ECG criteria
- P wave duration in lead II ≥ 120 ms (≥ 3 mm at a paper speed of 25 mm/s)
- Notched (bifid) P wave shape in lead II with an interpeak interval ≥ 40 ms; peaks may be of varying heights, and the second peak does not have to be higher than the first
- In lead I, the P wave is broad and often notched
- In V1, the terminal negative portion of the P wave is considered pathological if its depth is ≥ 1 mm and duration is ≥ 40 ms; the absolute value of the area of this phase is ≥ 0.04 mm·s (P-terminal force ≤ −0.04 mm·s). This sign reflects left atrial electrical abnormality and is not specific proof of anatomical LA enlargement
- The P wave amplitude in most leads is often not increased (unlike in P pulmonale), but a normal or increased amplitude does not rule out left atrial abnormality — the main reliance is on duration, morphology, and the terminal phase in V1
- The P axis may be normal or sometimes shifted to the left, but the axis direction is not a reliable diagnostic criterion for left atrial abnormality
What it looks like on ECG
P mitrale is best visualized in lead II, where the normal P wave is most distinct. When left atrial activation is delayed, the composite P wave curve may acquire a notched shape: the first peak is predominantly from the right atrium, and the delayed second peak is from the left atrium; the height of the second peak is not a diagnostic requirement, and it can be equal to or lower than the first. Between the humps lies a noticeable notch with an interpeak interval typically ≥ 40 ms.
| Lead | What to look for |
|---|---|
| II | duration ≥ 120 ms, often notched shape |
| I | broad, often notched P wave |
| V1 | deep and wide negative (terminal) phase of P, absolute value ≥ 0.04 mm·s |
| III, aVF | P amplitude is usually not increased |
The P wave amplitude in standard leads is often not increased and may remain within normal limits; however, a normal or increased amplitude does not rule out left atrial abnormality. Therefore, evaluation relies primarily on duration, morphology, and the terminal phase in V1 rather than P wave height. The P axis may be normal or sometimes shifted to the left, but axis direction is not used as an independent criterion. P wave notchings are sometimes also noticeable in lead I or V5–V6, but the primary leads for evaluation remain lead II (duration and morphology) and lead V1 (terminal negative phase).
Mechanism
The P wave is formed by the depolarization of both atria originating from the sinus node, with interatrial conduction via Bachmann's bundle; the initial part of the P wave predominantly reflects right atrial activation, while the terminal part reflects left atrial activation, with partial temporal overlap of these components. When conduction through the left atrial myocardium is slowed (due to wall hypertrophy, cavity dilation, or interatrial conduction disturbances), its contribution to the P wave shifts further in time from the right atrial component, decreasing the overlap. This can result in two peaks with a notch between them on the tracing, and the total P wave duration increases. Amplitude generally changes less than duration and morphology and may remain normal even with severe abnormality, making the amplitude criterion unreliable.
Causes
- Mitral valve disease — mitral stenosis and regurgitation (the classical cause that gave the sign its name)
- Arterial hypertension and left ventricular hypertrophy with diastolic dysfunction
- Chronic heart failure of any etiology with elevated left ventricular filling pressure
- Cardiomyopathies (dilated, hypertrophic)
- Prior atrial fibrillation and flutter — an associated marker of atrial remodeling; signs of left atrial abnormality may persist after restoration of sinus rhythm
- Aortic valve disease with secondary left atrial overload
- Coronary artery disease with left ventricular dysfunction
- Isolated interatrial conduction delay without structural left atrial enlargement
Differential diagnosis
- P pulmonale — the P wave is usually tall and peaked, > 2.5 mm in leads II, III, and/or aVF; duration is often not increased, but narrowness and absence of notching are not mandatory conditions. Primarily associated with right atrial electrical abnormality. P wave height alone does not reliably differentiate between right and left atrial enlargement — reliance is placed on duration and the terminal negative phase of the P wave in V1 rather than amplitude alone
- Interatrial block — isolated slowing of interatrial conduction, not always associated with structural LA enlargement. In partial interatrial block, the P wave is prolonged (≥ 120 ms); in advanced interatrial block, it is prolonged and biphasic (+/−) in the inferior leads (II, III, aVF). P mitrale morphology may actually be a manifestation of a conduction disturbance rather than anatomical LA enlargement; differentiation requires clinical data and echocardiography, which evaluates LA structure but does not rule out interatrial block
- Normal P wave variant in young and slender individuals — mild notching without an increase in total width ≥ 120 ms
- Electrode placement artifact or baseline wander altering P wave morphology — checked against adjacent complexes and other leads
- Biatrial enlargement — a combination of P mitrale features (width, notching in lead II) and P pulmonale features (tall P in V1 or II) simultaneously
Clinical significance and management
P mitrale is an electrocardiographic sign of left atrial electrical abnormality, not an independent clinical diagnosis or proof of anatomical LA enlargement. It requires correlation with the clinical picture and confirmation via echocardiography measuring LA size and volume. Its clinical significance lies in the fact that an enlarged left atrium and interatrial conduction disturbance are known predictors of atrial fibrillation and atrial tachyarrhythmias, as well as markers of increased left ventricular filling pressure in diastolic or systolic dysfunction.
