What do notched P (P wave) waves on an electrocardiogram (ECG) indicate?

Notched P Waves Indicate Left Atrial Abnormality

Notched P waves on ECG indicate left atrial abnormality, reflecting delayed left atrial activation and intraatrial conduction delay rather than simple enlargement. 1

Mechanism of Notched P Wave Formation

The notched or double-peaked appearance occurs because delayed left atrial activation causes the normally simultaneous right and left atrial peaks to separate and become more widely distinguishable. 1 This separation creates the characteristic "M-shaped" or bifid P wave morphology. 2

The underlying pathophysiology involves:

• Delay in Bachmann's bundle (the specialized interatrial conduction pathway), which is the most common mechanism 1
• Conduction delay within the left atrial myocardium itself 1
• Multiple contributing factors including atrial dilatation, muscular hypertrophy, elevated atrial pressure, and impaired ventricular distensibility 1, 2

Diagnostic Criteria for Left Atrial Abnormality

A widely notched P wave with ≥40 ms separation between peaks has equal diagnostic value to P wave duration ≥120 ms for identifying left atrial abnormality. 1

Key ECG criteria include:

• P wave duration ≥120 ms (present in the large majority of patients with left atrial abnormality) 1, 2
• Notch width ≥40 ms between peaks (creating the M-like appearance) 1, 2
• Increased P terminal force in lead V1 (product of amplitude and duration of terminal negative component) 1
• Left axis deviation of terminal P wave (30° to 90°) 1

A more liberal criterion using ≥20 ms peak-to-peak distance in the M shape has been associated with cardiovascular events and left atrial enlargement in patients with cardiovascular risk factors. 3

Clinical Significance and Associated Conditions

Notched P waves warrant evaluation for underlying cardiac pathology:

• Mitral valve disease (historically termed "P-mitrale") 2, 4
• Hypertension 2
• Heart failure 2
• Increased risk for atrial fibrillation 2

Patients with notched P waves (≥20 ms peak-to-peak distance) demonstrate significantly larger left atrial diameter (38.8 vs 36.8 mm) and higher left ventricular mass index compared to controls. 3

Recommended Terminology and Approach

Use the term "left atrial abnormality" rather than outdated terms like "left atrial enlargement," "P-mitrale," "atrial hypertrophy," or "atrial strain." 1, 2 This terminology is preferred because P wave changes reflect a combination of factors (dilatation, hypertrophy, elevated pressure, conduction delay) that cannot be distinguished from each other on ECG alone. 1, 2

Diagnostic Algorithm

When encountering a notched P wave:

1. Measure P wave duration - if ≥120 ms, this supports left atrial abnormality 1, 2
2. Measure notch width - if ≥40 ms between peaks, this is significant 1, 2
3. Assess P terminal force in lead V1 - increased amplitude and duration of terminal negative component supports the diagnosis 1, 2
4. Obtain echocardiography to assess left atrial size, function, and identify underlying structural heart disease 2
5. Evaluate for specific conditions: mitral valve disease, hypertension, heart failure 2
6. Assess arrhythmia risk, particularly for atrial fibrillation 2

Important Caveats

• Do not rely solely on P wave morphology - correlation with clinical findings and cardiac imaging is essential 2
• Recognize that intraatrial conduction delay can cause notched P waves even without actual atrial enlargement 1, 2
• Combined right and left atrial abnormalities may coexist, complicating interpretation 1, 2
• Normal variants and technical factors (electrode placement, paper speed, amplification) can affect P wave appearance 2, 5
• The sensitivity and specificity of P wave parameters for predicting left atrial enlargement are only moderate 5, 6