A rider has praised the skill of vets who performed potentially life-saving surgery on her horse’s extremely rare circulatory condition.

Five-year-old Hanoverian Luigi had been sent to Leahurst Equine Hospital at the University of Liverpool for investigation into a heart murmur when vets discovered three arteriovenous fistulas in his neck — abnormal connections between his carotid artery and jugular vein.

The vessels had become so enlarged that they were filling with blood and pressing on his throat when he put his head down, causing him to make a noise when breathing.

“As far as we know, he was born with the problem but it was only when he got to five that it started to become an issue,” explained Luigi’s owner Eve Rice, who bought the chestnut gelding as an unbacked three-year-old.

“I had him vetted when I first got him and nothing came up, but I took him to Germany and had to get him re-vetted and it was at that point that we discovered he had developed a heart murmur.

“When he went to Leahurst, they noticed that every time he put his head down there was a strong pulse in his neck and the swelling there would enlarge. They hadn’t seen it before and it took quite a few tests and scans to reach a conclusion.”

Normally, blood is pumped into the arteries by the heart and then travels through a fine network of capillaries into the veins. A fistula creates a shortcut straight from an artery to a vein and as a result the heart has to work harder to pump blood, putting it at an increased risk of failure.

“We get referrals for heart murmurs commonly but this is very unusual case, I have never seen another one, and there is only one other published example in a horse,” said Leahurst vet Harry Carslake.

He explained that when Luigi was first diagnosed, it was initially decided just to monitor him but his condition worsened and he started to make a noise when working. It was then decided that risky surgery would be necessary.

“The essential thing for us was to establish where the connections were — you can see from the images that he had a large and convoluted network of blood vessels. We located them using a combination of CT, ultrasound and fluoroscopy (where a contrast agent is injected into the artery),” Harry explained.

“In his case we found three abnormal connections, one large one next to his larynx and two others deeper into his head, which we knew we wouldn’t be able to access, but it was the larger one that was causing most of the problem.”

The three-hour procedure to tie-off the vessels was performed by specialist surgeon Debra Archer and involved a team of other specialists from the hospital, including experts in cardiology and imaging.

Eve said concern over the strain the fistulas were putting on Luigi’s heart had prompted her to go ahead with surgery and that while she was nervous “as they were clear it could go wrong”, she trusted the Leahurst team.

“It was very stressful, I dragged my neighbour round to watch Disney films with me while he was in surgery to try and take my mind of it!” she said. “When I went to see him afterwards, I expected him to be low but he was normal Luigi — in everyone’s faces trying to get cuddles.”

His recovery was also “quicker than expected” and Eve enlisted the help of physio Karris Reeves to help build his strength back up, as well as remaining in touch with Harry to monitor his progress.

“We were focused on making sure the cardio was really good. He lost quite a lot of muscle especially on his hindquarters, so the physio gave us pole exercises in hand and we did a bit of long-reining,” Eve said. “We also had to keep an eye on the pulse in his neck and make sure it wasn’t stronger and that his breathing stayed the same.

“It’s amazing how he’s come back from it such but he’s such a character and so full of life — he has always been fit as a fiddle,” she added, also joking that “while his show name is Lifetime, I now like to call him lifetime of debt!”

Continued below…

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Eve, who bought the son of Londonderry with inheritance money from her grandparents, is now carrying on with training with the aim to compete him at dressage in future.

“He’s quite quirky but very talented and while I want to compete him, I don’t want to rush,” she said. “He really is my horse of a lifetime and I wanted to find a way of thanking the entire team at Leahurst for taking the risk and giving him a better chance at life.”

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“Get tested” is the message from the chief executive of a charity committed to reducing cardiac deaths in young people.

Dr Steven Cox, of Cardiac Risk in the Young (CRY), is encouraging all people between the ages of 14-35 to have an ECG screening.

Many sporting organisations already have mandatory pre-participation tests and the British Horseracing Authority (BHA) is looking into whether it needs to adopt this practice.

