Effect of amiodarone, diltiazem, or both to achieve strict heart rate control in dogs with atrial fibrillation.

Dogs with atrial fibrillation (AF) live longer if heart rate is strictly controlled (SHRC, 24-h mean heart rate ≤ 125 beats per minute [bpm]). Amiodarone and diltiazem are used to lower ventricular response rate (VRR) in dogs with AF, but information about their relative efficacy for achieving SHRC is lacking. Describe heart rate response with sequential escalation of amiodarone or diltiazem monotherapy to combination therapy. Thirteen client-owned dogs with AF. Prospective cohort study with an adaptive treatment protocol. Dogs with AF with mean daily heart rate (MDHR) > 125 bpm (baseline, 24-h Holter) were randomized to receive amiodarone or diltiazem at standard doses. Dogs were reassessed 21 days later, and the other drug (amiodarone or diltiazem) was added if SHRC was not achieved. Dogs were then reassessed at 42 and 63-147 days after starting treatment. If MDHR >125 bpm at any repeat assessment, the second drug was added. Timing of final assessment was dependent on whether treatment was escalated. Monotherapy with amiodarone and diltiazem was initially used in 7 and 6 dogs, respectively. SHRC was achieved in 3 dogs (amiodarone, n = 2; diltiazem, n = 1) at first follow-up. At last follow-up, 11 of 13 dogs were receiving both drugs (median [range] 50 [20-153] days). Combination therapy of amiodarone and diltiazem did not result in SHRC (139 [94-160] bpm) in most dogs (7/11, 64%). Monotherapy with amiodarone or diltiazem or combination therapy of amiodarone/diltiazem did not achieve SHRC in most dogs in this study.