R wave = 1st positive deflection after P wave

Right precordial leads (V1-V2): R wave often prominent in infants (likely due to RV dominance & vertical position of heart)

• V1: R wave up to 26 mm may be normal in term newborn[1]
• Rapid ↓ in amplitude in 1stweek[2] ↓ with age

Mid-precordial lead V4: R wave often remains prominent from birth through adolescence[1]

Left-lateral precordial leads (V5-V6):R wave may be nearly absent in term newborn

• Gradually ↑ from birth to adolescence
• ↑ voltage in black (vs. white) adolescents[3]

References
1. Davignon A, Rautaharju P, Boisselle E, et al. Normal ECG standards for infants and children. Ped Cardiol 1:123, 1979.
2. Walsh SZ. The electrocardiogram during the first week of life. Br Heart J 25:784, 1963.
3. Rao PS. Racial differences in electrocardiograms and vectorcardiograms between black and white adolescents. J Electrocardiol 18:309, 1985.