Treatment of recent-onset atrial fibrillation with quinidine and flecainide in Thoroughbred racehorses: 107 cases (1987–2014)

Abstract

OBJECTIVE To compare the efficacy of quinidine and flecainide in treating naturally occurring, recent-onset atrial fibrillation (AF) in Thoroughbred racehorses. DESIGN Retrospective case series. ANIMALS 107 Thoroughbred racehorses. PROCEDURES Medical records of racehorses with AF that were treated with quinidine or flecainide between 1987 and 2014 were reviewed. Signalment, history, treatments, complications, and outcome data were collected. Horses were allocated to 2 groups according to the initial treatment: initial treatment with quinidine (group 1) or initial treatment with flecainide (group 2). Horses in group 2 that did not convert to sinus rhythm with flecainide were then administered quinidine (group 3). Complications, total quinidine dose, and duration of treatment were compared. Rates of conversion for horses treated with quinidine versus flecainide were also compared. RESULTS Overall rate of cardioversion was 91% (97/107). There was a significant difference in the rate of cardioversion for quinidine alone (91% [71/78]), compared with flecainide alone (41% [12/29]). In group 3, the conversion rate after the addition of quinidine treatment was 82% (14/17). Total quinidine dose and treatment duration did not differ significantly between groups 1 and 3. CONCLUSIONS AND CLINICAL RELEVANCE Overall rate of cardioversion for Thoroughbred racehorses with AF was similar to that in previous reports. Flecainide treatment was less effective than quinidine treatment, but the frequency of complications did not differ between quinidine and flecainide. Further investigation is suggested to evaluate the efficacy of flecainide for cardioversion in athletic horses.

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

“…In 1 study, naturally occurring AF with a duration of 12 days could be terminated by flecainide infusion, and in another study, flecainide was effective in 2 cases of naturally occurring AF . Furthermore, flecainide was reported to cardiovert 12 of 29 horses with recent‐onset AF . These findings suggest that the effects seen with flecainide are not restricted to experimentally induced AF.…”

“…Studies on flecainide for cardioversion of AF have provided conflicting results. High efficacy was seen in horses with experimentally induced AF, but flecainide was only effective in 1 of 10 horses with naturally occurring AF and in 41% of horses with recent‐onset AF . Reported adverse effects include agitation, restlessness, neck sweating, decreased borborygmi and several cases of ventricular proarrhythmia and sudden death .…”

Use of intravenous lidocaine to treat dexmedetomidine-induced bradycardia in sedated and anesthetized dogs

“…In recent years, drugs of this class have lost clinical importance due to availability of safer drugs. One exception is the current use of quinidine to treat atrial fibrillation in horses in places where transvenous electrical cardioversion is not available (Silva 2018;Takahashi et al 2018).…”

Fundamentals of arrhythmogenic mechanisms and treatment strategies for equine atrial fibrillation

“…Mitochondrial alterations have been implicated in arrhythmogenic mechanisms in several animal models 135–137 ; however, the clinical importance of such mechanisms in equine AF is unknown. Equine AF studies are mostly hospital‐based retrospective series 81–83,138–140 or prospective screening of racehorses 8,85 and it is difficult to draw conclusions for the whole population. The effect of age on equine AF appears to vary with breed and activity.…”