“If you have an undiagnosed cardiac condition, then you engage in sport, it increases your risk by three-fold,” Dr Cox told H&H.

“There is no reason a rider would be any different to someone playing rugby, cricket or football.”

The charity has free routine clinics at St George’s Hospital in Tooting, London, every few weeks as well as others around the country.

“People think of Fabrice Muamba on the football field (Fabrice, aged 23 at the time, had a cardiac arrest during an FA Cup quarter-final in 2012. His life was saved), and that affects their perception that it only happens to elite athletes,” said Dr Cox.

“The vast majority of sudden deaths which occur are at a grassroots level — someone who is aspiring in that sport.”

Dr Cox highlighted the case of 27-year-old amateur dressage rider Mair Schmeinck, who died from injuries sustained in a fall from her horse following a cardiac arrhythmia in October (horseandhound.co.uk, 25 January; letters, 12 May).

“One of the most important aspects of raising awareness of CRY after a tragedy like Mair’s is to make people aware of the signs and symptoms to look out for,” Dr Cox said.

“These include passing out and exercise-related chest pain which are both red flags as well as if there is a family history of young sudden cardiac death.

“However, only 20% of those people at risk of a young sudden cardiac death experience symptoms before the collapse.

“This is why screening is so important and why there are international recommendations for all young people to have heart checks before engaging in organised sport.”

The BHA and Liverpool Hon Moore’s University launched a cardiac study last month (news, 28 April).

It is focused on the effect rapid weight reduction through dehydration has on heart function in active jockeys. It is also hoped to help decide whether jockeys should undergo pre-participation cardiac screening.

The first free sessions were held at Malton and York racecourses earlier this month.

Dr Jerry Hill, the BHA’s chief medical adviser, recommends that jockeys take the time to get involved and have a free check-up.

“It will not only help important scientific research but might also unearth any existing, underlying conditions which our riders may not know they even had, allowing early intervention,” he said.

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Every horse has one, but do you know why the heart is a muscle, what causes it to beat and what rates are considered healthy? Here are 16 things you need to know about how the equine heart works — and what can go wrong.

1 The heart is a muscle that contracts in a rhythm, hence its regular beat.

2 It acts as an electric pump to move blood around the horse’s body and provide cells and tissues with oxygen.

3 Hearts have four chambers: the right and left atrium at the top, which both receive blood, and two ventricles at the bottom to pump blood.

4 When blood enters the right atrium, it is low in oxygen, so it passes into the right ventricle. From here it is pumped into the lungs via the pulmonary artery to be oxygenated.

5 Once this has happened, blood passes through the pulmonary veins to the left atrium and on to the left ventricle.

6 Now oxygen rich, blood is pumped into the horse’s body via the aorta, a major artery.

7 This heart cycle results in the distinctive beat you can hear.

8 To prevent blood flowing the wrong way, the upper and lower heart chambers and vessels are separated by valves.

9 These valves can leak slightly in fit, healthy horses. However, severe leakiness causes the heart’s chambers to enlarge, making the horse unsafe to ride. A heart scan is needed to determine this.

10 A pericardial sac filled with fluid surrounds the heart and cushions it.

11 A horse’s heart has massive capacity to increase the amount of blood it pumps. This is called cardiac output and is why horses can work at high speeds for a long time.

The following heart rates are normal for a fit, adult horse:

• At rest 24-40 beats per minute (bpm)
• In trot 80 bpm
• In canter 110 bpm
• A fast canter 140 bpm
• In gallop 180 bpm
• Maximum rate 240 bpm

12 There are two main forms of heart problems: (1) murmurs caused by abnormal blood flow and (2) arrhythmias, which are disturbances of the heart beat rhythm.

13 Functional flow murmurs are caused by large amounts of blood passing through the heart. These are common and are of no clinical significance.

14 Pathological murmurs are caused by heart disease. The murmur is graded according to its loudness, when it occurs in the heart’s cycle and where it can be heard the loudest.

15 If a pathological murmur is suspected, other signs of heart disease will be investigated and an ultrasound of the heart (echocardiogram) will be advised. This can identify problems such as abnormal function, leaky valves and a hole in the heart.

