A cardiac stress test, often referred to as an Exercise Tolerance Test (ETT), is a standardized medical procedure designed to evaluate how the heart and circulation system respond to physical exertion. The primary purpose of this test is to assess blood flow to the heart muscle and detect potential blockages in the coronary arteries that may not be apparent while a person is resting. During the procedure, the body is safely pushed to a level of increased effort, typically by walking on a treadmill or cycling, to increase the heart’s demand for oxygenated blood. For individuals unable to exercise, a pharmacological stress test uses medication to simulate the effect of physical activity on the heart. The results provide physicians with information to diagnose heart conditions, assess the effectiveness of existing treatments, and determine appropriate physical activity levels.
What Stress Tests Measure
Throughout the stress test, medical personnel continuously monitor several physiological data points to build a comprehensive picture of heart performance under duress. The most fundamental monitoring tool is the continuous Electrocardiogram (EKG), which tracks the heart’s electrical activity and rhythm for any changes that might signal a lack of oxygen. Changes in the EKG, particularly in the ST segment, are a primary focus for identifying potential cardiac issues.
Blood pressure is measured at regular intervals to observe the hemodynamic response to the increasing workload. A normal test shows a steady rise in systolic blood pressure as exercise intensity increases, reflecting the heart’s ability to pump blood effectively. The patient is also monitored for symptoms, such as chest discomfort, shortness of breath, or excessive fatigue, which can be indicators of underlying coronary disease.
The intensity of the exercise is increased incrementally, often following a standardized protocol such as the Bruce Protocol. The total duration of exercise achieved before termination is a crucial input, as it directly reflects the patient’s functional capacity and is used to calculate a standardized metric of fitness.
Defining a Successful Test Outcome
The definition of a “good score” on a stress test is centered on two main factors: achieving an adequate level of stress and demonstrating the absence of exercise-induced ischemia. A test is considered diagnostically adequate only if the patient reaches a predetermined Target Heart Rate (THR), which is generally set at 85% of the age-predicted maximum heart rate. The maximum heart rate is estimated by subtracting the patient’s age from 220 beats per minute.
Reaching this target rate is important because it ensures the heart has been sufficiently stressed to reveal blockages that only limit blood flow during peak demand. If a patient terminates the test early due to fatigue or symptoms before reaching the THR, the result may be considered inconclusive, potentially necessitating further testing.
A truly successful result, known as a “negative” test, indicates that no evidence of myocardial ischemia was present at maximum exertion. This negative finding is confirmed primarily by the EKG showing no significant ST-segment depression, which is the electrical signature of insufficient oxygen supply to the heart muscle. A successful test also includes the absence of angina, severe shortness of breath, or concerning blood pressure changes throughout the entire exercise period. The clinical interpretation of a negative test suggests a low likelihood of having significant blockages in the coronary arteries.
Understanding Metabolic Equivalents (METs)
While the clinical outcome determines the presence of coronary artery disease, the most quantifiable “score” a patient receives is the Metabolic Equivalent of Task (MET) score, which measures functional capacity and long-term prognosis. One MET is defined as the amount of oxygen consumed while sitting quietly at rest. The MET score achieved during the test represents how many times greater the patient’s peak oxygen consumption was compared to their resting state.
The achieved MET score is a powerful predictor of overall health and survival, independent of the diagnostic finding for blockages. Exercise capacity is often categorized by the maximum MET level sustained during the test. For instance, a score of 4 METs might represent a limited capacity, while a score of 7 METs is generally considered average for many adults.
Achieving a high MET score is an excellent prognostic sign. A common benchmark in cardiology indicates that achieving 10 METs or more is associated with a significantly reduced risk of future cardiac events. Patients who reach this level of workload tend to have a favorable long-term outlook. This metric allows physicians to counsel patients on their overall cardiovascular fitness level and the potential for improved longevity.
Interpreting Abnormal Findings
An abnormal or “positive” stress test result suggests that the heart is not receiving adequate blood flow during exercise, a condition called ischemia. The most definitive sign of a positive test is the presence of significant ST-segment depression or elevation on the EKG, which indicates that the heart muscle is experiencing an oxygen deficit under stress. These electrical changes often appear alongside symptoms like reproducible chest pain that intensifies as the workload increases.
Other abnormal findings include a failure of the systolic blood pressure to rise, or even a drop of more than 10 mmHg below the baseline pressure, which can indicate severe left ventricular dysfunction or extensive ischemia. The development of serious heart rhythm abnormalities during exercise is also a marker of a positive test outcome. When these abnormal indicators appear, the test is typically stopped immediately, leading to a conclusion that coronary artery disease is likely present. A positive test result often prompts the physician to recommend further imaging tests, such as a coronary angiogram, to visualize the extent and location of any arterial blockages.