CardiologyTier 2Topic

Myocardial perfusion scans

Principle

Nuclear medicine (SPECT or PET) imaging comparing myocardial blood flow at rest and during stress (exercise or pharmacological) - detects inducible ischaemia and assesses myocardial viability
An alternative to stress echocardiography and functional/anatomical CT for ischaemia assessment, chosen based on patient factors (ability to exercise, body habitus, local availability/expertise)
A radiotracer (e.g. technetium-99m sestamibi) distributes to myocardium proportional to blood flow; a region with a flow-limiting stenosis shows reduced tracer uptake during stress relative to rest ("reversible defect" = ischaemia); a defect present at both rest and stress ("fixed defect") suggests infarcted, non-viable tissue - though some fixed defects represent viable but severely hypoperfused (hibernating) myocardium requiring further characterisation

Pharmacological stress (adenosine, dipyridamole, dobutamine) is used when a patient cannot exercise adequately - adenosine/dipyridamole vasodilate normal coronaries preferentially, unmasking flow-limited territories by relative "steal"; dobutamine increases myocardial oxygen demand instead

Guides revascularisation decisions, particularly whether hypoperfused myocardium is viable (worth revascularising) or infarcted (unlikely to benefit)
A large reversible perfusion defect on stress testing identifies high-risk anatomy warranting invasive angiography, independent of symptom severity

Stable ischaemic heart disease, viability assessment before revascularisation, risk stratification post-MI

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