P wave

The P wave is the first positive deflection on the ECG
It represents atrial depolarisation

waves of the ecg

Characteristics of the Normal Sinus P Wave

Morphology

Smooth contour
Monophasic in lead II
Biphasic in V1

Axis

Normal P wave axis is between 0° and +75°
P waves should be upright in leads I and II, inverted in aVR

Duration

< 120 ms

Amplitude

< 2.5 mm in the limb leads,
< 1.5 mm 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.

: Click image for source

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).

: Click image for source

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”).

: Click image for source

Normal P-wave Morphology – Lead V1

The P wave is typically biphasic in V1, with similar sizes of the positive and negative deflections.

: Normal P wave in V1

Right Atrial Enlargement – Lead V1

Right atrial enlargement causes increased height (> 1.5mm) in V1 of the initial positive deflection of the P wave.

: Click image for source

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.

: Click image for source

Biatrial Enlargement

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:

: Reproduced from Wagner et al. (2007)

Common P Wave Abnormalities

Common P wave abnormalities include:

P mitrale (bifid P waves), seen with left atrial enlargement.
P pulmonale (peaked P waves), seen with right atrial enlargement.
P wave inversion, seen with ectopic atrial and junctional rhythms.
Variable P wave morphology, seen in multifocal atrial rhythms.

P mitrale

The presence of broad, notched (bifid) P waves in lead II is a sign of left atrial enlargement, classically due to mitral stenosis.

Bifid P waves (P mitrale) in left atrial enlargement

P Pulmonale

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).

Peaked P waves (P pulmonale) due to right atrial enlargement

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):

Accelerated Junctional Rhythm

When the PR interval is ≥ 120 ms, the origin is within the atria (e.g. ectopic atrial rhythm):

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:

Multifocal Atrial Rhythm

If ≥ 3 different P wave morphologies are seen and the rate is ≥ 100, then multifocal atrial tachycardia (MAT) is diagnosed:

Multifocal Atrial Tachycardia

Acknowledgements

Images and descriptions of the right and left atrial waveforms are reproduced from Chung and Nelson’s excellent “ECG – A Pictorial Primer“.

References

Chung DC, Nelson HM. ECG – A Pictorial Primer [internet]. Accessed 12/12/2011.
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. Review. PubMed PMID: 12028984; PubMed Central PMCID: PMC1123219 [Full Text].
Harrigan RA, Jones K. ABC of clinical electrocardiography. Conditions affecting the right side of the heart. BMJ. 2002 May 18;324(7347):1201-4. Review. PubMed PMID: 12016190; PubMed Central PMCID: PMC1123164 [Full Text].
Jenkins RD, Gerred SJ. ECGs by Example. Second Edition. Elsevier Churchill Livingstone 2005.
Phibbs, BP. Advanced ECG: Boards and Beyond (2^nd Edition), Saunders Elsevier 2006.
Surawicz B, Knilans TK. Chou’s Electrocardiography in Clinical Practice. 6th Edition. Saunders Elsevier 2008.
Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.

Author Credits

Words – Ed Burns
Pictures – Various sources (see references)
Web Editing – Ed Burns

The P wave