Left atrial rhythm: Diagnostic criteria and differentiation from nodal arrhythmias

Abstract

Left atrial rhythm is characterized by a left-to-right spread of atrial depolarization. The most frequent electrocardiographic pattern of this rhythm consists of upright or isoelectric P waves in lead I and inverted P waves in lead V 6. Two additional patterns, formed by negative P waves in leads I and V 6, with or without “dome and dart” P waves in lead V 1, are less common. A negative P wave in lead V 6 is the most sensitive and, in the absence of atrial inversion, the most specific sign of left atrial rhythm. The occurrence in left atrial rhythm of upright or isoelectric P waves in lead I is explained by the limitations inherent in Einthoven's triangle. Since the effective axis of lead I in Burger's triangle is inclined from the horizontal, a vector directed from left to right frequently gives rise to an upright or isoelectric deflection in lead I. The frequent appearance of inverted P waves in leads II, III and aVF in left atrial rhythm necessitates the establishment of criteria for differential diagnosis from nodal arrhythmias. Vectorial analysis of P waves in the horizontal plane as derived from the precordial leads provides the key for differentiation. In nodal and coronary sinus rhythms the horizontal P vector is directed to the left and slightly posteriorly, giving rise to negative or isodiphasic P waves in lead V 1 and to upright P waves over the left precordial leads. In left atrial rhythm the horizontal P vector points to the right and usually anteriorly, resulting in upright P waves in the right precordial leads and in negative P waves over the left precordial leads, particularly V 6.