LEFT ATRIAL ABNORMALITY/ENLARGEMENT

Atrial activation follows discharge of the sinus node. Normally, activation of the right atrium occurs first, followed by left atrial activation, sometimes leading to slight physiologic notching of the P wave. Delay of left atrial activation (interatrial block), as may result from left atrial distension, hypertrophy, scarring, or conduction delay, causes the P wave on the surface ECG to become broadened and often substantially notched, with an interpeak interval >0.04 seconds (figure 1 and waveform 1). The voltage of the terminal portion of the P wave is increased if there is left atrial enlargement (“P mitrale”), reflecting left atrial depolarization.

These ECG changes are most apparent in the inferior leads. Leads V1 and V2 show a deeply inverted or negative portion of the P wave (reflecting left atrial activation which is directed posteriorly) with an area that is greater than that of the initial upright portion of the P wave (reflecting right atrial activation, which is directed anteriorly). The negative portion of the P wave in V1 is >1mm wide and >1mm deep, with normal voltage and sweep speed settings. If the P wave in V1 is completely negative, this usually reflects left atrial abnormality, but may be normal or indicate an ectopic atrial focus. In lead II, the P wave is ≥.12 seconds. The left atrial vector may also increase toward the left (-30° to -90°) if left atrial enlargement progresses and becomes more pronounced, resulting in a negative terminal deflection of the P wave in leads III and aVF.

As noted below, a narrow but prominent, entirely negative P wave in lead V1 may also, paradoxically, occur in certain cases of right atrial abnormality, especially when the right atrium is located below the level of the V1 electrode. In such cases, tall, peaked P waves are usually seen in the inferior leads, more typical of right atrial abnormality (“P pulmonale”).

The following criteria suggest left atrial enlargement/abnormality when correlated with echocardiographic data:

• Negative phase of P in V1>0.04 sec — sensitivity 83 percent; specificity 80 percent
• Negative phase of P in V1>1 mm — sensitivity 60 percent; specificity 93 percent
• P-terminal force >0.04 mm/sec — sensitivity 69 percent; specificity 93 percent
• Notched P, interpeak interval >0.04 sec — sensitivity 15 percent; specificity 100 percent
• P wave duration >0.11 sec — sensitivity 33 percent; specificity 88 percent
• P wave/PR duration >1.6 — sensitivity 31 percent; specificity 64 percent