Chapter 13: Electrocardiographic Exercise Testing
What are the determinants of myocardial oxygen uptake?
A. Minute ventilation and the fraction of ventilation extracted by the tissues
B. Cardiac output and the peripheral arteriovenous oxygen difference
C. Intramyocardial wall tension, contractility, and heart rate
D. Stroke volume and heart rate
E. The maximal heart rate and the maximal systolic blood pressure
The answer is C. (Hurst’s The Heart, 14th Edition, Chap. 13) The determinants of myocardial oxygen uptake are intramyocardial wall tension (left ventricular pressure and end-diastolic volume), contractility, and heart rate (option C). Myocardial oxygen uptake can be estimated by the product of heart rate and systolic blood pressure (the double product or rate pressure product). This information is valuable clinically because exercise-induced angina often occurs at the same myocardial oxygen demand and thus double product. The higher the double product achieved, the better the myocardial perfusion and prognosis. VO2 max is determined by the maximal amount of ventilation (volume of expired gas [VE]) moving into and out of the lung and by the fraction of this ventilation that is extracted by the tissues: VO2 = VE × (FiO2 – FeO2) where VE is the minute ventilation and FiO2 and FeO2 are the fractional concentration of oxygen in the inspired and expired air, respectively (option A). The determinants of volume oxygen consumption [VO2] are cardiac output and the peripheral arteriovenous oxygen difference. Because maximal arteriovenous difference behaves more or less as a constant, maximal oxygen uptake serves as an indirect estimate of maximal cardiac output (option B). The product of stroke volume and heart rate is the cardiac output (option D). The rate pressure product is the product of the maximal heart rate (bpm) and the maximal systolic blood pressure (mm Hg), and it represents the internal workload or hemodynamic response (option E).
A 65-year-old male is referred to you for evaluation of possible coronary artery disease. He gives a 4-week history of chest discomfort with climbing two flights of stairs, where he had previously been able to climb several flights without difficulty. He denies any dyspnea, orthopnea, paroxysmal nocturnal dyspnea, or syncope. Two weeks ago, he was started on isosorbide mononitrate 30 mg po daily, metoprolol 25 mg po twice daily, and aspirin 80 mg po daily by his primary care physician. Physical exam is within normal limits. You plan to perform an exercise stress test. Which of the following sets of instructions is false?
A. Instruct the patient to hold metoprolol for at least 24 hours prior to the test