A cardiac stress test evaluates how the heart performs when working harder than usual. By increasing the heart’s workload in a controlled environment, physicians can uncover issues not apparent when a person is at rest. The primary purpose is to detect coronary artery disease (CAD), which involves blockages in the vessels supplying blood to the heart muscle.
The test also assesses overall blood flow, determines a patient’s safe limit for physical exertion, and evaluates treatment effectiveness. Monitoring includes the heart’s electrical activity, blood pressure, and physical symptoms. Selecting the correct test depends on assessing the patient’s physical ability and existing cardiac health markers.
The Foundation: Exercise Electrocardiogram
The most straightforward and accessible assessment is the Exercise Electrocardiogram (ECG), often called a treadmill stress test. During this procedure, the patient walks on a treadmill or pedals a stationary bicycle while continuously monitored by a 12-lead ECG. Exertion is gradually increased following standardized protocols to raise the heart rate to a target level, typically 85% of the age-predicted maximum.
The test monitors changes in the heart’s electrical activity, specifically shifts in the ST segment of the ECG tracing, which indicate poor blood supply. Blood pressure and heart rhythm are tracked, and the test stops if the patient develops concerning symptoms or reaches the target heart rate. The major limitation of this ECG-only test is its dependency on a normal baseline ECG tracing, as pre-existing electrical abnormalities can make the results uninterpretable.
Imaging for Deeper Insight: Stress Echocardiography and Nuclear Tests
When a physician requires a more detailed view of the heart muscle, or if the standard ECG test is inconclusive, imaging is incorporated. These advanced methods provide a functional assessment, visualizing the consequences of reduced blood flow. The two main imaging modalities are stress echocardiography and nuclear stress testing, offering distinct perspectives on heart function under strain.
Stress Echocardiography uses ultrasound technology to capture moving images of the heart before and immediately after peak stress. It focuses on detecting wall motion abnormalities—segments of the heart muscle that contract poorly or not at all under stress. This lack of normal movement indicates the muscle tissue is not receiving enough oxygenated blood due to a blockage.
In contrast, a Nuclear Stress Test, or Myocardial Perfusion Imaging (MPI), visualizes blood flow to the heart muscle. A small amount of a radioactive tracer is injected into the bloodstream at rest and again at peak stress. A specialized camera captures images showing the tracer distribution. Areas with adequate blood flow show normal uptake, while regions with reduced flow—known as perfusion defects—suggest a significant coronary blockage.
When Exercise Isn’t Possible: Pharmacological Stress
For patients unable to exercise sufficiently (e.g., due to severe arthritis or lung issues), the stress on the heart must be simulated using medication. This is pharmacological stress testing, where intravenous drugs mimic the physiological effects of exertion. These agents stress the heart enough to reveal underlying blood flow problems.
Two main drug types simulate stress. Vasodilators (e.g., adenosine or regadenoson) temporarily widen coronary arteries to increase blood flow. If a blockage exists, normal arteries widen significantly while diseased arteries cannot, creating a flow differential detected by imaging. A second class of drugs, like dobutamine, increases heart rate and contraction force, simulating physical activity. Because ECG changes alone are unreliable with drug-induced stress, this method is always combined with either echocardiography or nuclear imaging.
The Diagnostic Decision: Factors Guiding Test Selection
The decision regarding which stress test to order is guided by patient-specific and technical factors. Patient mobility is the primary consideration; if a patient can safely achieve the target heart rate through exercise, an exercise-based test is preferred for its accuracy and lower cost. If exercise is not possible, a pharmacological test must be chosen, narrowing options to an imaging-based test.
A patient’s baseline ECG tracing is a second determinant. If the resting ECG is abnormal (e.g., due to a left bundle branch block or a ventricular pacemaker), the standard ECG-only test is unreliable. In these cases, an imaging test (stress echo or nuclear MPI) is necessary because underlying ECG changes can mask or mimic signs of poor blood flow.
The pre-test probability of having CAD, based on symptoms and risk factors, also influences the choice. Patients with an intermediate probability benefit most from a stress test, often starting with the simplest exercise ECG. If the probability is higher, or if the initial test is inconclusive, a higher-accuracy imaging test is indicated to minimize the chance of a false negative result.
Technical factors help determine the final imaging modality. Stress echocardiography is preferred when a physician needs to evaluate the heart’s valves or structure, and it avoids radiation exposure. However, for patients with a larger body habitus or lung conditions that make ultrasound imaging difficult, the Nuclear Stress Test may be selected. The gamma camera used in MPI is less affected by these technical limitations, providing a clearer image of blood flow.