Effects of acute ischaemia on intramyocardial contraction heterogeneity; new ultrasound technologies to study an old phenomenon

The diagnostic approach of patients with coronary artery disease normally relies on the assessment of global and regional perfusion and contraction in the normal and pathological areas of the myocardium. However, experimental pathophysiological studies have clearly shown that microvascular flow, metabolic consumption and the strength of contraction do not behave homogeneously in the different transmural layers of the myocardium. In dogs with normal coronary arteries, for example, the consumption of oxygen and the concentration of high energy substrates such as kinase phosphate and ATP is greater in subendocardial layers. This transmural gradient is also preserved during increased myocardial consumption, such as in high rate atrial pacing. The myocardial blood flow behaves similarly, with a perfusion gradient favouring the subendocardium. In addition myocardial contraction has a transmural heterogeneity. When investigating myocardial contractility the anatomical arrangement of myocardial fibres has to be taken into account. Subendocardial and subepicardial fibres are mostly parallel to the long axis of the left ventricle and the midwall fibres are oriented circumferentially. During systole, these fibres contract and the longitudinal and transverse axes of the left ventricle shorten, while the ventricular wall thickness increases.