16 Physiological arrhythmias are heard at rest, but disappear when the heart rate increases due to exercise or excitement. It is common for an irregular rhythm — occasional pauses of the heart (missing a beat) — to be heard in fit horses. This is called second-degree atrioventricular blocks.

To find out more about your horse’s heart, and what you can do to keep it healthy, don’t miss the current issue of H&H Ask The Vet, which is on sale now

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Working at speed calls for a healthy, well-trained heart. Andrea Oakes discovers how technology can help us track heart performance and detect potential problems

Sports such as point-to-pointing and eventing make exceptional demands on a horse’s heart. This finely tuned muscle will beat 220-240 times per minute at a fast gallop, each time pushing up to 280 litres of oxygenated blood around the body.

XLEquine vet Imogen Burrows explains that the well-trained heart is a highly effective pump, with the capacity to store a large volume of blood and the muscular ability to propel it around the body. Rather than beating faster, it works harder.

“Training results in heart muscle enlargement (hypertrophy), accommodating the need for greater energy and oxygenation of muscles by pumping blood more efficiently,” says Imogen. “The heart rate will then be lower at given exercise levels.

“The average horse has a maximum heart rate of around 240 beats per minute (bpm). Untrained, he might reach this rate at a speed of eight metres per second (m/s), but the speed at which he reaches maximum rate when race-fit may rise to 12m/s.”

The time it takes for the heart to return to normal after extreme exercise is another indication of fitness.

“During recovery, the heart rate drops rapidly over the first couple of minutes, before returning to a resting rate of between 28bpm and 40bpm,” says Imogen. “The fitter the horse, the shorter the recovery time.”

Sophisticated monitoring

Trainers and riders have traditionally relied on what they see and feel to gauge fitness, but girth heart monitors can now deliver instant and precise heart rate measurements during exercise and recovery.

“This is a very effective way to assess a horse’s progress,” says Imogen. “Resting and maximal heart rates are not altered by training, and so are not useful fitness indicators, but heart rates at set speeds and recovery rates can be used to guide workload.”

Real-time heart monitoring can also aid the development of aerobic and anaerobic capacity.

“Increasing duration of exercise (endurance) requires predominantly aerobic energy production, whereas sprinting relies mainly on anaerobic processes,” says Imogen.

“Point-to-pointing requires both pathways, with anaerobic metabolism contributing only around 30% of energy.

“Training programmes can be tailored using a percentage of maximal heart rates (HRmax). Aerobic training begins when the horse is working at 60-70% HRmax, with a heart rate of between 144bpm and 168bpm. Pushing this slightly to 180-192bpm stretches aerobic capacity and improves efficiency of fuel utilisation during exercise. Anaerobic metabolism occurs at even higher rates.

“The instant feedback provided by a girth monitor allows the rider to adjust pace as necessary to keep the horse working comfortably within the correct heart rate zone,” adds Imogen.

Losing power

Different girth heart monitors can also help to detect health problems. An unusually high heart rate during exercise can indicate pain or sickness, while reduced performance or delayed recovery might point to a cardiac issue.

“Heart problems fall into two categories: abnormalities of rhythm (arrhythmias) or disruption of blood flow (murmurs),” says Imogen. “Collapse or sudden death is rare, however. In the vast majority of cases the muscles will run out of energy and the horse’s ability to perform will be impaired.

“Cardiac output — the ‘engine’ that powers the equine athlete — is a function of heart rate (the number of beats per minute) and stroke volume (the amount of blood pumped per beat).

“Because a significant heart problem reduces stroke volume, heart rate must increase to maintain cardiac output. The horse will reach his maximum heart rate at a much lower speed, resulting in poor performance.

“Fitness itself does not predispose the heart to problems, yet cardiac issues are more likely to be detected in the elite athlete,” adds Imogen. “This is because — while most leisure horses will be working well within their cardiac limits — the performance horse is being asked to exert himself beyond these moderate levels.”

