- P wave: 1st positive or negative deflection of cardiac cycle
- Cardiac event: atrial depolarization
- Right atrium depolarizes before left atrium due to proximity to sinus node
- Initial portion of P wave represents right atrial depolarization
- Terminal portion of P wave represents left atrial depolarization
P wave axis (frontal plane):
- Directed leftward & inferiorly
- Normal P waves: upright in I, II, & V4-V6; inverted in aVR
P wave amplitude:
- Lacks age-related variation [1,2]
- Changes less in children
- Lead II: similar to 2.5 mm adult normal maximum
- Average: 1.5 mm (50th percentile) & 2.5 mm (98th percentile) 
- 50th-95th percentile: 1.5-2.0 mm (birth), 1.8-2.9 mm (16 y/o) 
- 50th-98th percentile: 1.4-2.4 mm (birth), 1.9-2.9 mm (16 y/o) 
- Right precordial leads (V1-V2): not as extensively studied
- Positive P wave in V1 >1.5 mm (adult normal) rare in normal pediatric heart
P wave duration & dispersion:
- Dispersion = range of P wave durations on 12-lead ECG
- Age-dependent in fetus  & children  (likely from ↑ in atrial size with ↑ age)
- Duration progressively ↑ from early fetal gestation to adolescence [1,3,4]
- Birth: 51-65 ms (50th-98thpercentile) 
- 12-16 y/o: 81-95 ms (50th-98thpercentile) 
- Adult: 120-200 ms
1. Liebman J. Atrial hypertrophy. In: Liebman J, Plonsey R, Gillette P (eds): Pediatric Electrocardiography. Baltimore, Williams & Wilkins, 1982, p 140.
2. Davignon A, Rautuharuju P, Boisselle E, et al. Normal ECG standards for infants and children. Ped Cardiol 1:123, 1979.
3. Chia EL, Ho TF, Rauff M, et al. Cardiac time intervals of normal fetuses using noninvasive fetal electrocardiography. PrenatDiagn 25:546, 2005.
4. Kose S, Kilic A, Iyisoy A, et al. P wave duration and P dispersion in healthy children. Turk J Pediatr 45:133, 2003.