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

Increased spatio-temporal heterogeneity in ventricular repolarization is related to cardiac instabilities. Slowed adaptation of the QT interval to sudden abrupt changes in heart rate (HR), measured by the time constant of a first-order-like step response, has been identified as a marker of arrhythmic risk. Since abrupt HR changes are difficult to induce in patients, ramp-like HR variations observed in exercise stress test are considered in this work. The first-order-system time constant is estimated as the delay between the QT series response and the series of memoryless expected QT series from the instantaneous HR, ob-tained from a fitted hyperbolic regression model. The delay was estimated by minimizing the mean square error separately in the exercise and recovery phases. We applied this procedure to a subset of 251 patients of Tampere university hospital with different risk levels for Coronary Artery Disease (110 nonCAD, 39 low-risk, 12 mild-risk, 90 high-risk). The time constant values ranged from 17s to 49s, which are comparable with the time lags reported in other studies in response to step-like HR changes. The delays in exercise and recovery phases and the difference between these delays are markers with capacity for CAD risk stratification.