| Cardiac Stress Test |
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A cardiac stress test is a Medical Test performed to evaluate the Arterial Blood Flow to the Myocardium (heart muscle) during exercise compared to blood flow while at rest. In other words, it is a determination of the Myocardial perfusion reserve. Stress tests also reveal overall physical fitness. Stress test abnormalities reflect inadequacies of blood flow to the left ventricular heart muscle more than other regions of the heart. Such stenoses are the usual basis for stable or reproducible exercise-related Angina (chest pain) and reflect advanced Arterial Disease , which is a precursor to Heart Attack . However, stress tests do not reveal smaller Atheromas , nor do they reveal Vulnerable Plaques , which are the cause of nearly all heart attacks. Recent (late 1990s) clinical studies have demonstrated that the vulnerable plaques, which produce most myocardial infarctions, arise from coronary arteries with too little stenosis to be detected by stress test methods. Generally, stress tests are effective at detecting myocardial ischemia due to medium or high grade epicardial (outer surface of the heart muscle) arterial stenosis and they are effective at detecting previous heart attacks (regions in which blood flow is low enough that cell death has occurred and well-perfused muscle has been replaced by fibrous tissue). TEST OVERVIEW The patient either walks on a treadmill or is given IV medication which simulates exercise while connected to an EKG machine, usually the standard 10 connections used to record a 12-lead EKG. Patient symptoms and blood pressure response are repeatedly checked. Using EKG and blood pressure monitoring alone, the test is variously called a cardiac stress test, exercise stress test, exercise treadmill test, stress test or exercise EKG test. If radioactive isotopes are also used (commonly, Technetium Tc99m Sestamibi and rarely, Thallium-201), then it is usually called a nuclear stress test. Nuclear stress tests are extremely accurate in detecting and localizing normal and diseased bloodflow to cardiac cells. PURPOSE The American Heart Association recommends EKG treadmill testing as the first choice for patients with medium risks of coronary heart disease based on the risk factors of smoking, family history of coronary stenosis, hypertension, diabetes and high cholesterol. Perfusion ( Cardiolite ®) stress testing is appropriate for select patients, especially those with an abnormal resting EKG. More severe stenosis (probably greater than 70% occlusion) can produce abnormalities in both EKG waveforms and cardiac wall motion at rest or under stress Echocardiographic testing. Such high grade Narrowing s are typically the primary culprit responsible for those angina episodes which reproducibly occur at a given level of exercise. However, most Heart Attacks result from rupture of Atheroma lesions associated with only mild Narrowing (20% on average by intravascular ultrasound ( IVUS ) clinical studies), thus stress tests do not work well for detecting the vulnerable plaques which are responsible for most heart attacks. Like all tests, stress testing has problems with both Falsely Positive and Falsely Negative results compared with other clinical tests. Angiogram and/or intracoronary ultrasound (preferably in a hospital capable of Percutaneous Coronary Intervention {Link without Title} with stenting) can provide even greater information, but at the risk of complications associated with cardiac catheterization. VARIATIONS Some patients with abnormal resting EKGs, or those who are unable to walk safely, can be "exercised" pharmacologically instead of walking on a treadmill. The patient will typically receive a pharmaceutical such as dypridamole or adenosine (both vasodilators) while a cardiologist or physician's assistant reviews the electocardiogram (EKG) tracing and checks blood pressure periodically. A radiotracer (typically Tc99m Sestamibi although Thallium is possible) is injected during the simulated exercise portion. After a suitable waiting period, pictures are taken with a gamma camera. The pictures are then compared with the patient's resting images in order to assess the status of the patient's coronary arteries. DIAGNOSTIC VALUE The American Heart Association journal, ''Circulation'', describes: Treadmill test: sensitivity of 68%, specificity of 70% Nuclear test: sensitivity of 81%, specificity of 99% Unfortunately, the value of such tests is limited, especially for people without symptoms. According to United States data from 2004, for about 65% of men and 47% of women, the first Symptom of cardiovascular disease is heart attack or Sudden Death (death within one hour of symptom onset). RISKS Absolute contraindications to cardiac stress testing include acute Myocardial Infarction {Link without Title} (heart attack) within 48 hrs, unstable angina not yet stabilized with medical therapy, uncontrolled arrhythmia which may have significant hemodynamic responses (for example ventricular