Arrhythmias can also progress to congestive heart failure, characterized by coughing, accumulation of fluid in the chest and episodes of fainting. However, Frosty had never shown any hint of any of these signs.
tend to develop slower heart rates while at rest, but horses do it a bit differently. Their heart rates are naturally slow, so as they get fitter they appear to develop these arrhythmias as a way of regulating blood flow by skipping beats.”
Johnson says that an experienced veterinarian can usually identify physiological arrhythmias simply by listening, but to be certain, the test is to exercise the horse. “Arrhythmias that are purely physiological and considered normal resolve themselves when the horse’s heart rate increases through exercise or excitement,” he says. “At times when it’s critical for the heart to be pumping correctly, it does.”
Just as the referring veterinarian had done, the Missouri team worked Frosty hard on the longe, then listened to her heart again. Her arrhythmia was still there.
Next, the team hooked up an electrocardiogram (EKG) to generate a graphic printout of the electrical activity controlling Frosty’s heartbeats. The readout enabled the team to pinpoint her diagnosis to “intermittent periods of high grade 2nd degree block and 3rd degree block.” In other words, Frosty’s ventricles were consistently receiving delayed impulses from the upper chambers, and sometimes those impulses didn’t come at all. The EKG also showed evidence of “ventricular escape,” a coping mechanism that causes the ventricles to generate electrical impulses on their own.
In most cases, a horse with this type of arrhythmia would be extremely exercise-intolerant. “These horses will lose steam, not maintain power and fatigue quickly,” says Johnson. These arrhythmias can also progress to congestive heart failure, characterized by coughing, accumulation of fluid in the chest and episodes of fainting. However, Frosty had never shown any hint of any of these signs.
The team then performed an echocardiograph, to look at the mare’s heart via ultrasound, but they found no structural abnormalities that would account for the arrhythmia. They also drew blood to look for underlying disease or other inflammatory markers that can indicate heart disease--but again, Frosty appeared to be completely healthy.
Still, the veterinarians recommended a two-week course of the corticosteroid prednisolone plus an omega-3 fatty acid supplement and vitamin E as antiinflammatories. That, they said, would address any inflammation that might be the cause of the problem. They also asked McLean to rest the mare completely, with no forced exercise that could, in theory at least, lead to her fainting and injuring herself or a rider. They also wanted to see Frosty back at the university for a recheck in four to six weeks.