Regarding endothelial dysfunction, there are a number of compounds secreted by the healthy endothelium that are antithrombotic but that are altered in disease states and worsened in AFib.  The best studied is probably nitric oxide (NO).  The Figure shows basal NO production in atrial anatomic segments in controls (in dark blue) and in patients with AFib (in teal), and it can be seen that throughout the atria, but predominantly in the left atrium, there is a reduction in the production of NO [22]

Reiffel JA. Am J Med 2013; 126: 00-00.