PEDIATRIC CARDIOVASCULAR INTENSIVE-CARE - MYOCARDIAL PERFUSION

The heart in a person at rest has a high oxygen consumption that must be met by a high blood flow; during exercise or stress, oxygen consumption rises and flow must increase in proportion to avoid myocardial ischemia. At normal perfusing pressures, basal left ventricular flows can increase about 4-fold with maximal vasodilatation, that is, there is a large coronary flow reserve. This reserve can be reduced if basal flow is increased, for example, by tachycardia, anemia, catecholamines, hypoxemia and abnormal hemoglobins. It can also be decreased if maximal flow is reduced by disease of large or small coronary arteries, tachycardia, hypertrophy, or polycythemia. With abolition of coronary flow reserve, myocardial ischemia occurs. These changes occur predominantly in the subendocardium, and are usually patchy unless there is acute near total prolonged ischemia. Other causes of reduced subendocardial perfusion are a raised ventricular end-diastolic pressure and increased contractility. The immature heart may be at a particular disadvantage in terms of coronary reserve because of its high basal coronary flows.