<prompt> True regarding myocardial O2 demand?Inversely related to heart rateHas constant relation to external cardiac workDirectly proportional to duration of systoleIs negligible at rest  <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 onRate - Directly proportionalContractility - Directly proportionalIntramyocardial tension - Directly proportionalAfter load - Directly proportionalPreload - Directly proportionalWall thickness - Inversely proportionalThe oxygen demand of myocardium is not having the same change with varying external works. 