ECG criteria
- Amplitude in lead II: ≤2.5 mm (normal), >2.5 mm (increased) — sign of right atrial hypertrophy
- Duration: ≤120 ms measured in lead II (normal); >120 ms — sign of intra- or interatrial conduction block
- Shape in lead V1: positive, negative, or biphasic in the norm; biphasic with a predominant negative part — sign of left atrial enlargement (P mitrale)
- Morphology in lead II: monophasic, positive (normal in sinus rhythm); notched (fragmented) — sign of delayed atrial conduction
- P wave in leads III, aVL, V1–V2: assess polarity and morphology to determine the P axis and the origin of the rhythm
- PQ (PR) interval: from the onset of P to the onset of the QRS complex; normal 120–200 ms (3–5 small squares); <120 ms may indicate pre-excitation, >200 ms — first-degree AV block
What it looks like on ECG
The P wave precedes each ventricular QRS complex in normal sinus rhythm. In lead II, it is usually positive, smooth, dome-shaped, with an amplitude of 1–2.5 mm and a duration of up to 120 ms.
In lead V1, the P wave can be positive, biphasic (positive initial part, negative terminal part), or entirely negative — this is a normal variant. With left atrial enlargement, a prominent negative phase appears in V1 (P mitrale).
In leads aVL, I, it can be slightly positive or biphasic. In leads III, aVR, polarity depends on the orientation of the P axis.
A notched P wave (m-shaped in lead II) indicates delayed interatrial conduction and is often associated with left atrial enlargement or Bachmann's bundle block.
| Lead | Normal | Abnormal |
|---|---|---|
| II | Positive, 1–2.5 mm | >2.5 mm (RA hypertrophy), notched |
| V1 | Positive or biphasic | Pronounced negative phase (P mitrale) |
| aVF | Positive | Negative in inferior atrial premature complexes |
| V3–V4 | Slightly positive | Normal variant or atypical rhythm source |
Mechanism
The P wave reflects the electrical depolarization of the right and left atria. During sinus rhythm, excitation spreads from the sinus node (located near the superior vena cava orifice in the right atrium) downward and to the left across the atria, then reaches the AV node.
Depolarization of the right atrium occurs in the first half of the P wave, and the left atrium in the second half. The normal P wave axis (the direction of the depolarization vector) usually coincides with or is close to the normal QRS axis, as both vectors point downward and to the left.
The duration of the P wave corresponds to the time required for complete depolarization of both atria. Atrial enlargement or conduction disturbances within the intra-atrial conduction system prolong and deform the P wave.
Norm and variants
Amplitude: ≤2.5 mm in standard leads (I, II, III, aVL, aVF, V2–V6); ≤1.5 mm is allowed in V1 and aVL.
Duration: ≤120 ms (up to 0.12 s, or up to 3 small squares at the standard speed of 25 mm/s).
Shape: monophasic positive in leads II, aVF; monophasic negative in aVR; biphasic in I, aVL, V1, V2 — normal variants.
Polarity depending on the axis: with a normal P axis (−30° to +90°), the P wave is positive in I and II, and negative in aVR. A vertical P axis position (greater than +90°) may lead to a decreased amplitude in I and an increased amplitude in III.
In children: the P wave is more peaked, and the amplitude may be somewhat higher (up to 2–3 mm depending on age).
P pulmonale variant: tall, peaked P wave (>2.5 mm) in leads II, III, aVF in right atrial hypertrophy (COPD, pulmonary hypertension, Tetralogy of Fallot). The P axis is shifted to the right (>+90°).
Pathological changes
Enlarged P wave (P mitrale and P pulmonale):
- P mitrale: notched (M-shaped) P wave in leads I and II, duration >120 ms, negative or markedly biphasic component in V1; sign of left atrial hypertrophy.
- P pulmonale: tall, pointed P wave (>2.5–3 mm) in II, III, aVF; P axis shifted to the right; sign of right atrial hypertrophy.
Negative P wave in leads II, III, aVF: indicates an atypical rhythm source (atrial premature complex, ectopic atrial rhythm, junctional rhythm, retrograde conduction).
Absence of P wave before QRS: in complete AV dissociation, atrial fibrillation (wavy baseline instead of P), when P is superimposed on the QRS complex or T wave (hidden P, as in parallel AV dissociation).
