LANG, C.C.E., et al.: The Impact of QT Lag Compensation on Dynamic Assessment of Ventricular Repolarization: Reproducibility And The Impact of Lead Selection. In cardiac disease, abnormalities exist in the rate-corrected QT interval and the relationship between QT and heart rate. The QT/RR relationship is known to be dynamic and show circadian variation. The availability of automated methods for measurement of QT and RR intervals allows monitoring of the QT/RR relationship and may provide insights into arrhythmia onset. Using a method for analyzing 24-hour recordings that incorporates beat-by-beat QT and RR measurement and an automated mechanism for compensating for lag in adaptation of QT to changes in RR, the authors evaluated the impact of lag compensation on assessment of the QT/RR relationship, reproducibility, and the effect of lead selection in 15 normal subjects. The QT/RR relationship is continuously estimated from the lag compensated data over a 5-minute scrolling time frame. The relationship is expressed as an exponential formula, QT = QTo · RRJ where QTo is the QT interval at a standardized RR interval of 1 second and J is a variable exponent. We found that the use of lag compensation significantly improves the mean 24-hour correlation between QT and RR data (r = 0.87 vs 0.65). The 24- hour mean of QTo and J were highly reproducible (coefficients of variation 2% and 8%, respectively). The mean 24-hour QT/RR relationship for the population was QT = 0.415 · (RR)0.32. There was a small difference between leads in QTo and J. Compensating for QT adaptation lag provides a means of assessing the QT/RR relationship over long and short periods. This method allows investigation of the effect of acute interventions on the dynamic QT/RR relationship, which has previously been restricted by the presence of QT hysteresis.