Changes in electrocardiographic measurements were studied in growing Holstein cows. These cows, including neonatal and 12-year-old ones, were kept on the Stock Farm, Faculty of Agriculture, University of Tokyo. Electrocardiograms were recorded in the A-B bipolar lead and the standard limb lead. Those recorded in the A-B lead were used to measure the intervals of RR, PQ and QT, the durations of P, QRS and T waves, and the amplitudes of P, QRS and T waves. The mean electrical axis of the QRS complex was calculated from those recorded in the limb lead. The results obtained are summarized as follows.
1) The RR, PQ and QT intervals and the P and QRS durations increased with growth.
2) No changes were observed in P wave amplitude with age. The amplitude of the R wave was rather high till 30 days of age. The wave form of the QRS complex was of rS type. Afterward, it shifted to the QS type with no R wave. The amplitude of the S wave decreased gradually until 3 or 4 months of age. It was stabilized in about 0.8 mV. The amplitude of the T wave was fluctuated from 0 to 0.5 mV through all the stages.
3) The mean electrical axis of the QRS complex changed with age. The QRS vector axis was oriented forward to the left till 100 or 200 days of age. After that, it was oriented backward to the left.
4) Prolongation of PQ and QT intervals and forward deviation of the QRS vector axis were observed in cows of over the 4th calving.
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