The heart is the most important muscle in the body – without the heart, none of the other muscles would get the fuel that they need – oxygen. The heart is also a complex muscle that must operate constantly throughout the life of an animal. It must contract and relax in a rhythmical and predictable fashion to pump blood through the body. It does this by receiving electrical signals from two nodes within the heart: the sinoatrial (SA) node and the atrioventricular (AV) node.
The SA node is a collection of muscle fibers and nerve endings in the wall of the right atrium, the holding chamber for blood, where the rhythm of cardiac contraction is established. It is programmed to discharge or start the electrical circuit, which causes the heart to beat in the horse 28 to 44 times per minute at rest. During exercise, of course, the SA node discharges far more frequently, up to 240 times per minute. The interval between each discharge is very predictable in the normal horse – it is frequently likened to a metronome, which lets musicians keep a reliable beat to their music. The SA node establishes a rhythm and is known as the heart's pacemaker.
The AV node is a mass of muscle fibers located in the wall between the atria and the ventricles, the pumping chambers, that transmit the cardiac impulse from the SA node to the muscular walls of the ventricles. The electrical impulse travels through nerves called the Purkinje fibers throughout the ventricles in an explosive fashion, and causes the ventricles to contract. This final event is what causes blood to course through the body in its rhythmical, predictable way.
We hear the results of this cardiac activity as the heart sounds – primarily S1 and S2, which sound like 'lub-dub'. S1 is the sound that marks the approximate beginning of the contraction, and S2 is heard at the end of the contraction when the ventricles begin to relax.
The Importance of a Steady Heartbeat
The job of the heart is to pump blood filled with oxygen to the entire body – and certain areas of the body, especially the brain, cannot survive without oxygen for more than a few minutes. So if the heart were to stop completely, the brain and consequently the individual would die. If the heart develops an arrhythmia, that is it does not beat rhythmically, the result is a decreased blood flow and consequently lower oxygen levels delivered to the tissues.
As the heart beats, it performs two functions. In the period between S1 and S2 (from lub to dub), known as systole, the heart pumps blood from the ventricles into the body. We feel this pumping of blood as the pulse. In the period between S2 and S1 (from dub to lub), known as diastole, the heart relaxes – and this relaxation period is vital to the proper function of the heart as a pump. It is during diastole that the heart fills with blood returning from the body – without this relaxation period and return of blood to the heart, there would be nothing for the heart to pump out. If the electrical impulses to the heart become erratic, then the heart, in essence, becomes disorganized and doesn't know when to relax and when to contract. The heart pump loses its direction.
If your horse develops an arrhythmia, your veterinarian will want to evaluate your horse's cardiac system. As usual, the first step is to perform a careful physical examination. Your veterinarian can look for outright signs of cardiac failure that may occur when an arrhythmia is severe. The veterinarian will also listen carefully to evaluate the rhythm of the heart. As she listens, she will palpate the pulse, to see if every audible beat results in a spurt of blood coursing through the vessels. If there are skipped pulses, then it is likely that the electrical impulse to the heart may have been a useless one.
The definitive way to evaluate the rhythm of the heart is with an electrocardiogram, known as an ECG or EKG. The ECG takes advantage of the fact that the heart beats according to an electrical impulse that can be measured using a set of electrodes. The ECG produces a series of recognizable waveforms – labeled p, q, r, s, and t.
In humans, dogs, cats and many other smaller mammals, the ECG can tell us not just about the cardiac rhythm, but also about the size of various areas of the heart. The horse's heart, probably because it is so very large, is innervated slightly differently from other animals, receiving the signal through the ventricles all at once, rather than as a gradual spread. The result is that the ECG cannot be used for any measurements other than the cardiac rhythm.
Many horses, especially fit, athletic horses, have a normal arrhythmia called second degree atrioventricular block. This is characterized by a regularly irregular rhythm. That is, even though there is a break in the normal rhythm, it is very predictable. If you listen, you will hear 'lub dub, lub dub, pause, lub dub'. If you were to tap your foot to the rhythm, you would find that the pause is exactly the length of a normal beat, and the next beat would come in exactly on time.
