A nuclear stress test, also known as a Myocardial Perfusion Scan (MPI), is a diagnostic procedure that uses a radioactive tracer and specialized imaging equipment to visualize blood flow to the heart muscle. This non-invasive test helps medical professionals determine if the heart receives adequate blood supply at rest and during periods of increased demand, often to identify potential blockages in the coronary arteries or existing heart damage. The entire process involves two distinct imaging phases—one at rest and one under stress—to compare the heart’s function in different states.
Preparing for the Nuclear Stress Test
Preparation for a nuclear stress test involves strict adherence to dietary and medication instructions to ensure accurate results. Patients are typically instructed to fast, avoiding all food for about four to six hours prior to the test, though water is permitted. This fasting helps minimize digestive activity that could interfere with the quality of the heart images.
A strict restriction is the avoidance of caffeine, which must be stopped for 12 to 24 hours before the test, including decaffeinated products and certain over-the-counter pain medications. Caffeine and other stimulants can interfere with pharmacological stress agents, making it difficult to properly stress the heart. Patients must also consult their healthcare provider about temporarily stopping certain heart medications, such as beta-blockers, nitrates, and vasodilators (e.g., dipyridamole or adenosine-based drugs), as these can affect the test’s outcome. The total procedure often requires two to four hours, so patients should plan for a lengthy stay.
The Initial Resting Scan
The procedure begins with the placement of an intravenous (IV) line, typically in the arm, used for administering the radiotracer and any stress-inducing medications. Electrodes are placed on the chest, arms, and legs and connected to an electrocardiogram (EKG) machine to continuously monitor the heart’s electrical activity. Next, a small dose of a radioactive tracer, such as a Technetium or Thallium compound, is injected through the IV line.
The radiotracer travels through the bloodstream and is absorbed by the heart muscle cells in proportion to the blood flow they receive at rest. After a waiting period (15 to 60 minutes) to allow the tracer to fully circulate, the patient lies on a table for the first set of images. A specialized gamma camera or SPECT (Single-Photon Emission Computed Tomography) scanner rotates around the chest, capturing the emitted gamma rays to create a detailed three-dimensional image of the heart at its baseline. This initial scan establishes the resting blood flow pattern for comparison with the stress phase images.
Inducing Maximum Heart Stress
The test requires inducing maximum heart stress to compare blood flow between resting and active states. For patients who are physically able, the preferred method is exercise stress, typically using a treadmill or stationary bicycle. The patient walks while the speed and incline are gradually increased to bring the heart rate up toward a target level.
During the exercise, medical staff monitor the patient’s EKG, heart rate, and blood pressure to ensure safety and determine when peak exertion is achieved. Just before the patient stops exercising, a second dose of the radiotracer is injected into the IV line. This timing is crucial because the tracer must circulate and be taken up by the heart muscle when the coronary arteries are fully dilated from the physical activity.
If a patient cannot exercise due to physical limitations, a pharmacological (chemical) stress test is performed instead. Medications are administered through the IV to artificially simulate the effects of exercise by causing the coronary arteries to widen and increasing blood flow. Common pharmacological agents include vasodilators like adenosine, dipyridamole, or regadenoson.
Patients undergoing pharmacological stress may experience transient side effects such as flushing, mild chest discomfort, or a brief feeling of shortness of breath, which typically resolve quickly. The second dose of the radiotracer is injected during the peak effect of the medication, ensuring the heart is under maximum simulated stress for the imaging phase.
Post-Stress Imaging and Recovery
Immediately following the injection of the second radiotracer dose at peak stress, the patient moves to the imaging table for the second set of scans. This imaging must occur promptly, typically within 10 to 20 minutes, while the effects of the stress are still apparent. The gamma camera captures the tracer’s distribution in the heart muscle under stress, providing a picture of blood flow when the heart is working its hardest.
The medical team compares the resting scan and the stress scan to identify differences in tracer uptake. Good uptake in both scans indicates healthy blood flow. Reduced uptake only in the stress image suggests a flow-limiting blockage in a coronary artery.
After the final images are acquired, the patient is monitored until their heart rate and blood pressure return to baseline, a process that usually takes 10 to 15 minutes. Patients are encouraged to drink plenty of fluids to help eliminate the residual radioactive tracer and can typically resume normal activity immediately after leaving the facility.