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    P wave: atrial depolarisation, normal duration and amplitude, signs of atrial enlargement and absent P waves.

    The P wave is the first small, rounded deflection of each heartbeat on the ECG. It reflects depolarisation of the atria: the electrical impulse spreads from the sinus node across the right and then the left atrium, and the atria contract. A normal P wave before every QRS complex shows that the rhythm starts in the sinus node. The shape, size and duration of the P wave reveal enlargement of the atria, abnormal conduction between them and rhythms that start outside the sinus node.

    ECGAtriaSinus rhythm
    Duration< 120 ms
    Amplitude (lead II)< 2.5 mm (0.25 mV)
    Directionupright in I, II, aVF; inverted in aVR
    Lead V1often biphasic (+/−)
    One ECG beat with the P wave highlighted at the start of the cycle, before the PR segment and the QRS complex J point P PR seg. QRS ST seg. T PR interval ST interval QT interval
    Figure 1. The P wave (highlighted) is the first deflection of the cardiac cycle and corresponds to atrial depolarisation.

    Abnormal P waves

    • Tall, peaked P (≥ 2.5 mm in II), "P pulmonale": right atrial enlargement, as in chronic lung disease, pulmonary hypertension, tricuspid valve disease and some congenital heart defects
    • Wide, notched P (≥ 120 ms) and a deep negative terminal part in V1, "P mitrale": left atrial enlargement or delayed conduction between the atria (interatrial block), as in mitral valve disease, hypertension and heart failure
    • Inverted P in II, III, aVF or P after the QRS: the impulse starts low in the atria or in the AV junction
    • No P waves: atrial fibrillation (irregular fibrillatory waves instead of P), junctional rhythm, sinus arrest; sawtooth flutter waves in atrial flutter
    • Changing P shape from beat to beat: wandering atrial pacemaker or multifocal atrial tachycardia
    Normal P wave compared with a tall peaked P wave of right atrial enlargement and with absent P waves Normal P P under 2.5 mm Tall P P 2.5 mm or more No P no atrial activity
    Figure 2. Left: normal P wave. Middle: tall, peaked P wave (right atrial enlargement). Right: no P wave before the QRS, as in junctional rhythm or atrial fibrillation.
    Why it matters

    Enlarged atria and conduction delay between them are associated with a higher risk of atrial fibrillation and stroke. Absent P waves with an irregular rhythm suggest atrial fibrillation, which needs assessment of stroke risk and anticoagulation.

    References

    1. Hancock EW, Deal BJ, Mirvis DM, et al. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part V: electrocardiogram changes associated with cardiac chamber hypertrophy. Circulation. 2009;119(10):e251–e261. doi:10.1161/CIRCULATIONAHA.108.191097

    This article is for general information and does not replace a consultation with a doctor. ECG findings must be interpreted by a physician together with symptoms and other tests.

    Published on 2 July 2019