Myocardial ischaemia. Dilemma between information available and information demand.

The information available on the factors responsible for peroperative myocardial ischaemia justifies certain guidelines to the anaesthetist for preventive therapy. Contrary to present practice, the effect of different perturbations on myocardial oxygen consumption (MVO2) should be considered per beat rather than per unit time, because recovery for the energy expended during systole must occur during each diastole. During pacing-induced tachycardia MVO2/beat decreases as heart rate increases, suggesting that faster heart rates produce angina pectoris by a reduction in the duration of recovery, not an increase in oxygen demand. The effects of a change in heart rate induced by exercise are less obvious in this respect, but analysis shows that the MVO2/beat may increase because of the raised aortic systolic pressure, not heart rate.