The clinician should correlate the finding with the clinical picture (murmurs, dyspnoea, risk factors for valvular disease or LVH), order an echocardiogram to assess LA size and the valvular apparatus, look for concomitant signs of LV hypertrophy on the same ECG, and, if there is a history of arrhythmia paroxysms, account for the increased risk of AF during further follow-up.
Common interpretation errors
- Evaluating P wave width merely "by eye" without measuring in millimeters and converting to milliseconds
- Requiring the second peak of the P wave in lead II to be strictly taller than the first — this is not a criterion; a notched shape with an interpeak interval ≥ 40 ms is sufficient
- Confusing P mitrale with P pulmonale based on shape without assessing duration, the terminal phase in V1, and the clinical picture
- Failing to account for paper speed (12.5 mm/s versus 25 mm/s), which distorts the evaluation of duration in millimeters
- Concluding that anatomical LA enlargement is present based solely on P wave morphology without echocardiographic confirmation
P mitrale — broad and notched, like the letter "M" (mitrale): two humps — two atrial phases
1. What is the minimum P wave duration in lead II characteristic of P mitrale?
The classical criterion for left atrial electrical abnormality is a P wave width in lead II of at least 120 ms (3 mm at 25 mm/s).
2. Which statement regarding the notched shape of the P wave in lead II is correct?
The height of the second peak is not a mandatory criterion: it can be equal to or lower than the first; duration and an interpeak interval ≥ 40 ms are what matter.
3. What is required to confirm anatomical left atrial enlargement when P mitrale is found on an ECG?
The ECG reflects left atrial electrical abnormality but does not prove anatomical LA enlargement; this is confirmed by echocardiography measuring LA size and volume.
Frequently asked questions
How to distinguish P mitrale from P pulmonale?
P mitrale is a broad (≥ 120 ms) P wave in lead II, often notched, frequently with a normal amplitude. P pulmonale is a tall (> 2.5 mm) and peaked P wave in leads II, III, aVF. The former is associated with left atrial abnormality and the latter with right atrial abnormality, but amplitude and shape are not absolute differentiating features, and clinical evaluation is sometimes necessary.
Is P mitrale dangerous on its own?
The electrocardiographic sign itself is not life-threatening, but it indicates a left atrial electrical abnormality often associated with LA enlargement or interatrial conduction disturbance, which increases the risk of atrial fibrillation. The danger is determined by the underlying disease, not the P wave shape.
Is echocardiography necessary when P mitrale is detected?
Yes, echocardiography is the standard next step. It is the only method that allows measuring the size and volume of the left atrium, assessing the valve apparatus and left ventricular function, and confirming whether the ECG finding is associated with anatomical left atrial enlargement.
Can P mitrale be a normal variant?
Slight notching of the P wave without an increase in total width is found in some healthy individuals. A true P wave width of ≥ 120 ms is generally considered a pathological sign and requires further evaluation, although by itself it does not prove left atrial enlargement.
Does P mitrale mean the patient definitely has left atrial enlargement?
No. This is an ECG sign of left atrial electrical abnormality, which may reflect both anatomical left atrial enlargement and isolated interatrial conduction disturbance without chamber enlargement. These options can only be differentiated by echocardiography.
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Related terms
More in the section «Hypertrophy and Chamber Enlargement»
Sources
- Goldberger A. Clinical Electrocardiography: A Simplified Approach
- Hampton J. The ECG Made Easy
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
- Orlov V.N. Guide to Electrocardiography
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.