Coronary Oxygen Delivery to the Myocardium
Oxygen is supplied to the heart muscle (myocardium) by the coronary circulation. Coronary blood flow is determined by hemodynamic factors, such as perfusion pressure and vascular resistance. The latter is determined by vascular anatomy and structure, as well as by changes in the diameter of the vascular lumen resulting from contraction and relaxation of vascular smooth muscle (local regulation of blood flow).
The delivery of oxygen (DO2) to the myocardium (oxygen supply) is determined by two factors: coronary blood flow (CBF) and the oxygen content (concentration) of the arterial blood (CaO2).
O2 Delivery = CBF × CaO2
where CBF = mL/min and CaO2 = mL O2/mL blood
Therefore, the units for O2 delivery are mL O2/min. The normal oxygen concentration in arterial blood is about 20 mL O2/100 mL blood (0.2 mL O2/ml blood), or 20 vol%. CBF, expressed per 100g of tissue weight, is about 80 mL/min per 100g at resting heart rates. Therefore, the oxygen delivery to the heart under resting conditions is about 16 mL O2/min per 100g.
The maximal concentration of oxygen in the arterial blood is primarily determined by the oxygen that is bound to hemoglobin in the red blood cells (erythrocytes). Only a small amount of oxygen is dissolved in the plasma (~0.3% of total oxygen). Therefore, conditions that alter the concentration of red cells (i.e., the hematocrit), which is normally about 40% of the total blood volume, will alter the oxygen concentration and therefore the oxygen supply to tissues. The amount of hemoglobin within the red cells also determines the amount of oxygen carried by the blood. Anemia, for example, which can result from either a reduced hematocrit or reduced hemoglobin concentration within red cells, decreases oxygen delivery to tissues..
Ordinarily, the oxygen content of arterial blood changes relatively little. Therefore, the primary determinant of oxygen delivery in the absence of hypoxemia is coronary blood flow.
In coronary artery disease, several factors can reduce coronary blood flow. Stenotic lesions cause a narrowing of vessel, particularly the large epicardial coronaries (e.g., left anterior descending or circumflex arteries). The stenosis may be at a specific site, or it may be more uniform along the length of the vessel. In either case, the stenosis can limit maximal coronary flow (decreased coronary flow reserve.) Maximal flow is reduced because the fixed stenosis is in-series with the distal microcirculation. Diseased coronary vessels are more susceptible to vasospasm, which can lead to a temporary restriction of coronary flow at rest. This can occur during stressful conditions or during exercise in susceptible individuals. Finally, thrombus formation, particularly at the site of a ruptured atherosclerotic plaque, can partially or completely occlude a coronary vessel causing unstable angina or myocardial infarction.