Atrial Fibrillation in a Horse

 An 18 year-old gelding presented for a pre-lease exam. The horse was sound and displayed no clinical abnormalities. However, when the heart was auscultated the heart rhythm was abnormal. Normally, a horse's heart sounds like two distinct sounds, otherwise known as the "lub-dub". These two sounds correspond to the opening and closing of specific heart valves during contraction and filling of the heart's chambers . There are actually 4 heart sounds but typically we only describe the "lub-dub". In a normal horse, the number of lub-dubs that are counted in a minutes range between 28-40 lub-dubs or beats per minute and they should be equally spaced in time. The electrocardiogram (EKG) in Figure 1 is that of a normal horse. The red arrows correspond to the contraction of the ventricles and the green arrows correspond to the contraction of the atrial. Normally, the atria contract first followed by the contraction of the ventricle. The blue arrow corresponds to the time between ventricle contractions which should equal the heart rate or number of lub-dubs auscultated in a period of time. Notice that the blue arrows are equal in length between heart beats AND that there is one green arrow for every red arrow!!

Figure 1

The lub-dubs for this particular horse were irregular and there was no predictability of their irregularity hence the heart rhythm was described as "irregularly, irregular". This type of heart arrhythmia is consistent with a condition known as atrial fibrillation. In Figures 2 and 3 notice that the blue arrows which correspond to the time between heart beats vary significantly. Equally important, there appears to be many more green arrows (Figure 3) than red arrows. This would suggest that the atria is contracting more often than the ventricles, hence the term atrial fibrillation!!

Figure 2

Figure 3

 Atrial fibrillation in horses is not uncommon and young horses with this condition are often asymptomatic. Thoroughbreds are diagnosed most often and when it is diagnosed in younger horses, the cause for the arrhythmia is often not determined. However, in older horses underlying heart disease is often associated with atrial fibrillation.  The most common clinical complaint would be exercise intolerance and decline in performance. In addition, an elevated resting heart rate may be indicative of early congestive heart failure as well as persistently distended jugular veins (Figure 4). Diagnosis of atrial fibrillation requires an EKG and a cardiac ultrasound is recommended to determine if there is underlying heart disease. Often a 24 hr halter monitor (Figure 5) may be used to better describe the horse's 24 hr cardiac cycle. There are several methods available to try and "convert" a horse with atrial fibrillation. However, it is important to first determine that cardiac disease nor electrolyte disturbances are the root of the problem. The methods include various medications and electrical cardi-conversion.  Prognosis for horses with atrial fibrillation depends on whether the horse has underlying cardiac disease. In the case of cardiac disease, the prognosis is poor; however, in young horses with idiopathic atrial fibrillation, the prognosis can be good for full return to work as a sport horse!! This case re-inforces the need for a solid physical exam on a regular basis to make sure this type of disease does not go undetected!!
  

Figure 4

 

Figure5