Interatrial block (IAB) is the result of delayed conduction of the sinus impulse from right to left atrium, and is usually reflected in the electrocardiogram by the presence of a P-wave with a duration of 120 milliseconds (ms) or more [1]. Although poorly investigated in the past, the interest on this entity has substantially increased since an association with incident atrial tachyarrhythmias was described in the last decade of the past century [2].

Normal P wave configuration is positive in inferior leads, as the preferential route of left atrium depolarization is craniocaudal, through Bachmann bundle (BB). The two classic morphologies of IAB include partial interatrial block (P-IAB) and advanced IAB (A-IAB); although new patterns, reported as atypical IAB, have been recently described, illustrating the complexity of this entity [3]. In P-IAB, the impulse is conducted between the atria through the BB, but the conduction is delayed. This yields a wide and positive P-wave– usually bimodal – in inferior leads, as cranio-caudal depolarization is preserved. On the other hand, in A-IAB the BB is blocked (or the conduction delay is greater) and the impulse is conducted to the left atrium through mid or low- septal connections. This cranio-caudal depolarization of the left atrium results in a negative terminal wave in inferior leads.