P wave
Also calledP-wave
Definition
The first deflection of the cardiac cycle, produced by atrial depolarization; activation of the left atrium begins and ends later than that of the right. In sinus rhythm it is upright in leads I, II and aVF and inverted in aVR, normally lasts less than 120 ms and is no taller than 2.5 mm (0.25 mV). Right atrial abnormality makes it taller (more than 2.5 mm in lead II, often peaked), while left atrial abnormality makes it longer (120 ms or more) or notched; the older terms for these patterns are P pulmonale and P mitrale.
Clinical noteAn absent P wave or a P-wave axis outside 0 to +90 degrees points to a rhythm other than sinus; P waves unrelated to the QRS complexes mean the atria and ventricles are driven separately, as in AV dissociation or complete heart block.
Sources
- Hancock EW, Deal BJ, Mirvis DM, et al. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part V: electrocardiogram changes associated with cardiac chamber hypertrophy. Circulation. 2009;119(10):e251-e261. doi.org/10.1161/CIRCULATIONAHA.108.191097
- 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
- Meek S, Morris F. ABC of clinical electrocardiography. Introduction. II: Basic terminology. BMJ. 2002;324(7335):470-473. doi.org/10.1136/bmj.324.7335.470
Last reviewed 2 October 2026. Every definition in the EKG Dictionary cites the guideline, consensus statement or original paper it comes from.