Myocardial Oxygen Demand

The heart extracts more oxygen than other tissues while at rest (50-70%).  The heart’s normal compensation for increased oxygen demand is to increase coronary blood flow (400-600%).  Because cardiac tissue extracts at twice the rate of other tissues and since normal compensation for anemic states involves increased cardiac output, the heart is considered the organ which will determine the limit at which anemia is tolerated.

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The combination of blood pressure and heart rate provides a basic guideline for myocardial oxygen consumption the primary determinants of which are myocardial wall tension and contractility.  The goal for cardiac anesthesia is to render an appropriate anesthetic while avoiding stimulation of the stress response and subsequent myocardial oxygen uptake.  This proposition becomes a delicate balance of oxygen supply and demand in which the intent for the practitioner should be to increase the former and reduce the latter.

Myocardium (Supply & Demand During X-Clamping)

Supply
Non-Coronary Collaterals
Intrinsic Substance Stores
Glycogen

Demand
Electromechanical Activity
Wall Tension
Temperature

Mohrman DE, Heller LJ. Cardiovascular Physiology. 3rd ed. New York, NY.     McGraw Hill; 1991:175-188.