About Myocardial Oxygen Consumption true is:
D i.e. Directly propoional to mean aerial pressure- Myocardial O2 demand/ consumption is primarly determined by hea rate, contractility of myocardium (or ventricles) (i.e. inotropy), intramyocardial wall tension (which is directly propoional to after load or intra cavitatory pressure and preload or ventricular radius and inversely propoional to myocardial muscle mass or wall thickness) and ventricular work per beat (i.e. stroke volume X mean aerial pressure). For left ventricle mean aerial pressure of aoa and for right ventricle MAP in pulmonary aery is used.Myocardial O2 demand is not negligible at restQ. It has a constant relation (not directly propoional) to external cardiac work (during ejection phase of cardiac cycle). Myocardial O2 demand is directly propoional to hea rate but has no relation to duration of systoleQ.Myocardial oxygen consumptionBasal O2 consumption (determined by stopping the hea and aificially maintaining coronary circulation) is 2 mL/100 gm /min, which is considerably higher than that of skeletal muscleQ. O2 consumption by beating hea is 9 mL/100 gm /min at rest, which is the highest per unit mass consumption of O2Q.Due to very high O2 extraction, the aerio - venous O2 difference (ML/L) is also maximum in hea. So increase in O2 demand requires increase in coronary blood flow otherwise ischemia develops.Factors that enhance magnitude & /or rate of tension development will increase myocardial O2 demand. So increasing hea rate increases O2 demand by increasing rate (number of tension cycles developed/minute); whereas increasing inotropy (contractility) by increasing both magnitude & rate of tension developedQ.So myocardial O2 consumption is primarily determined by intramyocardialtension, contractile state & hea rateQ.Left ventricular stroke work is 7 times greater than that of right ventricle since the aoic pressure is 7 times greater than the pulmonary aery pressure. Theoratically, a 20% increase in stroke volume without a change in aerial pressure and a 20% increase in aerial pressure without a change in stroke volume should produce the same increase in 02 consumption But because of La plat relationship, pressure work (i.e, increase in after load or mean aerial pressure) produces a greater increase in 02 consumption than volume work (i.e., increase in preloadQ or stroke volume). This is why angina is more common in AS than AR (aoic regurgitation).
Inverse relation with hea rate
Inverse relation to mean systolic aerial pressure
Constant relation to external work done
Directly propoional to mean aerial pressure