ELECTROCARDIOGRAPHIC CHANGES IN THE GROWING MASTIN-ESPANOL

Serial electrocardiograms were recorded from 70 Mastin Espanol dogs in right lateral recumbency, aged between one day and three years, in or der to observe the changes in the waveform and intervals, QRS morpholo gy, cardiac rhythm and heart rate caused by their growth. Age and body weight 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 wa ve and duration of the QRS complex and ST segment. Q wave was observed in nearly all the recordings with different amplitudes, The S wave wa s only significant in one-day-old animals and QRS morphology showed si gnificant important changes from qrS and rS to qR and qRs morphologies during the first two weeks of life. The T wave increased its amplitud e 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 und er one month old and respiratory sinus arrhythmia was found from six m onths of age. Sex only influenced the duration and amplitude of the T wave. Males had higher mean values than females.