This article was first published in Horse & Hound magazine (27 November 2014)

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More articles on lameness and respiratory problems

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Lameness

Bone, joint and muscle injuries are the commonest causes of poor performance in dressage horses and show jumpers.

It is not unusual to have to nerve-block horses affected with subtle bilateral lameness in order to prove to their connections that lameness is indeed the problem.

By repeating standard exercise tests before and after, or following treatment with anti-inflammatory drugs like phenylbutazone (bute), it is often possible to prove that removing joint pain not only increases the maximum speed a horse can attain, but also greatly improves its willingness to exercise.

Respiratory problems

Inflammation of the lung and airways — the lower respiratory tract — is common in performance horses.

Vets are yet to quantify the effect of this inflammation on performance during low-intensity exercise, but we know that when infectious outbreaks occur in racing stables, there can be immediate and sometimes long-term effects on performance.

Exercise-induced pulmonary haemorrhage has detrimental effects on race performance, although its role in limiting the performance of other equine athletes, in whom it also occurs, is less certain.

Allergic lung disease causes more problems in older horses.

Although not quantified, it seems logical that low-grade disease could reduce performance either directly or indirectly by adversely affecting the horse’s well-being and its willingness to work.

The upper respiratory tract can also cause problems during exercise.

Displacement of the soft palate (gurgling), laryngeal paralysis (roaring) or collapse of the tissues around the larynx can all obstruct air flow to the lungs and limit the ability to carry oxygen. This forces the horse to slow down or fade quickly.

As affected animals do not always gurgle or roar, viewing the larynx with an endoscope during exercise is the only method for accurate diagnosis.

The heart

In a small percentage of horses the heart and cardiovascular system is the primary cause of poor performance.

Most commonly an abnormality of the heart rhythm, known as atrial fibrillation, is responsible.

Very rarely a severe heart valve leak can also reduce athletic performance but, surprisingly, valve leaks and heart murmurs are not associated with poor performance in most cases.

For the full article on poor performance, see the current issue of Horse & Hound (26 November, ’09)

Looking for more articles on lameness and respiratory problems?

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A new heart rate monitor that can be used by horse owners at home has been launched on the market.

Previously only vets have been able to monitor a horse’s heart rate but new equipment has been produced by Polar Electro to allow owners and riders to assess their horse’s physical condition.

The RS800CX GS Equine Heart Rate Monitor transmits uninterrupted, wireless live data while you are riding, providing a real-time heart rate as well as tracking speed, distance, pace and altitude.

The information is recorded and then the analysis can be downloaded onto a computer.

Manufacturers Polar Electro claim the product “might just give you the performance edge” as owners can use the device to understand how well a horse is building fitness, coping with stress and performing in competition.

And it can be used on a day-to-day basis to create bespoke exercise or recovery regimes.

Polar Electro also says it will encourage a “better connection” between rider and horse as trainers will know how much to ask of the horse and will be alert to illness, stress and fatigue.

The monitor costs £469.30 and is available at www.polarelectro.co.uk

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Swedish research has scientifically demonstrated that an increase in a rider’s heart rate can lead to an increase in their horse’s heart rate.

This confirms what horse people have long suspected — that a lack of rider confidence can worry a horse.

A team from the Department of Animal Environment and Health at the University of Agricultural Sciences in Uppsala, published their findings in the July 2009 edition of The Veterinary Journal.

The horses tested were of various ages, breeds and sexes, and the handlers from varying equestrian backgrounds.

Each participant was asked to ride or lead their horse between two set points. On the fourth and final time, they were told that an assistant would open an umbrella as the combination passed. In reality, the umbrella was not opened.

The horse had no idea that an umbrella might open so the researchers knew any change in the horse’s heart rate would be down to the change in the person’s, resulting from the anticipation of the umbrella being opened.

The horses’ heart rates increased as the humans’ did when the horses were both ridden and led, suggesting the horses could sense the change even when the only human contact was through a lead rein.

H&H vet Karen Coumbe said: “This shows how sensitive horses can be to human responses and be very aware of people’s moods.”

