A 65-year-old man comes to the clinic due to periodic substernal chest pain over the last 6 months. The pain episodes occur during strong emotion, last for 10-15 minutes, and resolve spontaneously. The patient has a long history of hypertension and diabetes mellitus type 2. His right foot was amputated 2 years ago due to diabetes complications. Myocardial perfusion imaging is performed to evaluate for possible obstructive coronary artery disease. Resting imaging reveals uniform distribution of the radioactive isotope throughout the myocardium. Imaging after administration of an adenosine analog reveals nonuniform radioactive isotope uptake in the myocardium, consistent with myocardial ischemia. Which of the following best describes the mechanism of the change observed during the stress portion of the study?

Question

A 65-year-old man comes to the clinic due to periodic substernal chest pain over the last 6 months.  The pain episodes occur during strong emotion, last for 10-15 minutes, and resolve spontaneously.  The patient has a long history of hypertension and diabetes mellitus type 2.  His right foot was amputated 2 years ago due to diabetes complications.  Myocardial perfusion imaging is performed to evaluate for possible obstructive coronary artery disease.  Resting imaging reveals uniform distribution of the radioactive isotope throughout the myocardium.  Imaging after administration of an adenosine analog reveals nonuniform radioactive isotope uptake in the myocardium, consistent with myocardial ischemia.  Which of the following best describes the mechanism of the change observed during the stress portion of the study?

Quizplus · Accepted Answer

11ec25c8_d344_c93b_bc81_39175bc8729a_MD0003_00 This patient's clinical presentation (recurrent substernal chest pain elicited by strong emotion and resolving after several minutes) is suggestive of stable angina. Pharmacologic stress testing allows evaluation for obstructive coronary artery disease (CAD) in patients who are unable to perform adequate exercise (eg, due to amputation or severe osteoarthritis). This patient's coronary artery vasodilator stress test (eg, adenosine, dipyridamole) reveals myocardial ischemia consistent with obstructive CAD. Adenosine and its synthetic analogs (eg, regadenoson, apadenoson) stimulate adenosine A2A receptors on vascular smooth muscle cells, causing coronary vasodilation and increased myocardial blood flow. There is a several-fold augmentation of blood flow in nonobstructed coronary arteries. Blood flow is increased in stenosed coronary arteries as well but to a much lesser extent. This relative blood flow difference is magnified from rest, causing a detectable reduction in radioactive isotope uptake by myocardial cells in areas supplied by a stenotic coronary artery (appears as an ischemic defect on myocardial perfusion imaging). (Choice B) An increase in heart rate and/or myocardial contractility leads to an increase in myocardial oxygen demand, and is the underlying physiologic basis for exercise stress testing and dobutamine (beta-1 receptor agonist) stress testing. Dobutamine stress testing is typically used in patients who require pharmacologic stress testing but have contraindications to vasodilator stress testing (eg, hypotension, obstructive lung disease). (Choices C and D) Both increased afterload and increased systemic venous return create increased left ventricular wall tension and increased myocardial oxygen demand. In patients with CAD, the resulting coronary vasodilation will lead to a greater increase in blood flow in normal coronary arteries compared to stenosed coronary arteries (similar to a pharmacologic vasodilator stress test); however, diagnostic stress testing for CAD does not make use of these physiologic mechanisms. (Choice E) Diseased (stenotic) coronary arteries do not constrict during a pharmacologic vasodilator stress test, but instead dilate to a lesser degree than normal coronary arteries. Educational objective: Pharmacologic vasodilator stress testing (eg, adenosine) causes a marked increase in blood flow in normal coronary arteries and a relatively small increase in blood flow in stenotic coronary arteries. The difference in blood flow allows for diagnosis of obstructive coronary artery disease due to reduced uptake of radioactive isotope in ischemic myocardium. References: Stress protocols and tracers. (http://www.ncbi.nlm.nih.gov/pubmed/17174798) Coronary artery blood flow: physiologic and pathophysiologic regulation. (http://www.ncbi.nlm.nih.gov/pubmed/10626079)

A 65-Year-Old Man Comes to the Clinic Due to Periodic