A nuclear stress test takes images of blood flow through your heart at rest and under stress, then compares the two to reveal whether any areas of the heart muscle aren’t getting enough blood. The entire visit typically takes three to four hours, though much of that time is spent waiting for a small amount of radioactive tracer to be absorbed by your heart cells between imaging sessions. Here’s what to expect at each stage.
How to Prepare Before the Test
You’ll be asked to avoid caffeine for 12 to 24 hours beforehand. This includes coffee, tea, chocolate, energy drinks, and some medications. Caffeine is structurally similar to the drugs used during the test and acts as a direct competitor, blocking them from working properly. If caffeine is still in your system, it can mask areas of reduced blood flow and make the test less accurate.
Most facilities also ask you to skip food for several hours before the test, wear comfortable clothes and walking shoes, and bring a list of your current medications. Some heart or blood pressure medications may need to be paused beforehand, but your doctor’s office will give you specific instructions on that.
The Resting Images Come First
When you arrive, a technologist places an IV line in your arm and injects a small dose of a radioactive tracer. The tracer travels through your bloodstream and gets taken up by heart muscle cells that have good blood flow and active metabolism. Cells that are healthy and well-supplied with blood absorb more tracer; cells that are damaged or starved of blood absorb less.
After the injection, you wait roughly 30 to 60 minutes for the tracer to settle into your heart tissue. Then you lie on a table while a specialized camera rotates around your chest, detecting the low-level energy the tracer emits and building a detailed map of blood flow through every region of your heart. These resting images serve as the baseline. You need to stay still during imaging, which takes about 15 to 20 minutes.
The Stress Portion: Exercise or Medication
The second half of the test puts your heart under stress so the imaging team can see how blood flow changes when your heart is working hard. For most people, this means walking on a treadmill that gradually increases in speed and incline, or pedaling a stationary bike. You’ll be hooked up to an ECG monitor throughout, and a technologist will be checking your heart rate and blood pressure at regular intervals. The goal is to push your heart rate to a target based on your age.
If you can’t exercise due to joint problems, mobility issues, or lung disease, you’ll receive a medication through your IV instead. The most commonly used drug today is regadenoson, which selectively widens coronary arteries and mimics the blood flow changes that exercise would cause. It doesn’t make your heart pound the way a treadmill does. Instead, it works by increasing blood flow through healthy arteries while areas fed by narrowed arteries can’t respond as well, creating a visible difference on the images. Some people feel flushed, short of breath, or mildly headachy for a minute or two after the injection, but these effects pass quickly.
At peak stress, whether from exercise or medication, the technologist injects a second dose of tracer through your IV. This captures the “stressed” pattern of blood flow in your heart.
The Second Round of Images
After another waiting period for the tracer to distribute, you return to the camera table for a second set of images. These post-stress images are compared side by side with the resting set. The entire comparison is what gives the test its diagnostic power: it shows not just whether there’s a problem, but what kind of problem it is.
What the Results Mean
Your cardiologist looks at the two sets of images and categorizes any abnormalities into one of two main patterns.
A reversible defect means an area of the heart shows reduced blood flow during stress but looks normal at rest. This indicates living, viable heart tissue that isn’t getting enough blood when demand increases, the hallmark of a narrowed coronary artery. These areas are the ones most likely to benefit from treatment, whether through medication, stenting, or bypass surgery.
A fixed defect means the same area shows reduced blood flow on both the stress and resting images. This pattern typically indicates scar tissue from a prior heart attack. Because the muscle in that region is no longer alive, restoring blood flow to it generally won’t improve its function.
Normal results, where tracer uptake is even across the heart in both sets of images, mean blood flow is adequate at rest and under stress. The test catches coronary artery disease with a sensitivity of roughly 80 to 90 percent and a specificity of about 65 to 80 percent, meaning it’s quite good at detecting disease when it’s present and reasonably good at ruling it out.
Radiation Exposure
The tracer does expose you to a small amount of radiation. A standard nuclear stress test delivers a median effective dose of about 11 to 12 millisieverts, which is roughly equivalent to three to four years of natural background radiation compressed into one day. For context, a standard chest X-ray delivers about 0.02 millisieverts. The exposure is meaningful enough that the test isn’t ordered casually, but it’s well within the range considered acceptable when there’s a clinical reason to check your heart’s blood supply.
Newer protocols and camera technology are pushing doses lower, with a target of 9 millisieverts or less for a growing share of labs.
After the Test
Once imaging is complete, the IV comes out and you can go home. There are no activity restrictions. Drink plenty of fluids for the rest of the day to help flush the tracer from your system; it’s essentially gone by the next morning. For the remainder of the day, try to keep some distance from young children and pregnant women as a precaution, since you’ll emit a very low level of radiation until the tracer clears.
Results are usually ready within a few days. Your doctor will contact you to discuss the findings and whether any follow-up testing or treatment changes are needed.
Who Should Not Have the Test
The exercise portion of a nuclear stress test is not safe for everyone. Per American Heart Association guidelines, absolute contraindications to the treadmill component include a heart attack within the past two days, unstable chest pain that hasn’t been controlled with medication, uncontrolled heart failure, severe symptomatic aortic valve narrowing, acute inflammation of the heart muscle or its lining, and acute aortic dissection. For many of these patients, the pharmacological (medication-based) version of the test may still be an option, since it stresses the blood vessels rather than the whole cardiovascular system.
People with severe asthma or COPD may not be able to receive the standard vasodilator medications, since those drugs can trigger airway tightening. In those cases, an alternative medication that works by a different mechanism can sometimes be used instead.