Patients who are undergoing high-risk procedures (especially vascular procedures which require cross-clamping of the aorta) may require additional evaluation for CAD, such as exercise treadmill testing. Patients with unstable angina, new electrocardiographic (ECG) changes (especially ischemic), or high cardiac risk indices should also be considered for further cardiac evaluation. Patients who are unable to exercise (e.g., those with peripheral vascular disease) or who have baseline ECG abnormalities may require a dipyridamole-thallium scan to detect ischemia.
Patients with evidence of ischemic disease on noninvasive testing may require coronary angiography with an eye toward coronary artery bypass grafting (CABG) prior to high-risk surgery. CABG, of course, carries its own operative risks, but patients who have had successful CABG reduce their perioperative cardiac risk to approximately that of the normal population.
What techniques can be used to identify patients at risk for perioperative cardiac complications?
The history, physical examination, and ECG are important. Coronary angiography is costly and invasive, carries its own risk, and does not assess function. Radionuclide ventriculography, arm ergometry, and Holter monitoring have been advocated, but their predictive values are inadequate. The exercise treadmill test estimates cardiac risk and functional status but often cannot be done, especially in patients with peripheral vascular disease. Stress echocardiography (often done with dobutamine or dipyridamole) may become a valuable way of predicting perioperative complications in patients who cannot exercise but so far has been studied in only a limited number of patients. Vasodilator myocardial imaging remains the most popular technique overall for assessing perioperative cardiac risk. (See Chapter 37 for an algorithm on using these techniques.)
How is vasodilator myocardial imaging done?
The technique involves administration of a vasodilator (usually dipyridamole) and a contrast agent (thallium-201), followed by myocardial perfusion scanning. Intravenous dipyridamole dilates normal coronary arteries by enhancing their sensitivity to adenosine, a potent coronary artery dilator. Stenotic or occluded vessels remain unaffected. After dipyridamole is infused, thallium-201 is injected, and myocardial scanning is done immediately and 4 hours later. Patients with adequate myocardial perfusion have normal scans. Patients with defects on the initial scan and no defects after 4 hours have ischemic myocardium. Perfusion defects which do not change after 4 hours usually indicate old infarction.
Adenosine-thallium scanning appears to be equally accurate in predicting perioperative risk. It may have an advantage over dipyridamole-thallium scanning because adenosine is rapidly metabolized and so side effects are transient. However, adenosine causes a higher incidence of chest pain.
Which patients should be evaluated for CAD prior to surgery? Photo Gallery
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