Stress Testing

Stress testing is one of the most widely used tools in cardiology. It provides diagnostic and prognostic information in patients with suspected or known coronary heart disease, and is a useful tool for assessing the adequacy of therapy. Stress testing can also objectively assess the functional capacity of a patient, and help determine the nature of a patient's functional limitations.

Indications

The most common (and, perhaps, most useful) indication for stress testing is to determine whether a patient's symptoms relate to underlying coronary artery disease. For patients in whom the diagnosis of CAD is already certain, stress testing may be useful to:

• assess risk and determine long-term prognosis
• assess exercise capacity
• evaluate the efficacy of therapy
• assist in therapeutic decision making (i.e., determine who may benefit from cardiac catheterization and revascularization)
• detect exercise-related arrhythmias
• localize a region of ischemia in order to target percutaneous revascularization

Contraindication

There are small but real risks associated with stress testing, including myocardial infarction ( MI ), serious arrhythmias, and death (approximate rate of 1 in 2,500 tests). Most of these complications occur in patients with certain high-risk markers; therefore, patients should be carefully screened before undergoing exercise testing to ensure that no contraindications exist (see Table 6-1).

Testing Madalities

Stress testing can be performed in several ways (Table 6-2). The stress can be induced by exercise (either walking on a treadmill or riding a bicycle) or by pharmacological means with dobutamine or coronary vasodilators. Treadmill exercise is the best-standardized modality and allows for more flexible protocols, as speed and incline can be varied independently. The Bruce protocol is the most commonly used and consists of incremental increases in the speed and slope of the treadmill every three minutes. Bicycle testing may, however, be better tolerated in patients who have orthopedic or balance problems. The aim of the exercise is to increase myocardial oxygen demand (MV0₂). In patients with coronary artery disease (CAD), the increased MV0₂ may exceed the ability of the coronary arteries to supply oxygenated blood, resulting in ischemia.

Dobutamine produces an increase in heart rate, contractility, and blood pressure, thereby mimicking exercise and increasing MV0 ₂. Dipyridamole or adenosine cause coronary vasodilation preferentially in normal coronary arteries. This results in a flow mismatch with blood flow in normal coronary arteries increased relative to diseased coronary arteries. Pharmacological stress testing with these agents yields sensitivities and specificities comparable to exercise stress testing.Pharmacological stress is not without risk; dobutamine can induce ventricular arrhythmias and precipitate myocardial ischemia, while dipyridamole can cause bronchospasm.