- PR interval: period from beginning of P wave to beginning of QRS (ie, P wave + PR segment)
- Cardiac event: transmission from atrium (adjacent to sinus node) to ventricle (adjacent to Purkinje-network fibers)
PR interval duration:
- In general: ↑ PR interval with ↑ age 
- Average based on age:
- 18-22 weeks GA: 102 ms
- Term: 110 ms
- Birth: 107 ms
- 1 month: 98 ms
- ↓ PR interval from birth-to-1 month primarily due to autonomic factors 
- ↑ PR interval after 1-month due to slowing of AV nodal conduction related to intrinsic changes in AV node & ↑ in atrial size
- Lower limit of normal:
- Children: 90 ms
- Adults: 120 ms
- Upper limit of normal:
- Young infants: 140 ms
- Adolescents: 180 ms
- Adults: 200 ms
Short PR interval:
- Due to conduction over (1) accessory AV connection (more common) or (2) fasciculoventricular pathway 
- May be difficult to recognize with short PR interval
- Suspect if: 
- PR interval <100 ms
- No Q waves in left-lateral precordial leads (V5-V6)
- LAD (QRS in frontal plane)
Prolonged PR interval:
1st-degree AV block
- Due to conduction slowing at atrium, AV node, or His-Purkinje system in children 
- Occurs at only one of these sites in children with heart disease 
1. Chia EL, Ho TF, Rauff M, et al. Cardiac time intervals of normal fetuses using noninvasive fetal electrocardiography. PrenatDiagn 25:546, 2005.
2. Perry JC, Giuffre, Garson A Jr. Clues to the electrocardiographic diagnosis of subtle Wolff-Parkinson-White syndrome in children. J Pediatr 117:871, 1990.
3. Sallee D III, Van Hare GF. Preexcitation secondary to fasciculoventricular pathways in children: a report of three cases. J Cardiovasc Electrophysiol. 10:36, 1999.
4. Sarubbi B, Mercurio B, Ducceschi V, et al. A “multisite” atrioventricular block. Ital Heart J 5:64, 2004.
5. Sherron P, Torres-Arraut E, Tamer D, et al. Site of conduction delay and electrophysiologic significance of first degree atrioventricular block in children with heart disease. Am J Cardiol 55:1323, 1985.