NUCLEAR AND ECHO STRESS TESTING
As discussed by Dr. Frederic B. Gerewitz, M.D.
Treadmill stress testing, in which a patient's heart function is evaluated while exercising on a treadmill, has been in use for about 50 years, but its use first with nuclear imaging or later with echocardiography has made stress testing a far more effective diagnostic and prognostic tool.
Nuclear stress testing was well established when echo stress testing emerged and, while initially the two modalities were competitive, they are now two good alternatives, according to Dr. Fred Gerewitz of Associated Cardiovascular Consultants. "Head-to-head studies have found similar accuracy," Dr. Gerewitz said.
Both modalities have advantages and disadvantages. Stress echo takes less time, is less costly and allows the cardiologist to assess valvular function. "On the downside," Dr. Gerewitz said, "10 to 15% of patients do not image well." In that category Dr. Gerewitz included patients with:
- Chest deformities
Additionally, those with heart muscle damage are more difficult to assess by stress echo.
"Those with normal body build and no cardiovascular disease are good candidates."
With nuclear stress testing, which uses Thallium and/or Cardiolite/Myoview, Dr. Gerewitz continued, it doesn't matter if a patient has emphysema, and chest deformities and obesity pose less of a problem. "But nuclear doesn't look at valvular function. Stress echo does."
For many patients, treadmill testing is not an option, and that is where pharmacologic stress testing comes in. "Pharmacologic testing uses vasodilators such as adenosine or persantine, or inotropes such as dobutamine."
Dobutamine is used in echo stress testing, but vasodilators are not. All the drugs are effective in nuclear stress testing, but the vasodilators are diagnostically superior for use with nuclear imaging, Dr. Gerewitz explained. "Dobutamine and the vasodilators work on different principles." Dobutamine increases contractility and heart rate, while vasodilators create "flow imbalances" based on "relative coronary flow reserves." These differences may be the reason why echo stress is not suitable for use with vasodilators. "It may not be suitable for picking up discrepancies in coronary flow reserves."
In patients with severe asthma, dobutamine is used in both nuclear and echo testing because vasodilators may activate asthma. While adenosine can make a slow heart rate worse, dobutamine can worsen a heart rate that is high at rest. Therefore, dobutamine is generally avoided in patients with atrial fibrillation or sinus tachycardia.
Dr. Gerewitz said that either echo stress testing or nuclear stress testing can be used for both diagnostic and prognostic purposes -- that is, they can determine the presence of disease on the one hand, and, on the other, determine how much at risk a patient may be for an adverse cardiac event. Between 35 and 40 percent of the nuclear stress tests administered by ACC physicians are pharmacologic. Also, using the pharmacologic approach, viability testing can determine if heart muscle can be rejuvenated and start working normally. (Occasionally viability testing is done in conjunction with conventional treadmill testing.)
Stress testing and medications
Patients scheduled for stress testing may or may not have to continue with their regular medications. Dr. Gerewitz said if a patient on a beta blocker, which blunts the sensitivity of the test, is undergoing a stress test for diagnostic purposes, the beta blocker should be held prior to the test. However, if the test is for prognostic purposes, then the beta blocker can be used since the purpose is to determine how the patient feels and how he or she is doing on the treatment regimen.
Dr. Gerewitz elaborated on the role of prognostic stress testing at ACC. "We do a lot of that. Family practitioners more often use stress tests for diagnostic purposes, but, in our patient population, we frequently already know they have coronary artery disease. We want to know how well they are functioning."
Dr. Gerewitz emphasized that high-risk patients who undergo stress testing represent only one subset of patients at risk. "For a larger group of patients, those with blockages of 40 to 50%, stress tests are not good at picking those up." He said plaques in those individuals may be unstable and can cause sudden heart attacks. "The hunt is on for who is at risk. Fifty-percent of heart attack victims never make it to the hospital."
Looking to the future of stress testing, however, Dr. Gerewitz sees great promise. He described pharmacologic stress testing using Magnetic Resonance Imaging (MRI). "It's a totally new cardiac imaging modality not yet readily available and with not many trained physicians." Dr. Gerewitz said large human trials have not been completed, but animal studies suggest that the approach "holds promise for lots of different things," among them the diagnosis of congenital heart conditions and myocarditis, and coronary plaque imaging. CT angiography is another promising technique.
Physicians can also assure their patients that stress testing is safe. "Only five in 100,000 tests have a bad outcome," Dr. Gerewitz said.
- In properly selected patients nuclear stress testing and echo stress testing have been found to be equally accurate.
- Stress testing may be conducted with patients either continuing or holding regular medications, depending upon the goal of the testing.
- Only 5 in 100,000 stress tests (0.005%) have an adverse outcome.
Dr. Frederic B. Gerewitz
received his medical degree from SUNY-Downstate Medical Center in Brooklyn, N.Y. He completed a fellowship in Cardiology at the Philadelphia Heart Institute, Presbyterian Medical Center, Philadelphia, PA. A fellow of the American College of Cardiology and a member of the American College of Physicians and the American Medical Association, he has co-authored several articles published in medical and scientific journals.
A take-home message about... Nuclear and echo stress testing
- Stress testing is valuable not only for diagnosis but also for monitoring patients.
- Most individuals can undergo some form of stress testing.
- MRI and CT hold the potential to give insights into new groups of cardiac patients.
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