What Causes Racehorse Lungs to Bleed?

 

           Exercise-induced pulmonary hemorrhage (EIPH), or bleeding from the lungs, occurs world-wide in the performance horse.  However, it is observed rarely in other species.  EIPH is an important cuase of exercise intolerance.  It occurs primarily during spring racing in Quarter Horses, Standardbreds and Thoroughbreds.  It also is observed in a variety of other evens such as barrel racing, cutting, reingin and roping competitions.  EIPH also has been reported in the racing camel, racing Greyhound and a few human athletes.

           EIPH in horses has a long history in racing, dating to at least the 18th century.  Bleeding Childers, born about 1716, was one of the first documented bleeders.  This horse was the great-grandsire of the Thoroughbred foundation sire Eclipse, but was retired to stud because of frequent bleeding episodes.  His name was subsequently changed to Bartletts Childers.  In 1766, Herod, another foundation sire, broke a blood vessel in the lung and lost the Great Subscription Purse at York, England.

           EIPH results from strenuous sprint exercise and/or pathologic changes in the equine athlete.  It is defined as the presence of blood in the tracheobronchial tree (system of tubes in the lungs) following strenuous exercise.  EIPH generally occurs soon after training begins, and tends to increase in incidence with age.  Present evidence suggests that high vascular pressures cause stress failure of the pulmonary capillaries, resulting in hemorrhage and edema (excess collection of fluid) in the gas-exchange region of the lung.

           Endoscopic surveys (using an instrument to visualize the interior) of the airways in horses after a race have demonstrated that a large number of horses suffer from EIPH.  The incidence has been reported to be as high as 75 percent by endoscopy; however, less than five percent bleed from the nose.

           Endoscopic examination of the airways of horses after racing reveals a 95 percent incidence of bleeding in horses examined at least two times.  Bronchoalveolar lavage studies (washing of the lungs) suggest that hemorrhage occurs in essentially all horses during racing or training.  Complete recovery from an episode of bleeding might take four to six weeks, depending on the severity of the condition.

 

Factors to consider

           Although EIPH has been recognized for some 300 years, many unanswered question still exist about the causes and mechanism of bleeding, the appropriate treatment, and management of horses that are bleeders. Within the cardiopulmonary system, three factors need to be considered: the heart, the blood vessels, and the blood that circulates through the vessels.

At rest, the heart beats about 25-30 times per minute and ejects about a liter (a little more than one quart) of blood with each beat. During exercise, the demands of the body, particularly skeletal muscle, increase significantly, and heart rate increases to 220-230 beats per minute.

Stroke volume — the amount of blood ejected by the heart with each beat — increases by as much as 50 percent to 1.5 liters. Blood pressure increases in all four chambers of the heart, both the filling chambers (the atria) and the ejection chambers (the ventricles). The atria are some-times called the primer pumps, and the ventricles, the power pumps.

The pressure increases proportionately more in the atria than in the ventricles; it also increases proportionately more in the right ventricle, which supplies blood to the pulmonary circulation, than in the left ventricle. These high pressures in the heart result in profound increases in the vascular pressures in the pulmonary circulation, which, in turn, result in rupture of the small capillaries.

The time for contraction and relaxation of the heart is significantly less (0.25 seconds) during exercise than at rest (two seconds). The high right and left atrial pressures during exercise suggest that the heart might be responsible for the extremely high pressures in the pulmonary circulation. The equine heart appears to be stiff, and its rate of relaxation could be too slow during the high heart rates that accompany exercise to accommodate the large volume of returning blood from the veins, thereby causing the high atrial pressures.

The vessels that rupture in the lung during exercise are now believed to be the capillaries. There is strong evidence that the extremely high pressures in the pulmonary circulation during exercise result in mechanical failure of these tiny vessels, resulting in hemorrhage into the alveoli (air-containing spaces where gas is exchanged with the blood).

In the lung, the alveolar gas and blood are separated by an extremely thin membrane, called the blood-gas barrier.  At rest, about 25-30 liters of blood flow each minute through the pulmonary circulation, when the blood gives off carbon dioxide and picks up oxygen. During exercise, cardiac out-put increases significantly, and blood flow through the lungs increases about tenfold to nearly 300 liters per minute. Blood pres-sure in the pulmonary vessels increases four- to fivefold, eventually resulting in rupture of some of the thin pulmonary capillaries that are located in the alveolar walls. These walls need to be thin for maximum efficiency of gas exchange, but they also must be strong enough to withstand the high pressures placed upon them during strenuous exercise.

