Intrinsic left atrial histoanatomy as the basis for reentrant excitation causing atrial fibrillation/flutter in rats

Background

Although the pulmonary veins are accepted as preferential trigger sites for paroxysmal atrial fibrillation/flutter (AF/AFL), the intrinsic basis for reentrant excitation is undetermined in persistent AF/AFL.

Objective

To identify histoanatomic substrates for reentrant AF/AFL in rats.

Methods

Spatiotemporal patterns of impulse propagation were visualized optically on the posterior surface of the atria in di-4-ANEPPS-stained Langendorff-perfused rat heart/lung preparations. The relevant histology was also analyzed.

Results

Burst (S1-S2) pacing at the right atrium provoked AF/AFL in 15 of 19 hearts, and most cases developed by organized reentrant excitation through the coronary sinus (CS) and left atrial (LA) roof, with nonorganized irregular propagation in 3 cases. The reentrant circuit developed along 2 pathways of propagation: a slower pathway at the LA roof (conduction velocity, 42.4 ± 16.6 cm/s) and a faster pathway along the CS (conduction velocity, 53.3 ± 9.2 cm/s). Upon extra stimulus (S2) after consecutive S1 pacing, the impulse at the roof propagated retrogradely from the CS, resulting in reentrant propagation anchored by the atrial septum and posterior LA. Histologic quantification revealed significantly lower myocardial density in the posterior LA and the septum than elsewhere in the atria. Moreover, myocytes in the LA roof, than in the CS, were of lower density, more randomly arranged in the direction of conduction, and characterized by more disorganized distribution of connexin 43 over the entire cell membrane, which is consistent with the slower impulse propagation there.

Conclusion

The intrinsic histoanatomic heterogeneity in the LA would constitute a pro-reentrant substrate responsible for perpetuating AF/AFL.

Introduction

Atrial fibrillation/flutter (AF/AFL) is a common rhythm disorder¹ that is initiated by ectopic firing and then persists by complex reentrant impulse propagation within the atria.2, 3 Of various regions in the atria, the muscle sleeves of the pulmonary veins (PVs) have been pinpointed as important sites of origin for paroxysmal AF and are therefore regarded as effective therapeutic targets for catheter ablation to terminate and preclude the arrhythmia.⁴ However, ablation of the PVs has limited efficacy for persistent AF,⁵ which is mediated by reentrant propagation within the atria.2, 3 Several lines of evidence suggest that the left atrium (LA), especially the posterior wall including the LA roof, can be a culprit site for AF/AFL reentrant activity. For example, high-resolution optical mapping in isolated failing sheep hearts revealed that vortex-like reentry emerges through the posterior LA.⁶ Clinical studies involving electroanatomical mapping also revealed reentrant activity via conduction block at the posterior LA.⁷ Furthermore, a role for the LA roof in reentrant AF was suggested by suppression after ablation of this region in both clinical and experimental studies.8, 9, 10 However, compared with that of the PVs, the histoanatomy of the LA underlying reentrant propagation in AF/AFL has not been fully described. This is because persistent AF principally occurs as a result of many forms of functional and structural atrial remodeling produced by various underlying heart diseases.2, 3 Nevertheless, certain histoanatomic substrates responsible for reentrant propagation in AF/AFL, if residing even in the LA of nondiseased rat atria, may be a clue to understanding the mechanism of persistent AF/AFL in humans. To address this issue, we sought to induce AF/AFL in ex vivo perfused hearts from healthy rats and we conducted functional and histoanatomic studies of the whole atria by combining optical imaging of impulse propagation with histologic examination. We used excised rat hearts rather than in situ hearts of larger animals because (1) less complicated spatiotemporal patterns of AF/AFL can be detected readily owing to the small size of the atria,¹¹ (2) the entire atrial histology can be assessed systematically on a single glass slide, and (3) excision of the heart is essential for spatiotemporally precise fluorescence analysis of the posterior aspects of the LA. We unveiled intrinsic histoanatomic features of the LA, especially of the LA roof, that contribute to the formation of reentrant circuits in AF/AFL.