Exercise-induced pulmonary haemorrhage (EIPH)

Exercise-induced pulmonary haemorrhage (EIPH) refers to bleeding (haemorrhage) from blood vessels within the lung (pulmonary) which occurs during strenuous exercise (exercise-induced). Whilst it has been documented for over 300 years that a small percentage of racehorses show blood at the nostrils after racing (e.g. Bleeding Childers, Herod), it was not until the introduction of fibreoptic endoscopes (devices for looking into the airways; they are passed via the nostrils in the conscious horse) that it became clear that blood observed at the nostrils during or following strenuous exercise nearly always originated from the lung.

With the more widespread use of endoscopy in veterinary practice and veterinary research, it is now clear that 40-75% of Thoroughbred horses will have some blood in their trachea (windpipe) after racing. The degree of bleeding varies considerably between individual horses, with horses showing visible blood at the nostrils often being referred to as "bleeders".

Whilst originally thought to be a Thoroughbred "problem", it is now clear that any breed or type of horse undertaking strenuous exercise may experience EIPH. The condition has been observed in Thoroughbreds following flat racing and steeplechasing, Standardbred racing (trotting or pacing), polo, show jumping, cross-country and barrel racing. EIPH has also been shown to occur in racing greyhounds, camels and humans after intense exercise.

The bleeding that occurs as a result of intense exercise in horses is not randomly or uniformly distributed throughout the whole lung, but affects the dorso-caudal (uppermost and rearmost) part of the lung (See Figure). In a young two year old horse in training or young riding horse, with strenuous work (fast canter and gallop and possibly jumping), the very tips of the lung will tend to be affected. As the horse ages and works more a larger amount of lung is affected and the bleeding tends to become worse. It may be that continual bouts of EIPH cause structural changes in the lung as a result of the repair process and this may explain the trend for more frequent and severe bleeding with age.

As the amount of blood in the trachea after intense exercise can vary greatly between horses and even in the same horse over a period of time, various grading systems have been developed to describe the amount of blood seen endoscopically after exercise. The system we use grades the bleeding on a scale of 0 (no blood in the trachea) to 5 (the whole trachea is covered with blood). This scoring is undertaken 30-90 minutes after exercise. In one survey, involving 223 flat racehorses in Great Britain, we found the prevalence of EIPH after flat racing to be 40% in two-year olds, 65% in three-year olds and 82% in horses older than three. In addition, in the older horses the more severe grades of bleeding occurred more frequently.

Despite the fact that it is now 25 years since it was generally accepted that horses bleed in their lungs after intense exercise, the cause is still not known. The most popular theory is that the bleeding occurs because of very high stresses (known as transmural pressures) acting across the walls of the tiny capillaries (small blood vessels) in the horses's lung during exercise leading to stress failure. The blood is only separated from the air spaces within the lung by a very thin membrane, which facilitates uptake of oxygen by the blood. During exercise horses develop tremendously high pressures in the pulmonary blood vessels and it is hypothesised that this could be sufficient to rupture the vessel walls. This is the basis behind the use of the drug Lasix (frusemide) in North America. Lasix is a diuretic, a drug which causes increased urine production hence loss of water from the blood circulation. This causes the blood pressure both at rest and during exercise to be lower. However, the scientific evidence that bleeding is due to high pressure is poor and as yet there are no scientific studies that prove that Lasix either reduces or prevents EIPH, indeed in some surveys of the condition in the USA in the 1980s, horses racing on Lasix were no less likely to bleed than untreated animals. In addition, high blood pressure theories of EIPH cannot immediately explain why the bleeding occurs in the upper and rear part of the lung. Perhaps one reason for the popularity of Lasix is that it has been shown to improve performance because a treated horse will be carrying around 20-30 kg less weight.

We have recently proposed that EIPH results from the high impact of the front legs on the ground during fast cantering, galloping and jumping. When the front legs hit the ground during galloping, the shoulder is pushed hard onto the rib cage. When the foot is planted on the ground, the force is transmitted to the lung and a shock-like "wave" passes through it. Because of the shape of the lung this "wave" becomes amplified and most intense in the rear and upper part of the lung. The damage is similar to that experienced in the lungs of people in car accidents where they are hit hard in the front of the chest. In this situation, the damage and bleeding that occurs in the lung is not usually at the front of the lung, but at the back. A similar situation could exist between the back and top part of the lung and the chest wall.

In a survey, 26% of flat trainers and 54% of National Hunt trainers thought that bleeding affects racing performance. In a survey carried out by one of us (Colin Roberts) it was found that the incidence of EIPH following racing was not different between a random sample of horses and poorly performing animals. In a study under laboratory conditions performed at the University of Sydney, with horses exercising on a treadmill, it was shown that placing 200ml of blood in the left and right lungs significantly reduced performance. This amount of blood was thought to be equivalent to Grade 3 (the middle grade of bleeding) on our scale.

There is evidence that EIPH may cause permanent alterations in the blood supply to the affected parts of the lungs. Fortunately, EIPH only affects a relatively small amount of the total lung. However, the presence of blood in the airways may of itself also lead to inflammation. Inflammation is the process that occurs when body tissues are damaged. For example, in the case of a cut in the skin, the inflammatory response results in increased blood flow to the area, with redness, swelling, heat and pain. Some studies have suggested that inflammation may occur in the lung as a result of EIPH.

At present, without knowing the true cause of EIPH it is extremely difficult to make recommendations on management to prevent, reduce or treat the condition. Many suspect that airway inflammation, following infection or as a result of poor stable hygiene (allowing contamination with moulds, dust and ammonia), may accentuate EIPH. Others believe that upper airway conditions such as laryngeal hemiplegia ("roaring") may worsen bleeding.

A current Horserace Betting Levy Board Grant to the Animal Health Trust is funding a two year study to investigate the effects of changes in airway resistance on the occurrence of EIPH. If the theory of pulmonary capillary stress failure due to high transmural pressures is correct, then negative pressures in the airways outside the blood vessels could increase the overall pressure (stress) across the vessel wall and exacerbate bleeding. In horses with increased airway resistance due to upper or lower airway disease, the incidence and severity of EIPH would therefore be expected to rise,

In addition, we are also investigating our theory that EIPH may be caused by locomotory impact and attempting to devise better techniques for being able to measure how much blood enters the airways and from exactly which part of the lung. Only once we have a clear understanding of the causes of EIPH and access to techniques to accurately quantify the bleeding within the lung will it be possible to improve our management of this condition.

For those interested in further reading, there are two recent publications which summarise the outcome of two recent workshops on EIPH:

Exercise-Induced Pulmonary Haemorrhage Workshop. C.A.Roberts and H.H. Erickson. Equine Exercise Physiology 5, Equine Veterinary Journal, Supplement 30 (1999), pages 642-644.

Exercise-Induced Pulmonary Haemorrhage Study Group. (June 1999) The Horserace Betting Levy Board.

Dr David Marlin
Centre for Equine Studies
August 2000

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