CARDIAC STRESS TESTING
Normally, the coronary arteries dilate to four times their usual di-ameter in response to increased metabolic demands for oxygen and nutrients. Coronary arteries with atherosclerosis, however, dilate much less, compromising blood flow to the myocardium and causing ischemia. Therefore, abnormalities in cardiovascular function are more likely to be detected during times of increased demand, or “stress.” The cardiac stress test procedures—the ex-ercise stress test, the pharmacologic stress test, and, more recently, the mental or emotional stress test—are noninvasive ways to eval-uate the response of the cardiovascular system to stress.
The stress test helps determine the following: (1) CAD, (2) cause of chest pain, (3) functional capacity of the heart after an MI or heart surgery, (4) effectiveness of antianginal or antiarrhythmic med-ications, (5) dysrhythmias that occur during physical exercise, and (6) specific goals for a physical fitness program. Contraindi-cations to stress testing include severe aortic stenosis, acute myo-carditis or pericarditis, severe hypertension, suspected left main CAD, HF, and unstable angina. Because complications associ-ated with stress testing can be life-threatening (MI, cardiac arrest, HF, and severe dysrhythmias), testing facilities must have staff and equipment ready to provide advanced cardiac life support.
Mental stress testing uses a mental arithmetic test or simulated public speech to determine whether an ischemic myocardial re-sponse occurs, similar to the response evoked by a conventional treadmill exercise test. Although its use for diagnostic purposes in patients with CAD is currently investigational, preliminary re-sults indicate that the ischemic and hemodynamic measures ob-tained from mental stress testing may be useful in assessing the prognosis of patients with CHD who have had a positive exercise test (Krantz et al., 1999).
Stress testing is often combined with echocardiography or ra-dionuclide imaging (discussed later). These techniques are per-formed during the resting state and immediately after stress.
In an exercise stress test, the patient walks on a treadmill (most common) or pedals a stationary bicycle or arm crank. Exercise in-tensity progresses according to established protocols. The Bruce protocol, for example, is a common treadmill protocol in which the speed and grade of the treadmill are increased every 3 minutes. The goal of the test is to increase the heart rate to the “target heart rate.” This is 80% to 90% of the maximum predicted heart rate and is based on the age and gender of the patient. During the test, the following are monitored: two or more ECG leads for heart rate, rhythm, and ischemic changes; BP; skin temperature; phys-ical appearance; perceived exertion; and symptoms including chest pain, dyspnea, dizziness, leg cramping, and fatigue. The test is terminated when the target heart rate is achieved or when the patient experiences chest pain, extreme fatigue, a decrease in BP or pulse rate, serious dysrhythmias or ST segment changes on ECG, or other complications. When significant ECG abnormalities occur during the stress test (ST segment depressions), the test re-sult is reported as positive and further diagnostic testing is required.
In preparation for the exercise stress test, the patient is instructed to fast for 4 hours before the test and to avoid stimulants such as tobacco and caffeine. Medications may be taken with sips of water. The physician may instruct patients not to take certain cardiac medications, such as beta-blockers, before the test. Clothes and sneakers or rubber-soled shoes suitable for exercising are to be worn. Women are advised to wear a bra that provides adequate support. The nurse describes the equipment used and the sen-sations and experiences that the patient may have during the test. The nurse explains the monitoring equipment used, the need to have an intravenous line placed, and the symptoms to report. The type of exercise is reviewed, and patients are asked to put forth their best exercise effort. If the test is to be per-formed with echocardiography or radionuclide imaging, this information is reviewed as well. After the test, patients are mon-itored for 10 to 15 minutes. Once stable, they may resume their usual activities.
Physically disabled or deconditioned patients will not be able to achieve their target heart rate by exercising on a treadmill or bi-cycle. Two vasodilating agents, dipyridamole (Persantin) and adenosine (Adenocard), administered intravenously, are used to mimic the effects of exercise by maximally dilating the coronary arteries. The effects of dipyridamole last about 15 to 30 minutes. The side effects are related to its vasodilating action and include chest discomfort, dizziness, headache, flushing, and nausea. Adenosine has similar side effects, although patients report these symptoms as more severe. A unique property of adenosine is that it has an extremely short half-life (less than 10 seconds), so any severe effects rapidly subside. Dipyridamole and adeno-sine are the agents of choice used in conjunction with radionu-clide imaging techniques. Theophylline and other xanthines, such as caffeine, block the effects of dipyridamole and adenosine and must be avoided before either of these pharmacologic stress tests.
Dobutamine (Dobutrex) is another medication that may be used for patients who cannot exercise. Dobutamine, a synthetic sympathomimetic, increases heart rate, myocardial contractility, and BP, thereby increasing the metabolic demands of the heart. It is the agent of choice when echocardiography is used because of its effects on altering myocardial wall motion (due to enhanced contractility). In addition, dobutamine is used for patients who have bronchospasm or pulmonary disease and cannot tolerate having doses of theophylline withheld.
In preparation for the pharmacologic stress test, patients are in-structed not to eat or drink anything for at least 4 hours before the test. This includes chocolate, caffeine, caffeine-free coffee, tea, carbonated beverages, or medications with caffeine (eg, Anacin, Darvon). If caffeine is ingested before a dipyridamole or adeno-sine stress test, the test will have to be rescheduled. Patients tak-ing aminophylline or theophylline are instructed to stop taking these medications for 24 to 48 hours before the test (if tolerated). Oral doses of dipyridamole are to be withheld as well. Patients are informed about the transient sensations they may experi-ence during infusion of the vasodilating agent, such as flushing or nausea, which will disappear quickly. The patient is instructed to report any other symptoms occurring during the test to the cardiologist or nurse. An explanation of echocardiography or radionuclide imaging is also provided as necessary. The stress test may take about 1 hour, or up to 3 hours if imaging is performed.