A Cardiolite stress test is a specific type of Myocardial Perfusion Imaging (MPI), a non-invasive diagnostic procedure used to evaluate blood flow to the heart muscle. The test utilizes a small amount of a radioactive tracing agent, often called Cardiolite or its generic name, Technetium Tc99m Sestamibi. This tracer is injected into the bloodstream, traveling through the coronary arteries and being absorbed by heart muscle cells in proportion to the blood flow they receive. By taking images of the heart after the tracer injection, both at rest and during induced stress, healthcare providers can visualize areas that may not be receiving adequate blood supply.
Diagnostic Goals of Myocardial Perfusion Imaging
A primary reason a physician orders a Cardiolite stress test is to detect the presence and extent of Coronary Artery Disease (CAD). CAD involves the narrowing or blockage of the coronary arteries, which can restrict the supply of oxygen-rich blood to the myocardium, or heart muscle. The test provides a functional assessment, showing how significant any suspected blockages are by observing their effect on blood flow.
The procedure is highly effective in assessing myocardial perfusion, the process of blood flowing through the heart tissue. By comparing the rest and stress images, the test distinguishes between two types of abnormalities: ischemia and infarction. Ischemia refers to a temporary lack of sufficient blood flow that can be reversed with treatment. Infarction indicates an area of permanent heart muscle damage, typically from a previous heart attack.
The results are also used for risk stratification in patients with known heart issues. Identifying the amount of heart muscle affected by reduced blood flow helps predict the risk of future adverse cardiac events. The information gathered assists in guiding management decisions, such as whether a patient might benefit more from medication or an interventional procedure.
Step-by-Step: The Stress and Imaging Procedure
The Cardiolite stress test is typically conducted in two separate phases, sometimes split across two days. The process begins with establishing an intravenous (IV) line for tracer injection and monitoring. During the first phase, the patient is at rest, and an initial, smaller dose of the Technetium Tc99m Sestamibi tracer is injected.
After a waiting period (usually 20 to 60 minutes) allowing the tracer to circulate and be absorbed, the first set of images is acquired. The patient lies still while a specialized Single-Photon Emission Computed Tomography (SPECT) camera rotates around the chest to detect the radiation. These rest images establish a baseline of blood flow under normal conditions.
The second phase involves inducing stress in the heart, achieved through either physical exercise or pharmacological agents. Exercise stress, typically on a treadmill, is preferred for patients who can reach a high enough heart rate to challenge the coronary arteries. During exercise, the patient’s heart rate, blood pressure, and electrocardiogram (ECG) are continuously monitored.
For patients unable to exercise effectively due to physical limitations, a pharmacological stress agent simulates the effects of exertion. Medications like Regadenoson (Lexiscan), Dipyridamole, or Adenosine cause the coronary arteries to dilate, mimicking increased blood flow demand. Dobutamine is another agent that can be used to increase heart rate and contractility directly.
At the point of peak stress, a second, higher dose of the Cardiolite tracer is injected into the IV line. This tracer is rapidly distributed, reflecting the blood flow at its maximum capacity. Immediately following the stress component, the patient is moved back to the SPECT camera for the second set of images. Comparing the blood flow patterns captured in the rest and stress images reveals potential flow abnormalities caused by blockages.
Preparing for the Test and Understanding the Findings
Preparing correctly for the test is important for obtaining accurate results. Patients are usually instructed to fast for at least four hours before the test, though clear liquids are often permitted. A crucial preparation step is the complete avoidance of caffeine for 24 hours prior to the procedure, as it can interfere with pharmacological stress agents or artificially alter the heart rate.
Patients are advised to wear loose-fitting, comfortable clothing and supportive walking shoes, especially if exercise is anticipated. Certain heart medications, such as beta-blockers or nitrates, may need to be temporarily withheld; patients must consult their physician for specific instructions. Smoking should also be avoided on the day of the test because it can negatively impact image quality.
The interpretation of the images focuses on the distribution of the radiotracer within the heart muscle. A normal result shows an even, uniform distribution of the tracer across all segments of the heart in both the rest and stress images, indicating healthy blood flow.
An abnormal finding can be categorized as either a reversible defect or a fixed defect. A reversible defect appears as an area of reduced tracer uptake on the stress images, which then fills in and appears normal on the rest images. This pattern is highly suggestive of flow-limiting Coronary Artery Disease (CAD). A fixed defect shows reduced or absent tracer uptake in the same segment of the heart on both the rest and stress images. This finding typically signifies a scar or area of dead tissue from a prior heart attack.