Intra-Atrial Block: Definition and Relationship to Atrial Fibrillation and Other Adverse Outcomes

Abstract

In 1916, Bachmann first reported on the inter-auricular time interval. However relatively little attention was paid to this ECG measurement for decades. Then, in 1956, Samuel Bradley and Henry JJ Marriott reported on intra-atrial block (IAB) in 4,500 ECGs.As defined by them, IAB was a P wave duration of 0.12 sec or longer. Since that time, others have defined IAB as 0.11 sec or longer or 0.12 sec or longer. Several authors have suggested subcategories, such as first-, second-, and third-degree patterns and some have defined specific intra-atrial and inter-atrial pathways. These are of electrocardiographic interest but have not been substantiated as related to different clinical outcomes. Many disorders have been associated with IAB. More importantly, however, IAB has been associated with several adverse outcomes, including sinus node dysfunction, atrial tachyarrhythmias - especially atrial fibrillation, thromboembolic events, and increased mortality. This brief review will detail the above to emphasize to ECG readers the importance of not overlooking IAB in their interpretations.

Keywords: Inter-atrial block; Intra-Atrial Block: Recognition and Significance; Intra-atrial block; P wave duration Running Head; atrial conduction.

Figure 1.. A schematic of normal intra and inter atrial conduction (solid arrows represent normal conduction velocity), panel A; slow intra and inter atrial conduction – represented by the squiggly lines, panel B; and slow intra atrial conduction with block across Bachmann’s bundle, panel C. Conduction across the less frequently apparent coronary sinus (CS) and fossa ovalis (FO) fibers, which are likely used to achieve conduction to the left atrium in the setting of Bachmann’s bundle block, is not illustrated although their approximate locations with respect to Bachmann’s bundle is shown.

Figure 2.. Three examples of IAB. Panel A shows a widened but smooth P wave. Panel B shows a widened and notched P wave (as well as some premature atrial complexes). Panel C shows a widened and notched P wave with the terminal forces being negative in the inferior leads. Panel A is most compatible with delay mainly in the right atrium. Panel B is most compatible with delay between the right and left atria, with the second part of the P wave (second notch) most likely representing delayed left atrial activation. Panel C is most suggestive of delay in Bachmann’s bundle, with the terminal forces going away from the inferior leads and upwards towards the left atrium due to lower septal activation rather than conduction across Bachmann’s bundle in the upper septum.

Figure 3.. Two tracings showing types of atrial dissociation. Panel A shows right and left atrial dissociation with independent rhythms and therefore two sets of P waves marching through each other. Only the right atrial P wave (sinus rhythm) conducts to the ventricles and results in QRS complexes. Panel B shows a similar phenomenon but in this case the patient is post heart transplant with one set of P waves conducting to the ventricles that originate in the transplanted right atrium and the second set comingfrom the recipient’s atrial rim. Arrows are place to indicate the non-conducted P waves.