[Rate-pressure product and myocardial oxygen requirements]

The relationship between the rate-pressure product, myocardial oxygen consumption and the production or uptake of lactate by the myocardium was studied in 45 patients under basal conditions and rapid atrial pacing (except in 3 cases with atrial fibrillation). The underlying pathology was ischemic heart disease in 25 cases, non obstructive cardiomyopathy in 9 cases, and obstructive cardiomyopathy in 11 cases. Contrary to what has been reported in normal subjects, a significant linear relationship between the rate-pressure product or indexed rate-pressure (multiplied by body surface area) and myocardial oxygen uptake. The divergence of these results may be due to the method of measuring coronary sinus blood flow, these authors having used the N 20 whereas in this study the flow was measured by thermodilution. In particular, it is possible that this divergence results from abnormalities of ventricular volume, compliance and contractility in the patients under study: these abnormalities may disturb the relationship between the rate-pressure product and myocardial oxygen consumption in normal subjects. A double linear relationship, different in each of these groups, was observed at rest between the indexed rate-pressure product and oxygen uptake, and the uptake or production of lactate: in myocardial ischemia without ischemia at rest, in non obstructive cardiomyopathy in sinus rhythm. The significance of these relations differed in each of these groups. Under atrial pacing, the increase in indexed rate-pressure product compared to basal conditions was proportional to the presence of absence of ischemia. Variations of the indexed rate-pressure product seem to be related to myocardial oxygen consumption and not to myocardial oxygen requirements. The variations of the rate-pressure product, during the administration of drugs, for example, do not reflect myocardial oxygen needs. During exercise ECG, the maximal increase in the indexed rate-pressure product reflect the functional reserve of the myocardium.