Normal electrocardiographic QT interval in race-fit Standardbred horses at rest and its rate dependence during exercise

Objectives: Cardiac repolarization, measured as QT and T-peak to T-end (TpTe) intervals on the ECG, is important, as irregularities caused by diseases, ventricular hypertrophy, drugs and genetic defects can trigger arrhythnnias which predispose human patients to syncope and sudden cardiac death. In horses, repolarization is not well described and therefore QT analysis cannot yet be used diagnostically. Therefore, we sought to describe reference values for the normal QT and TpTe intervals in Standardbreds and to determine the best method for heart rate (HR) correction. Animals: 30 Standardbreds. Methods: QT and TpTe intervals were measured during rest and exercise and plotted against HR converted to R-peak to R-peak interval (RR). Data were fitted with relevant regression models. Intra- and inter-observer agreement was assessed using Bland-Altman analyses. Results: Data were best described by a piecewise linear model (r(2) > 0.97). Average prediction error of this model was smaller than for both Bazett and Fridericia corrections. Coefficient of repeatability of intra- and inter-observer variability was 8.76 ms and 5.64 ms respectively and coefficient of variation was 1.77% and 2.76% respectively. TpTe increased with RR in stallions. Conclusions: The QT interval in Standardbred horses shortens with decreasing RR interval (increasing HR) as in humans, but in a markedly different order as it clearly follows a piecewise linear model. The equine QT interval can be measured easily and there is small intra- and inter-observer variability. This model of the equine QT interval provides clinicians with a method that could support a diagnosis of repolarization disturbances in horses. (C) 2013 Elsevier B.V. All rights reserved.