64 71 Myocardial blood flow.

Myocardial (coronary) blood flow: about 4% of cardiac output, regulation, coronary reserve and ischaemia.

Myocardial (coronary) blood flow is the blood supply of the heart muscle through the coronary arteries. At rest it is about 4% of cardiac output, roughly 200–250 mL/min. The heart extracts a very large share of the oxygen from coronary blood even at rest, so when its demand rises it can obtain more oxygen mainly by increasing flow. During heavy exercise, oxygen demand of the left ventricle rises about sixfold and coronary flow about fivefold. Most coronary flow occurs during diastole, when the contracting muscle does not compress the vessels; a fast heart rate shortens diastole.

HeartCoronary circulationIschaemia

Share of cardiac output≈ 4%

At rest≈ 200–250 mL/min

Heavy exerciseup to ≈ 5 × resting flow

Main flow phasediastole

Figure 1. Resting distribution of a cardiac output of about 5 L/min (after Chapman and Mitchell, 1965). The highlighted bar shows heart (coronary) blood flow.

This application estimates regional blood flow as a percentage of cardiac output and converts it into mL/min from the calculated cardiac output. In clinical practice, organ blood flow is measured invasively or with imaging (for example indicator dilution, Doppler ultrasound, PET or MRI perfusion).

Regulation

Coronary flow is matched to the heart's metabolism by local factors (adenosine, nitric oxide, potassium channels sensitive to ATP), by autoregulation that keeps flow stable over a range of pressures, and by the autonomic nervous system. The coronary flow reserve, the ability to increase flow above resting level, falls when arteries are narrowed or the microcirculation is damaged.

Reduced coronary flow

Narrowing of coronary arteries by atherosclerosis, spasm, thrombosis or microvascular disease causes myocardial ischaemia: angina, ischaemic ECG changes, arrhythmias and, if flow stops, myocardial infarction. Low blood pressure, very fast heart rate, severe anaemia and left ventricular hypertrophy also reduce oxygen supply relative to demand. In the model of this application, a share below 4% of cardiac output is flagged as a sign of possible coronary circulatory insufficiency.

Seek emergency care

for chest pain or pressure lasting more than a few minutes, especially with breathlessness, sweating, nausea or pain spreading to the arm, neck or jaw.

References

Duncker DJ, Bache RJ. Regulation of coronary blood flow during exercise. Physiol Rev. 2008;88(3):1009–1086. doi:10.1152/physrev.00045.2006
Vander A, Sherman J, Luciano D. Human Physiology: The Mechanisms of Body Function. 8th ed. Boston: McGraw-Hill; 2001. Fig. 14-9 (after Chapman CB, Mitchell JH. Sci Am. 1965). Data: BioNumbers BNID 109465

This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.

Published on 30 April 2024