Section: USMLE

53) A 68-year-old man sustains a myocardial infarct resulting from thrombotic occlusion at the origin of the left circumflex artery. Cardiac catheterization demonstrates that the patient has a left dominant coronary circulation. In which of the following areas of the heart has ischemic necrosis most likely occurred?

1. Apex of left ventricle and anterior portion of septum
2. Lateral left ventricular wall and posterior portion of the septum
3. Lateral wall of the left ventricle only
4. Posterior portion of the septum only
5. Right ventricular wall

A right dominant coronary circulation is present when the posterior descending branch originates from the right coronary artery (80% of individuals). On the contrary, the posterior descending artery originates from the left circumflex artery in a left dominant circulation (20% of individuals). The posterior descending branch gives blood to the posterior half of the interventricular septum. Occlusion of the left circumflex artery in a left dominant circulation will therefore lead to ischemic necrosis in the left ventricular wall and the posterior interventricular septum.

The apex of the left ventricle is dependent on the anterior descending branch; thus, occlusion of the left circumflex does not affect this portion of the left ventricle.

Infarction of the lateral (free) wall alone will result from occlusion of the circumflex in a right dominant circulation.

An isolated infarct of the posterior interventricular septum arises from occlusion of the posterior descending branch.

Isolated infarcts of the right ventricular wall are very rare and would be caused by occlusion of branches of the right coronary artery.