Nuclear Medicine Cardiac Stress Test

Authors: Dr Ghee Chew*
                            Dr Timothy Cain *

What is a nuclear medicine (NM) cardiac stress test?

A nuclear medicine (NM) cardiac stress test assesses the blood supply to the heart and provides information about how the heart is working. Images (or pictures) are taken of the heart while at rest and after the heart is stressed. The stress is in the form of exercise on a treadmill or exercise bike, or if this is not possible, by using medication to increase the blood flow to the heart. Differences in blood flow to different parts of the heart muscle can be shown more clearly when the blood flow is increased after exercise.

Before the images are taken, a radioactive medication (radiopharmaceutical) is injected into a vein, usually in the arm, which then passes through the blood stream and concentrates in the heart. While the patient is lying down on a scanner bed, a gamma camera or scanner is positioned in front of the heart. The camera or scanner rotates around the chest, and three-dimensional images of the heart are formed from the gamma rays passing through the body having been released from the radiopharmaceutical.

In most cases, the radiopharmaceutical used is called ^99mTc sestamibi or ^99mTc tetrafosmin. In some facilities and in certain circumstances, a third agent called thallium-201 might be used instead.

After images are taken while at rest, the stress test is carried out. The stress and rest scans are then compared.

Why would my doctor refer me to have this procedure?

This test is ordered if there is concern about the blood supply to the heart, which can lead to a heart attack. Usually this is because of symptoms such as chest pain and shortness of breath that can be associated with heart disease.

How do I prepare for a NM cardiac stress test?

It is important that you let staff at the hospital or radiology practice where you are having the test done know if you are (or think you could be) pregnant or if you are breast-feeding.

Women who are breast-feeding and people who are the primary or sole carer of small children will need to make special preparations after the test, such as to stop breast-feeding for a short time and/or to avoid close contact with young children. This is due to the small amount of radioactivity released from the body after the test. You should discuss this with your referring doctor or the NM practice where you will have the test carried out. See InsideRadiology: nuclear medicine for further information for breast-feeding patients and those in close contact with children about the precautions to take after nuclear medicine tests.

To prepare for the stress part of the test, it is recommended that you dress appropriately for physical exercise in loose comfortable clothing and comfortable footwear, as the stress test might involve walking on a treadmill or riding an exercise bike. The stress test might consist of using a medication to increase blood flow to the heart.

There are some medications you might be taking that work by slowing the heart rate. These medications might need to be stopped before the stress test. You will need to tell the radiology facility where you are having the test what medications you are taking. If necessary, they will contact your doctor to discuss whether any of these medications should be stopped for the stress test.

It is important that you do not have a large meal before the test. All forms of beverages and foods containing caffeine (e.g. coffee, tea, colas, cocoa) should be avoided for 24 hours before the test, as they can interfere with the effect of a stress medication called dipyridamole (Persantin). Decaffeinated teas or coffee and herbal teas should also be avoided, as there might still be traces of caffeine.

If you are already taking a regular prescription of dipyridamole or Persantin to prevent strokes, then this might not be used to stress the heart for the purpose of this test. If it is used as a stress agent, your regular dose of Persantin might need to be temporarily stopped for a short period before the study.

The NM specialist who carries out the test will need to know what medications you are taking. It is a good idea to bring a list of medications with you on the day of the test.

What happens during a NM cardiac stress test?

The study is carried out in two parts.

Part 1: The NM specialist will need to assess your heart under “stress”. This is done by exercising on a treadmill or exercise bike. If you are unable to exercise sufficiently, a medication to mimic exercise will be used.

For the stress test, an intravenous cannula (a thin plastic tube) will be put into a vein in your arm. You will also have electrocardiograph (ECG) leads placed on your chest, which are connected to a heart monitor. The NM specialist will closely monitor your heart with the ECG and regular blood pressure checks.

When your heart has reached a target work capacity, you will be injected with a radiopharmaceutical into the intravenous cannula. You will then be asked to rest in the waiting room for approximately half an hour before the first set of heart images are taken. During this time, you might be given a drink of water or milk, or a small tub of ice cream to eat. This helps make the images of your heart clearer.

Imaging involves lying on the scanning bed of a gamma camera for approximately 20 minutes while the camera rotates around your heart. In most imaging centres, the gamma camera also has an inbuilt CT scanner. A low energy CT would usually be performed with your cardiac nuclear scan. This CT scan which results in a radiation exposure of less than a quarter of a regular chest CT is added because it significantly improves the quality of the nuclear scan. This increases the scan time by about 2 minutes or less. During this time, three ECG leads will again be placed on your chest and connected to a monitor as the scan is timed according to the rhythm of your heart. It is preferable for you to have your arms above your head during the scan, but scanning with the arms by your sides is also possible.

You will then be given an appointment time to return in the afternoon for Part 2 (usually 3–4 hours after Part 1). In between Parts 1 and 2, you can have caffeine and a light lunch. Part 2 of the test assesses your heart at “rest”. During this time, it is important that you avoid heavy physical activity: for example, a slow 5-minute walk to the cafeteria is acceptable, but a brisk 30-minute walk is not. The tubing in your arm will remain in place during this time.

Part 2: You will be given a second dose of the same radiopharmaceutical through the same plastic tubing in your arm. The tubing is then removed. Again, you will be asked to sit in the waiting room for approximately half an hour and you will be given another drink or small tub of ice cream. Your second set of images is then taken similar to your scan in the morning in approximately the same time.

