ECG signal

Dr Diana R Holdright
MD, FRCP, FESC, FACC, MBBS, DA, BSc

Consultant Cardiologist

Dr Diana Holdright
 
 

Cardiac Investigations - Myocardial Perfusion Scanning

A myocardial perfusion scan is used to evaluate blood flow to the heart muscle and establish how well the heart muscle is functioning. Under stress the heart may not receive enough blood, which can result in symptoms such as angina and breathlessness. As such, in this test the heart is put under a controlled amount of stress, usually in the form of exercise, following which a radioactive tracer called an isotope is injected into a vein, which allows pictures to be produced that show the blood supply to the heart and how well the heart muscle is performing.

The tracer enters heart muscle cells, the amount being proportional to the blood supply the heart muscle is receiving. Areas supplied by a narrowed coronary artery will take up less of the tracer during exercise compared with at rest, giving rise to a “perfusion deficit”, and scarred areas of heart muscle from a previous heart attack will not take up any tracer either at rest or with exercise. As such, the scan gives detailed information about the blood supply to the heart at rest and with stress, and the heart muscle function can also be assessed. 

During the test an injection, usually a drug called adenosine, is given into a vein in the arm to dilate the coronary arteries while gentle exercise (pedalling on a stationary bike) is undertaken. Towards the end of the exercise period the tracer, typically thallium or technetium, is injected and approximately 45 minutes later the heart is scanned. During this time the patient will lie still with both arms above the head while the camera moves around the chest, picking up the location and concentration of the isotope in the heart.  There is then a break, typically around 90 minutes although this may vary, following which another radioactive injection may be given. It takes approximately one hour for the tracer to perfuse sufficiently to allow the images to be taken and during this time the patient is able to leave the clinic and have a light lunch. Upon return the patient is scanned for a second time. The whole process takes between three and five hours.

In terms of preparation the most important point is to avoid all caffeine-containing drinks and foods for 12 hours prior to the test, as caffeine blocks the actions of adenosine and will therefore reduce the accuracy of the test. It is important to note that caffeine is not only found in coffee and tea, but also chocolate, and some painkillers and cold and ‘flu remedies, amongst other things: 

Drink Caffeine (mg/100ml) Drink Caffeine (mg/100ml)
Espresso 173.6 Pepsi 10.7
Fresh Coffee 45.4 Green Tea 10.6
Red Bull 32.0 Diet Pepsi 10.1
Monster 32.0 Coca-Cola 9.7
Instant Coffee 24.1 Chocolate Slim Fast 8.4
Tea 19.9 Hot Chocolate 3.8
Diet Coke 12.7 Lipton Iced Tea 2.8
Lucozade 12.1 Decaf Coffee 2.4
Dr. Pepper 11.8 Cocoa 2.1
 
Food Caffeine (mg/100g) Medicine Caffeine (mg/pill)
Dark Chocolate 71.4 Panadol Extra 65
Milk Chocolate 21.4 Solpadeine Plus 30
Chocolate Cereal 3 Benylin Cold and ‘Flu 25

(please note that this table is not exhaustive)

Patients should take along a list of all drugs and inhalers, and dipyridamole (Persantin, Asasantin) should be stopped 48 hours before the scan. After the test, the small amount of radioactive chemicals in the body decay over time and will be excreted in the urine and faeces. Since the scan involves radiation, pregnant women should not have the test, and close contact with pregnant women and children should be avoided after the scan, the time limit depending on the isotope used. Rarely the tracer can be detected by airport security detectors for up to four weeks after the test; patients should ask their doctor to issue them with an explanatory letter if they are planning air travel during this time.

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Cardiac Investigatins - Myocardial Perfusion ScanningCardiac Investigatins - Myocardial Perfusion Scanning