If you were to look at an ECG of a horse with second degree AV Block, you would see normal 'pqrst' complexes until the time when a pause occurred. At that time, all you would see is a 'p' wave. Then, the normal 'pqrst' would return.
What is happening is that the normal innervation to the SA node, which comes from the vagus nerve, has such a strong input in the horse, that it suppresses some of the impulses that are trying to reach the AV node. So, we see a 'p' wave, but no 'qrst'. Because there is no real abnormality in the heart, the next impulses go through just fine.
In the normal horse, a little bit of excitement or exercise should make the second degree AV block disappear. The exercise or excitement makes the horse's adrenaline levels increase and the vagal influence go down – thus the arrhythmia goes away. This is a benign arrhythmia. An ECG is not usually recommended in these cases.
Occasionally, a horse will have second degree atrioventricular block that doesn't disappear with exercise. This usually indicates that the arrhythmia in this case is not benign, and an ECG and perhaps an echocardiogram is warranted.
Atrial fibrillation is marked by rapid randomized contractions of the atrial muscles, causing an irregular, often rapid, ventricular rate. The cardiac impulse becomes confused at the level of the SA node. For reasons that are not entirely clear, the signal from the SA node does not travel smoothly through the atria to the AV node and then to the ventricles. Rather, multiple signals come from various areas of the atria that begin to act like many small SA nodes. Only a few of these signals manage to get to the AV node and then to the ventricles. As a result, the atria never relax or contract properly, and the heart does not get the chance to pump as efficiently as it should.
In the majority of horses, there is no discernable underlying cause for the atrial fibrillation, but the horse's large heart and high input from the vagal nerves are factors. Some studies have shown that horses with atrial fibrillation have large atria and stretching injuries (scarring) or inflammation in those atria. These problems likely promote the conduction disturbance of atrial fibrillation, whereby the electrical pathway encircles the atria, rather than exiting the atria as normal, to the ventricles.
Some horses do have heart murmurs and underlying valvular disease that precipitate the atrial fibrillation. Other factors that may contribute to atrial fibrillation include:
Effect on Performance
Horses have an enormous cardiac reserve. This means that their hearts are capable of delivering far more blood than needed for mild to moderate work. Consequently, horses usually don't show signs of exercise intolerance due to atrial fibrillation until they start to exercise at high levels, as in racing, the three-day eventing or polo.
The astute veterinarian can usually be suspicious of atrial fibrillation just by listening to the heart (auscultation) and will notice that the rhythm is irregularly irregular. Veterinarians often refer to it as sounding like sneakers in a dryer or bongo drums.
The definitive diagnosis is made with an electrocardiogram (ECG). Remember that the p wave reflects electrical activity in the atrium due to the SA node, and if the SA node isn't firing properly, then there won't be a recognizable p wave. Instead, there are multiple fibrillation or f waves – perhaps 20 or 30 of them where there ought to be only one 'p' wave.
Most horses do not have any underlying cardiac pathology that causes atrial fibrillation. However, long-standing atrial fibrillation can lead to cardiac disease.
The treatment for atrial fibrillation is potentially dangerous. So, if your horse is debilitated or is at high risk for toxicity for other reasons, you may choose not to treat. If your horse doesn't need to do high-intensity work, you may never notice that he has a problem.
The treatment of choice is a drug called quinidine. Quinidine has to be given with a nasogastric tube every 2 hours throughout the treatment period. This drug can have toxic effects on horses, such as colic, founder, collapse, swelling around the throat, and abnormal heart rhythms, other than atrial fibrillation. For these reasons, horses should only be treated at facilities where they can be monitored continually, preferably with a 24-hour ECG monitor.
The prognosis is excellent, meaning that there is a 95 percent chance of recovery, if the following conditions are in effect:
The prognosis is guarded to good (meaning that there is an 80 percent chance of recovery, but a 60 percent chance that the atrial fibrillation will return) if these conditions are present:
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:
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.