When should a P wave be considered as P pulmonale in the electrocardiogram (ECG) in patients with suspected pulmonary hypertension or cor pulmonale?

P Pulmonale ECG Criteria

A P wave should be considered P pulmonale when it measures ≥2.5 mm in amplitude in leads II, III, or aVF, with prominent initial positivity (≥1.5 mm) in leads V1 or V2, while maintaining normal P wave duration. 1, 2

Specific Diagnostic Criteria

Amplitude Requirements

• P wave amplitude ≥2.5 mm in lead II is the primary criterion for identifying right atrial abnormality (the preferred modern term over "P pulmonale") 1, 2
• P wave amplitude ≥1.5 mm in leads V1 or V2 provides additional specificity, particularly when the initial component is prominent and positive 2
• The frontal P-axis should be approximately 75° or greater 1

Morphologic Characteristics

• The P wave appears tall and peaked rather than broad 2, 3
• P wave duration remains normal (distinguishing it from left atrial abnormality, which shows prolonged duration) 2
• The morphology is most prominent in inferior leads (II, III, aVF) 1

Clinical Context and Interpretation

When to Apply These Criteria

• These criteria are most relevant in patients with suspected pulmonary hypertension, cor pulmonale, or chronic lung disease 1, 4
• The ECG finding should prompt evaluation for underlying pulmonary or right heart pathology 2, 3

Performance Characteristics

• Specificity is high (approaching 100% in men, 94% in women when both lead II and V1 criteria are met), meaning P pulmonale reliably confirms right atrial enlargement when present 5
• Sensitivity is poor (only 48-66%), meaning absence of P pulmonale does NOT exclude right atrial enlargement or pulmonary hypertension 5
• The ECG lacks sufficient sensitivity to serve as a screening tool for pulmonary arterial hypertension, with only 73% sensitivity for right-axis deviation and 55% for right ventricular hypertrophy even when mean pulmonary artery pressure is 50 mm Hg 1, 2

Prognostic Significance

• P wave amplitude in lead II ≥0.25 mV (2.5 mm) carries important prognostic information in established pulmonary arterial hypertension, associated with a 2.8-fold greater risk of death over 6 years 1, 2
• Each additional 1 mm of P wave amplitude in lead III corresponds with a 4.5-fold increased risk of death 1

Critical Pitfalls to Avoid

Common Diagnostic Errors

• Do not rely on P pulmonale alone to screen for pulmonary hypertension - its poor sensitivity means many patients with significant disease will have normal ECGs 1, 2, 5
• Do not use P pulmonale to exclude right atrial enlargement - absence of these findings does not rule out the diagnosis 5
• Verify findings across multiple leads rather than relying on a single lead, as this improves diagnostic accuracy 2

When ECG is Insufficient

• In patients with severe tricuspid regurgitation, ECG findings may be unreliable 1
• Transthoracic echocardiography should always be performed when pulmonary hypertension is suspected, as it provides superior sensitivity and can measure right atrial volume directly 1, 3
• Right heart catheterization remains the gold standard for confirming pulmonary hypertension before initiating specific therapy 1, 6

Recommended Diagnostic Algorithm

When P pulmonale is identified on ECG:

1. Confirm the diagnosis by verifying amplitude criteria in multiple leads (II, III, aVF, V1, V2) 2
2. Assess for additional ECG signs of right heart strain (right axis deviation, right ventricular hypertrophy, ST-T changes in right precordial leads) 1
3. Obtain transthoracic echocardiography to evaluate tricuspid regurgitation velocity, right atrial size, right ventricular function, and estimate pulmonary artery pressure 1, 6
4. Perform ventilation/perfusion scanning to exclude chronic thromboembolic pulmonary hypertension 1
5. Consider right heart catheterization for definitive diagnosis if echocardiography shows intermediate or high probability of pulmonary hypertension 1, 6