Atrial fibrillation (AF), an electrical disorder of the heart rhythm, is the most common pathological arrhythmia in horses.
With this condition, the atria fail to contract but instead quiver or fibrillate, and impulses are conducted intermittently
through the atrioventricular (A-V) node to the ventricles. The resulting electrocardiogram (ECG) has irregular R-to-R intervals,
a lack of P waves and undulations in the baseline—known as F waves—that are caused by the fibrillating atria (see Figure 1).
Figure 1. An ECG tracing showing atrial fibrillation, characterized by a lack of P waves, irregular R-to-R intervals and undulating
baseline F waves. (Photo courtesy of Laura Faulker, VMD, New Bolton Center.)
Although AF often develops in horses with advanced structural heart disease—particularly in horses with valvular insufficiency
that results in atrial dilatation—horses often develop this arrhythmia with minimal or no detectable signs of heart disease.
"We see a broad spectrum of horses with AF, including a young racehorse that has no other cardiac disease that we can detect
but has acute onset of AF," says Virginia Reef, DVM, DACVIM, University of Pennsylvania's New Bolton Center. "Such horses
are considered similar to what they call lone AF in humans, which is AF in an otherwise healthy heart." Reef also sees horses that have AF associated with underlying structural
disease. Mitral regurgitation is the most common valvular insufficiency causing left atrial enlargement, and these horses
are at more risk of developing AF, she says.
"In older horses with AF that aren't engaged in vigorous work, we suspect that they might have underlying myocardial disease
even if we can't definitively identify it," Reef says.
If a horse with atrial fibrilation (AF) has a highly athletic job, such as polo, the horse needs to be treated for AF to
perform safely, experts say. (GETTY IMAGES/THOMAS HOTTNER)
A small group of horses also present with AF and congestive heart failure (CHF). "That category of horse we would treat very
differently," says Reef. For horses with CHF, AF is not the primary problem. Instead, it's the underlying heart disease that
is causing the CHF, and treatment focuses on rate control and palliative treatments rather than rhythm conversion.
Which horses are affected—and how
AF typically affects full-grown horses. "Fortunately, AF is rare in foals, though occasionally you'll have a newborn foal
have a transient arrhythmia," Reef says. "You need a critical atrial mass to get AF, so it isn't a problem in foals, basically,
unless the foal has a very diseased heart." For the same reasons, AF is not a problem typically seen in ponies, unless their
hearts are enlarged.
Just as horses with AF vary between those with healthy hearts and those with CHF, the presenting complaint in horses with
AF can vary widely as well. High-level athletes, such as race horses, event horses, polo ponies, fox hunters, barrel racers,
endurance horses and many show jumpers, experience exercise intolerance and a drop in performance. Horses that perform at
lower level cardiovascular work, such as dressage horses, show hunters, Western pleasure horses and trail or pleasure horses,
may not have any signs of AF that their owners can detect. The reason for these different presenting signs involves the physiology
of horses with AF.
During the cardiac cycle, the atria depolarize and contract, and then, subsequently, the ventricles depolarize and contract.
The atria prime the pump, or the ventricles, which are responsible for moving blood to the lungs and to the body.
"If the atria are fibrillating, you've essentially lost that prime to the pump," says Reef. "At rest that does not make any
difference whatsoever, though during exercise it makes a big difference, as priming of the pump might contribute 15 to 20
percent of cardiac output."
If the horse is doing intense athletic work, optimum cardiac output is important. Cardiac output is determined by both heart
rate and the amount of blood moving out of the ventricles at each heartbeat (stroke volume). AF decreases stroke volume during
exercise, resulting in a higher heart rate at each level of exercise.
Young, healthy horses in normal sinus rhythm can have heart rates of 240 beats/min or even higher during maximal exercise.
Horses with AF, however, have a heart rate typically 40 to 60 beats/min higher at each level of exercise, so they often reach
that maximal heart rate at a submaximal exercise level.
"With that irregularity of rhythm, and the lack of that priming of the pump, it seems that horses in AF cannot do their job
if their heart rate exceeds 220 beats/min, for whatever the exercise level that they're doing," says Reef.
At New Bolton Center, if an owner is considering not treating a horse for AF because it is not exhibiting clinical signs,
then an exercising ECG is obtained to ensure the horse is safe to perform its job while it is experiencing AF. Some horses
do not follow the general rule of being just 40 to 60 beats/min higher than average, and these animals can have dangerously
high heart rates just trotting. Others exhibit malignant ventricular arrhythmias in addition to AF during exercise. Cardioversion
is recommended for these horses if their owners wish to continue to use them as performance animals.