Equine Sports Medicine

Cardiac Arrhythmias in the Horse

The heart is the most important muscle in the body - without the heart, none of the other muscles would be able to get the fuel that they need - oxygen. How does the heart know when and how to pump the blood to the body? It receives electrical signals that tell it, in a very rhythmical and predictable fashion, when to contract, and thus pump blood out of the body.

Overview

How does the heart's electrical circuit work?
There is an area in the heart, called the sinoatrial (SA) node, that is programmed to discharge (start the electrical circuit), 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 is known as the heart's pacemaker. Schematic of the innervation of the heart You can think of it as the conductor, and the rest of the heart as the orchestra. In order for a recognizable symphony to emerge, all the players must follow the conductor - and for a recognizable, useful heart rhythm to emerge, the rest of the heart must follow the beat of the SA node. After the SA node, that is located in the right atrium (the holding chamber for blood) discharges, the electrical current travels through nerves to the atria, and then to the area between the atria and the ventricles (the pumping chambers), called the atrioventricular (AV) node. The electrical impulse then 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'. In the horse, we can also hear S3 and S4 if we listen carefully.

Why is it so important for the heart to beat so steadily and rhythmically?
The job of the heart is to pump the blood out to the body - and certain areas of the body, especially the brain, cannot survive without the oxygen in the blood for more than a few minutes. So if the heart just stops completely, then the brain and consequently the individual will die. If the heart beats abnormally, the consequences are not quite as severe, but it does result in decreased blood flow and consequently lower oxygen content in the blood. As the heart beats, it performs two functions. In the period between S1 and S2, known as systole, the heart is actively pumping blood from the ventricles out into the body. We feel this pumping of blood as the pulse. In the period between S2 and S1, 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 very disorganized, and doesn't know when to relax and when to contract. The heart pump has lost its direction.

How do we evaluate the rhythm of the heart?

  • As usual, the first step is to perform a careful physical examination. The veterinarian can look for outright signs of cardiac failure (see the article on valvular heart disease) that may occur when an arrhythmia (an abnormal rhythm) is severe. The veterinarian will also listen carefully to evaluate the rhythm of the heart. As your veterinarian 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'.
  • The 'p' wave reflects the flow of electricity through the atria, and tells us that the atria have received a normal impulse, and have contracted.
  • The 'q, r, s' waves tell us that the electrical circuit has traveled through the atrioventricular node, and through the ventricles, and has resulted in the ventricles contracting.
  • The 't' wave is the most important one, in a way - it tells us that the electrical impulse has traveled all through the heart, and that the heart is now 'repolarizing', or getting ready for the next impulse to start in the SA node, travel through the atria, go through the AV node, and through the ventricles.
  • Figure 2: Normal ECG

  • 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 we cannot use the ECG for any measurements other than the cardiac rhythm.

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What Arrhythmias Are We Likely to Hear in the Horse?

2šAV Block
  • Many horses, especially fit, athletic horses, have an arrhythmia called Second Degree Atrioventricular Block.
  • When your veterinarian listens to your horse, this will sound like 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 2šAV 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 with 2šAV Block? 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 2šAV Block disappear. The exercise or excitement makes the horse's adrenaline levels go up, and the vagal influence go down - thus the arrhythmia goes away.
  • Once in a while, 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.
  • Does your horse need to have an ECG to determine if the 2šAV Block is benign? Probably not. This arrhythmia is so common in horses, and the field test (that is, listening to the heart again after trotting the horse for a minute or two) is so dependable, that we usually do not recommend that the average horse have an ECG performed.
  • If the arrhythmia does not disappear when the horse is trotted, then an ECG and perhaps an echocardiogram is warranted.

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Atrial Fibrillation

What is happening in the heart during atrial fibrillation?
In atrial fibrillation, the cardiac impulse becomes confused at the level of the SA node. For reasons that are not entirely clear - again, most likely because of the very large size of the horse's heart, and because of high input from the vagal nerves - 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. The result? The atria never relax or contract properly, and the heart does not get the chance to pump as efficiently as it should.

What causes atrial fibrillation?