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Do heart murmurs really affect performance horses?

Should you risk buying a horse with a heart problem and what are the chances of a murmur being debilitating? Recent research has given some answers.

A study I carried out last year that was published in the Journal of Veterinary Internal Medicine concluded that race-fit thoroughbred horses with heart murmurs did not appear to perform any differently from those who were unaffected.

Hard evidence

As the results of this study showed no consistent associations between racing performance and the presence — or grade — of heart murmur, its authors concluded that unless the heart itself fails, there should be no adverse effects on performance.

Different murmurs

A murmur is a noise that can be heard with a stethoscope at a time when the heart should normally be silent, for example between the normal heart sounds.

Most healthy sport horses have a heart murmur of some sort and only occasionally will the murmur mean there is a serious heart problem. But just knowing that a horse has a murmur tells you nothing.

The vet must decide whether the problem is caused by normal blood flow or whether a heart valve might be leaking. If a leak is suspected, then the specific valve must be identified because some are more likely to cause problems than others.

The significance of valve regurgitation depends on its severity and the likelihood of valve function deteriorating and the leak worsening over time.

Modern technology

Specialised ultrasound equipment can be used to visualise normal and abnormal blood flow in the heart, allowing an accurate diagnosis of the source of the murmur to be made.

In the vast majority of horses, diagnosis of valvular regurgitation certainly does not mean the end of athletic activity and a significant proportion of affected horses are successfully sold on.

What are the implications?

Heart murmurs are very common in sport horses, as is poor performance. But, conversely, cardiac disease is a very uncommon cause of reduced performance.

When it is, however, it is almost always an abnormal cardiac rhythm that is to blame, not a murmur. So be careful that a horse with a cardiac murmur is not written off until you have all the relevant facts.

For the full article on heart murmurs, see the current issue of Horse & Hound (28 May, ’09)

Looking for an equine vet near you?

Heart attacks in horses are a rare but extremely traumatic event that can occur without warning during ridden exercise or when the horse is exercising at liberty in the field.

The problem is not exclusive to top sport horses, such as three-day eventers and racehorses; any horse in any discipline can be affected. In a 2005 study funded by the International League for the Protection of Horses 25% of sudden death fatalities were pleasure horses, and exercise doesn’t even have to be that strenuous or fast.

Healthy horses are on average 50 times more likely to die during exercise than healthy people, but the causes of death are very different. The human casualties who collapse during marathons are much more likely to have succumbed because of an inherited disorder of the heart muscle or of the electrical activity in the heart. These conditions do not exist in horses to our knowledge.

Why do horses die during exercise?

When the sudden death of an apparently healthy horse occurs during exercise, almost half are due to massive fatal haemorrhage (bleeding) from arteries or veins of either the chest or abdomen.

The remaining, largest and most perplexing group of sudden death fatalities has no distinct abnormalities to explain the demise at post-mortem, except non-specific signs of sudden cardiac failure.

Unfortunately, the underlying reason for a large blood vessel in the abdomen or chest breaking and causing major bleeding in a horse is usually not clear.

Before routine worming became the norm, sudden deaths were attributed to artery damage caused by migrating larvae of an equine parasite (strongylus vulgaris). However, since worming became much more effective, thereby reducing potential larval damage to arteries, the prevalence of exercise-related fatal haemorrhage does not seem to have reduced.

It makes sense that, if a blood vessel is weak, tearing is most likely to occur during exercise because that is when there is the highest pressure within it. When the horse’s heart rate increases with work, the pressure in the arteries increases dramatically, which could potentially cause a weakened vessel wall to burst.

During fast exercise in horses, the pressure within vessels, particularly those going to and from the lungs, is much higher than in people. This probably partly explains why healthy horses die more often than people during exercise.

Horses in general have very healthy hearts because we have bred them for speed, not for looks. They are not affected by congenital or inherited heart disease, unlike cats, dogs and humans. Also horses rarely drink beer, breakfast at McDonalds or smoke so are not affected by the build-up of fatty substances in their coronary arteries like some humans are.

