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Cardiac tests - nuclear perfusion imaging

Cardiologists Dr Clive Handler and Dr Gerry Coghlan conclude their series on tests commonly used in primary care

Cardiologists Dr Clive Handler and Dr Gerry Coghlan conclude their series on tests commonly used in primary care

This test is of value in a small number of clinical situations, and so is only available in a few hospitals in the UK. The widespread availability of open-access exercise testing to assess myocardial ischaemia, transthoracic echocardiography to assess left ventricular function and cardiac anatomy, stress echocardiography to assess myocardial ischaemia and hibernating myocardium, as well as the availability of – and short waiting times for – coronary angiography in most district general hospitals, has relegated nuclear perfusion imaging to a small minority of patients.

Principles

The uptake of radioactive chemical into the left ventricular muscle depends on the blood supply. No radionuclide will be taken up into dead heart muscle, and less will be taken into an area of heart muscle supplied by a narrowed coronary artery than an area of normal heart muscle supplied by a normal artery. These differences become apparent only when the heart is subjected to stress using exercise or drugs.

The test

Most tests are done to assess myocardial ischaemia, and are similar to an exercise stress test on a cycle or treadmill. In addition, a radioactive nuclear chemical (usually thallium) is injected into an arm or hand vein at peak exercise. The patient then lies down under a gamma camera and the uptake of radionuclide is imaged. Further images are taken after around four hours to investigate reperfusion. The report is subjective.

Indications

Nuclear perfusion imaging may be used to investigate reversible ischaemia in patients with resting ECG abnormalities (pre-excitation, left bundle branch block), which complicate interpretation of ST-segment changes. The sensitivity and specificity are not significantly different from those of an exercise test nor are they superior to stress echocardiography.

Limitations

The accuracy of the test is affected by many factors. False-negative scans occur with inadequate stress, patients on anti-ischaemic medication, collateral circulation, poor quality imaging and breast attenuation. False-positive scans occur in patients with conduction abnormalities and cardiomyopathies, and because of technical problems. Nuclear material, if used repeatedly, carries a risk. The test is invasive, costly and time consuming. Although a single dose of radiopharmaceutical used is probably not very dangerous, patients may be reluctant to have an injection of these substances when much less hazardous, cheaper and quicker alternative tests are available.

The report

This should indicate the probability of coronary heart disease. Identifying the location of coronary artery disease is difficult and often unreliable in view of the limitations of ‘relative perfusion'. For example, a patient with severe triple-vessel coronary artery disease and homogenously reduced myocardial blood supply may have no relative perfusion defect identified, and so the scan may be reported as ‘normal.' There is also variability in coronary arterial blood supply to the inferior wall of the left ventricle from either or both the right and circumflex arteries, so attempts to localise coronary artery disease with nuclear perfusion imaging are often inaccurate. Interpretation of scans is open to error.

Dr Clive Handler is consultant in pulmonary hypertension at the Royal Free Hospital, London

Dr Gerry Coghlan is consultant cardiologist at the National Pulmonary Hypertension Unit, Royal Free Hospital, London

This article is based on a chapter in Management of Cardiac Problems in Primary Care by Dr Clive Handler and Dr Gerry Coghlan, published by Radcliffe, which is offering a 20% discount on this book to Pulse readers (usual price £29.95+p&p, offer price £23.96+p&p). To claim, order via www.radcliffe-oxford.com or by telephone on 01235 528820. Please quote discount code ‘Pulse'. Offer expires 31 May 2009.

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