- Represents ventricular depolarization
- Ventricular depolarization sequence similar to adults:
- (1) Septal & paraseptal
- (2) Ventricular free wall
- (3) Posterobasal septum &posterior ventricles
- Depends on:
- (1) Relative ventricular mass (RV & LV)
- (2) Cardiac position
- (3) Body mass & habitus
- (4) Tissue impedance (ie, effective resistance)
- <31 weeks gestation: LV > RV size 
- Preterm infant (<37 weeks): less prominent RV dominance than term infant , unless RVH 2/2 lung disease of prematurity (ECG may assist in following chronic residual lung disease in such cases) 
- Term infants at birth: RV > LV size 
- Prominent R waves in right precordial leads (V1-V2)
- Deep S waves in left-lateral precordial leads (V5-V6)
- Term infants at 1 month: LV > RV size
- Notched R waves in V1 rarely seen in neonates, but common in normal infants >2 months (sign of RV conduction delay in adults)
- Term infants at 6 months: LV-to-RV size reaches adult ratio , but ECG differs from adult pattern 
- Variation from adult pattern possibly from more vertical orientation of heart in infants, which results in more prominent mid-precordial voltage & less voltage in left-lateral precordial leads (V5-V6)
- RV dominance regression pattern in V1: 
- (1) Notch in upstroke of R wave (young infants >2 months)
- (2) Notch in downstroke of R wave
- (3) S wave (older infants)
- Term & preterm infants: very short 
- Children have higher frequency QRS signals than adults & therefore may require higher bandwidth recording (250 Hz) to accurately measure QRS duration 
- Mean QRS duration: generally ↑ with age after birth (likely 2/2 ↑ in ventricular mass)
- 18-22 weeks gestation: 47 ms 
- Term: 53 ms 
- Birth: 50 ms [6,9]
- Adolescence: 70 ms [6,9]
QRS axis (frontal plane):
- Healthy preterm infants: leftward & posteriorly 
- Term infants: rightward & inferiorly (due to RV dominance) 
- Changes to adults pattern (leftward & inferiorly) within 1st year
1. Emery JL, Mithal A. Weights of cardiac ventricles at and after birth. Br Heart J 23:313, 1961.
2. Sreenivasan VV, Fisher BJ, Liebman J, et al. Longitudinal study of the standard electrocardiogram in the healthy premature infant during the first year of life. Am J Cardiol 31:57, 1973.
3. Walsh EP, Lang P, Ellison RV, et al. Electrocardiogram of the premature infant at 1 year of age. Pediatrics 77:353, 1986.
4. Onat T, Ahunbay G. Regression of right ventricular dominance in the electrocardiogram after birth: a longitudinal follow-up of a healthy cohort from birth to 3 years. PediatrCardiol 19:197, 1998.
5. Emery JL, Mithal A. Weights of cardiac ventricles at and after birth. Br Heart J 23:313, 1961.
6. Davignon A, Rautaharju P, Boisselle E, et al. Normal ECG standards for infants and children. Ped Cardiol 1:123, 1979.
7. Rijnbeek PR, Kors JA, Witsenburg M. Minimum bandwidth requirements for recording of pediatric electrocardiograms. Circulation 104:3087, 2001.
8. Chia EL, Ho TF, Rauff M, et al. Cardiac time intervals of normal fetuses using noninvasive fetal electrocardiography. PrenatDiagn 25:546, 2005.
9. Macfarlane PW, McLaughlin SC, Devine B, et al. Effects of age, sex, and race on ECG interval measurements. J Electrocardiol 27(Suppl):14, 1994.