That comes as no surprise to Ontario Veterinary College professor Dr. Peter Physick-Sheard, who has been studying how equine hearts function in his research with some other equine athletes – Standardbred racehorses. “I tell my students that if the horse’s heart is beating steadily like a metronome, step back – because he might fall over on them,” Physick-Sheard says. Irregularities in heart rhythm are normal in horses; hearing lub dub, lub dub, pause, lub dub is perfectly natural. However, there are some types of irregularities that can cause concern.
Physick-Sheard explains that each contraction or beat of the heart is preceded by electrical activity initiated in a special area of the heart (called the pacemaker) triggering biochemical changes that signal the heart muscle to contract. ECGs (electrocardiograms) detect that electrical signal. “Every wiggle on the ECG screen means a cardiac contraction,” explains Physick-Sheard. The data from an ECG can also indicate whether or not the heart muscle is healthy or whether the heart’s internal conduction system is normal, and can provide other information about heart function.
The Wide Range of Normal
It’s important to know what normal heart rhythm looks like in a horse to help diagnose and treat problems when horses are not healthy. “What we’ve learned is that variations in heart rhythm don’t necessarily mean disease, and disease is not necessarily associated with variations in heart rhythm,” he explains. “The normal variation is huge. It’s absolutely normal for all horses to show some rhythm disturbances day to day. In fact, an absolutely steady, regular rhythm is abnormal.”
Physick-Sheard’s theory about this – a theory he admits is unproven, but makes sense logically – is that “it comes down to engineering, to a question of instability in feedback loops. A horse has a very low resting heart rate and a high vagal tone. If you are a pygmy shrew with a very fast heart rate of 1,000 beats per minute, when you need to adjust cardiac output, you have the opportunity to adjust heart rate every 1,000th of a minute. So you can make small adjustments each time. A resting horse can only vary heart rate and output every two seconds or so. That’s a long time between beats, so you have to make bigger adjustments and the system is always playing catch-up. The result is variation in rhythm at rest. These variations tend to disappear rapidly when heart rate rises in response to a stimulus, but when the heart is slowing down again you tend to get the same sort of instability in rhythm you observed at rest.”
That theory is supported by the fact that most of these rhythm variations are benign and smooth out with light exercise. To explain the situations where the variations can be more problematic, Physick-Sheard categorizes the variations as originating in either the top part of the heart (the atria) or the bottom part (the ventricles).
About 95% of the variations observed originate in the atria. The atria can beat irregularly without affecting the regular beating of the ventricles, so the horse is still getting a steady supply of blood. “With only three exceptions, these are benign and disappear when the horse has some light exercise,” he says. Those three exceptions are atrial fibrillation (when the heart beats very irregularly), supraventricular tachycardia (when the atria beat rapidly and cause the ventricles to also beat very fast), and third-degree heart block. The first two can be treated and managed. The third is potentially fatal and can’t be treated, but is fortunately very rare and usually only seen in older horses and sometimes in donkeys.
The other five per cent of variations in heart rhythm begin in the ventricles. “About 95 per cent of these are probably abnormal,” says Physick-Sheard. “While we don’t know to what degree they are clinically abnormal, they represent deviations from normal impulse formation and conduction in the heart.”
In his study of Standardbred racehorses, Physick-Sheard found that about 18% had some obvious abnormal ventricular arrhythmias immediately after racing. However, they recovered spontaneously and went on to race successfully in the following weeks. He and his students are following up with these horses to see if problems develop as time goes on.
“There are times when ventricular abnormalities reflect clinical disease,” he explains. “Not only is this serious for the horse, but it’s also a serious hazard for the rider. These arrhythmias can be associated with progressive cardiac disease leading to fainting spells or the horse’s death. While we don’t get too excited about most atrial heart rhythm changes, we are more concerned about the ventricular ones.”
The cardiac problems that can lead to the ventricular arrhythmias include bacterial or viral infections, damage to the heart muscle or accidental poisoning. It is also very often caused by the horse becoming dehydrated or having his electrolyte levels “out of whack.” In these cases, treating the underlying cause will resolve the arrhythmia as well.
“Any horse with a ventricular arrhythmia should not be exercised or worked until you know what’s going on,” Physick- Sheard adds.
There are rarely indications for the specific treatment of rhythm disturbances in horses. “Most are best treated by managing the underlying cause,” says Physick-Sheard. “Exceptions are atrial fibrillation (which can be treated by the use of drugs such as quinidine sulphate, or by transvenous electrical cardioversion), and intense ventricular rhythm disturbances that are immediately life-threatening. In that case the choice of drug will depend very much on the suspected underlying cause of the disturbance.” The use of antiarrhythmic agents in emergency circumstances carries the risk of causing fainting or actually worsening the disturbance, but this is fairly uncommon. Surgery for arrhythmias is never an option.
While knowing the meaning of these different heart rhythm variations can be important in diagnosing and treating horses, Physick-Sheard admits: “We just don’t know enough about what’s going on “under the hood”. There’s so much variation among horses depending on their fitness levels, the type of work they do, even their genetic background, that it’s hard to pinpoint what is normal so that we can better recognize what is abnormal.” His ongoing research, and that being done at other universities and veterinary colleges around the world, may provide us with the additional data needed to get a clearer picture of the workings of the equine heart.
Setting the Standard
The results of two studies on the prevalence of exercise-associated arrhythmias in dressage horses and showjumpers were presented at the International Conference on Equine Exercise Physiology in November, 2010. The purpose of the studies was to establish electrocardiographic reference values to aid in determining if exercise associated arrhythmias contribute to poor performance in sport horses, as most previous studies have only been conducted on racehorses.
In the first test, 21 normal performing dressage horses aged 5-16 years were examined clinically and via echocardiographic (ECG) during rest, during a dressage test, and during a one-hour recovery period. The test found that supraventricular premature complex (SVPC, a common arrhythmia) occurred rarely during rest, in 28.6% of the test subjects during exercise, and in 61.9% of the horses during the recovery period. Ventricular premature complex (VPC, another common arrhythmia) was seen in one horse; mild valvular regurgitation (leaky valve) was seen in 52% of the horses. No significant associations between arrhythmia and age, gender or valvular regurgitation were observed.
It concluded that while arrhythmias are relatively infrequent in dressage horses, they can occur during exercise and recovery. Also, maximum heartrate in dressage horses was found to be low compared to both showjumpers and racehorses. It was determined that additional studies of dressage horses with moderate-to-severe valvular regurgitation at rest and/or poor performance are needed to further determine the significance of cardiac arrhythmias.
The second study involved 34 healthy showjumping horses aged 4-13 years. ECG exams were performed during rest, during a showjumping course, and during a 45-minute recovery. The results showed that 39% of the horses had one or more mild or moderate valvular regurgitations, but no associations were found between valvular regurgitation and arrhythmias. SVPCs were found in 32% of the horses at rest, 89% during exercise and 54% during recovery. VPCs were demonstrated in 18% of the subjects during exercise and 7% during recovery. The researchers concluded that the studies showed a high prevalence of SVPCs both during and after exercise, but few VPCs.
Establishing these “normal” reference values sets the groundwork for future studies of equine cardiac function and provides valuable information for veterinary practitioners.