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Choosing the Right Stress Test

Stress testing is an excellent tool used to assess patients for a host of cardiovascular conditions. Such testing is widely available, safe and non-invasive. It allows assessment of exercise capacity, stress-induced arrhythmias, hemodynamic significance of valve disorders and correlation of symptoms with myocardial ischemia. Combined with imaging modalities, stress tests can provide the incremental diagnostic and prognostic information for patients with an intermediate probability of CAD and can identify individuals with a high risk for severe (left main or 3-vessel) CAD. Many patients can be adequately risk stratified for the presence of severe coronary artery disease with a simple exercise treadmill test while others will require a more sophisticated stress test combined with imaging techniques.

Doctor going over charts

Who should undergo a simple exercise treadmill test (ETT) without imaging?
Used in conjunction with other clinical data, ETTs are very effective at prognosticating a patient risk of death or non-fatal myocardial infarction. Patients appropriate for this test should be able to exercise safely on a treadmill to reach their goal heart rate [85% x (220 - age)]. Further, it is important to have a normal resting electrocardiogram. The utility of the test is greatly diminished and not recommended with the following conditions:

  • Left bundle branch block on ECG
  • Use of Digoxin
  • Left ventricular hypertrophy
  • Ventricular paced rhythm
  • Pre-excitation - WPW
  • Widespread ST abnormalities


My patient cannot exercise. How should they be stressed?

Patients who are unable to exercise due to peripheral vascular disease, intrinsic lung disease, poor exercise tolerance, arthritis or other reasons will need to be stressed by pharmacologic means. The two main agents used are dipyridamole (Persantine) and dobutamine.

  • Persantine is a vasodilating agent generally used with nuclear imaging. Diseased coronary arteries will not vasodilate in the same fashion as healthy arterial tissue, thereby providing a flow pattern that differs between stress and rest. It is sometimes combined with low-level exercise protocols to reduce the non-cardiac side effects and improve image quality. The only contra-indication would be poorly controlled COPD (active wheezing).
  • Dobutamine, a beta-adrenergic agonist, increases heart rate and contractility in a dose-related fashion when infused intravenously and simulates exercise within the coronary arterial vasculature. The agent is used in both nuclear and echocardiographic imaging.


How does nuclear imaging compare with an echocardiography?

Both nuclear and echocardiographic imaging have significantly advanced our diagnostic capabilities in the field of cardiology. Each of theses tests have inherent pros and cons that are important to recognize when ordering.

Nuclear imaging uses radiotracers (thallium-201 or technetium-99m sestamibi [Cardiolite]) to evaluate myocardial perfusion and function, and has greatly advanced the ability to detect and assess the extent of coronary ischemia. To detect ischemia or infarction, a radioisotope is injected at rest and after stress to produce images of myocardial regional uptake, which is proportional to regional blood flow. Normally, with maximal exercise or pharmacologic stress, myocardial blood flow is greatly increased above the resting condition. If a fixed coronary stenosis is present, myocardial perfusion in the territory supplied by the stenosis cannot be increased, which will create a flow differential and uneven distribution of the tracer.

Pros

  • Stress myocardial perfusion imaging has a sensitivity of >90% for detecting patients at risk of cardiac death or MI. 
  • Nuclear perfusion studies can also provide a measure of left ventricular function and wall motion.
  • Rest nuclear imaging can be helpful in assessing myocardial viability.


Cons

  • While accurate images can be obtained in most patients, artifacts due to breast and diaphragmatic tissue attenuation, and LBBB can lead to false-positive interpretation.
  • Lack of hemodynamic data
  • Heart valve structure and function is not accessed


Echocardiography visualizes the heart directly in real time using ultrasound, providing convenient assessment of the cardiac chambers, myocardium, valves, pericardium, and great vessels. Exercise or Dobutamine Stress Echocardiography (SE) can be used to detect the presence, location, and severity of inducible myocardial ischemia as well as for risk stratification and prognosis. During stress-induced ischemia, decrements in contractile function are directly related to decreases in regional subendocardial blood flow. Normally, with exercise, or dobutamine infusion, left ventricular wall motion becomes hyperdynamic. The hallmark of ischemia is the development with stress of new, or the worsening of preexisting, wall motion abnormalities which can precede ischemic EKG changes. The lack of improvement with stress in an already hypokinetic segment indicates infarction. Stress-induced left ventricular cavity enlargement, systolic dysfunction, or mitral regurgitation may also suggest CAD. 

Pros
  • Helpful to evaluate non-diagnostic regular treadmills or nuclear stress tests with possible artifactual findings (tissue attenuation or LBBB).
  • Highly specific – Less false positive test
  • Good assessment of valvular function with stress strengths

Cons
    • Poor viability data
    • Less sensitive then nuclear imaging – more false negatives
    • Technically difficult in obese patients


    My patient has an abnormal resting ECG. Now which test do I order?

    Patients with abnormal resting ECGs usually require nuclear or echocardiographic imaging to improve both the sensitivity and specificity of the study. If the patient has no limitations to exercise, we recommend a treadmill test as the stress portion of the study. Even in patients with abnormal baseline ECGs, exercise provides useful prognostic information. The only exception is the patient with a LBBB undergoing a nuclear study. These patients should receive persantine to minimize the risk of a false positive test. With respect to imaging, as nuclear scans are more sensitive, we prefer to start with nuclear imaging to increase sensitivity of detecting CAD and minimize the risk of a false negative test. As each of these tests provides different information, it is important to choose the one that answers the clinical question posed. We are always available to answer any of your questions.

    Southcoast Physicians Group An affiliate of Southcoast Physicians Group.