How does it work?
During exercise, healthy coronary arteries dilate (open) more than a diseased / blocked artery, causing more blood supply to heart muscle supplied by the normal artery. Contrary, blood flow is reduced in the narrowed / diseased arteries. This reduced flow causes reduced oxygen supply to the involved heart muscle, producing symptoms like chest discomfort or unexpected shortness of breath and abnormalities on the ECG.
When a "perfusion radiotracer" is injected intravenously, it is taken up by the heart muscle in relation to the blood flow. Therefore, areas of the heart with adequate blood flow quickly picks up the tracer, while those with reduced blood flow have slow and reduced tracer uptake or not at all. A scanning camera takes multiple three dimensional images of tracer uptake by the heart.
Further analysis of these images helps to identify the location, severity and extent of reduced blood flow to the heart muscle (ischaemia).
How is a Nuclear Cardiac Stress Test performed?
The test is actually divided into three parts:
Imaging after stress test
Imaging at rest.
Cardiac stress testing is normally performed using a treadmill exercise test and is the test of choice. It demonstrates physiologic effects of exercise on the blood pressure and heart rate. It also helps give the physician an idea about the patient's exercise tolerance. If the patient is unable to exercise secondary to physical limitations such as severe arthritis, artificial limbs, generalized weakness, paralysis, unsteady gait, etc., the physician may choose a pharmacological form of test, by slowly giving a medication intravenously. The commonly used pharmacological agents are dipyridamole, dobutamine and Adenosine. If possible, some form of pharmacological stress testing may also be combined with a brief period of slow treadmill exercise.
The resting heart rate and blood pressure are recorded. Sticky electrodes are attached to the chest and connected to the ECG cables, of the Stress test machine. A 12-lead resting ECG is recorded on paper. The treadmill is then started at a relatively slow speed. The treadmill speed and it's slope or inclination are increased every three minutes according to a preprogrammed protocol. Bruce is the commonest protocol at our centre.
Each three minute interval is known as a Stage (Stage 1, Stage 2, Stage 3, etc). The patient's blood pressure is recorded during the second minute of each Stage. The ECG is constantly displayed on the monitor. It is also recorded on paper at end of each stage. The physician pays particular attention to the heart rate, blood pressure, changes in the ECG, irregular heart rhythm, and the patient's appearance and symptoms. The test may be stopped prior to achievement of the target heart rate if the patient develops significant chest discomfort, shortness of breath, dizziness, unsteady gait, etc., or alarming changes are noted on the ECG or heart rhythm. It may also be stopped if the blood pressure (BP) is beyond acceptable limits. Please let the physician and staff know if you feel unwell.
In case your physician decides that a pharmacological stress is more appropriate, you would receive intravenous infusion of the relevant drug.
Two sets of isotope images are obtained. One at rest, and one following exercise.
Depending upon the isotope used and the protocol for resting images may be obtained before the stress test, or a few hours later.
The scanning camera rotates around the patient's chest, stopping to take individual pictures. The patient needs to lay flat and still during the scanning period which takes approximately 11 to 20 minutes. The pictures or images are stored into the a particular patient, the t, or a few hours later. computer, which reconstructs them as "slices" of a three dimensional heart. Your physician has an opportunity to view a three dimensional representation of your heart and by comparing the physician can identify disease and assess its severity.
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