What does a hump in the P (P wave) on an electrocardiogram indicate?

P-Wave Hump (Notched/Bifid P-Wave)

A hump or notch in the P-wave (creating an M-shaped or bifid appearance) indicates left atrial abnormality, most commonly caused by delayed intraatrial conduction through Bachmann's bundle, and should prompt evaluation for underlying conditions such as mitral valve disease, hypertension, heart failure, or left ventricular hypertrophy. 1

Primary Diagnostic Criteria

The notched P-wave represents left atrial abnormality when specific measurements are met:

• P-wave duration ≥120 ms is the most consistent finding and is present in the large majority of patients with left atrial abnormality 2, 1
• Widely notched P-wave with ≥40 ms separation between peaks creates the characteristic M-like or bifid appearance and has equal diagnostic value to P-wave duration 2, 1
• Increased P terminal force in lead V1 (product of amplitude and duration of terminal negative component) is the most frequently used criterion with sensitivity of 76% and specificity of 92% 1
• Left axis of terminal P-wave (−30° to −90°) provides additional supporting evidence 2

Pathophysiologic Mechanisms

The notched appearance reflects specific underlying processes:

• Delayed intraatrial conduction through the specialized interatrial pathway (Bachmann's bundle) is the primary mechanism, causing the right and left atrial peaks that are normally fused to become widely separated 2, 1
• Elevated atrial pressure from impaired ventricular distensibility contributes to P-wave changes 2, 3
• Left atrial dilatation and muscular hypertrophy occur in combination with conduction delay 2, 1

Clinical Significance and Risk Stratification

A notched P-wave carries important prognostic implications:

• Predicts cardiovascular events including stroke, heart failure, coronary artery disease, and sudden death when the peak-to-peak distance is ≥20 ms (hazard ratio 1.83) 4
• Correlates with left atrial enlargement as patients with notched P-waves have significantly larger left atrial diameters (38.8 vs 36.8 mm) and higher left ventricular mass index 4, 5
• Indicates left ventricular diastolic dysfunction with significant correlation to pseudonormal or restrictive mitral inflow patterns 5
• May be the earliest electrocardiographic sign of hypertensive heart disease, often appearing before voltage criteria for left ventricular hypertrophy 2

Diagnostic Approach

When you identify a notched P-wave, follow this systematic evaluation:

1. Measure P-wave duration in lead II—values ≥120 ms strongly suggest left atrial abnormality 1
2. Assess the notch width—separation ≥40 ms between peaks is diagnostically significant 1
3. Evaluate P terminal force in lead V1—calculate the product of amplitude and duration of the terminal negative component 1
4. Use multiple criteria simultaneously rather than relying on a single finding to enhance diagnostic confidence 1, 6

Underlying Conditions to Evaluate

Systematically assess for these specific cardiac conditions:

• Mitral valve disease (stenosis or regurgitation) 1, 7
• Hypertension with left ventricular hypertrophy 2, 1
• Heart failure with impaired ventricular distensibility 1, 3
• Dilated or hypertrophic cardiomyopathy 2
• Risk for atrial fibrillation development 1

Next Steps in Management

After identifying a notched P-wave:

• Order echocardiography to directly measure left atrial size and assess left ventricular diastolic function, as this remains the gold standard 1, 5
• Treat underlying conditions aggressively—manage hypertension, optimize heart failure therapy, and evaluate valvular disease 1
• Monitor for arrhythmias, particularly atrial fibrillation, as left atrial abnormality increases this risk 1

Critical Pitfalls to Avoid

• Do not use outdated terminology such as "P-mitrale," "atrial hypertrophy," or "atrial enlargement"—the preferred term is "left atrial abnormality" because P-wave changes reflect a combination of factors including dilatation, hypertrophy, elevated pressure, and conduction delay 2, 1, 6
• Never rely on P-wave morphology alone—always correlate with clinical findings and confirm with echocardiography 1
• Recognize that normal variants and technical factors can affect P-wave appearance 1
• Consider combined atrial abnormality—if you also see tall peaked P-waves in lead II >2.5 mm or prominent initial positivity in V1/V2 ≥1.5 mm, both right and left atrial abnormalities may coexist 2, 6
• Remember that interatrial conduction delay can cause bifid P-waves even without true atrial enlargement 1