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What are the major determinants of myocardial oxygen supply and demand?

The myocardium normally extracts 65% of the oxygen in arterial blood compared with extraction of 25% by other tissues. An increase in myocardial demand for oxygen can be met only by an increase in coronary blood flow, not by increased oxygen extraction.

The major determinants of myocardial oxygen supply include the following:

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  Coronary anatomy . Myocardial blood supply derives from the left and right coronary arteries. The right coronary artery normally supplies the right atrium and most of the right ventricle and in right dominant circulation (85% of people) gives rise to the posterior descending artery, which supplies part of the interventricular septum and the inferior wall of the left ventricle. It also usually supplies the sinoatrial node (60% of patients) and the atrioventricular node (85% of patients). The left coronary artery bifurcates into the left anterior descending artery and the circumflex arteries. The left anterior descending artery supplies the septum of the left ventricle and the anterior wall, whereas the circumflex artery supplies the lateral wall and in left dominant circulation (15% of people) gives rise to the posterior descending artery.

  
  
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  Coronary perfusion pressure (CPP). CPP is determined by the difference between aortic pressure and ventricular pressure. During systolic contraction, the pressure in the left ventricle approaches aortic pressure, so no coronary perfusion occurs during this time. The left ventricle is almost entirely perfused during diastole, whereas the right ventricle is perfused during both systole and diastole. CPP for the left ventricle is the difference between the aortic diastolic pressure (ADP) and the left ventricular end-diastolic pressure (LVEDP):

  
  
  <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='CPP = ADP − LVEDP’>CPP = ADP − LVEDPCPP = ADP − LVEDP
  CPP = ADP − LVEDP
  
  
  
  
  
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  Heart rate. Because nearly all perfusion of the left ventricle occurs during diastole, the duration of diastole is also a significant determinant of coronary perfusion. Increases in heart rate result in a disproportionately greater decrease in diastolic time resulting in decreased coronary perfusion.

  
  
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  Arterial oxygen content. Hemoglobin concentration and oxygen saturation need to be optimized to maintain adequate supply. The optimal hemoglobin concentration and transfusion trigger remain a matter of debate.

The major determinants of myocardial oxygen demand include the following:

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  Basal oxygen requirements. Basal requirements account for about 20% of the oxygen requirements.

  
  
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  Left ventricular wall tension (T). T is directly proportional to the intraventricular pressure (P) and ventricular radius (r) and inversely proportional to ventricular wall thickness (h): T = Pr/2h. Increases in either preload (increased radius) or afterload (increased pressure) increase T and oxygen demand. The increase in oxygen demand secondary to an increase in afterload is usually compensated by an increase in CPP and oxygen supply.

  
  
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  Heart rate. Increases in heart rate increase oxygen demand, while decreasing oxygen supply, increasing the potential for myocardial ischemia.

  
  
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  Contractility. Decreased myocardial contractility is associated with smaller oxygen requirements. However, if CPP is decreased, the oxygen supply is decreased.