Stress Function Index, a Novel Index for Risk Stratification and Prognosis Using Stress Echocardiography

Abstract

The purpose of the study was to define an appropriate parameter for risk stratification and prognosis of patients undergoing stress echocardiography. Among stress echocardiography variables, peak wall-motion score index (WMSI) and ejection fraction (EF) have been shown to be independent and significant predictors of cardiovascular morbidity and mortality. Data on the impact and importance of each parameter in risk stratification and prognosis are limited. We evaluated 1560 patients (59 +/- 13 years; 51% men) undergoing stress echocardiography (36% treadmill, 64% dobutamine). Peak WMSI was derived from the cumulative sum of 16 left ventricular segments divided by sum of visualized segments at peak stress. The ratio of peak WMSI to EF was calculated for the entire cohort. Based on this ratio and using the receiver operating characteristic curve, patients were divided into 3 groups: low- (< 1.9), intermediate- (1.9-3.1), and high- (> 3.1) risk subgroups. Follow-up (2.8 +/- 1.1 years) for confirmed myocardial infarction (n = 26) and cardiac death (n = 38) were obtained. Stress echocardiography effectively risk stratified patients into low- (0.7%/y), intermediate- (2.0%/y), and high- (4.4%/y) risk subgroups (P < .0001) based on the ratio of peak WMSI to EF. Cox proportional hazard model showed that risk stratification based on the ratio of peak WMSI to EF (global chi2 = 106.05; P < .0001) provided incremental value beyond that provided by risk stratification by peak WMSI (global chi2 = 79.23; P < .0001) or risk stratification by EF alone (global chi2 = 87.12; P < .0001). The ratio of peak WMSI to EF (stress function index) provides best incremental prognostic value and effectively risk stratifies patients into low-, intermediate-, and high-risk subgroups and is better than risk stratification by either peak WMSI or EF alone. The results of stress echocardiography should routinely combine peak WMSI and EF for effective risk stratification.