Cardiac Arrhythmias in the Horse
Dr. Melissa Mazan
VPCs are ventricular premature contractions – sporadic abnormal beats that occur in an otherwise normal rhythm. The majority of the heart's electrical impulses come from the correct SA node. However, interspersed among these normal beats is a beat that arrives too early – because an area in the ventricle has fleetingly taken over the job of the SA node, and fires a signal for the heart to contract.
The cause depends on the number of VPCs per minute. Unlike atrial fibrillation, the presence of multiple VPCs may signal that there is serious underlying cardiac or systemic disease. VPCs may indicate that there is an area of the heart that is damaged, perhaps from a viral infection. Or, some systemic disease (such as severe colic) may have caused the body's electrolytes to become deranged, and this may in turn cause the heart to develop an arrhythmia.
However, horses may have occasional VPCs without there being any problem with the heart at all.
As usual, diagnosis begins with a good physical examination and auscultation of the heart. Your veterinarian may note an occasionally irregular rhythm – the occasional beat will arrive early. If the veterinarian palpates a pulse while listening to the heart, he will find that the early beat does not generate a pulse. In some cases, these early beats arrive as a run of abnormal beats.
If your veterinarian hears these early, irregular beats are heard, he will perform an ECG, and will note that interspersed among the regular qrs complexes will be waves that look different – often wider or taller.
In order to tell if your horse has clinically significant runs of VPCs, or if they are just an incidental finding, your veterinarian may choose to monitor your horse's heart over a 24 hour period using a special, small unit that is attached to the horse's abdomen with a girth. He may also recommend that your horse have a treadmill ECG performed. If your horse develops runs of VPCs during exercise that coincide with a decrease in performance, then treatment for this cardiac arrhythmia is certainly warranted.
It depends on how frequently the VPCs arrive. If the VPCs are infrequent, then they may not need to be treated at all. If there are runs of VPCs, then the best treatment is rest – for one to two months. VPCs can also be managed with anti-arrhythmic drugs such as lidocaine if the horse can be carefully monitored in a hospital. In some cases, for instance if the VPCs are a sequela to viral disease, anti-inflammatory agents, such as corticosteroids or non-steroidal anti-inflammatory drugs (such as Banamine®) may be useful.
The prognosis is excellent once the underlying cause is removed. If the horse has a serious infection of the heart (myocarditis, endocarditis, pericarditis), and the VPCs persist, the prognosis is poor to fair. If heart failure is present, the prognosis is grave without intensive support.