tachycardia), symptomatic severe aortic stenosis, aortic dissection, pulmonary embolism, pericarditis. Major side effects from cardiac stress testing can include palpitation, chest pain, shortness of breath, headache, nausea, or fatigue. Adenosine and dipyridamole can cause mild drug-induced hypotension. However, hypotension caused by exercise stress testing or dobutamine is almost always abnormal and concerning for severe coronary disease. Stress tests using radiological agents confer a definite (albeit low) long term risk of cancer, but patients undergoing such examinations often receive little or inaccurate information about these risks. For comparison, the annual background radiation per annum a person receives is approximately 3 mSv. A chest xray is approximately 0.1 mSv. A coronary angiogram (cardiac catheterization) has an effective dose of 3-20 mSv (depending on operator skill, type of intervention, etc). A routine chest helical MDCT is around 5-7 mSv. A cardiac CT (with retrospective EKG gating) is around 8-13 mSv (Morin). A sestamibi scan is approximately 12 mSv. A thallium scan is approximately 25 mSv. A Thallium scan corresponds the dose of 250 chest x rays, or an extra cancer risk of about 1 in 16000 exposed patients (A. de González). The lifetime risk of fatal cancer development is 4%/Sv or 0.004%/mSv or about 0.1% for a thallium scan. Therefore, frequent usage of these tests has to balance the benefits against the risks of radiation. Another major risk of stress testing, whether by exercise or pharmacological agents, is the possibility of inducing an MI, especially in patients with severe multi-vessel coronary artery disease. This risk, however, is substantially lower than the risk (about 1%) of major complications (such as inducing a heart attack, stroke, peripheral artery clot and embolism) from cardiac catheterization. The choice of pharmacologic stress agent to be used (dobutamine, adenosine, dipyridamole) depends on factors such as concurrent medications and diseases. Dobutamine is usually used when a patient has asthma or severe COPD, takes the medication theophylline or has ingested coffee or chocolate (anything with caffeine), or has 2nd or 3rd degree AV block (a type of heart block). Adenosine or dipyridamole is generally used when a patient has poorly controlled hypertension, glaucoma, or has left bundle branch block (LBBB, another type of heart block). It is well known that patients with LBBB can have false positive septal ischemia if dobutamine is used as a pharmacologic agent in nuclear stress test. FURTHER RESEARCH Magnetic resonance imaging ( MRI ) has expanded the choice of modalities available for cardiac stress testing. MRI has superior spatial resolution (on the order of around 1.5 mm for cine imaging and 2.5 mm for perfusion imaging), and temporal resolution (around 40 ms for cine imaging), compared with that of a nuclear or PET stress test (spatial resolution of around 9mm for nuclear and 6mm for PET). The increased spatial resolution allows for more sensitive detection of ischemia, which initially starts at the thin subendocardial layer, due to stenotic epicardial supply vessels. First-pass stress perfusion cardiac MR imaging is performed using a rapid bolus injection of gadolinium based contrast and rapidly obtaining T1 weighted images of the myocardium at every R-R interval after pharmacologic stress induced with adenosine. The stress and resting first-pass perfusion MRI data can then be analyzed using a convolution model (such as the Marquard-Levenberg least-squares algorithm) to determine the quantitative global myocardial perfusion reserve (Michael Jerosch-Herold). Delayed hyper-enhancement imaging can be done after 10-15 minutes of contrast injection to evaluate for regions of infarction or fibrosis which has increased signal due to the slower washout of contrast from these areas (Thomson LE). Stress cardiac MRI perfusion testing thus is sensitive enough to detect subtle ischemia and myocardial infarctions even if they are limited only to the subendocardial level. The major problem again is that they still do not detect the "vulnerable plaques" which is the major cause of most heart attacks. Stress testing, even if done in time, will detect only some of these people before symptoms, debility or death. Stress testing methods, though more effective than a resting EKG, only detect medium to high grade flow limitations; this assuming the testing is fully and aggressively performed. However, most acute artery flow disrupting events leading to heart attacks are due to rupture of "vulnerable plaques". Most of the "vulnerable plaques" cause less than 40% Lumen narrowing, a degree of Stenosis too small for most stress testing methods to detect. Historically, through the mid-1980s, it was believed that detecting these high grade stenoses was the key to recognizing people who would have Heart Attacks in the future. However, there was also long-standing experience that some people could exercise all the way to maximum predicted heart rate, have