Cardiac stress test (or Cardiac diagnostic test) is a test used in medicine and cardiology to measure the heart’s ability to respond to external stress in a controlled clinical environment.
The stress response is induced by exercise or drug stimulation. Cardiac stress tests compare the coronary (heart) circulation while the patient is at rest with the same patient’s circulation observed during maximum physical exertion, showing any abnormal blood flow to the heart’s muscle tissue (the myocardium). This test can be used to diagnose ischemic heart disease, and for patient prognosis after a heart attack (myocardial infarction).
The cardiac stress test is done with heart stimulation, either by exercise on a treadmill, pedaling a stationary exercise bicycle ergometer or with intravenous pharmacological stimulation, with the patient connected to an electrocardiogram (or ECG). People who cannot use their legs may exercise with a bicycle-like crank that they turn with their arms.
The level of mechanical stress is progressively increased by adjusting the difficulty (steepness of the slope) and speed. The test administrator or attending physician examines the symptoms and blood pressure response. With use of ECG, the test is most commonly called a cardiac stress test, but is known by other names, such as exercise testing, stress testing treadmills, exercise tolerance test, stress test or stress test ECG.
Nuclear stress test
The best known example is myocardial perfusion imaging. Typically, a radiotracer (Tc-99 sestamibi, Myoview or Thallous Chloride 201) may be injected during the test. After a suitable waiting period to ensure proper distribution of the radiotracer, scans are acquired with a gamma camera to capture images of the blood flow. Scans acquired before and after exercise are examined to assess the state of the coronary arteries of the patient.
Showing the relative amounts of radioisotope within the heart muscle, the nuclear stress tests more accurately identify regional areas of reduced blood flow.
Stress and potential cardiac damage from exercise during the test is a problem in patients with ECG abnormalities at rest or in patients with severe motor disability. Pharmacological stimulation from vasodilators such as dipyridamole or adenosine, or positive chronotropic agents such as dobutamine can be used. Testing personnel can include a cardiac radiologist, a nuclear medicine physician, a nuclear medicine technologist, a cardiology technologist, a cardiologist, and/or a nurse.
The American Heart Association recommends ECG treadmill testing as the first choice for patients with medium risk of coronary heart disease according to risk factors of smoking, family history of coronary artery stenosis, hypertension, diabetes and high cholesterol.
Perfusion stress test (with 99mTc labelled sestamibi) is appropriate for select patients, especially those with an abnormal resting electrocardiogram.
Intracoronary ultrasound or angiogram can provide more information at the risk of complications associated with cardiac catheterization.
The common approach for stress testing by American College of Cardiology and American Heart Association indicates the following:
Treadmill test: sensitivity 73-90%, specificity 50-74% (Modified Bruce Protocol)
Nuclear test: sensitivity 81%, specificity 85-95%
(Sensitivity is the percentage of sick people who are correctly identified as having the condition. Specificity indicates the percentage of healthy people who are correctly identified as not having the condition.)
The value of stress tests has always been recognized as limited in assessing heart disease such as atherosclerosis, a condition which mainly produces wall thickening and enlargement of the arteries. This is because the stress test compares the patient’s coronary flow status before and after exercise and is suitable to detecting specific areas of ischemia and lumen narrowing, not a generalized arterial thickening.
According to American Heart Association data, about 65% of men and 47% of women have as their first symptom of cardiovascular disease a heart attack or sudden cardiac arrest. Stress tests, carried out shortly before these events, are not relevant to the prediction of infarction in the majority of individuals tested. Over the past two decades, better methods have been developed to identify atherosclerotic disease before it becomes symptomatic.
These detection methods have included either anatomical or physiological.
Examples of anatomical methods include:
- CT coronary calcium score
- Intima-media thickness (IMT)
- Intravascular ultrasound (IVUS)
- Examples of physiological methods include
- Lipoprotein analysis
The anatomic methods directly measure some aspects of the actual process of atherosclerosis itself and therefore offer the possibility of early diagnosis, but are often more expensive and may be invasive (in the case of IVUS, for example). The physiological methods are often less expensive and more secure, but are not able to quantify the current status of the disease or directly track progression.
The choice of pharmacologic stress agents used in the test depends on factors such as potential drug interactions with other treatments and concomitant diseases.
Pharmacologic agents such as Adenosine, Lexiscan (Regadenoson), or dipyridamole is generally used when a patient cannot achieve adequate work level with treadmill exercise, or has poorly controlled hypertension or left bundle branch block. However, an exercise stress test may provide more information about exercise tolerance than a pharmacologic stress test.
Commonly used agents include:
Vasodilators acting as adenosine receptor agonists, such as Adenosine itself, and Dipyridamole (brand name “Persantine”), which acts indirectly at the receptor.
Regadenoson (brand name “Lexiscan”), which acts specifically at the Adenosine A2A receptor, thus affecting the heart more than the lung.
Dobutamine. The effects of beta-agonists such as dobutamine can be reversed by administering beta-blockers such as propranolol.
Lexiscan (Regadenoson) or Dobutamine is often used in patients with severe reactive airway disease (Asthma or COPD) as adenosine and dipyridamole can cause acute exacerbation of these conditions. If the patient’s Asthma is treated with an inhaler then it should be used as a pre-treatment prior to the injection of the pharmacologic stress agent. In addition, if the patient is actively wheezing then the physician should determine the benefits versus the risk to the patient of performing a stress test especially outside of a hospital setting. Caffeine is usually held 24 hours prior to an adenosine stress test, as it is a competitive antagonist of the A2A adenosine receptor and can attenuate the vasodilatory effects of adenosine.
Aminophylline may be used to attenuate severe and/or persistent adverse reactions to Adenosine and Lexiscan.