What is a heart attack?

The term “heart attack” refers to the abnormal rhythm of the heart (ventricular fibrillation) that results in its total failure to pump oxygenated blood to the brain. Once the brain’s oxygen supply fails, death follows soon afterwards.

We do not yet understand the triggers that cause the majority of equine fatalities to develop ventricular fibrillation during exercise. At post-mortem there are no obvious changes to the heart.

It seems likely that high body temperatures, low oxygen levels, increased acidity in the blood and changes to the concentration of certain electrolytes, particularly potassium, that occur normally during exercise in horses might make the abnormal rhythms more likely to develop and be sustained, but the actual trigger is as yet unclear.

What does seem clear is that affected horses are usually clinically normal and do not have any previous history of heart murmurs or other problems.

While there are still so many unanswered questions, vets and scientists are striving to understand this condition better. The Home of Rest for Horses is currently helping to improve the knowledge of vets by funding a three-year non-invasive research project into heart failure to the tune of £63,000.

H&H, 28 July 2005

According to vet Karen Coumbe, heart rate, pulse and respiration are “some of the first veterinary checks to be made when assessing any potential problem, so it is important to know what is ‘normal’ for your horse”.

The best place to feel for the heartbeat is where the girth lies, behind the elbow on the near side of the horse. The pulse is the pressure beat of the blood as the heart pumps it around the arteries and is usually the same as the heart rate.

How to take the pulse

The pulse is easiest to find after exercise — at rest, it is slow and can be difficult to detect. It can be felt by gently pressing a superficial artery against unyielding tissue, such as bone, with the flat of the fingers.

• Count the beats for 15sec and multiply by four to find the rate per minute
• Feel for the tubular structure of the facial artery, which runs under the jaw, and press gently
• The axillary artery can be found just below the elbow on the inside of the forelegs.
• A pounding digital pulse is indicative of laminitis or pus in the foot. It can be found either side of the back of the fetlock. Never ignore this sign.
• The coccygeal artery can be felt either side of the tail under the dock

What is normal?

At rest, the pulse and heart rate of a healthy adult horse is generally between 32 and 42bpm. The heart and pulse rate can increase rapidly to more than 200bpm during hard work, so always allow time for the pulse to return to normal before checking after work. The fitter the horse, the quicker the recovery time will be.

A persistently raised pulse in a resting horse suggests problems. Look for obvious signs of pain or lameness and always check the temperature for indications of fever. It can also be an indicator of heat exhaustion, shock or heart disease.

If the horse is showing signs of colic with an elevated pulse of more than 80 beats per minute (bpm) there is something seriously wrong and urgent veterinary attention should be sought.

A consistently lower than normal pulse may also be an indication of shock, hypothermia or possibly poisoning. If appropriate, warm the horse with rugs and seek veterinary advice.

• This article was orginally published as part of the Accident & Emergency series (25 September) in Horse & Hound. Don’t miss this Thursday’s issue, which focuses on how to cope with a rider who has suffered a broken arm in a fall.

Atrial fibrillation is a disorder of theheart, which causes its regular rhythm to become interrupted and go out of synch. A potentially serious condition, atrial fibrillation is most often seen in large, athletic horses with big hearts, but can also be caused by a heart defect.

The healthy heart

The heart is divided into two parts: the left side, which supplies the muscles and vital organs with oxygenated blood, and the right side, which must pump blood to the lungs to replenish them with oxygen. Each side has a muscular pumping chamber, the ventricle, and a collecting chamber or atrium.

The heart is instructed to beat by an electrical impulse that originates high in the wall of the right atrium. This impulse travels quickly through the walls of both collecting chambers (atria) and causes them to contract, pushing any remaining blood into the pumps.

The electrical impulse is then delayed for almost 0.5sec before it reaches the pumping chambers, enabling all the blood to arrive in the ventricles before they contract and ensuring maximum efficiency.

This electrical activity can be recorded as an electrocardiogram (ECG), which is a map of the journey of the electrical impulse through the heart.

