1. Introduction

The ST-segment depression/heart rate (ST/HR) hysteresis [Lehtinen et al., 1996a, 1996b] is a recently introduced novel computerized method for integrating the exercise and recovery phase ST/HR analysis for improved diagnosis and prognostication of coronary artery disease. It is a continuous variable, which extracts the prevailing direction and average magnitude of the hysteresis in ST depression against HR during the first three consecutive minutes of postexercise recovery (Fig. 1).

Figure 1. Plots of ST-segment depression against HR during both the exercise and first three minutes of postexercise recovery (i.e., ST/HR diagrams) illustrating the computerized determination of the ST/HR hysteresis from a single lead. In these diagrams, ST depression is plotted in an upward direction on the vertical axis, and the negative values represent ST elevation. Typical ST/HR diagrams are shown for a clinically normal subject (left panel) with an unambiguously negative hysteresis in ST depression against HR (i.e., clockwise hysteresis loop in recovery) and for a patient with coronary artery disease (right panel) with an unambiguously positive hysteresis in ST depression against HR (i.e., counterclockwise hysteresis loop in recovery). In a graphic sence, the ST/HR hysteresis was obtained by dividing the area of hysteresis by the DHRrec.

2. Review of the Results

In a clinical population of 347 patients referred for routine bicycle exercise ECG test in Tampere University Hospital (Tampere, Finland), the ST/HR hysteresis has been shown to have significantly better diagnostic performance (area under the receiver operating characteristics curve = AUC = 0.89) in detection of coronary artery disease than the end-exercise ST depression (AUC = 0.76, p < 0.0001), recovery ST depression (AUC = 0.84, p = 0.0063) or ST/HR index (AUC = 0.83, p = 0.0023) [Lehtinen et al., 1996b, Lehtinen 1997c]. Furthermore in that study population, the superior diagnostic performance was shown to be relatively insensitive to the ECG lead selection [Viik et al., 1997; Lehtinen 1999, Viik et al., 1999] and to ST-segment measurement point [Lehtinen et al., 1997]. In a study population of 61 middle-aged asymptomatic subjects, which were exercise tested twice with bicycle ergometry, the agreement of interpretation between the repeated measurements were significantly higher for ST/HR hysteresis (97%) than that for end-exercise ST depression (74%, p = 0.0010) or ST/HR index (79%, p = 0.0045) [Lehtinen et al., 1997]. The ST/HR hysteresis performs well especially in female population [Viik et al., 1998a; 1998b] and it has been found to be an independent predictor of mortality in 2480 thallium SPECT exercise tested patients, which were followed 6.2 years in Cleveland Clinic (Cleveland, OH, USA), even after adjusting for ST/HR index and thallium perfusion defects [Cole et al., 2000].

3. Conclusion

The studies made suggest that the ST/HR hysteresis can significantly improve not only the diagnostic performance, but also the prognostication of coronary artery disease. In order to facilitate the worldwide validation, the method should be implemented in computerized exercise ECG analyzers thus becoming accessible to clinical users.

Acknowledgements

The financial support of Academy of Finland and Ragnar Granit Foundation is gratefully acknowledged.

References