Stress Testing and Cardiac Imaging
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Direct answer
Stress testing and cardiac imaging detect inducible myocardial ischaemia or anatomical coronary stenosis. The choice among exercise ECG, stress echocardiography, nuclear perfusion imaging and CT coronary angiography depends on pretest probability, the patient's ability to exercise and the resting ECG. An exercise ECG showing horizontal or downsloping ST depression of 1 mm or more is the simplest functional test, while CT coronary angiography assesses anatomy and has a high negative predictive value.
What you must remember
- Exercise ECG target: 85 per cent of age-predicted maximum heart rate, calculated as 220 minus age; ST depression of 1 mm or more is positive, with false positives in left bundle branch block, left ventricular hypertrophy and digoxin use.
- Stress echocardiography: wall motion is compared at rest and peak stress; dobutamine substitutes for exercise, with atropine added when the heart rate response is inadequate.
- Nuclear perfusion (SPECT): technetium-99m sestamibi or thallium-201 is used; a reversible defect indicates inducible ischaemia, while a fixed defect indicates scar.
- Pharmacological stress: adenosine and regadenoson are vasodilators; adenosine is avoided in asthma, bronchospasm and second- or third-degree heart block.
- CT coronary angiography: excellent for excluding obstructive disease in low to intermediate probability; limited by heavy calcification, irregular rhythm and high heart rate, and it involves contrast and radiation.
- Uninterpretable resting ECG: left bundle branch block, ventricular pacing and pre-excitation make exercise ECG unreliable, so an imaging stress test is chosen.
A 60-year-old man with exertional angina and left bundle branch block
A 60-year-old man has typical exertional angina, and his resting ECG shows left bundle branch block, which makes exercise ST analysis unreliable. The pretest probability is high, so the examiner moves to an imaging stress test rather than a plain treadmill.
- Choose the modality: Stress echocardiography with dobutamine or nuclear perfusion is chosen.
- Read a positive result: A positive result, meaning inducible wall motion abnormality or a reversible perfusion defect in a territory, leads to invasive coronary angiography.
- Do not over-reassure: A negative result in a patient with high probability does not fully exclude disease, so symptoms and risk are reviewed.
- Consider CT angiography: In a patient with low to intermediate probability and no known coronary disease, CT coronary angiography may be chosen instead, because a normal study has high negative predictive value.
- Do not treat from calcium alone: A high coronary calcium score confirms atherosclerosis but does not establish that the stenosis is ischaemic, so the score alone should not drive treatment.
- Weigh local access: The decision is made on the combination of symptoms, risk, resting ECG and local availability, which in India often limits access to nuclear imaging outside tertiary centres.
Where the examiner tests the interpretation
Several traps recur in examiner questions.
- Trap: diagnosing infarction from a fixed SPECT defect Correct: a fixed defect can be an attenuation artefact from breast tissue in the anterior wall or from the diaphragm in the inferior wall, so attenuation must be named first.
- Trap: trusting a negative exercise ECG in every patient Correct: exercise ECG is less reliable in women and in patients who do not reach target heart rate, so a negative test in that setting is weaker.
- Trap: equating calcium score with ischaemia Correct: coronary calcium scoring is a marker of plaque burden, not of ischaemia, which is the same distinction made for CT.
- Trap: ignoring radiation in repeated testing Correct: echocardiography and cardiac MRI avoid ionising radiation, and the examiner may ask which test to choose in a young woman who needs repeated assessment.
Frequently asked questions
What heart rate target is used in an exercise stress test?
Exercise ECG aims for 85 per cent of the age-predicted maximum heart rate, calculated as 220 minus age. Failure to reach this target reduces sensitivity, so a negative test in that case is less reliable.
Why is dobutamine used in stress echocardiography?
Dobutamine is a beta-1 agonist that increases myocardial contractility and heart rate, substituting for exercise in patients who cannot walk on a treadmill. Atropine is added when the heart rate fails to rise adequately.
What does a fixed perfusion defect on SPECT indicate?
A fixed defect on SPECT, present at both stress and rest, indicates myocardial scar from prior infarction. A reversible defect that resolves at rest indicates inducible ischaemia, and breast or diaphragmatic attenuation can mimic defects.
When is CT coronary angiography preferred to a functional stress test?
CT coronary angiography is preferred when pretest probability is low to intermediate and the patient has no known coronary disease, because its high negative predictive value rules out obstruction. It is less suitable with heavy coronary calcification or an irregular heart rhythm.