Compensatory regional contraction following coronary artery occlusion depends on subendocardial hyperaemia and hyperkinesis

The effects of circumflex coronary artery occlusion on regional myocardial performance and blood flow in the left ventricular anterior wall was studied, using 16 thoracotomized pentobarbital-anaesthetized cats. Two pairs of ultrasonic crystals were placed in the midwall; one segment (longitudinal) parallel to the subendocardial fibres, the other (circumferential) being aligned to the fibres of the outer half-layer. A shunt line from the right subclavian artery to the main left coronary artery was unrestricted in eight cats (Group A) and clamped in another eight cats (Group B) until coronary perfusion pressure was clearly reduced with only a minor reduction in shunt flow, but without changes in global cardiac function. After circumflex coronary occlusion hyperkinesis was most pronounced in segments parallel to subendocardial fibres (longitudinal), also validated as a marked leftward shift in the end-systolic pressure-length relation of these segments. Such a shift may indicate decreased regional afterloading along the cardiac major axis. Hyperkinesis of longitudinal segments was attenuated in Group B where a fixed shunt stenosis hampered subendocardial and mid-myocardial hyperaemia. The lesser hyperkinesis of Group B was associated with decreased left ventricular systolic pressure and cardiac output. Thus, impaired compensatory contraction outside an acute ischaemic region was produced by a significant coronary stenosis which precluded the subendocardial hyperaemia and hyperkinesis of that region.