The above scanning procedure applies to the use of the radioactive medication ^99mTc sestamibi or ^99mTc tetrafosmin, which is usually the case in most NM facilities. In the event the radioactive medication used is thallium-201, the scan is carried out immediately after it has been given without the half hour wait, and you won’t be given anything to eat or drink before the scan. Also, if thallium-201 is used, you might be asked to return the next day for another set of images, but there will be no further injections involved.

Are there any after effects of a NM cardiac stress test?

If you are to do a treadmill exercise stress test as part of this scan, you might feel tired after the exercise, but the radiopharmaceuticals themselves do not cause any side-effects.

If you are unable to have a stress test on a treadmill, then one of three types of medications, dipyridamole (Persantin), adenosine or dobutamine, might be given intravenously (directly into a vein) to increase blood flow in your heart.

Dipyridamole works by causing the heart arteries to dilate (open fully). There are multiple potential side effects:

• headache;
• a warm sensation in the face;
• might make asthma worse;
• in cases of significant coronary artery disease, it might induce a heart attack, although the risk of this adverse event is low.

If symptoms of headache persist, a caffeinated beverage such as tea or coffee is recommended and should relieve the headache. You might be given another medication called aminophylline, which acts to reverse the side-effects of dipyridamole if they do not resolve quickly.

An alternative drug, adenosine, works in a similar manner to dipyridamole. Side-effects similar to dipyridamole might be experienced. Symptoms of chest pain or pressure can also occur, but these side-effects go away quickly once the adenosine has been given.

These side-effects should disappear shortly after the stress test and before you leave the NM facility (see below).

If you are breast-feeding or caring for young children, see the “How do I prepare” section for more information about special precautions you might need to take.

How long does a NM cardiac stress test take?

The time it takes to achieve the target heart rate during the “stress” component of the test will vary depending on the patient. It usually takes no longer than 15 minutes. When the waiting interval is included, the first scan should be completed in approximately 1 hour.

There is another 3 to 4-hour interval, during which you are free to leave the imaging department. The “rest” scan will take a total of about an hour from when you receive the second dose of radiopharmaceutical to the completion of the scan.

What are the risks of a NM cardiac stress test?

The main risks of the test relate to the stress component.

If you are having an exercise stress test, there is a small risk of sustaining a heart attack (myocardial infarction) or cardiac ischaemia (angina) if you have significant coronary artery disease and you work too hard on the treadmill or exercise bike. Your heart will be monitored to identify any abnormal ECG changes and the attending NM specialist will moderate the exercise to minimise this risk.

Dobutamine is a short-acting medication that is designed to increase the pumping capacity of the heart, mainly by increasing the heart rate and, to a lesser extent, increasing the strength of the cardiac contractions. It might result in the sensation of palpitations (irregular heart beat), which is normal. Some patients might experience light-headedness and nausea. There is a risk of inducing a fast and abnormal cardiac rhythm (e.g. atrial fibrillation, ventricular tachycardia, ventricular fibrillation), which could adversely affect heart function requiring urgent therapy. These cardiac rhythm disturbances are unlikely with the doses of dobutamine used. If you have significant coronary artery disease, there is also a small risk of inducing a heart attack (myocardial infarction).

Occasionally, the target heart rate cannot be reached with the maximum allowable dose of dobutamine. You might then be given a second medication called atropine, again in small doses. Atropine can cause the symptom of dry mouth, and might cause confusion in some patients. It is also not to be given if you have the eye condition, glaucoma.

The overall risk of sustaining a heart attack from a stress test is approximately 2 to 3 in 10,000, and this risk is the same as for an exercise ECG stress test carried out by a cardiologist. There is a very small risk from the ionising radiation emitted by the radiopharmaceutical.

What are the benefits of a NM cardiac stress test?

The NM cardiac stress test is a non-invasive (simple), low-risk study that allows your doctor to assess the presence of significant coronary artery disease and how well your heart muscle is working. It is very good at assessing the likelihood of the risk that you will experience a significant cardiac event (heart attack) and the importance of other clinical findings.

Who does the NM cardiac stress test?

The stress test is usually carried out by a NM specialist doctor with the assistance of a nurse who is also trained in cardiac resuscitation. The scan is carried out by a NM technologist. The images are interpreted by the NM specialist doctor.

Where is a NM cardiac stress test done?

The stress component of the test is carried out in a facility with the appropriate exercise equipment (treadmill or exercise bike) and the appropriate resuscitation equipment (oxygen supply, medications and defibrillator).

Most public or private facilities with NM services should have the capacity to carry out this test.

When can I expect the results of my NM cardiac stress test?

The time it takes for your doctor to receive a written report will vary. The private radiology practice, clinic or hospital where your procedure is carried out will be able to tell you when your doctor is likely to receive the report.

It is important that you discuss the results with your doctor, so they can explain what the results mean for you.

References

Colquhoun D. CSANZ Position Statement: Clinical Exercise Stress Testing in Adults. 2014 Aug. Available from: http://www.csanz.edu.au/wp-content/uploads/2014/08/Clinical_Exercise_Stress_Testing_2014-August.pdf

*The author has no conflict of interest with this topic.

Page last modified on 24/8/2018.