Premature atrial contraction (PAC)
Also calledPAC · atrial premature beat · APB · atrial premature complex · PAB
Definition
An early atrial depolarization from an ectopic focus outside the sinoatrial node, seen as a premature P wave whose morphology differs from the sinus P wave. The QRS that follows is typically narrow but may be wide if aberrant conduction occurs, and a very early PAC may reach the AV node while it is refractory and not be conducted at all. The pause after a PAC is typically non-compensatory (less than twice the sinus cycle length) because the premature impulse resets the sinus node; when it does not reset it, a compensatory pause follows.
Clinical noteA non-conducted (blocked) PAC is one of the causes of an unexpected pause on a rhythm strip, and frequent blocked PACs can mimic bradycardia.
Sources
- Yu R, Qi J, Wang X. Pediatric premature atrial contractions: clinical characteristics, diagnosis, and management: a literature review. Am Heart J Plus. 2026;68:100811. doi.org/10.1016/j.ahjo.2026.100811
- Viljoen C, Smith R, Chin A. Reviewing the causes of electrocardiographic pauses. Cardiovasc J Afr. 2017;28(4):257-260. doi.org/10.5830/CVJA-2017-041
- Samesima N, God EG, Kruse JCL, et al. Brazilian Society of Cardiology guidelines on the analysis and issuance of electrocardiographic reports - 2022. Arq Bras Cardiol. 2022;119(4):638-680. doi.org/10.36660/abc.20220623
- Mason JW, Hancock EW, Gettes LS. Recommendations for the standardization and interpretation of the electrocardiogram: Part II: electrocardiography diagnostic statement list. Circulation. 2007;115(10):1325-1332. doi.org/10.1161/CIRCULATIONAHA.106.180201
Last reviewed 2 October 2026. Every definition in the EKG Dictionary cites the guideline, consensus statement or original paper it comes from.