True regarding myocardial O2 demand?
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
After load - Directly proportional
Preload - Directly proportional
Wall thickness - Inversely proportional

The oxygen demand of myocardium is not having the same change with varying external works.
Inversely related to heart rate
Has constant relation to external cardiac work
Directly proportional to duration of systole
Is negligible at rest