Coronary flow and flow reserve in children.

Aortic blood pressure affects coronary blood flow, but within the normal physiological blood pressure range coronary blood flow is constant. The coronary flow is pulsatile, being maximal in the early diastole. There is a smaller systolic flow component. The low systolic pressure in the right ventricle favours systolic flow. The proportion of systolic flow is greater in the right than in the left coronary artery. Heart diseases in children cause several haemodynamic and functional changes that are likely to affect myocardial perfusion. Newborns with severe valvular aortic stenosis may have a retrograde systolic flow in the left coronary artery. Children with dilated cardiomyopathy have a reduced coronary flow related to myocardial mass. Coronary flow reserve (CFR) is defined as the ratio of maximal coronary blood flow, as induced by reactive hyperaemia or administration of vasodilators, divided by resting flow. Coronary flow can normally increase 2.5-4-fold. CFR is reduced if basal flow is increased due to myocardial hypertrophy, strain or hypoxaemia. Very low CFR values measured with positron emission tomography are reported in neonates with surgically treated congenital heart disease. Measurement of coronary flow velocity with the intracoronary Doppler guide wire may be regarded as a reference or "gold standard" in the evaluation of coronary flow velocity and CFR. Coronary flow and CFR in children is a largely unexploited field, and has vast potential for future research.