Oxygen in the myocardium

Oxygen in the myocardium

Normally, there is a balance between oxygen demand and supply in the myocardium. The oxygen demand is determined by the ventricular volume, pressure in the left ventricle, tension of the wall, heart rate, and contractility. The supply of oxygen depends on factors such as aortic pressure, coronary vascular resistance, coronary blood flow, regional myocardial distribution, and the oxygen difference between arteries and veins. Ischaemia describes the situation in which the oxygen demand exceeds the supply. This can occur when the demand becomes higher, e.g. during exercise (increased HF) or with myocardial hypertrophy (increased ventricular contractility). An imbalance in the oxygen supply/demand can also be due to a decrease in the oxygen supply, which is mainly determined by the coronary blood flow. Transient imbalance between

oxygen supply and metabolic demand may present itself in a predictable cascade of events: biochemical abnormality, diastolic and systolic myocardial dysfunction, electrical instability, and ultimately angina pectoris. The stable form of ischaemia, mostly caused by a stable obstruction in a coronary artery, is characterized by a predictable pattern of symptoms related to a certain level of exercise or stress. A stable angina pectoris can be the clinical presentation of this form of ischaemia. The unstable form of ischaemia occurs when there is a reduced blood flow secondary to vasospasm, or obstruction by a stenosis and/or thrombus formation as a result of plaque rupture. This may result in unpredictable clinical events, e.g acute dyspnoea, arrhythmia, transient angina pectoris at rest, or even a myocardial infarction.

1

Coronary blood flow is determined by

2

Oxygen blood supply to the heart depends on