Coronary arteries

The heart is perfused by right and left coronary arteries arising directly from the take off of the thoracic aorta. There are some differences in the nomenclature used by anatomists and clinicians when describing the coronary arteries. The left coronary artery leaves the aorta and is referred to as the 'left main stem' before its bifurcation. It then divides into the circumflex artery and an anterior branch, the anterior inter-ventricular artery or 'left anterior descending' (LAD) artery. The branches of the circumflex artery are referred to as obtuse marginal (OM) branches and individual branches as OM1, OM2, etc. (Fig. 8.2).

Classically, the right coronary artery gives rise to the marginal branches of the right ventricle (RV), before terminating as the posterior inter-ventricular artery referred to by clinicians as the 'posterior descending artery' (PDA). There are some anatomic variations in size and extent of distribution of the right coronary artery and not all right coronary arteries terminate as a PDA. In 10-15% of cases the right coronary artery is small and the PDA may arise as a terminal branch of the left circumflex artery. When the PDA arises from the left circumflex, it is referred to as a left dominant circulation. If the right coronary artery gives rise to the PDA, the right coronary is said to be dominant. It should be noted, however, that regardless of the origin of the PDA, the bulk of the blood supply to the heart is still borne by the left coronary artery.

Coronary blood flow, which amounts to about 225ml/min at rest, can increase to just over 1 l/min with exercise. During systole, particularly during the phase of isovolumetric contraction, the intra-myocardial coronary arteries are compressed and coronary flow is reduced. Coronary blood flow thus occurs mainly during diastole. Conditions which result in a low dia-stolic blood pressure or which increase the intra-myocardial

Figure 8.2. Right anterior oblique angiographic view of left coronary artery and major branches. Note the stenotic lesion in the LAD and the obstructed OM artery arising from the circumflex. The PDA, which in this case arises from the right coronary artery, is filled in a retrograde fashion by dye from the LAD.

Figure 8.2. Right anterior oblique angiographic view of left coronary artery and major branches. Note the stenotic lesion in the LAD and the obstructed OM artery arising from the circumflex. The PDA, which in this case arises from the right coronary artery, is filled in a retrograde fashion by dye from the LAD.

tension during diastole (e.g. an increase in end-diastolic pressure) can compromise coronary blood flow. In these situations, the subendocardial muscle is particularly vulnerable.

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