RÉSUMÉ
Evaluation of the cardiovascular system of canine pediatrics requires awareness of the anatomical and physiological changes occurring from birth to six months of age. The aim of this study was to report electrocardiographic changes in the littermate mongrel dogs from birth to six months. Serial six-lead electrocardiograms were recorded from ten normal littermate mongrel dogs kept on right lateral recumbency, aged between birth and six months. Heart rate significantly decreased until the age of six months, reaching values of 99 +/- 5 beats per min. Age as well as body weight caused a gradual increase in duration of the P wave, P-R and Q-T intervals, S-T segment and also amplitude of the R and T waves. Sinus rhythm was commonplace in dogs under 18-week-old and sinus arrhythmia was found from 16 weeks of age. Sex only influenced the amplitude of the R and Q waves. Males had higher mean values of the R wave amplitude [0.991 +/- 0.050] than females [0.740 +/- 0.039], but females had higher mean values of the Q wave amplitude [0.256 +/- 0.013] than males [0.217 +/- 0.010]. The mean electrical axis [ +/- SD] of QRS waves in this study was 69.61° +/- 21.47°. The possible dependence of the electrocardiographic alterations on age should be taken into consideration in practical veterinary medicine when working with dogs of different ages