Accumulation of atrial adipose tissue is associated with atrial fibrillation (AF). However the underlying mechanisms remain poorly understood. We examined the relationship between fatty infiltrations in the atrial myocardium and the development of AF structural/fibrotic substrate.

Atrial samples, collected in 92 patients during cardiac surgery and in a sheep model of persistent AF, were subjected to a detailed histological analysis. A transition from fatty to fibro-fatty infiltrates with persistence of AF was tested in the AF sheep model using a cardiac magnetic resonance imaging.

In sections of human right atrial samples, subepicardial fatty infiltrations were commonly observed in the majority of patients. A clear difference in the appearance and fibrotic content of these fatty infiltrations was observed. Fibro-fatty infiltrates predominated in patients with permanent AF (no AF 37±24% vs paroxysmal AF 50±21% vs permanent AF 64±23%, P<0.001). An inverse correlation between fibrotic remodeling and the amount of subepicardial adipose tissue is suggested. A shift from pure fatty to dense fibro-fatty infiltrations was observed in the left atria of AF sheep. Secondly, cellular inflammation, mainly consisting of functional cytotoxic T lymphocytes, was observed together with adipocyte cell death in human atria.

AF is associated with a transformation of fatty into fibro-fatty infiltrations, a process in which cytotoxic lymphocytes might be involved. The replacement of fatty infiltrates by fibrosis could be an early step in the remodelling of the subepicardium and the formation of the AF substrate.

The author hereby declares no conflict of interest