About myocardial oxygen demand, true is

Myocardial O2 demand and consumption is directly propoional to hea rate and systemic aerial pressure. Myocardial efficiency is the least when hea is contracting against a greater afterload or aerial resistance. It is less efficient with increasing hea rate. And, it is most economical with greater stroke volume (with afterload and hea rate kept constant). Even in quiescent hea, myocardium extracts 75-80% O2 delivered to it; it is highest in the body. Myocardial O2 usage is most closely related to the total tension developed by the ventricular muscle. The term tension-time index (TTI) is used to express this parameter. TTI = mean systolic pressure x duration of systole x hea rate