Characterization of Impaired Ventricular Repolarization by Quantification of QT Delayed Response to Heart Rate Changes in Stress Test

Cristina Pérez1, Esther Pueyo1, Juan Pablo Martínez2, Jari Viik3, Pablo Laguna4
1University of Zaragoza,, 3Tampere University of Techonology, 4Zaragoza University


Motivation and aim: The adaptation time of the QT interval to sudden abrupt heart rate (HR) changes modeled as a first-order system response an ECG marker to stratify patients for arrhythmic risk. In such a first-order model, this time constant, tau, is the same as the delay in responding to a ramp-like HR change. Thus, the time lag between the actual QT series and the expected memoryless HR-dependent QT series can be estimated from the exercise and recovery phases, being much more feasible than from sudden step-like HR changes.

Materials: 39 stress test ECG recordings were selected from FINCAVAS study and divided into three groups, according to their likelihood for Coronary Artery Disease (CAD): low-risk, mild-risk and high-risk patients.

Methods: The memoryless expected HR-dependent QT interval time series was calculated by fitting four regression models to the [QT(n);RR(n)] data pairs in three stationary QT-to-RR dependent windows. The time lag, tau_*, was estimated by a Mean Square Error fit between the real QT(n) response ramp and the memoryless expected HR-dependent QT(n-tau_*) ramp, separately in exercise, tau_e , and recovery, tau_r , phases.

Results and conclusion: The hyperbolic model provided the best fitting, and therefore, the selected one. The average time lags for exercise and recovery phases (tau_e = 26.65s and tau_r = 69.97s, respectively, in the low-risk group) are in line with those of a previous study estimated from step-like HR changes in daily activities (tau_e = 34.79s and tau_r= 48.40s). Results show that the delay tau_e increases for higher risk patients; and the difference, Delta_tau, between delays tau_e and tau_r is remarkably larger for low-risk patients, being much reduced when CAD risk increases. Interestingly, the delay tau_r manifests a significantly reverted behavior. In conclusion, the proposed markers tau_e, tau_r and Delta_tau show potential for CAD risk stratification.