Right atrial enlargement

ECG: Sinus rhythm with right atrial enlargement secondary to a volume overload of the left-to-right atrial shunt: normal P-wave duration, tall (3 mm) and peaked P-wave in lead II, greater amplitude in leads II, III than in lead I, exclusive positive deflection in V1; there are tall R waves in the right precordial leads suggestive of a right ventricular hypertrophy by diastolic overload;

Comments: As seen previously, right atrial activation begins and ends prior to that of the left atrium. In right atrial enlargement, the right atrial component of the P-wave is increased both in terms of amplitude and duration. Right atrial depolarization is prolonged, superimposed and added to the left atrial depolarization.

Various abnormalities allow defining a right atrial enlargement:

• The duration of the P-wave usually remains normal. Indeed, while the activation time of the right atrium is extended, this prolongation is superimposed at the end of the left atrial activation and thus has no impact on the total duration of the P-wave. When the right atrium is extremely enlarged and dilated, the time required to pass through the right atrium is prolonged after the end of the activation of the left atrium, thereby increasing the duration of the P-wave.
• The predominance of right atrial vectors tends to shift the resultant vector of the P-wave downward and forward. The electrical axis of the P-wave can be slightly deviated to the right between 60 and 90° such that the P-wave in leads II and III is taller than the P-wave in lead I.
• The P-wave is tall and exceeds 2.5 mm in lead II (2 mm in lead III or aVF). The amplitude of the initial component of the P-wave is particularly increased. At times, the amplitude of the P-wave can be maximal in lead III.
• The P-wave can have a triangular pattern in lead II: narrow, tapered and peaked P-wave (relatively narrow base and increased amplitude).
• An exclusive positive deflection is frequently observed in V1 (tall and peaked P-wave which can exceed 2 mm) with a steep ascending branch or biphasic pattern, with a predominance of the positive phase (the positive component of the P-wave in V1 primarily corresponds to the activation of the right atrium) and an amplitude  > 1.5 mm.
• Signs of right ventricular hypertrophy are frequently also seen (depending on etiology); in this particular setting, it is called pulmonary P-wave since it results from a severe lung disease (severe respiratory failure); abnormalities are thus predominant in the limb leads (lead II).

The electrical signs of right atrial enlargement in the early years of life differ very little from those observed in adults. It should be noted that the P-wave can be tall in infants and physiologically reach 2 to 2.5 mm in lead II. Similarly, the amplitude of the initial phase of the P-wave in V1 can reach 2 mm.

The primary causes of right atrial enlargement are right valve diseases (tricuspid and pulmonary), pulmonary arterial hypertension (primary, pre- or post-capillary), certain congenital heart diseases (pulmonary stenosis, Tetralogy of Fallot, tricuspid atresia) and certain lung embolisms. An increase in voltage of the P-wave in lead II can constitute a normal variant in certain thin and slim patients.

Take-home message: Right atrial enlargement is reflected on the ECG by an increase in the amplitude of the P-wave (> 2.5 mm in lead II) which can display a triangular pattern (peaked and narrow), a lack of prolonged P-wave duration, an exclusively positive pattern in V1 or with strong positive predominance. The diagnosis of right atrial enlargement is hence essentially based on the analysis of lead II and of V1.