<prompt> True about cardiac O2 demand isDirectly proportional to mean arterial pressureInversely proportional to heart rateInversely proportional to cardiac workHas a constant relation to the external work done by the heart  <response>: Oxygen demand depends on the work being done. The work done is to pump the blood which is in the ventricles preload against the resistance in the aorta afterload. So, myocardial oxygen demand depends on- Rate- Directly proportional- Contractility- Directly proportional- Intramyocardial tension- Directly proportionalo After load- Directly proportionalo Preload- Directly proportionalo Wall thickness- Inversely proportional- The oxygen demand of myocardium is not having the same change with varying external works. 