no abnormal symptoms and completely normal stress test results, only to die of a massive Heart Attack within a few days to weeks. From the 1960s to 1990s, despite the success of stress testing identifying many who were at high risk for Heart Attack , its failure to correctly identify many others was a conundrum, discussed in medical circles but ''unexplained''. The high grade Stenoses which are detected by stress test methods are often, though not always, responsible for recurring symptoms of Angina . Cardiac stress tests do detect some individuals who already have with very advanced Coronary Arterial disease and stenosis, some of whom did not recognize that they had advanced disease. However, stress test results (especially stress perfusion cardiac MRI which can detected subtle diffuse subendocardial decreased perfusion due to microvascular disease) are also sometimes abnormal in some people who do not have high grade Narrowing s of their coronary arteries as visualized by Coronary Angiography , which provides more accurate information and partial visualization of the Coronary Artery Lumen s. This was long viewed as a False Positive result, with some of these individuals diagnosed as having Syndrome X , i.e meaning clear recurring signs of angina, though with smooth open Coronary Artery lumens on Coronary Angiography . The actual underlying issues responsible for this apparent conundrum are now better understood, see Atheroma and Microvascular Disease . In the 1950s, heart attacks were commonly attributed to Coronary Thrombosis , a clot closure of a Coronary Artery , based on post mortem examination findings. In the late 1950s to early 1960s, this concept became replaced by the concept of Stenosis based on the angiographic view of the lumens of the coronary arteries. In turn the angiographic view led to promotion of cardiac stress testing to detect Stenoses , i.e. the severe ones more commonly present in people experiencing recurrent Angina with physical exertion. By the early to mid-1990s, it became more widely recognized that rupture of more rapidly evolving and unstable Atheroma , ''hidden'' within the walls of the coronary arteries, called "vulnerable plaques", even though they often produce little or no Stenosis of the coronary lumen, is the primary event which produces most Heart Attack s; thus back to the Coronary Thrombosis view, though with more sophistication of understanding some of the complexities. Two clinical trials published in the late 1990's, focusing on the relation between plaque structure, lumen stenosis and Myocardial Infarction , in which each individuals coronary anatomy was tracked with both Angiography and IVUS found that 75% or greater stenotic areas were responsible for only about 14% of heart attacks. The typical heart attack occurred at an artery location with extensive, eccentric plaque within the wall but a luminal stenosis of only 20%. This finding added further evidence to the importance of the concept of vulnerable plaques. The detection of these vulnerable plaques using high resolution CT, MRI, IVUS, OCT (Optical Coherence Tomography), and molecular imaging is currently hotly researched. For CT, as of 2005, 64-slice multidetector machines are providing the best artery and lumen images, yet still do not clearly reveal which plaques are vulnerable. It is hope that perhaps with better resolution and ability to characterize the content of the plaques that an imaging modality may in the future be able to indicate which plaques is "vulnerable" as it is clear that detecting stenosis itself, however subtle, is not enough. Unfortunately, cardiac stress tests are only capable of detecting medium to high grade limitations of blood flow to the left ventricular heart muscle which may produce recurring Angina , not the Atheroma which produce Heart Attack s. Stress test methods do not evaluate blood flow to non-left-ventricle heart muscle. Thus stress test results are often Falsely Negative for many people, in terms of predicting who is at high risk for Myocardial Infarction due to atheroma or ruptured "vulnerable plaques". It has become clear that stress testing recognizes most people at risk for Heart Attack s too late, unfortunately only after the disease and symptoms of the disease have developed. By the time, a majority of people would already have at least medium stenosis of coronary vessels with development of atheroma or have already had heart attacks or died. It is hoped that research in higher resolution imaging techniques will allow for earlier detection and characterization of subtle atheroma and to initiate lifestyle changes and optimal medical therapy in "vulnerable patients" before they develop symptoms. SEE ALSO REFERENCES Circulation, Fletcher et al. AHA Exercise Standards for Testing. 201:104:1694. National Guideline Clearinghouse. Cardiac Stress Test Supplement. ICSI:2003Nov.26p.87. 1. 2. 3. 4. EXTERNAL LINK Stress Test information from Children's Hospital Heart Center, Seattle. |
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