Effect of XAF‐1407 on atrial effective refractory period (aERP): (a) aERP measurements on baseline day before (−5 min) and 60, 120, and 180 min after end of drug administration. (b) aERPs 1 h after end of drug administration and successful cardioversion on procedure days, compared to respective values at baseline. (c) Rate dependency in aERP duration as a function of cycle length at baseline (N = 11, n = 11) and at day 3 (N = 9, n = 9). (d) Rate dependency in aERP duration as a function of cycle length at baseline (N = 11, n = 11) and at day 29 (N = 5, n = 5). *P < .05, significantly different as indicated. N, number of horses; n, number of measurements