Heart rhythm

In a healthy state, the electrical impulses fire off regularly from the right atrium, providing a constant rhythm. As a horse becomes fitter, the rate of firing is damped down because his larger, more muscular heart does not need to beat so frequently.

In atrial fibrillation, the electrical activity of the collecting chambers is completely chaotic. As a result, instead of beating regularly in a co-ordinated pumping motion, the atria beat out of synch.

Meanwhile, the ventricles are being bombarded by electrical “nonsense”. There is no longer a regular heart rhythm with the odd missed beat – instead, there is a chaotic rhythm, which the vet can recognise by taking a pulse or listening with a stethoscope.

During atrial fibrillation there is no useful contraction of the atria, so there is no effective turbo charge to fill either ventricle. Instead, the pumps have to fill under gravity. This is not as disastrous as it sounds,because 70% of filling does occur passively under gravity. At rest and during slow paces, the heart can easily make up for this inefficiency by beating a little faster every minute.

Symptoms and treatment

Affected horses usually appear normal at rest. However, at all levels of exercise,the heart must beat faster to make up for reduced efficiency. In horses, which work well below maximum cardiac capacity, the rider may be unaware of the problem.

It is in the disciplines which require all of the heart’s capacity that atrial fibrillation becomes obvious such as hunters, eventers, racehorses and point-to-pointers.

When atrial fibrillation is affecting performance, the condition may be treated with quinidine. Treatment is effective and normal rhythmis restored in eight out of nine horses. Unfortunately, the drug must be administered repeatedly by a stomach tube and can have unpleasant side effects, including,in a handful of cases, death.

As a result, when the chaotic rhythm is not affecting your horse’s performance, your vet may decide to leave well alone.

Unfortunately, the problem can recur, and for every eight horses returned to normal rhythm, one will remain in atrial fibrillation. When treatment fails, horses can continue successfully in a less demanding sporting discipline, so failure may still not mark the end of their working life.

Treatment is always delayed for at least 48hr after the onset of the abnormal rhythm. In a number of horses,the heart willrevert to normal of its own accord, so the vet will always give it the opportunity to do this by itself.

Self-correcting atrial fibrillation is common in smaller flat racing Thoroughbreds. In these horses, by the time the horse has walked back from the racetrack to his stable, the heart rhythm may have returned to normal.

It is important that you know your own horse’s normal temperature, pulse and respiration so you can spot any changes.

The average healthy horse has a pulse of 35-42 beats per minute at rest, although this will vary in individual animals depending on a variety of factors such as age, breed and degree of fitness. This can increase to more than 200 beats per minute when a horse is galloping hard.

The heartbeat in the horse is most easily felt on the left side of the lower chest where the girth would go, just behind the elbow. It can also be felt where the facial artery passes under the jaw.

Other good places to feel the pulse, particularly in the headshy horse, are:

• inside the forelimb just below the elbow
• either side of the back of the fetlock
• either side of the tail under the dock

What causes a raised pulse rate?

If your horse he is nervous or alarmed, for example when a vet or a stranger approaches him, then his pulse will shoot up. When he calms down, his pulse will quickly return to its normal rate. Anxiety is the most common reason for an increased pulse.

Another healthy reason for a raised pulse is exercise. If your horse has just been worked, his pulse will be faster than normal – the increased pulse rate will depend on the amount of exercise, the period of recovery and how fit he is.

If your horse is injured while competing, do not be alarmed if his pulse is fast when first measured. Always allow time for his pulse to return to normal.

Warning signs

A persistently raised pulse is more serious. Reasons for this can include:

• Pain
• Fever
• Heat exhaustion
• Shock
• Heart disease

If your horse has a persistently raised pulse check the following:

• Look for obvious signs of pain
• Check his temperature to see if he has a fever
• Check for any signs of lameness
• Is he showing any signs of colic

What if his pulse is slower than usual?

A lowpulse usually indicates a relaxed, fit and hopefully healthy horse, but it can be associated with serious conditions such as shock, hypothermia or even poisoning.

