Myocardial ischemia: lack of coronary blood flow, myocardial oxygen supply-demand imbalance, or what?

Abstract

This opinionated article reviews current concepts of myocardial ischemia. Specifically, the historical background is briefly presented. Then, the prevailing paradigm of myocardial oxygen-supply-demand imbalance is criticized since demand is a virtual parameter that cannot be measured and data on measurements of myocardial blood flow and contractile function rather support matching between flow and function. Finally, a concept of myocardial ischemia that focusses on the reduction of coronary blood flow to below 8-10 µl/g per beat with consequences for myocardial electrical, metabolic, contractile and morphological features is advocated.

Keywords: coronary blood flow; hibernation; myocardial infarction; myocardial ischemia; reperfusion; stunning.

Fig. 1.

Relationship of coronary blood flow to perfusion pressure. There is an autoregulatory plateau that is shifted upward with increased cardiac function and metabolism and conversely shifted downward with decreased cardiac function and metabolism. Below a perfusion pressure of about 40-50 mmHg flow is pressure-dependent modified. Modified from Mosher et al. (67) with permission.

Fig. 2:

Relationship of regional contractile function to regional myocardial blood flow. There is an almost linear relationship between function and flow such that during exercise-induced ischemia both flow and function are proportionately reduced and conversely proportionately increased by drug treatment. This flow-function relationship can persist for hours during short-term hibernation and can only be briefly shifted upwards by inotropic stimulation. With stunning (postischemic contractile dysfunction) and microembolization, contractile function is decreased in the absence of a detectable flow reduction. With chronic hibernation which results from repetitive stunning, matching of flow and function is restored. From Heusch (36) with permission.

Fig. 3:

In both, nonischemic and ischemic myocardial regions flow and function are proportionately changed, e.g., by beta blockade. In the nonischemic region, beta blockade reduces heart rate and contractile function, and flow follows function by metabolic vasomotion. The ischemic myocardium receives more blood flow through the stenosis and through collaterals at reduced heart rate. Here, function follows flow. From Heusch (39) with permission.