1: Coronary circulation

Topic 1 Coronary circulation

Coronary blood flow

Oxygen consumption of myocardium at rest = 8–10 ml/min/100 g.

The heart receives ~5% of total cardiac output.

Myocardial oxygen extraction is high (~75%), and therefore there is little extraction reserve. Furthermore, the myocardium does not have great capacity for anaerobic glycolysis.

During exercise myocardial oxygen consumption can increase to > 40 ml/min/100 g.

Increased demand is met by increasing coronary blood flow.

Coronary blood flow is subject to autoregulation which is closely coupled to and driven by myocardial oxygen consumption (mVO₂). Autoregulation is lost when coronary perfusion pressure drops below 60 mmHg.

There is local metabolic control of coronary blood flow, thought to be largely mediated by adenosine and nitric oxide. There is additionally sympathetic innervation of alpha and beta adrenoceptors.

Coronary blood flow varies within the cardiac cycle and most flow occurs during diastole. During systole, extravascular compression reduces intramyocardial flow. The greatest resistance to perfusion is the subendocardial layer where the extravascular compressive forces are greatest and vascular pressure is reduced. Compressive forces are lower in the right ventricle and the drop in flow during systole is less pronounced.

Wave intensity analysis, using pressure- and flow-sensitive wires in human coronary arteries, has identified six predominant waves influencing phasic flow. It is postulated that a dominant backward-propagating ‘suction’ wave, generated by a fall in resistance of the coronary microvasculature with myocardial relaxation, is largely responsible for diastolic flow.

Tags: Cardiology Churchills Ready Reference

Jun 5, 2016 | Posted by admin in CARDIOLOGY | Comments Off

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