Time‐dependent antiarrhythmic effects of flecainide on induced atrial fibrillation in horses

Background, Objective: Pharmacological treatment of atrial fibrillation (AF) in horses can be challenging because of low efficacy and adverse effects. Flecainide has been tested with variable efficacy. To test whether the efficacy of flecainide is dependent on AF duration. Animals and Methods: Nine Standardbred mares. Factorial study design. All horses were instrumented with a pacemaker and assigned to a control or an AF group. On day 0, all horses were in sinus rhythm and received 2 mg/kg flecainide IV. Atrial fibrillation subsequently was induced in the AF group by pacemaker stimulation. On days 3, 9, 27, and 55, flecainide was administered to all horses, regardless of heart rhythm. Results: Conclusions and Clinical ImportanceAll horses in AF cardioverted to sinus rhythm on days 3 and 9. On day 27, 5/6 horses cardioverted, whereas only 2/6 cardioverted on day 55. The time from the start of flecainide infusion to cardioversion (range, 3-185min, log transformed) showed linear correlation with the cumulative duration of AF (r(2)=.80, P<.0001). Flecainide induced abnormal QRS complexes in 4/6 AF horses and 1/3 controls. A positive correlation was found between heart rate before flecainide infusion and number of abnormal QRS complexes (0.14, P<.05). One horse suffered from cardiac arrest and died after flecainide infusion. Flecainide is effective for cardioversion of short-term induced AF, but the effect decreases with AF duration. Controlling heart rate may minimize adverse effects caused by flecainide, but the drug should be used with great caution

Effect of flecainide on atrial fibrillatory rate in a large animal model with induced atrial fibrillation

Background: Atrial fibrillatory cycle length has been considered one of the indices of atrial electrical remodelling during atrial fibrillation (AF), which can be assessed from surface ECG by computer-assisted calculation of atrial fibrillatory rate (AFR). Horses have been suggested as a bona fide model for AF studies since horses too, develop lone AF, however data on AF characteristics in horses are extremely sparse and non-invasive characterization of AF complexity using surface ECG processing has not been reported. Aim: The aim was to study characteristics of induced AF and its modification by flecainide. Methods: The study group consisted on 3 horses with spontaneous persistent AF and 13 with pace-induced AF. Seven horses were treated with saline (control) and eight with flecainide (2 mg/kg). ECGs were analysed using spatiotemporal cancellation of QRST complexes and calculation of AFR from the residual atrial signal. Results: At AF onset, AFR was 295±52 fibrillations per minute (fpm) in the horses with induced AF treated with flecainide, 269±36 fpm in the control group (ns), and 364±26 fpm in the horses with spontaneous persistent AF (P<0.05 compared to the control group). Flecainide caused a decrease in AFR in all animals and restored sinus rhythm in the animals with induced AF. In the control animals, AFR increased from 269±36 fpm to a plateau of 313±14 fpm before decreasing to 288±28 fpm during the last 10% of the AF episodes preceding spontaneous conversion (P<0.05). Conclusion: AFR in horses with induced AF resembles AFR in humans with paroxysmal AF. Flecainide caused a rapid decrease in AFR in all horses, further supporting the method to be a non-invasive technique to study the effect of antiarrhythmic compounds