  • In the majority of horses, there is no discernable underlying cause for the atrial fibrillation. Once again, the horse's large heart and high input from the vagal nerves is to blame - we theorize that these inputs cause the signal from the SA node to become disorganized.
  • Some horses do have heart murmurs and underlying valvular disease that precipitate the atrial fibrillation.
  • Other factors that may contribute to atrial fibrillation include
    • Viral disease
    • Low potassium levels in the blood - this can occur secondary to using Lasix (a drug used in racehorses that are 'bleeders').
    • Anemia
    • Colic

What effect does atrial fibrillation have on the horse's performance?
Horses have an enormous cardiac reserve - meaning that their hearts are capable of delivering far more blood than they need for mild to moderate work. What does this mean for the horse? It means that horses usually don't show signs of exercise intolerance due to atrial fibrillation until they start to exercise at high levels - for instance, racing, the three-day eventing, or polo.

How do we diagnose atrial fibrillation?
The astute veterinarian can usually be suspicious of atrial fibrillation just on auscultation. The astute veterinarian will notice that the rhythm (unlike, for instance, 2šAV Block) 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. Remember that the 'p' wave reflects electrical activity in the atrium due to the SA node - 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.

Figure 3: Atrial fibrillation

My horse doesn't race or play polo - he's just a pleasure horse. Does his atrial fibrillation need to be treated?
In most instances, the answer is 'yes'. Most horses do not have any underlying cardiac pathology that causes a-fib. However, longstanding a-fib can lead to cardiac disease.

Sometimes, the answer is 'no'. The treatment for a-fib (see below) 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.

How do we treat atrial fibrillation?
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 very toxic effects on horses, such as colic, founder, collapse, swelling around the throat, and abnormal heart rhythms (other than a-fib). For these reasons, we advise that horses only be treated at facilities where they can be monitored continually, preferably with a 24-hour ECG monitor.

What is the prognosis for recovery from atrial fibrillation?
The prognosis is excellent (meaning that there is a 95% chance of recovery) if:

  • The resting heart rate is less than 60 beats per minute
  • The atrial fibrillation has been present for less than 4 months
  • There is no associated heart murmur, or the murmur is of low intensity (grade 2 out of a possible 6, or less).

The prognosis is guarded to good (meaning that there is an 80% chance of recovery, but a 60% chance that the atrial fibrillation will return) if:

  • The heart rate is greater than 60 beats per minute
  • The atrial fibrillation has been present for more than 4 months
  • There is an associated heart murmur that is grade 3 out of 6 or more.

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Ventricular Premature Complexes (VPCs)

What is a VPC?
VPCs refer to 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.

What causes VPCs?

  • This depends on how many VPCs are occurring every 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.

How are VPCs diagnosed?

  • As usual, diagnosis begins with a good physical examination and auscultation of the heart. The veterinarian will 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 these early, irregular beats are ausculted, then the veterinarian will perform an ECG, and will note that interspersed among the regular 'qrs' complexes will be ones that look different - often wider or taller.
  • Figure 4: Ventricular premature contractions

  • 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. She 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.

How do we treat VPCs?
Again, 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. The horse should be monitored periodically using the ECG during that time. 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.

What is the prognosis for VPCs?
The prognosis is excellent once the underlying cause is removed.

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Ventricular Tachycardia

What happens to the heart with V-tach?
Ventricular tachycardia (or V-tach, pronounced 'Vee-tack') is a much more serious 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 act as pacemakers. In less serious cases, one area in the ventricle will 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.

What happens to the horse with V-tach?
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. 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.

How is V-tach diagnosed?
Once again, the key to diagnosis is a good physical examination. Our 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 will look wide and bizarre.

What causes V-tach?
V-tach is caused either by severe systemic disease, or underlying cardiac disease (for instance, 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 sepsis (systemic bacterial infection). Horses that have serious respiratory disorders and become hypoxemic (have low blood oxygen) are also susceptible to V-tach.

How is V-tach treated?
In the best possible scenario, once the underlying cause is treated, then the V-tach goes away. Do we 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 (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 very toxic side effects, and can even be associated with sudden death. For this reason, they should always be given under veterinary supervision.

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Other than second degree atrioventricular block, cardiac arrhythmias are uncommon in horses. The most common arrhythmia that can cause clinical signs, atrial fibrillation, is usually not associated with any underlying organic heart disease, but is a consequence of the horse's natural anatomy and physiology. However, if your horse demonstrates exercise intolerance, or if your veterinarian suspects that he has a cardiac arrhythmia, it is a simple matter to perform an ECG to determine what is happening at the level of electrical activity in the heart. Early and effective treatment of even relatively benign arrhythmias such as atrial fibrillation will protect the heart muscle, and help your horse to live a longer, healthier, more productive life.