Two digital electrocardiographs and their computer-based analysis programs produce clinically significant differences in values and reference intervals for dog electrocardiograms

Objective: To compare measured ECG intervals and amplitudes obtained using electrocardiographs adapted for veterinary or human use and to develop ECG reference intervals for endurance-trained dogs. Methods: Standard limb lead ECG recordings were obtained simultaneously in February and early March of 2021 using 2 digital electrocardiographs and a convenience sample of 121 endurance-trained sled adult dogs. Visually verified computer-based ECG measurements for the 2 electrocardiographs were compared using the Wilcoxon signed-rank test, Deming regression, and Bland-Altman plots. We assigned acceptable median values for differences (effect sizes) of < 10 milliseconds for P-wave duration, PQ interval, QRS duration, JT interval, and QT interval and < 10% for P-, Q-, R-, S-, and T-wave amplitudes. Results: Statistically significant differences between the 2 electrocardiographs existed for the majority (7 of 9) of global ECG indices and the majority (10 of 15) of lead I-, II-, and III-wave amplitudes. Clinically important differences based on assigned effect sizes were identified for lead II and III Q- and R-wave amplitudes, which the veterinary electrocardiograph measured 11% to 23% higher. The veterinary electrocardiograph identified a higher proportion of dogs that may have left or right ventricular enlargement. Reference intervals developed for 7 of 9 global ECG indices were narrower for the veterinary electrocardiograph. Conclusions: The low-pass filter on the veterinary electrocardiograph attenuates higher frequencies on the canine ECG to a lower extent than the human electrocardiograph. Clinical Relevance: Reference intervals for dog ECGs depend on the digital electrocardiograph, its high-frequency filtering characteristics, and its computer-based analysis program.