True about cardiac O2 demand is
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 proportional
o   After load- Directly proportional
o   Preload- Directly proportional
o   Wall thickness- Inversely proportional
-        The oxygen demand of myocardium is not having the same change with varying external works.
Directly proportional to mean arterial pressure
Inversely proportional to heart rate
Inversely proportional to cardiac work
Has a constant relation to the external work done by the heart