Evolution of the electrocardiogram in young dogs during the first 12 weeks of life.

In 36 normal pups the evolution of the ECG during the first 12 weeks was studied. Sixteen pups were randomly selected, healthy pups; 20 were normal littermates from dog families with congenital heart disease. The direction of P, QRS, and T vectors was determined, and the modal QRS axis (vector) was constructed from lead I and AVF in the frontal, from lead I and V10 in the transverse, and from lead AVF and V10 in the sagittal plane. Except for the T wave directly after birth, no marked difference in the ECG evolution between the two groups was found. After birth the modal QRS vector was almost exclusively directed to the right. The modal QRS vectors in the first, second, and third weeks were significantly different from each other. By the twelfth week all vectors were directed to the left and the majority was oriented leftward and caudally. In the serial ECG tracings the change from right ventricular to left ventricular dominance was seen to be progressive. The R/S ratio in the left chest leads increased from less than 1.0 at week 1 to a multiple of 1.0 after the sixth week. The changes occurring in scaler ECG's were mirrored in VCG's recorded in selected pups.