Atherosclerotic lesions of the coronary arteries can limit blood flow to the myocardial regions supplied by the affected vessels. In some patients, certain medications can cause a redistribution of blood flow away from the ischemic areas, exacerbating existing myocardial ischemia. Which of the following drug effects is most likely to produce this phenomenon?

Question

Atherosclerotic lesions of the coronary arteries can limit blood flow to the myocardial regions supplied by the affected vessels.  In some patients, certain medications can cause a redistribution of blood flow away from the ischemic areas, exacerbating existing myocardial ischemia.  Which of the following drug effects is most likely to produce this phenomenon?

Quizplus · Accepted Answer

Hemodynamically significant atherosclerotic lesions (>70% occlusion) reduce the amount of blood supplied by a coronary vessel to the corresponding myocardial territory. In response, the myocardium releases locally acting, endogenous substances (eg, adenosine, nitric oxide) that cause dilation of coronary arterioles, leading to a downstream reduction in vascular resistance and recruitment of additional blood flow to that region. Arterioles that supply areas of significantly ischemic myocardium can become maximally dilated at rest, limiting their capacity to further increase blood flow. Pharmacologic stress agents (eg, adenosine, dipyridamole) are used during myocardial perfusion imaging to simulate exercise-induced coronary arteriole dilation and assist in identifying areas of ischemic myocardium. Most of the time, administration of these drugs will result in a significant increase in blood flow to nonischemic myocardium and no change in blood flow to ischemic myocardium (because additional blood flow is recruited proximally). In some patients with extensive coronary artery disease, the recruitment of additional upstream blood flow is limited due to proximal atherosclerotic disease (reduced coronary flow reserve). In such a setting, these drugs can cause an absolute decrease in blood flow to ischemic myocardium, as most of the limited blood supply is redirected toward the newly vasodilated areas of nonischemic myocardium. This redistribution of blood flow leading to worsening ischemia is referred to as coronary steal. (Choices B, D, and E) Decreased myocardial contractility, negative chronotropy, and systemic venous dilation (leading to reduced preload) all decrease myocardial oxygen demand through a reduction in cardiac work. These mechanisms act to improve myocardial ischemia globally and will not result in a redistribution of blood flow away from areas of ischemic myocardium. (Choice C) The epicardial vessels are the large coronary arteries of the heart (eg, right coronary, left circumflex). Although dilation of these large arteries plays a role in increasing myocardial blood flow, coronary blood flow regulation is mostly mediated by the arterioles. Dilation of large coronary arteries would improve cardiac blood flow and would not trigger coronary steal. Educational objective: Increases in resting blood flow to ischemic myocardium are primarily mediated by locally-acting substances (eg, adenosine, nitric oxide) that trigger coronary arteriolar vasodilation. Pharmacologic arteriolar vasodilators (eg, adenosine, dipyridamole) mimic the vasodilation that occurs with exercise and may cause redistribution of blood flow from ischemic to nonischemic areas of myocardium (ie, coronary steal).

Atherosclerotic Lesions of the Coronary Arteries Can Limit Blood Flow