Cardiac stress test

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A Cardiac Stress Test is a medical test performed to evaluate arterial blood flow to (and indirectly the amount of oxygen that will reach) the myocardium (heart muscle) during physical exercise, compared to blood flow while at rest. As an exercise test, the results can also reflect overall physical fitness. These tests do not assess Emotional stress, or other connotations of the term stress.

Stress test abnormalities (i.e. if it is 'positive') are an indication of marked imbalances of relative blood flow to different portions of the Left Ventricular muscle tissue of the heart. This is important because the Left Ventricle of the heart performs the greatest amount of work involved in pumping blood around the body. Blood flow imbalances within the heart muscle of the other three heart chambers are not detected this way.

Usually, only High Grade Stenoses (i.e. severe narrowing) of the larger coronary (surface of the heart) arteries can be detected. Severe stenoses, e.g. those greater than 75% occlusion, are usually the result of advanced Atherosclerosis (i.e. hardening of the arteries, or arterial disease) and these High Grade Stenoses are the usual cause for both 'stable' or reproducible exercise-related Angina (chest pain) and for 'positive' stress tests. Less severe stenoses are automatically compensated for, by vasodilation (widening or relaxing) of the ventricular arterioles during exercise, and do not usually produce enough of an imbalance of relative blood flow, to be detectable by stress test methods.

What most laypersons do not realize, is that stress tests do not detect Atheromata (plaques of Cholesterol and other fats, etc) present throughout the heart or other body arteries, nor do they reveal the Vulnerable Plaques, which are the cause of most Heart Attacks or Myocardial Infarctions. Recent (late 1990's) clinical studies have shown, that the vulnerable plaques which produce most myocardial infarctions are commonly present within many regions of the coronary (surface of the heart) arteries, yet are typically relatively flat, i.e. not protruding into the artery lumen sufficiently to produce enough stenosis (usually less than 50%, average 20% by some IVUS studies) to be detectable by stress test methods. Thus, over the last 20 years or so, newer approaches in research, have increasingly focused on IMT to detect plaque at earlier stages of progression, before the artery lumen becomes severely stenosed, or compromised.

So the major limitation of the stress test approach, is that it needs a high grade stenosis as an indicator of heart attack risk. High grade stenosis, while being a good indicator of advanced arterial disease, is not the major cause for the majority of heart attacks. This issue is also reflected in the results of the COURAGE trial, which demonstrated that 'intensive pharmacologic therapy and lifestyle intervention', which they called 'Optimal Medical Therapy' (not opening stenoses with Angioplasty, per-se) produced the more effective results in survival and quality of life.

Test overview The patient either walks on a treadmill, or is given IV medication which simulates exercise, while connected to an ECG machine, usually the standard 10 connections, used to record a 12-lead ECG. The level of exercise is increased by 3 minute stages, of progressively increased Grade (% incline) and Speed (mph, km/h, etc). The patient's symptoms, and blood pressure response are repeatedly checked. When using ECG and Blood Pressure monitoring alone, the test is variously called a Cardiac Stress test, Exercise Stress test, Exercise Treadmill test, Exercise Tolerance test, Stress Test or Exercise ECG test.

If Radioactive Isotopes are also used (commonly, Technetium Tc99m Sestamibi and rarely, Thallium-201), then it is usually called a Nuclear Stress Test. Given the ability to visualize the relative amounts of radioisotope within different regions of the heart muscle, nuclear stress tests are more accurate in detecting regional areas of decreased blood flow to myocardial (heart muscle) cells. However, diffuse global ischemia (decreased blood flow that is evenly spread out) may still not be recognized, because absolute blood flow is not quantitatively measured, only regional comparative variations.

Risks Absolute contraindications to cardiac stress testing include acute myocardial infarction (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 of major complications (such as inducing a heart attack, stroke, peripheral artery clot and embolism) from cardiac catheterization (about 1%).

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. The adverse effects associated with the use of pharmacologic stress test agents can be reversed upon completion of the test. For drugs that promote adenosine (including dipyrimadole or adenosine itself), adenosine antagonists that constrict blood vessels such as theophylline or caffeine can be given. The adverse effects of beta-agonists like dobutamine can be reversed with the administration of a beta-blocking agent such as propranolol.

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Nuclear Medicine

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