STRESS TESTING AND NUCLEAR IMAGING
Stress ECG and stress imaging studies are widely used noninvasive procedures that provide important information on cardiac function and the presence of hemodynamically significant coronary artery disease (CAD). The correct use of stress testing is critically important for the cost-effective management of patients with known or suspected CAD. When the most appropriate procedure is performed, it provides important diagnostic and prognostic information that determines the optimal management strategy to be undertaken for that individual. Stress testing is also used in patients with known CAD to determine exercise “prescriptions” before cardiac rehabilitation (Fig. 10.1).
FIG 10.1 Evaluation for Hemodynamically Significant Coronary Artery Disease (CAD) in Clinically Stable Patients.
FIG 10.2 Testing to Detect Myocardial Ischemia.
Exercise stress testing (EST) involves subjecting a patient to increasing levels of exercise with continuous ECG monitoring for myocardial ischemia and arrhythmias. Although the sensitivity and specificity of stress ECG for the detection of CAD are low (range: 55%–75%) compared with more advanced testing (including the use of imaging), stress ECG is widely available, relatively inexpensive, and can provide important prognostic information about the patient. Generally, diagnostic treadmill stress testing is done on patients with a low or intermediate pretest likelihood of having CAD. However, EST can also be used in patients with known CAD to evaluate the effectiveness of current therapies, to ascertain overall functional capacity, to determine general prognosis, and/or to provide an exercise prescription. In children with congenital heart disease, EST can be used to quantify functional capacity.
The sensitivity of EST for detecting CAD is proportional to the heart rate (HR) achieved during exercise. Thus, in preparation for the study, patients are usually asked to transiently discontinue medications that affect HR response (e.g., β-blockers or calcium channel blockers). Patients should fast for at least 4 hours before the test. Exercise is done on a treadmill, or alternatively, using a bicycle ergometer. In special circumstances, arm ergometry and isometric hand exercises can be used. There are several different protocols for EST. All of them start exercise at a given rate and incline angle, and then gradually increase one or both parameters until an adequate HR and exercise endurance are achieved. Generally, exercise is continued until the patient reaches a target HR of at least 85% of the maximum predicted HR (MPHR) for the age of the patient (220 beats/min − age in years ± 10 to 12 beats/ min). Studies that have correlated ECG changes with CAD generally involve reaching this target HR.
Once a patient reaches the target HR, they should continue to exercise until fatigued or until signs or symptoms develop. If a patient exceeds a double product (HR × systolic blood pressure) of 25,000 or attains an exercise level of at least 5 metabolic equivalents as a secondary target, the test may be considered adequate. Hemodynamic instability, gross ECG changes, or severe patient symptoms are also indications to terminate the procedure. At the end of exercise testing, the patient slowly reduces the intensity of exercise. Vigorous exercise results in increased blood flow and pooling in the extremities, and a “step-down” phase (low-level exercise) allows the patient to re-equilibrate before ceasing exercise. After exercise termination, patients are monitored in a supine position until they are no longer tachycardiac (i.e., HR <100 beats/min) if not back to baseline HR. Importantly, if there were any ECG changes or symptoms experienced by the patient during the study, posttest monitoring should be continued with any necessary treatments until these have been resolved, even if hemodynamics (HR and blood pressure) have returned to acceptable levels. The posttest monitoring serves to reveal any arrhythmias or ST-segment changes that may develop and that may be late signs of ischemic disease (Fig. 10.2).
The ECG must be interpreted with certain caveats. Although the standard 12-lead configuration can be used, in many instances, a modified 12-lead configuration is substituted. This involves placing limb leads more proximally than is done for a standard ECG (e.g., electrodes are placed on the shoulders rather than the arms). This modification results in ST-segment changes being accentuated and more easily detected during stress, but it is also a baseline stress ECG that differs from a supine ECG done with standard lead placement.
The presence of myocardial ischemia during the test is suggested if previously normal ST segments show flattened or downsloping depression >1 mm below the baseline in three consecutive beats. An important issue concerns ST-segment changes that can occur in some individuals simply because of the increased respiratory rate that accompanies exercise. A prestress or poststress ECG performed with hyperventilation should be done to allow comparison of ECG changes that are associated with an increased respiratory rate.
The prognostic information obtained from a treadmill stress test is often useful for deciding on the next diagnostic or therapeutic step for a given patient. Of the several methods used for prognosis after EST, the most widely used is the Duke Treadmill Score. The time of exercise, the presence (or absence) of ST-segment changes during the study, and patient symptoms are used to determine a score that correlates with event-free survival.
Bicycle-based studies use a comparable approach to provide similar information. The patient maintains a steady, pedaling rate over a period of time with regular increases in the intensity required for pedaling. At comparable HRs, a higher level of physiological stress (reflected by metabolic equivalents) is present in individuals walking on a treadmill than individuals pedaling a bicycle. However, the data available for comparing these two forms of exercise are limited. Caution should be used in translating clinical information between forms of exercise.
Contraindications to exercise include unstable coronary syndrome, decompensated heart failure (HF), severe obstructive valvular or hypertrophic cardiomyopathic disease, untreated life-threatening arrhythmias, and advanced atrioventricular block. Under certain circumstances, exercise testing under rigorously controlled conditions is performed on patients with aortic stenosis to determine their suitability for aortic valve replacement. Severe baseline hypertension (>220/120 mm Hg) or the presence of large arterial aneurysms are also contraindications, as are systemic illnesses such as acute pulmonary embolus and aortic dissection. Exercise studies should be used cautiously in individuals with an implantable cardiac defibrillator (ICD), particularly if their underlying ECG shows a prolonged QRS interval (due to an underlying bundle branch block or paced rhythm), because in this circumstance, the defibrillator may “recognize” the rapid HR induced by exercise as ventricular tachycardia. Arrhythmias such as uncontrolled atrial fibrillation may also make interpretation of exercise stress ECGs difficult or impossible, and patients with these arrhythmias should be considered stress imaging study.