A 43-year-old man comes to the office due to occasional chest discomfort over the last 6 weeks. He thinks he is most likely experiencing musculoskeletal pain but is concerned due to family history of heart disease. The patient has no medical conditions and does not use tobacco. He leads an active lifestyle and exercises every day. He undergoes treadmill exercise stress testing. Baseline blood pressure is 122/75 mm Hg and pulse is 54/min. After 10 minutes of exercise, his blood pressure is 155/80 mm Hg and pulse is 150/min. He has no chest pain and ECG shows no abnormalities. Compared to pretest conditions, which of the following is the single most important limiting factor for left ventricular myocardial blood supply during the test?
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Myocardial perfusion is provided by the right and left coronary arteries arising directly from the aortic root. In contrast to most other vascular beds in the body, left ventricular myocardial perfusion occurs mainly during diastole. During ventricular systole, intraventricular pressure and wall stress in the left ventricle exceed the aortic perfusion pressure (eg, 120 mm Hg), preventing effective coronary perfusion. Wall tension is highest in the subendocardial region, making this area of the myocardium particularly susceptible to ischemia. Relaxation of the left ventricle during diastole decreases intraventricular pressure to about 10 mm Hg (which is much lower than aortic diastolic pressure [~80 mm Hg]), providing a driving force that allows for adequate perfusion.
Increased heart rate shortens the time of ventricular relaxation (duration of diastole); therefore, the time available for maximal coronary blood flow decreases and consequently becomes the major limiting factor for coronary blood supply to the myocardium.
(Choice B) During exercise, mechanisms such as flow-mediated dilation and release of vasodilators such as adenosine and nitric oxide ensure adequate coronary blood supply.
(Choices A, E, and F) Left ventricular contraction force, intraventricular pressure, and ventricular wall stress all increase during exercise to provide adequate blood flow to actively contracting skeletal muscle. However, because there is already minimal left ventricular perfusion occurring during systole, the increase in these forces does not cause further limitation in left ventricular myocardial perfusion.
(Choice C) Diastolic aortic pressure is the driving force for coronary blood flow during diastole, and coronary blood flow is reduced in patients with low diastolic pressures. This patient's diastolic pressures are similar while at rest and during exercise and would not contribute to decreased coronary blood supply.
Educational objective:
The high systolic intraventricular pressure and wall stress of the left ventricle prevent myocardial perfusion during systole; therefore, the majority of left ventricular myocardial perfusion occurs during diastole. Shorter duration of diastole is the major limiting factor for coronary blood supply to the left ventricular myocardium during periods of tachycardia (eg, exercise).