If you have a healthy horse with a low pulse, do not worry, but with an ill looking horse you should do the following:

• Start by taking his temperature
• If your horse is shocked, he will have a weak pulse
• If there is no obvious reason talk to your vet

The horse’s heart is a pump that supplies all its muscles and organs with oxygen andnutrients and carries away waste products.

The heart is divided into two sides. The left side takes oxygenated blood from the lungs and supplies muscles and vital organs, such as the brain and kidney.

The de-oxygenated blood then returns to the right side to be pumped to the lungs, where it is replenished with oxygen. Each side has a muscular pumping chamber, the ventricle, and a collecting chamber, the atrium.

With each beat, a typical adult horse pumps approximately one litre of blood out of each side of the heart – the stroke volume. With a resting heart rate of 30-40 beats a minute, this means the total blood pumped in one minute is 30-40 litres.

The heart can increase the quantity of blood being pumped around the body to meet the demands of exercise in two ways.

• Increase the amount of blood pumped with each beat. When exercise begins, adrenaline makes the heart muscle contract more forceful, sending more blood around the body with each beat.
• Increase the heart rate. In Thoroughbreds, the resting heart rate is between 28 and 36 beats a minute, while the maximum heart rate is between 225 and 240 a minute. The huge range of heart rate is the most important mechanism for increasing cardiac output during exercise, particularly as work intensity increases.Just as muscles for movement become stronger with training, so, does the heart muscle. Research has shown that the amount of heart muscle, and the width and area of the left heart chamber, increased dramatically with training.Increased heart volume allows it to hold more blood at rest, and during exercise, while increased muscle also allows the more powerful “fit” heart to eject more of this blood with each beat.New technologies enable us to measure the horse’s heart rate both at rest and during exercise. This information can provide a helpful insight into a horse’s fitness and potential performance.

Murmurs are audible by a vet in up to 60% of horses. The vast majority can be considered normal, functional murmurs, and do not indicate any underlying heart or vascular disease (ie: disease of the blood vessels).

The difficulty lies in establishing which cases are functional murmurs and which are caused by a heart problem – pathological murmurs – which could affect the horse’s athletic ability and safety.

In addition, there is a third group of murmurs – physiological murmurs – which are caused by conditions not involving the heart, such as anaemia or pregnancy.

Pathological murmurs can be caused by:

• Leakage between two or more of the heart chambers, usually due to a defective valve. Valvular insufficiency or incompetence is most common.
• Disease of the heart wall muscle – myocardial disease.
• Abnormal narrowing of the heart valve – stenotic valve.

Assessing a murmur

If a horse is showing clear signs of congestive heart failure, such as appearing dull or lifeless, coughing or heaving, then the presence of a loud heart murmur is clinically significant.

However, if a problem arises while the horse appears healthy and well, the owner will want to know if the murmur is due to an underlying problem which is going to affect the horse’s performance.

The vet will take into account the horse’s age and recent performance when assessing a murmur. The vet will then carry out a clinical examination paying particular attention to:

• General condition
• Colour of mucous membranes
• Capillary refill time
• Strength and tone of the arterial pulse
• Filling of the jugular veins
• Nasal discharge
• Respiratory characteristics

The vet will also listen to the horse’s chest with a stethoscope. This will provide valuable information, but the horse must be in a relaxed state. Any true resting heart rate of more than 50 beats per minute could be considered abnormal.

This examination is done to evaluate the heart rhythm and classify the murmur according to:

• Timing and duration: where the murmur occurs in the cardiac cycle and for how long.
• Character: type, intensity and sound.
• Point of maximal intensity and radiation: where on the chest wall it can be heard best.

This classification can help establish the origins of a murmur and if it is likely to be associated with a cardiac abnormality or not. Many functional murmurs are localised, of short duration, “musical” and of low intensity, while pathological murmurs associated with serious cardiac problems tend to be louder, harsher and last longer.

Finally, the vet may refer your horse to a specialist centre for an echocardiography or ultrasound examination of the heart. This will provide more detailed information about the state of the horse’s heart in difficult or unusual cases.