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disease in the population under study (for a simplified approach to Bayes' theorem see FAQs). Diagnostic testing is most effective in patients with an intermediate prevalence of disease estimated from the patient's age, gender and chest pain history, together with consideration of coronary risk factors (Fig. 13).

Sensitivity

The overall sensitivity of SPECT imaging using either 201Tl or 99mTc-sestamibi is 85-90%. SPECT detects stenosis in individual vessels with a sensitivity for the left anterior descending artery of 80%, right coronary artery 80% and circumflex 70%. In those patients in whom SPECT cannot be performed (unable to lift left arm, unable to lie still, body weight considerations), quantitative planar studies yield a similar sensitivity (on a per patient basis) although the identification of individual vessel stenoses is reduced.

Sensitivity varies depending upon:

Table 3. Indications for MPI for detection of coronary artery disease (CAD)

Intermediate Pretest Probability of CAD Abnormal Resting Electrocardiogram

• Non-specific ST-T Abnormalities

• Left Ventricular Hypertrophy

• Conduction Disturbance

• Ventricular Pacing

• Pre-excitation (WPW syndrome). Non-diagnostic Treadmill Stress Test

• Inability to Reach a 85% Maximal Predicted Heart Rate

• Extent of Coronary Disease

The sensitivity of detecting CAD increases from 80% in patients with single vessel disease to 90% in double vessel disease and 95% in triple vessel disease. The ability to correctly identify a patient as having multivessel disease (by detecting perfusion defects in two or more coronary artery territories) is 65% (Fig. 14).

• Severity of Coronary Disease

Angiographically, stenoses are classified as moderate if they compromise 50-70% of the lumen and severe if >70%. Since impairment of coronary flow reserve is related to the severity of coronary stenosis, the sensitivity of perfusion imaging will vary with stenosis severity and ranges from 60% with moderate stenosis to 90% with severe stenosis.

The identification of inducible ischemia with perfusion imaging depends upon creating coronary flow heterogeneity. Patients who are only able to perform a low workload on the treadmill have a submaximal increase in coronary flow. The sensitivity of detecting CAD is reduced in patients unable to attain 70% of their predicted maximum heart rate. In those able to exceed 70% but unable to reach 85% of predicted maximum, it appears that the detection of individual coronary stenoses is decreased while sensitivity (on a per patient basis) is maintained. If patients cannot perform adequate exercise, pharmacologic vasodilatation with dipyridamole or adenosine should be employed. In patients with significant reactive airway disease dobutamine is an alternative with similar sensitivity to exercise.

Sensitivity may also be adversely affected by drugs which reduce myocardial oxygen demand and/or improve coronary flow (nitrates, beta blockers, calcium channel blockers). Beta blockers and non-dihydropyridine calcium channel blockers (verapamil, diltiazem) decrease myocardial oxygen demand and reduce the sensitivity of MPI with treadmill exercise and dobutamine whereas nitrates and dihydropyridine calcium channel blockers (nifedipine, amlodipine) may lower the sensitivity of all stressors because they dilate conductance arteries. If possible, nitrates should be withheld on the day of the stress test and in clinically stable patients rate-limiting drugs should be held for 24-36 hours prior to a treadmill or dobutamine test. As indicated earlier, since methylxanthines block adenosine receptors, patients undergoing perfusion scintigraphy with either dipyridamole or adenosine must be

Figure 13. 85 year old male with atypical chest pain. The MPI study was performed for diagnostic purposes. There is extensive severe reduction of activity in the septum (long arrow), anterior wall (arrowhead) and apex (short arrow) at stress with normalization at rest. A subsequent angiogram showed an occluded LAD with filling of the LAD via collaterals from the RCA. Note the dilation of the LV cavity at stress (transient ischemic dilation) which is evident in all three planes.

Figure 13. 85 year old male with atypical chest pain. The MPI study was performed for diagnostic purposes. There is extensive severe reduction of activity in the septum (long arrow), anterior wall (arrowhead) and apex (short arrow) at stress with normalization at rest. A subsequent angiogram showed an occluded LAD with filling of the LAD via collaterals from the RCA. Note the dilation of the LV cavity at stress (transient ischemic dilation) which is evident in all three planes.

on a caffeine-free diet for approximately 24 hours and avoid methylxanthine containing medications during this time.

The sensitivity of MPI in detecting CAD in women is reduced compared to men. Reasons for this include: (1) left ventricular cavity size in women tends to be smaller. Small lesions may therefore be harder to resolve with SPECT. (2) Women are less likely to perform an adequate level of treadmill exercise. (3) Disease severity in women is generally less than in men and, therefore, sensitivity will decrease as stenosis severity decreases. (4) Breast attenuation causes greater variability in the appearance of the anterior wall. True anterior wall defects may be attributed to breast attenuation thus reducing sensitivity.

While the "gold standard" for the detection of CAD is coronary arteriography, it is important to appreciate that the anatomic data from angiography may not always be concordant with the physiologic data provided by perfusion imaging. Coronary flow reserve using Doppler flow velocity technique correlates better with SPECT

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