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
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
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
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