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survival in a group of patients of mean age 68 with a normal stress echocardiogram was 97.4 per cent.
1 The Seattle Heart Watch program: a study, initiated by Drs. Robert A. Bruce et al. in 1971, involving community
physicians in hospitals, offices, and the medical department of the Boeing Company. It tested the feasibility, utility
and reproducibility of results of symptom-limited exercise testing in ambulatory cardiac patients and apparently
healthy subjects. A database of more than 10,000 individuals was developed over a period of 10 years.
ICAO Preliminary Unedited Version — October 2008 III-1-10
This was better than their age and gender matched peers. Another study found a one per cent six year
annual mortality in a large group of patients of mean age 54 years with a normal exercise
echocardiogram. But a third, which was the largest that has assessed long term survival and outcome
following a normal stress echocardiogram, concluded that prognosis was “not necessarily benign”. The
patient mean age was older (68 years).
This technique is being increasingly used and has the benefit that there is no radiation burden. However, it
has to be carried out in an experienced centre and more long term outcome data are needed.
Myocardial Perfusion Imaging
A more widely available investigation is myocardial perfusion imaging (MPI). The largest experience
with MPI has been obtained with thallium-201, a radionuclide with a half-life of 72 hours which decays
to mercury-201. The standard dose is 80 MBq1; approximately four per cent are cleared in the first pass
through the coronary circulation. The radiation dose is quite high and is equivalent to 18mSv2, exceeding
the radiation dose received during coronary angiography by a factor of two or three although with the
most modern equipment, doses are often lower. It behaves as potassium in the exercising myocardium
being taken up by the myocardial cells via a sodium-potassium adenosine triphosphatase (ATP-ase)-
dependent mechanism.
Exercise is now being supplanted by pharmacological agents, commonly adenosine, as the means of
myocardial stress. It causes maximal vasodilatation, the heart rate response being limited. A
pharmacological agent is preferred in the presence of left bundle branch block. Imaging takes place
following maximum stress and three hours later to permit redistribution of the isotope. Other stressor
agents include dipyridamole and dobutamine. Other radionuclides such as technetium-99m-2-methoxyisobutyl-
isonitrile (MIBI) provide better resolution for a smaller radiation burden.
The power of MPI in the prediction of outcome has been established and surpasses exercise ECG
although it, too, has incomplete specificity and sensitivity in diagnostic terms. The exercise ECG can be
expected to be of the order of 68 per cent sensitive and 77 per cent specific; thallium scanning is a few
percentage points better on both counts. Both modalities depend crucially on the prevalence of coronary
disease in the population being studied. In one study of 3573 patients with angiographic coronary artery
disease and a normal MPI, the incidence of death or myocardial infarction was 0.9 per cent per annum
over a mean of 28 months. A more recent review of the outcome of 7376 consecutive patients with a
normal exercise or adenosine MPI, hard events (cardiac death, myocardial infarction) were more common
with increasing age, male gender, diabetes and known coronary artery disease, but the highest event rate
was 1.4 to 1.8 per cent per annum over the two-year study period. Many regulatory authorities would
regard these figures as failing to provide adequate confidence for certification.
The incremental prognostic value of sequential investigation of patients suspected of suffering from
coronary artery disease has also been evaluated. The addition of exercise ECG to the clinical examination
and resting ECG adds significant predictive power, while the addition of MPI improves it further. The
hierarchical prognostic gain from adding exercise ECG, exercise single photon-emission computed
tomography (SPECT) thallium-201 imaging and coronary angiography has been reviewed and
demonstrated that imaging quadrupled the prognostic power but coronary angiography provided no
additional improvement over exercise ECG.
1 MBq: Megabecquerel = 1000 kBq. 1 Bq = 1 event of radiation emission per second, named after Antoine Henri
Becquerel, French physicist (1852 – 1908).
2 mSv: millisievert. 1 Sv = 1 J/kg is the dose equivalent for the biological effect of radiation. The unit is named
after Rolf Maximillian Sievert, Swedish medical physicist (1896-1966).
ICAO Preliminary Unedited Version — October 2008 III-1-11
Myocardial Perfusion Imaging is an expensive investigation with a significant radiation burden. This is of
　
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