Prolonged P wave (>120 ms): may be a result of interatrial conduction block (Bachmann's block), delayed conduction through the left atrium, or atrial hypertrophy.
Notched (fragmented) P wave: indicates asynchronous atrial activation; frequently seen in left atrial hypertrophy, Bachmann's block, or focal atrial myocardial damage.
Biphasic P wave with a predominant negative phase in V1: sign of left atrial enlargement (P mitrale shape).
Causes of P wave enlargement and deformation
Left atrial hypertrophy:
- Arterial hypertension, left ventricular hypertrophy
- Mitral stenosis and regurgitation
- Dilated cardiomyopathy
- Restrictive cardiomyopathy
Right atrial hypertrophy:
- COPD, chronic cor pulmonale
- Congenital heart disease (atrial septal defect, Tetralogy of Fallot)
- Primary pulmonary hypertension
- Prolonged right atrial hyperactivation
Atrial conduction disturbances:
- Bachmann's block (partial interatrial conduction block)
- Conduction slowing due to fibrosis, atrial inflammation
Atypical rhythm sources:
- Atrial premature complex (negative P before QRS or superimposed on T)
- Ectopic atrial rhythm
- Junctional rhythm (P absent or hidden in QRS/ST)
Other causes:
- Pregnancy (widening, normal increase in heart volume)
- Hyperthyroidism (high metabolism)
- Acute pulmonary embolism (rightward axis shift of P, sometimes tall P in III)
- Infectious myocarditis, pericarditis
Differential diagnosis
U wave and P wave: The U wave appears after T, while P precedes QRS. With accelerated AV conduction or pre-excitation, P may be very close to QRS, but it begins before QRS.
P on T superposition (P wave on T wave) and hidden P: when the PQ interval is <120 ms (pre-excitation, increased excitability), the P wave may be hidden in the end of the T wave of the previous cycle. Attempting to prolong the AV interval (e.g., a vagal maneuver in paroxysmal tachycardia) can reveal a distinct P.
f wave in atrial fibrillation and P wave: in atrial fibrillation, instead of a distinct P wave, chaotic baseline undulations are visible (f-waves, from fibrillation). The normal P disappears.
P wave and R wave (in lead aVR): in aVR, a normal P wave is negative. A positive wave in aVR can be mistakenly interpreted as part of R if attention is not paid to the temporal sequence.
Artifacts and P wave: muscle tremor or patient movement can mimic P deformation. Comparison with other cycles and recording conditions helps distinguish true changes from artifacts.
Clinical significance and management
The P wave is used to identify the rhythm and assess atrial function. A normal P wave preceding each QRS confirms sinus rhythm and normal AV conduction.
Broad and/or notched P wave:
- Indicates atrial hypertrophy or dilation, as well as potential conduction slowing
- May be a risk marker for atrial fibrillation (especially with left atrial enlargement)
- Requires clinical correlation: echocardiographic assessment of atrial dimensions, finding the cause (arterial hypertension, valvular heart disease, COPD)
Negative P wave in leads II, III, aVF:
- Indicates an atypical rhythm source
- Requires determining the type of arrhythmia and searching for the cause (hypokalemia, hypoxia, digoxin toxicity, primary arrhythmia)
Absence or concealment of the P wave:
- Requires clarification: whether P is located within the QRS complex or ST segment (as in junctional rhythm), superimposed on T (pre-excitation), or absent altogether (atrial fibrillation)
- Atrial fibrillation requires searching for the cause and assessing ventricular response rate
Management tactics:
- When signs of atrial hypertrophy are detected — order an echocardiogram, monitor blood pressure, treat the underlying condition (hypertension, heart defect)
- For an atypical P wave and rhythm disturbances — determine the type of arrhythmia, consider consulting an arrhythmologist, and order Holter ECG monitoring if necessary
- For a prolonged PQ interval — rule out AV block; for a shortened PQ — consider pre-excitation (WPW syndrome)
Common interpretation errors
- P and U confusion: the U wave often goes unnoticed, especially in hypokalemia or with certain medications. P always precedes QRS, and U appears after T; they can be distinguished by temporal sequence and morphology.
- Re-evaluation of a subtle P wave in V1: normally, P in V1 may be biphasic or even completely negative — this does not necessarily mean P mitrale. One must look at the amplitude of the negative phase (>1.0 mm) and the morphology in lead II (notching, prolongation).
