Describe the effects of a tachycardia on myocardial oxygen supply and demand in a normal heart.

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College Answer

The main points expected were the determinants of myocardial oxygen supply. These include
arterial oxygen content and coronary blood flow. Coronary blood flow depends on coronary
perfusion pressure and coronary vascular resistance and that most left coronary blood flow
occurs in diastole. Tachycardia reduces diastolic time and hence left coronary blood flow. In
comparison blood flow in the right coronary artery is continuous both in systole and diastole
and is little affected by heart rate. A correctly labelled diagram of left and right coronary
blood flow attracted extra marks. Unfortunately most diagrams were inaccurate, not labelled
and had no units on the axes. Systolic compression particularly reduces blood supply to the
left ventricular subendocardium which is most susceptible to ischaemia. Extra marks were
given for describing metabolic autoregulation, the high oxygen extraction, explaining that
oxygen supply cannot be increased by increasing oxygen extraction in the coronary
circulation and describing the driving pressure differences in both coronary arteries in
systole and diastole.

A description of the determinants of myocardial oxygen demand was also required (e.g. left
ventricular, preload, contractility, afterload and tachycardia This part of the question was
particularly poorly answered.