Myocardial Oxygen Extraction
The amount of oxygen delivered to the myocardium is greater than the amount that is actually taken up (oxygen consumed) by the myocardium to support oxidative metabolism. Typically, the myocardium extracts approximately 50% of the oxygen supplied by the arterial blood. This oxygen extraction is determined by the ratio of oxygen consumption to coronary blood flow as described by the Fick Principle. Oxygen extraction is, by definition, the difference between the arterial and venous concentrations of oxygen (CaO2 – CvO2) and therefore has the units of mL O2/100 ml blood.
Compared to most organs of the body (see table below), the oxygen extraction of the heart is relatively high. The oxygen extraction of the heart is typically 10-12 vol% (mL O2/100 ml blood).
|skeletal muscle (resting)||2-5|
Theoretically, the maximal amount of oxygen that can be extracted is 20 vol %. In reality, however, the maximal oxygen extraction is around 15-16 vol % because of the kinetics of oxygen dissociation from hemoglobin. Therefore, the heart is extracting one-half to two-thirds of the physiologically available oxygen under normal operating conditions. Because of this, the heart must tightly couple oxygen supply and demand in order to ensure adequate tissue oxygenation. In the absence of coronary artery disease (CAD), coronary blood flow increases almost proportionately to increases in myocardial oxygen consumption (MVO2) thereby preventing tissue hypoxia and functional impairment. Local regulation of blood flow adjusts coronary blood flow to the metabolic demands of the contracting myocardium.
In the presence of CAD, coronary blood flow may not supply adequate oxygen to meet the metabolic demands of the contracting heart. This will increase the oxygen extraction and decrease the venous oxygen content. This leads to tissue hypoxia and angina. If the lack of blood flow is because of a fixed stenotic lesion in the coronary artery (because of atherosclerosis), blood flow can be improved within that vessel by 1) placing a stent within the vessel to expand the lumen, 2) using an intracoronary angioplasty balloon to stretch the vessel open, or 3) bypassing the diseased vessel with a vascular graft. If the insufficient blood flow is caused by a blood clot (thrombosis), a thrombolytic drug that dissolves clots may be administered. Anti-platelet drugs and aspirin are commonly used to prevent the reoccurrence of clots. If the reduced flow is because of coronary vasospasm, then coronary vasodilators can be given (e.g., nitrodilators, calcium-channel blockers) to reverse and prevent vasospasm.