P wave
Summary
The P wave on ECG represents atrial depolarization; it is normally < 120 ms wide (< 3 small squares) and < 2.5 mm tall in lead II. Absent P waves suggest atrial fibrillation; prolonged P waves indicate atrial enlargement or delay.
Detail
The P wave is the first deflection on the ECG following the baseline and represents the spread of electrical activity through the atria. Normal P wave duration is 80-120 ms (3 small squares) and amplitude is 0.5-2.5 mm. P wave morphology varies by lead: in lead II, it is typically biphasic or upright; in AVR, it is typically downward. A bifid (notched) P wave in lead II ('P mitrale') suggests left atrial enlargement; a tall, peaked P wave suggests right atrial enlargement ('P pulmonale'). Absent P waves suggest atrial fibrillation (replaced by fibrillation waves). Prolonged P-R interval (> 200 ms) indicates first-degree AV block or slowed conduction. Inverted P waves in lead V1 are normal, but in lead II suggest ectopic atrial rhythms or lead reversal. Understanding P wave morphology is critical for diagnosing arrhythmias and chamber enlargement.
Sources
- First Aid for USMLE Step 1
- Marriott's Practical Electrocardiography
- Harrison's Principles of Internal Medicine
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