an immediate fall in local coronary blood flow to the ischaemic region, accompanied by a sharp elevation in NADH redox level. Local contractile force in the ischaemic region was also reduced. The non-ischaemic region showed little or no change following occlusion. Response to heart rate elevation before ligation was increased work, elevation of NADH redox levels, and increased coronary flow. Following ligation, this response was attenuated in the ischaemic region, but not abolished. It is concluded that ischaemic myocardium retains the capacity for inotropic response even when intracellular O2 levels are low.