- Incorrect measurement of P duration: duration is measured from the onset to the end of the wave, including both phases of a biphasic P. Mistakes are often made by taking only the visible part, especially if P begins at the end of a small grid square.
- Ignoring P-axis position: a negative P wave in II–III with a positive P in aVL can be a variant with a vertical P axis, but it may also indicate an ectopic source. The QRS axis must be checked and compared with the clinical picture.
- Confusion between P mitrale and intra-atrial block: both are accompanied by P notching and prolongation, but P mitrale predominantly in leads II and V1 has a characteristic notched (M-shaped) appearance, whereas intra-atrial block presents with a more uniform prolongation. Clinical information (arterial hypertension, mitral stenosis vs. cardiomyopathy) helps clarify the diagnosis.
Normal P: ≤2.5 mm in height, ≤120 ms in width, monophasic in II (P pulmonale — peaked upward; P mitrale — notched in II and negative in V1).
1. On a patient's ECG in lead II, the P wave is notched (M-shaped), with a duration of 140 ms, and the negative phase predominates in V1. What does this indicate?
A notched P in II, prolonged (>120 ms), with a predominant negative phase in V1 are classic signs of left atrial hypertrophy and enlargement (P mitrale). P pulmonale is characterized by a tall, peaked P in II–III without notching.
2. What is the normal duration of the P wave?
According to ACCF/AHA standards and ESC guidelines, normal P wave duration does not exceed 120 ms (0.12 s). At a paper speed of 25 mm/s, this corresponds to 3 small squares (each square = 40 ms). Prolongation >120 ms indicates intra-atrial conduction delay.
3. A patient with COPD. On the ECG in leads II, III, aVF, the P wave is tall and peaked (3 mm), and in V1 it is positive and tall. The P axis is shifted to the right (+110°). This is a sign of:
A tall, peaked P wave (>2.5 mm) in II, III, aVF, positive and tall in V1, with a P axis >90° are typical signs of P pulmonale, indicating right atrial hypertrophy in COPD and chronic pulmonary hypertension. P mitrale would be notched and have a negative phase in V1.
Frequently asked questions
How to distinguish P mitrale from P pulmonale on ECG?
P mitrale (left atrial hypertrophy): notched or M-shaped P wave in leads I and II (with two peaks), duration >120 ms, prominent negative phase in V1. P pulmonale (right atrial hypertrophy): tall, peaked P wave in II, III, aVF (>2.5 mm), often ≤120 ms, P axis shifted to the right (>+90°), may be positive and tall in V1.
Can the P wave be negative normally?
Yes. A negative P is normal in leads aVR and V1. In leads III and aVL, it can be slightly negative with a vertical P axis position. However, a negative P in leads II and aVF in the presence of QRS complexes in the same leads indicates an atypical rhythm source and requires clarification.
What does a broad (prolonged) P wave mean?
A P wave >120 ms indicates intra-atrial conduction delay or atrial enlargement (most commonly left atrial). Causes: arterial hypertension, left atrial hypertrophy, interatrial conduction block (Bachmann's block), inflammation, or fibrosis of the atrial myocardium. Requires an echocardiogram to assess atrial dimensions.
Why might the P wave be hidden within the QRS complex or T wave?
With a shortened PQ interval (pre-excitation, WPW syndrome) or in parallel AV dissociation, the P wave may completely coincide in time with the QRS complex or be superimposed on the beginning or end of the T wave. In this case, P is not visible on the ECG, but can be revealed by prolonging the AV interval (vagal maneuvers, adenosine).
Is an abnormal P wave a sign of heart disease?
An abnormal P wave requires clinical correlation. Left atrial enlargement is often associated with arterial hypertension or valvular heart disease and can be a risk marker for atrial fibrillation. However, not all P wave changes indicate severe disease; some variants may be individual anatomical features. An echocardiogram and clinical context are essential for proper interpretation.
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Related terms
More in the section «Basics: waves, intervals, axes»
Sources
- Goldberger AL, Goldberger ZD, Shvilkin A. «Goldberger's Clinical Electrocardiography», 10th ed., 2018
- Surawicz B., Knilans T. «Chou's Electrocardiography in Clinical Practice: Acute and Chronic», 6th ed., 2008
- Orlov V. V. «Guideline on Electrocardiography», 8th ed., 2017
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.