Gated SPECT myocardial perfusion scintigraphy: a multi-faceted tool for the evaluation of heart failure.

Compared to the large volume of data available on the prognostic utility of myocardial perfusion scanning (MPS) in unselected patients referred for stress testing and several subgroups therein, there is only a relatively scant amount of literature pertaining specifically to the use of MPS to determine prognosis in patients with heart failure. One could be excused for being surprised at this state of affairs, since heart failure is widely prevalent, and frequently encountered by cardiologists, internists, and specialists in cardiac imaging, and MPS is one of the most widely utilized cardiac assessment tools. In this issue of the Journal Candell-Riera and colleagues report on the prognostic value of clinically indicated Tc-99m gated SPECT MPS in a prospective cohort of patients with left ventricular ejection fraction (LVEF) B40%. Of 6114 patients referred to the authors’ laboratory for MPS over a 6-year period, 365 had LVEF B40%. After excluding patients with nonischemic LV dysfunction and censuring patients who had early (with 60 days of MPS) revascularization, 167 patients with ischemic cardiomyopathy were followed up for an average of 2.3 ± 1.2 years to identify the variables associated with cardiac death. Thirty patients had stress testing precluded by severe heart failure and therefore, underwent only rest gated MPS. The mortality rate over this relatively short period of follow-up was substantial: 22% (36/167) all-cause mortality, of which 17% (29/ 167) were cardiac deaths. As expected, the majority of deaths in this population were from progressive heart failure (23), and most of the remaining were sudden cardiac deaths. MPS was read using a standard 17-segment LV model with visual, semi-quantitative scoring for perfusion and regional function. Residual myocardial viability was defined as preserved perfusion (normal to severe hypoperfusion) in C3 segments with severe dysfunction (severe hypokinesis, akinesis, or dyskinesis). For comparison with prior studies, this translates into an extent of viable but dysfunctional myocardium involving at least 18% of the LV myocardium, and approximates the threshold of critical mass that is generally accepted as clinically significant (i.e., predictive of recovery of function if revascularized, or of an adverse prognosis if not revascularized). 1 Perfusion assessment however, was based on visual rather than the quantitative (albeit relative) methods previously established as most accurate for detection of viability, and nitrate-enhancement was not performed. 2 On univariable analysis, patients who died from cardiac causes were older, less often able to undergo exercise testing, and had a greater prevalence of myocardial viability. Among patients who were able to undergo exercise testing, the survivors had greater exercise intensity and duration, with a lower prevalence of combined ischemia and viability, but not ischemia alone. Numerous other clinical and electrocardiographic criteria, and notably all coronary angiographic criteria, were similar between these groups. In a multivariable model that included the entire study group of 167 patients, myocardial viability and the inability to exercise were associated with cardiac death. In patients able to exercise, exercise intensity \75 W, exercise duration B5 minutes (cut-offs determined by ROC curve analysis), myocardial viability and ischemia were associated with cardiac death. The addition of the exercise MPS to the gated MPS variables significantly improved the chi-square of the model, thus establishing incremental prognostic value. The Candell-Riera study, like any clinical study, has limitations when scrutinized for scientific rigor, and many of these are acknowledged by the authors. The results however are biologically tenable, and consistent with our understanding of the relationships between exercise capacity, myocardial viability, and outcome in heart failure. It is also one of the few prospectively conducted studies of the prognostic utility of MPS, for See related article, pp. 212–221