There is a subset of horses that need electrical cardioversion because quinidine places the horses at more risk. "Within that
patient population, quinidine allows the heart to conduct more of the fibrillating impulses to the ventricles," Reef says.
"And then the horses' heart rates can get way too high and they can be at risk for other arrhythmias."
Horses normally have very high resting vagal or parasympathetic tone that slows impulse conduction through the A-V node. But
this influence is lessened in very excitable or nervous horses, and, thus, their resting heart rate in AF may be elevated.
These horses may also have sympathetic-induced ventricular arrhythmias. Quinidine enhances conduction through the A-V node,
which can result in dangerously high heart rates in certain horses. Therefore, excitable, nervous horses, or those with ventricular
arrhythmias, are ideal candidates for electrical cardioversion.
"TVEC is used in horses because administration of external defibrillator paddles, as per human medicine, is not successfully
used in horses," says Reef. With TVEC, two electrodes are passed via two separate catheters through the jugular vein into
the horse's heart. One is directed to the left branch of the pulmonary artery, so it is sitting over the left atrium, while
the other inserted into the right atrium.
"The idea with TVEC is to have the electrical current go across the fibrillating atria to reestablish that normal rhythm,"
Reef says. The catheters must be placed properly; placement is done under echocardiographic guidance, and thoracic radiographs
are obtained to ensure the catheters are properly placed.
The amount of time it takes to place a catheter, administer anesthesia, perform the TVEC procedure and recover the horse varies
from facility to facility, says Reef: "It's a shorter time frame than a quinidine cardioversion, but it takes many more people
to do it." The procedure requires both expertise and specialized equipment, such as the proper defibrillator, so TVEC procedures
are performed only at specialized facilities such as teaching hospitals and advanced equine facilities. This need for specialized
equipment and several personnel makes TVEC more expensive than pharmacologic cardioversion.
"Some of the risks with TVEC are just the fact that horses have to be under general anesthesia. And with horses, more than
with other species, there are risks of general anesthesia, especially during recovery," says Faulkner.
"The key is to weigh both the client's concern for the TVEC procedure—which includes the risk of general anesthesia, recovery,
all those issues—versus the drug administration," says Reef.
Prognosis after AF conversion
The prognosis is generally good for affected horses without more severe heart disease. "An important point is that if you
successfully cardiovert a horse in AF back to normal sinus rhythm by any method, the horse should be able to go back to a
healthy state," says Reef. "That is one of the great things about treating horses with AF—you can restore the horse to its
previous performance activity. It is a very successful treatment. The horse's ability to convert—to go back to 'normalcy'—and
go back to race successfully or to whatever athletic performance event it is required to perform is independent of the way
you decided to treat it. Once the horse's heart rhythm is normal, the horse is good to go."
Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and
veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.