Last updated 10/4/25
Introduction
The P wave reflects atrial depolarization and provides critical clues about sinus rhythm, atrial size, conduction pathways, and supraventricular arrhythmias. Though often overlooked, P-wave analysis helps identify atrial enlargement, ectopic atrial rhythms, AV conduction disease, and pre-excitation syndromes.
Normal P Wave Morphology
- Duration: ≤120 ms (3 small boxes)
- Amplitude: ≤2.5 mm in inferior leads (II, III, aVF)
- Axis: Normally 0° to +75° (upright in I and II, variable in III; inverted in aVR)
- Shape: Smooth, monophasic in most leads; biphasic in V1 is normal if terminal negative portion ≤1 mm deep and <40 ms wide.
- Lead V1: Best to assess atrial enlargement — initial positive = RA, terminal negative = LA.
From LITFL.com
Abnormal P Wave Patterns
Right Atrial Enlargement (RAE)
- ECG Criteria
- Tall, peaked P >2.5 mm in II/III/aVF.
- P >1.5 mm in V1–V2.
- Also known as “P pulmonale.”
- Common Causes: Pulmonary hypertension, tricuspid disease, COPD, congenital heart disease.
- Pearl: Axis may shift rightward; often associated with RAD.
Left Atrial Enlargement (LAE)
- ECG Criteria
- P >120 ms in II with notched/bifid peaks (≥40 ms apart).
- Terminal negative P in V1 >1 mm deep & >40 ms wide.
- Also known as “P mitrale.”
- Common Causes: Mitral stenosis/regurgitation, LVH, HOCM, hypertension.
- Pitfall: LAE may appear with normal amplitude but prolonged duration.
Biatrial Enlargement
- ECG Criteria
- P tall (>2.5 mm) and wide (>120 ms) in II.
- V1: prominent initial + terminal − portions.
- Causes: Combined valvular disease, pulmonary hypertension, LVH.
More on ECG-Topic Atrial Enlargement
Pearl: If you see flattening of the p wave with PR prolongation, think hyperkalemia.
P Wave & Rhythm Diagnosis
Pattern | Key ECG Feature | Clinical Pearl |
Sinus rhythm | Upright P in I, II; negative in aVR | Consistent morphology & PR interval |
Low atrial / ectopic atrial rhythm | Inverted P in inferior leads | Often slower than sinus; may follow ablation or surgery |
Wandering atrial pacemaker | ≥3 P morphologies, variable PR | HR <100 (if >100 → multifocal atrial tachycardia) |
Atrial flutter | “Sawtooth” baseline; P waves absent/replaced | Flutter waves most visible in II, III, aVF, V1 |
Atrial fibrillation | No discrete P; fibrillatory baseline | Irregularly irregular RR |
Junctional rhythm | P absent, inverted after QRS, or short PR | Retrograde atrial activation |
Premature atrial contraction (PAC) | Early, abnormally shaped P with non-compensatory pause. | Appears out of place on the rhythm lead. |
ECG Example - Sinus Rhythm
- Normal P wave axis: up in II, II, aVF and down in avR
- Fixed PR intervals
- P:QRS ratio 1:1
ECG Example - Ectopic Atrial Rhythm
- Opposite P wave axis: down in II, II, aVF and up in avR
- Fixed PR intervals
- P:QRS ratio 1:1
ECGExample - Wandering Pacemaker
- Note the ≥ 3 different p wave morphologies in the rhythm lead
ECG Example - Atrial flutter
- Note the sawtooth pattern, P:QRS > 1
- P-like flutter waves have similar morphology within each lead
ECG Example - Atrial fibrillation
- Irregular rhythm, varying R-R intervals
- May see small fibrillations that could resemble p waves do not have the same morphology in each lead
ECG example - Junctional rhythm
From LIFTL
- Opposite P wave axis: down in II, II, aVF and up in avR
- Fixed short PR intervals
- P:QRS ratio 1:1
ECG example - Premature Atrial Contractions (PACs)
- Focus on lead II rhythm strip - Note the breaks in the rhythm preceded by a beat with an inverted P wave. These are PACs.
Pearl: If you only have few seconds: look at lead II and V1. It can tell you if there is atrial enlargement and if the rhythm is sinus or not.
Key Points
- P wave = atrial depolarization: Normal ≤120 ms wide, ≤2.5 mm tall in inferior leads, axis 0°–+75° (up in II, III, aVF and down in aVR)
- RAE: tall peaked P (>2.5 mm in II/III/aVF or >1.5 mm in V1–V2); LAE: wide/notched P in II or deep terminal negative V1; Biatrial: tall + wide in II.
- Lead II & V1 are the quickest screen for atrial enlargement and sinus vs non-sinus rhythms.
- Loss or flattening of P can signal early hyperkalemia; PR depression isn’t always pericarditis (consider atrial ischemia).
- Rhythm tips: ≥3 P morphologies = wandering atrial pacemaker; retrograde/absent P with short PR = junctional; sawtooth = flutter; no discrete P + irregular RR = AF.
Related Posts
- Atrial Enlargement (LAE, RAE, BAE)
- ECG Exercise on Atrial Enlargement
- The PR Interval
- Pericarditis
- Wolff-Parkinson White
- Lown–Ganong–Levine
References
- Brady WJ, Mattu A, Tabas JA. Critical Decisions in Emergency and Acute Care Electrocardiography. Wiley-Blackwell; 2009.
- Burns E, Buttner R. P wave abnormalities. Life in the Fast Lane ECG Library. Updated 2024.
- Chou TC, Surawicz B. Chou’s Electrocardiography in Clinical Practice. 6th ed. Saunders/Elsevier; 2008.
- Davila C. The ECG. Updated ed. Self-published; 2024.
- Garcia TB. 12-Lead ECG: The Art of Interpretation. 2nd ed. Jones & Bartlett Learning; 2013.
- Mattu A, Brady WJ. ECGs for the Emergency Physician 1. BMJ Books; 2003.
- Mattu A, Brady WJ. ECGs for the Emergency Physician 2. BMJ Books; 2008.
- Podrid PJ. Podrid’s Real-World ECGs: A Master’s Approach to the Art and Practice of Clinical ECG Interpretation. Vol 6. Cardiotext Publishing; 2016.
This post is for the education and not medical advice.