Is Your Stress Test the Wrong Test?
ardiovascular disease remains the leading cause of death. According to the American Heart Association, for about 65% of men and 47% of women who are developing coronary artery disease the first symptom of their disease is a heart attack or sudden death. Most physicians still rely on a Cardiac Stress Test to find evidence of this silent killer before it strikes. For the patient, a “normal” stress test is usually taken as a reassuring sign that all is well. Yet the evidence is clear that in the vast majority of those with coronary disease a stress test is very likely to be normal right up until suddent death or a heart attack strikes.
A cardiac stress test can be a very useful tool when performed properly and in the right circumstances. But, as a screening test for detecting developing plaque in a coronary artery its usefulness is severely limited. To understand why this is so it is important to understand what a stress test is designed to find versus how heart attacks usually happen.
The major limitation of all cardiac stress tests is that it requires a high level of blockage in one or more coronary arteries to produce an abnormal result. The underlying principle of a stress test is very simple: increase the heart rate, either with exercise or drugs, and look for evidence that a portion of the left ventricle muscle does not get enough additional blood flow to match the increase in demand as the heart rate rises. This will usually require at least a 65% narrowing in one of the arteries on the surface of the heart before the amount of blood flow through that artery can not increase enough to meet the demand and a portion of the muscle begins to starve.
At that point the treatment options are to re-open the artery with a stent or bypass surgery. Either way, the treatment is invasive and the patient has already been at very high risk for a heart attack for many years.
This limitation in stress testing does not depend on what type of stress test is done. An Exercise Treadmill Test looks for changes in the ECG as the heart rate rises. A Stress-Echo Test uses ultrasound to image the heart after exercise to detect portions of the left ventricle that do not move as well as expected because of lack of blood flow. A Nuclear Stress Test uses an injection of Thallium or Technetium radio-isotopes and a gamma-ray camera to image the distribution of blood flow in the heart muscle before and after exercise. All of these techniques still depend upon there being a limitation of blood flow to a portion of the left ventricle. This simply does not occur unless there is a high-grade blockage in the artery.
The nuclear stress test is generally considered to be the most accurate type of stress test. It has a sensitivity of about 81%, meaning that it will miss about 19% of high-grade blockages. (As noted above, it will miss almost all of the blockages that narrow an artery less than about 65%) It has a specificity of about 90%, meaning that 10% of the abnormal nuclear stress tests will turn out not have a significant blockage.
A nuclear stress test also involves substantial radiation exposure. Many physicians remain unaware that a nuclear stress test is likely to expose their patients to more radiation than any other cardiac test, including CT scans. A nuclear stress test done with thallium will average 25 mSv of radiation. A test done with technetium-sestamibi will be around 12 mSv. For comparison, the average person receives about 3 mSv / yr of radiation from natural sources. The lifetime risk of cancer is about 0.004% per mSv or about 0.1% per thallium scan. While this risk is low for any single nuclear stress test, many patients undergo numerous stress tests over a period of years. Frequent use of nuclear stress tests has to balance the benefits against the risks of radiation.
A stress test is a useful tool in certain circumstances such as deciding whether an episode of chest pain is from blocked artery. Some patients, particularly diabetics, may not experience chest pain even when an artery is blocked and a stress test can be helpful in those patients when a blockage is suspected in the absence of symptoms. However, by the time your stress test turns positive, the opportunity for preventing severe coronary disease is long past. A stress test is not suitable for early detection and treatment or screening.
The stress test’s poor ability to detect impending problems stems from the fact that most heart attacks happen in arteries that are less than 40% narrowed. This is too little of a blockage to cause the reduction in maximal coronary blood flow that is required for detection by stress testing. Research in the 1990’s showed that the primary event in a heart attack is rupture of a plaque in the artery wall. This causes a blood clot to form at the site where the surface of the plaque has split open. It is the sudden formation of a clot, not the plaque itself, which stops blood flow through the artery. The typical heart attack occurs in an artery where there is extensive plaque within the wall of the artery but a narrowing of the lumen of only 20%.
More effective tools have been developed to detect the presence of plaque many years before a stress test is likely to be abnormal. High definition CT scanners can painlessly and non-invasively look directly at the arteries to see the presence of calcium deposits in the artery wall that are associated with developing plaque. The amount of calcium is an excellent indicator of the severity of heart attack risk. CT Angiography, a scan that uses an intravenous dye injection and even higher resolution scanning, can measure the degree of narrowing in an artery. CT Angiography can be very helpful in deciding whether an abnormal stress test result is a false-positive or truly indicates a narrowing that needs further treatment.
A CT coronary calcium scan will generally expose a patient to only 0.5-2.0 mSv and a CT Angiography is typically 1-8 mSv. These doses are significantly lower than those associated with nuclear stress tests.
It is clear that stress testing detects only late stage coronary artery disease when the opportunity for prevention has already been lost. While there is a role for stress tests in deciding on the significance of symptoms or assessing the status of a patient with known coronary disease, it is not a screening test for detecting early plaque. Unfortunately, a normal stress test often causes patients with substantial cardiac risk to forgo treatment and have a false sense of security. Utilizing newer techniques to directly image the arteries in patients at intermediate or high risk can detect early stage disease, when treatment is easier and more effective, at lower cost and less risk than stress testing.