Electrocardiographic changes in the growing Mastín Español.

Serial electrocardiograms were recorded from 70 Mastín Español dogs in right lateral recumbency, aged between one day and three years, in order to observe the changes in the waveform and intervals, QRS morphology, cardiac rhythm and heart rate caused by their growth. Age and bodyweight caused a gradual increase in the duration and amplitude of the P wave, duration of the PR, QT and RR intervals, amplitude of the R wave and duration of the QRS complex and ST segment. Q wave was observed in nearly all the recordings with different amplitudes. The S wave was only significant in one-day-old animals and QRS morphology showed significant important changes from qrS and rS to qR and qRs morphologies during the first two weeks of life. The T wave increased its amplitude until the age of two months and changed its polarity (from negative to positive) from five months of age onwards. The heart rate decreased until the age of seven months, reaching stabilised values of 110 +/- 7.3 beats per minute. Sinus tachycardia was commonplace in animals under one month old and respiratory sinus arrhythmia was found from six months of age. Sex only influenced the duration and amplitude of the T wave. Males had higher mean values than females.