Left atrial enlargement

ECG: Sinus rhythm with a major alteration of the P-wave pattern consistent with left atrial enlargement: broad P-wave > 140 ms, bifid P-wave in leads I, II, III, aVF and from V3 to V6, enlarged negative component in V1; a likely left ventricular hypertrophy with increased R wave voltages in aVL and V4 and in S wave voltage in V3, lateral repolarization disturbances;

Comments: in left atrial enlargement, the left atrial component of the P-wave is increased in terms of voltage and duration. Various abnormalities allow establishing a left atrial enlargement:

• the prolongation of the activation of the left atrium induces an increase in P-wave duration beyond the pathological threshold of 110 ms, with the amplitude remaining generally unchanged.
• the predominance of left atrial vectors tends to shift the activation vector to the left and backward. The electrical axis of the P-wave in the frontal plane can be deviated to the left (< 15°) with a wide and tall P-wave in lead I and of lower-voltage or biphasic in leads II and III.
• the P-wave often displays an abnormal, notched, bifid or slurred morphology (possible M or double hump pattern). Indeed, the morphology of the P-wave is modified by an individualization of the late left atrial component causing a double-peaked bifid pattern, the second being slightly higher than the first which is particularly visible in leads I, II, aVL and in the left precordial leads from V3 to V6.
• in the horizontal plane, the P-wave forces are usually directed to the left and backward. The P-wave in V1 can be prominent with a biphasic pattern. The area of the negative terminal component exceeds that of the positive initial component with a duration > 40 ms and an amplitude > 1 mm. The index proposed by Morris (amplitude of the negative deflection in V1 in mm multiplied by its duration in seconds) is positive if greater than 0.03.

The ECG does not allow differentiating atrial hypertrophy from a conduction disorder within the atrium or a dilated atrium. Indeed, if the atrial mass is dilated, the P-wave is widened due to the increase in the duration of the depolarization associated with the greater distance needed to travel by the activating wave. The electrocardiographic term “atrial enlargement” can hence correspond to a significant thickening of the atrial wall, to a dilation of the cavity, to a marked intra-atrial conduction disorder or a combination of these various elements.

The primary causes of left atrial enlargement are mitral stenosis (sometimes associated with right ventricular hypertrophy), hypertrophic cardiomyopathy (associated with left ventricular hypertrophy), ischemic cardiomyopathy, mitral regurgitation and diseases with an increase in left ventricular pressure (aortic stenosis, aortic insufficiency, hypertension). The most characteristic pattern is often observed in mitral valve diseases, hence the use of the term “mitral P-wave”.

Take-home message: Left atrial enlargement is reflected on the ECG by an increase in the duration of the P-wave (> 110 ms) with a possible bifid pattern, a left axis deviation of the P-wave (< 15°), a positive Morris index (increase in duration > 40 ms and amplitude > 1 mm of the terminal negativity of the P-wave in V1).