Three treatment options are available for AF in horses, with the exception of horses with CHF:
1. Pharmacologically converting the arrhythmia, with the most common drugs being either quinidine gluconate or quinidine sulfate
2. Performing a transvenous electrical cardioversion (TVEC) procedure, which was originally designed at the University of
Guelph, Ontario, and also developed at the University of Ghent, Belgium
3. Not treating the arrhythmia and leaving the horse in AF.
As for the first two options, Reef says that there are no data or controlled studies that conclude that one is better than
the other. "Though there are some known risks with the use of quinidine, there are also risks with TVEC," she says. And the
third option? "Many horses doing low level exertion can perform their jobs just fine in AF," Reef says.
How do you determine which treatment to recommend to the owner? If a horse has a highly athletic job such as racing, three-day
eventing, polo, rigorous fox hunting or upper level show jumping, that horse needs to perform safely at a relatively high
level of cardiovascular work. "One needs to choose either the pharmacologic or the electrical cardioversion, because those
horses could not do their job without it," Reef says.
In addition, there is a subset of horses that, although their performance is not that demanding, cannot do their job safely
while in AF because they have excessively high heart rates or ventricular arrhythmias. "When the heart rate gets very high,
they are also more at risk for developing ventricular arrhythmia," says Reef. That situation could possibly lead to sudden
death, though it is rare. Still, you don't want people riding these horses, Reef says.
"If you have a horse that is a low-level competition horse, you need do an exercise test to determine if it is safe to use
as intended," Reef says. The owner may decide not to treat the horse if it's safe for his or her purposes. But if the horse
has a high heart rate or ventricular arrhythmias, if it's involved in rigorous exercise and needs to compete at the top of
its game, if it's a sales prospect, or if the owners want to do what's best for the horse long-term, an owner may decide to
convert the arrhythmia even if the horse isn't experiencing any clinical signs, Reef says.
Reef says that if a horse is carefully monitored, quinidine is reasonably safe when used by experienced clinicians. And the
pharmacologic use of quinidine administration is probably half the cost of TVEC.
"Any antiarrhythmic drug that one would administer to convert AF—quinidine being the most common—has the potential of making
the arrhythmia worse," says Reef. There are other side effects, most commonly relating to the gastrointestinal tract, that
one has to look out for.
For AF, the administration of quinidine sulfate has been approved for more than 30 years. The average dose is 22 mg/kg (10
g/450-kg horse) given every two hours until a normal sinus rhythm is achieved, the horse experiences an adverse reaction or
a total of four doses has been administered. Most horses will tolerate four of those doses every two hours and convert sometime
in the following two hours. So if you calculate that four-dose regimen, the average horse will convert within eight to 10
Several other medications have been tried to treat AF in horses, but those other drugs have more side effects and are less
efficacious than quinidine.
"There are a fair number of private practices that will use quinidine, but what we do at New Bolton Center is more closely
monitored," says Laura Faulkner, VMD, of the New Bolton Center. "We have a continuous ECG on the horses all the time during
treatment, as well as once they are discharged home. We also take periodic plasma quinidine concentrations to see if the quinidine
concentration is within the therapeutic range or is getting too high and close to a toxic level, and then we adjust our treatments
accordingly. That is why we feel quite comfortable doing quinidine cardioversion because we can monitor the horses so closely."
Reef adds, "New Bolton Center is one of the few places that can actually evaluate plasma quinidine concentrations in a real-time
situation and use the information to treat the horse most effectively. The quinidine blood-to-tissue equilibration occurs
very quickly—within 30 minutes. If the horse has not converted with the initial treatment every two hours and is tolerating
the treatment well, we continue with quinidine administration every half-life (i.e., every six hours), to maintain a steady-state
plasma and tissue concentration of the drug. This regimen is often successful in horses with chronic AF. By maintaining a
steady-state plasma concentration of the drug, you are giving the heart time to convert."
In private practice, or in facilities that can't monitor plasma quinidine concentrations, clinicians use a pulsatile treatment
with quinidine—give several doses, then stop and not treat the horse overnight, then resume the pulsatile treatment in the
morning. With this method, the horse is more likely to get toxic concentrations of the drug in its tissues and blood and it
may experience more side effects.