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.
Ventricular tachycardia (or V-tach, pronounced 'Vee-tack') is a much more serious, although frequently treatable, variation of VPCs. In V-tach, there are no longer any normal beats. Instead, the heart completely ignores the signals sent by the SA node, or perhaps, if the SA node is diseased, the signals are either absent, or so distorted that they are of no use. Instead, portions of the ventricles (instead of the normal atria) act as pacemakers. In less serious cases, one area in the ventricle may continually act as a pacemaker. More serious signs occur when many different areas of the ventricles take turns acting as the pacemaker. What ensues is a chaotic, very rapid, irregularly irregular rhythm.
If the heart doesn't regularly relax to fill with blood, and isn't receiving the proper signals to contract regularly, then the body will not receive the amount of blood that it needs to nourish the muscles, brain, and internal organs with oxygen. The result? Your horse may appear weak and anxious, and he will often have an elevated respiratory rate. If the condition is severe, he may show respiratory distress, and may develop pulmonary edema (water in the lungs), which may manifest as a whitish foam at the nostrils. In severe cases, the horse may collapse or die.
The key to diagnosis is a good physical examination. Your veterinarian will look for weak pulses, an increased heart rate (often over 80 beats per minute), and pale mucous membranes. Often, the heart sounds will be louder than usual.
The definitive diagnosis is made with an electrocardiogram. In V-tach, none of the complexes look normal. The 'qrs' complexes look wide and bizarre.
V-tach is caused either by severe systemic disease, or underlying cardiac disease, like a valvular disorder that has caused the heart to become deformed, or an inflammation of the cardiac muscle itself due to some infection, often viral. We most frequently see V-tach in horses with severe gastrointestinal disorders – for instance, after colic surgery – or with some form of systemic bacterial infection (sepsis). Horses that have serious respiratory disorders and become hypoxemic (have low blood oxygen) are also susceptible to V-tach.
In the best possible scenario, once the underlying cause is treated, the V-tach goes away. But we don't always have time to wait for the underlying cause to be treated. Reasonable criteria for waiting include: The V-tach is relatively slow (less than 80 beats per minute).
The horse shows no signs of shock.
The horse is not in respiratory distress.
The V-tach is unifocal, which means it emanates from only one area in the ventricles.
If the V-tach is multifocal, if the horse shows signs of impending collapse, respiratory distress, or has a very high heart rate, the arrhythmia itself can be life threatening and must be treated.
Lidocaine given intravenously is the drug of choice for treating V-tach. Quinidine can also be used intravenously. It is important to remember, however, that both drugs can have toxic side effects, and can even be associated with sudden death. For this reason, they should always be given under veterinary supervision. Other drugs used experimentally include diltiazem and propanolol. Use of these drugs is only recommended for specialists.
It is important that you get your horse's heart checked regularly by your veterinarian, especially horses that are retired from intense exercise. In these horses, the early sign of exercise intolerance may be absent. In addition, any horse that has a persistently elevated (over 45 beats per minute) heart rate should be checked. Ask your veterinarian to listen to your horse's heart during any evaluation or examination.