18 Normal infants were chosen for this investigation. Their E. C. G. (Standard limb leads, Unipolar limb leads, Unipolar precordial leads) were followed up successively recorded for every lead. Electrocardiograms taken under the same conditions were analysed to study electrocardiographic changes with the growth of the infant. The following results were obtained: 1) R-R Interval, Heart Rate and Sinus Arrythomia.
Decreasing Heart Rate was observed during period from 10 days to 2-3 weeks after birth and this condition discontinued at about 4 weeks of age. Maximum average Heart Rate was found at 2 month of age, followed by a decrease in Heart Rate as the age increased. Incidence of Sinus Arrythomia was 21, 6% and there was no change with the growth of the child. 2) Duration of P wave.
Duration of P wave increased on every lead with the advancement of age. Only the duration of PIII wave showed no definite changes, which indicated indefinite variation in the same infant. 3) PQ Interval. At every lead PQ Interval increased with advancing age with a temporaly decrease of PQ (VI) and PQ (V5) Interval at the 2nd month. Direct co-operation was establiched between change of PQ(II) and of R-R Interval. The following formula was set up: PQ(II)=0.236 R-R+1.01±0.578 4) QRS Interval. QRS Interval became longer with each increasing month at every lead, and there was significant direct co relation between change of QRS and R-R Interval. The next formula was established: QRS(II)=0,0819 R-R+1, 70±0,597 5) QT Interval. QT Interval increased at every lead with each increasing month, and there was direct co-relation between change of QT(II) and of R-R Interval with growth. This co-relation was indicated by the next formula: QT(II)=0,1231 R-R+16,66±0,59 6) Venticular Activation Time. (V. A. T.) During the first 3 months V. A. T. (VI) was greater than V. A. T. (V5), both became equal at 4th month and then this relation was inverted. This change might be due to the rapid increase of left ventricular thickness.