P Wave Overview
- The P wave is the first positive deflection on the ECG
- It represents atrial depolarisation
Characteristics of the Normal Sinus P Wave
- Smooth contour
- Monophasic in lead II
- Biphasic in V1
- Normal P wave axis is between 0° and +75°
- P waves should be upright in leads I and II, inverted in aVR
- < 120 ms
- < 2.5 mm (0.25mV) in the limb leads,
- < 1.5 mm (0.15mV) in the precordial leads
Atrial abnormalities are most easily seen in the inferior leads (II, III and aVF) and lead V1, as the P waves are most prominent in these leads.
The Atrial Waveform – Relationship to the P wave
- Atrial depolarisation proceeds sequentially from right to left, with the right atrium activated before the left atrium.
- The right and left atrial waveforms summate to form the P wave.
- The first 1/3 of the P wave corresponds to right atrial activation, the final 1/3 corresponds to left atrial activation; the middle 1/3 is a combination of the two.
- In most leads (e.g. lead II), the right and left atrial waveforms move in the same direction, forming a monophasic P wave.
- However, in lead V1 the right and left atrial waveforms move in opposite directions. This produces a biphasic P wave with the initial positive deflection corresponding to right atrial activation and the subsequent negative deflection denoting left atrial activation.
- This separation of right and left atrial electrical forces in lead V1 means that abnormalities affecting each individual atrial waveform can be discerned in this lead. Elsewhere, the overall shape of the P wave is used to infer the atrial abnormality.
Normal P-wave Morphology – Lead II
- The right atrial depolarisation wave (brown) precedes that of the left atrium (blue).
- The combined depolarisation wave, the P wave, is less than 120 ms wide and less than 2.5 mm high.
Right Atrial Enlargement – Lead II
- In right atrial enlargement, right atrial depolarisation lasts longer than normal and its waveform extends to the end of left atrial depolarisation.
- Although the amplitude of the right atrial depolarisation current remains unchanged, its peak now falls on top of that of the left atrial depolarisation wave.
- The combination of these two waveforms produces a P waves that is taller than normal (> 2.5 mm), although the width remains unchanged (< 120 ms).
Left Atrial Enlargement – Lead II
- In left atrial enlargement, left atrial depolarisation lasts longer than normal but its amplitude remains unchanged.
- Therefore, the height of the resultant P wave remains within normal limits but its duration is longer than 120 ms.
- A notch (broken line) near its peak may or may not be present (“P mitrale”).
Normal P-wave Morphology – Lead V1
The P wave is typically biphasic in V1, with similar sizes of the positive and negative deflections.
Right Atrial Enlargement – Lead V1
Right atrial enlargement causes increased height (> 1.5mm) in V1 of the initial positive deflection of the P wave.
NB. This patient also has evidence of right ventricular hypertrophy.
Left Atrial Enlargement – Lead V1
Left atrial enlargement causes widening (> 40ms wide) and deepening (> 1mm deep) in V1 of the terminal negative portion of the P wave.
Biatrial enlargement is diagnosed when criteria for both right and left atrial enlargement are present on the same ECG. The spectrum of P-wave changes in leads II and V1 with right, left and bi-atrial enlargement is summarised in the following diagram:
Common P Wave Abnormalities
Common P wave abnormalities include:
The presence of broad, notched (bifid) P waves in lead II is a sign of left atrial enlargement, classically due to mitral stenosis.
The presence of tall, peaked P waves in lead II is a sign of right atrial enlargement, usually due to pulmonary hypertension (e.g. cor pulmonale from chronic respiratory disease).
Inverted P Waves
P-wave inversion in the inferior leads indicates a non-sinus origin of the P waves. When the PR interval is < 120 ms, the origin is in the AV junction (e.g. accelerated junctional rhythm):
When the PR interval is ≥ 120 ms, the origin is within the atria (e.g. ectopic atrial rhythm):
Variable P-Wave Morphology
The presence of multiple P wave morphologies indicates multiple ectopic pacemakers within the atria and/or AV junction. If ≥ 3 different P wave morphologies are seen, then multifocal atrial rhythm is diagnosed:
If ≥ 3 different P wave morphologies are seen and the rate is ≥ 100, then multifocal atrial tachycardia (MAT) is diagnosed:
- Chung DC, Nelson HM. ECG – A Pictorial Primer [internet].
- Edhouse J, Thakur RK, Khalil JM. ABC of clinical electrocardiography. Conditions affecting the left side of the heart. BMJ. 2002 May 25;324(7348):1264-7. PMID: 12028984; PMC1123219
- Harrigan RA, Jones K. ABC of clinical electrocardiography. Conditions affecting the right side of the heart. BMJ. 2002 May 18;324(7347):1201-4. PMID: 12016190; PMC1123164
ECG Library Basics
LITFL Further Reading
- ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
- ECG A to Z by diagnosis – ECG interpretation in clinical context
- ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
- 100 ECG Quiz – Self-assessment tool for examination practice
- ECG Reference SITES and BOOKS – the best of the rest
- Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric
- Wagner GS. Marriott’s Practical Electrocardiography 12e
- Chan TC. ECG in Emergency Medicine and Acute Care
- Dubin D. Rapid Interpretation of EKG’s
- Mattu A. ECG’s for the Emergency Physician
- Hampton JR. The ECG In Practice, 6e