Review articleThe clinical importance of electrocardiographic changes during pharmacologic stress testing with radionuclide myocardial perfusion imaging
Introduction
Stress radionuclide myocardial perfusion imaging (MPI) by use of single photon emission computed tomography (SPECT) is a commonly used noninvasive method for the evaluation of suspected or known coronary artery disease (CAD). The results of perfusion imaging play a fundamental role in providing information to diagnose and risk-stratify patients.1, 2, 3, 4 When SPECT MPI is combined with exercise stress testing, it is generally understood that the electrocardiographic (ECG) data alone do not add to either the diagnostic or prognostic value.5, 6, 7, 8, 9, 10 In patients who are unable to exercise, MPI is used with either vasodilator or dobutamine stress. The presence of ECG changes during pharmacologic stress testing is less frequent, and the diagnostic and prognostic implications are less well understood. This review will examine the available data on ECG changes during pharmacologic stress testing with MPI, particularly with regard to the detection of CAD and prognosis with respect to subsequent cardiac events.
Section snippets
Pharmacologic stress agents
Adenosine and dipyridamole are potent coronary vasodilators, which are both used with SPECT MPI. Adenosine is produced by both the myocardial smooth muscle and endothelium and binds to several tissue receptors, including the cardiac-specific A2A receptor, which ultimately causes coronary vasodilation. Dipyridamole works indirectly by blocking the cellular reuptake of adenosine, thereby increasing adenosine levels. These agents are often used in patients who are unable to adequately perform a
Potential mechanisms of ECG changes during vasodilator stress
Dipyridamole and adenosine cause a 3- to 6-fold increase in baseline myocardial blood flow.17 In those patients with hemodynamically significant coronary artery stenosis, the portion of the vessel distal to the stenosis is already maximally dilated and, therefore, has a minimal response to vasodilator agents as a result of its limited flow reserve. However, in those areas of the myocardium that are supplied by “normal” coronaries, coronary blood flow increases substantially with vasodilation.18
Diagnostic implications of ECG changes during vasodilator stress MPI
The sensitivity and specificity for the detection of CAD during exercise stress testing are 50% to 70% and 60% to 80%, respectively.23 Exercise stress testing in combination with SPECT MPI has improved sensitivity (90%) and specificity (70%) over exercise alone.24 Although the extent of ST-segment depression has been found to be a predictor of severe CAD,25 the overall diagnostic accuracy of SPECT MPI for the detection of CAD has been consistently higher than the ECG response during exercise
Prognostic value of ECG changes during vasodilator MPI
Previous studies have suggested that, as with exercise MPI, vasodilator stress MPI also provides powerful risk stratification.1, 2, 3, 4, 54 For this reason, vasodilator stress imaging has been used as an alternative to exercise MPI in the evaluation and diagnosis of CAD in those patients who are unable to achieve an adequate exercise stress response.
Separate studies have suggested that the cardiac event rate with normal vasodilator MPI may be approximately 2-fold higher (1.3%-2.3%) as compared
Prognostic value of ECG changes during dobutamine MPI
It has previously been demonstrated that dobutamine and exercise MPI are comparable diagnostic studies for the detection of reversible perfusion defects in patients with suspected CAD.63 The annual cardiac event rates, however, are quite different between these two stress modalities. In a report by Calnon et al,64 the annual cardiac event rate with a normal dobutamine MPI study was 2.3%. The presence of ST-segment depression during dobutamine infusion with a normal MPI study carries a
Conclusion
Radionuclide MPI has become a reliable tool for the detection and surveillance of suspected or known CAD, regardless of whether it is used in combination with exercise or pharmacologic stress agents. The presence of an abnormal electrocardiogram, especially during a pharmacologic stress test, may indicate an elevated risk of multivessel CAD and higher annualized cardiac event rates. Therefore the presence of ischemic ECG changes should prompt further evaluation, even in the presence of normal
Acknowledgment
The authors have indicated they have no financial conflicts of interest.
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