The force of gravity acting on the pulmonary blood has been postulated to be the primary determinant for regional differences in blood flow in the lung, particularly in a large animal like the horse. This would result in decreased per-fusion at the top of the lung, and increased perfusion at the bottom of the lung. Vascular pressures also should be higher in blood vessels at the bottom of the lung than at the top. However, the lesions of EIPH are most pronounced at the top of the lung.

Recent studies show that, at rest, vessels at the bottom of the lung receive no more blood flow than those at the top. During exercise, the increase in blood flow to the lung is, in fact, greater at the top than at the bottom of the lung. This increase in blood flow at the top of the lung during exercise could account for the characteristic location of EIPH in this region.

 

Exercise-induced splenic contraction

          The blood consists primarily of red blood cells and plasma. The spleen has the capacity to store 35-50 percent of the total red cells in the body, and does so when the horse is resting. A major factor contributing to the athletic ability of the horse is exercise-induced splenic contraction. Red blood cells are ejected from the spleen and placed in the circulation. This greatly augments circulating of blood volume and hemoglobin concentration.

The splenic capacity for red cell storage and subsequent release during exercise is related to the type of horse; e.g., Quarter Horses have much higher splenic weights than draft horses. The red blood cell fraction of the blood increases linearly from 40 percent at rest to 60-65 percent during exercise.

This splenic infusion of red cells increases the oxygen transport capacity, maximum oxygen consumption, cardiac output, and pulmonary vascular pressures; however, there might be an upper point where the improved oxygen-carrying capacity is offset by an increase in blood viscosity. The elevated viscosity of the blood could result in an increase in vascular pressure in the pulmonary capillaries and con-tribute to EIPH. Small airways disease Small airways disease is also a general feature in bleeders — this has been observed with imaging and post-mortem studies. These horses often have bilaterally symmetrical, dark discoloration of the dorsocaudal regions of the caudal lung lobes. These lesions have a complex pathogenesis that might be related directly to previous bouts of bleeding. Local inflammatory lesions causing small airways disease could predispose a horse to EIPH and enhance stress failure of the pulmonary capillaries and bleeding.

On the other hand, EIPH might first occur in healthy horses and lead to secondary inflammatory lesions and small airways disease. The presence of chronic pulmonary inflammatory processes associated with an increase in lung volume during high intensity exercise could be responsible for increased blood vessel fragility and the occurrence of bleeding in the lungs. At this time, it is not certain if the lesions found in the lungs of bleeders are the primary cause of EIPH or the secondary consequence of bleeding in the lungs.

In summary, EIPH might be the result of multiple factors that predispose the equine athlete to bleeding from the lungs. These factors include:

·        a large and powerful cardiac pump, which might not function optimally at high cardiac output and high heart rate;

·        pulmonary capillaries, which might not be strong enough to contain the blood at the high pressures that develop during exercise;

·        an increase in blood viscosity, which could augment pulmonary pressure as the spleen contracts and infuses large numbers of red blood cells into the circulation during exercise;

·        and inflammatory lesions in the airways, which might lead to small airways disease.

 

Medication – a li’l dab’ll do ya

            Bleeding, as noted in the accompanying article, was first documented in racehorses nearly 300 years ago.  Horses still bleed in response to the rigors of racing, but unlike in those days of yore, when they do it today, their horsemen have more choices on what to do about it.

            Medication is often the preferred, but increasingly controversial, choice.  Most racing jurisdictions today permit at least some use of drugs to alleviate exercise-related pulmonary hemorrhage (bleeding), and they also allow medication for the treatment of another of the most common maladies of all species and sports.  The two most common types of permitted medication are furosemide, for bleeind, and phenylbutazone, for soreness.

            Furosemide (sold under the trade name Lasix) is a diuretic that stimulates urinary output and the absorption of excess fluid from the tissues, relieving the condition known as edema.  Furosemide has other effects, including the reduction of blood pressure in the aorta and pulmonary artey – that is why it is used to treat bleeders.  Furosemide can enhance racing performance, however, particularly on first use in a horse starting a race.

            Phenylbutazone (Butazolidin, commonly called “bute”) is a nonsteroidal anti-inflammatory medication that reduces inflammation, swelling, pain and fever.  An analgesic similar to aspirin, bute is neither a preventative nor an anesthetic, nor does it enhance performance.

            RCI suggest that a racetrack’s official veterinarian maintain a “Bleeder/Lasix list of all horses which have demonstrated external evidence of exercise-induced pulmonary hemorrhage or the existence of hemorrhage in the trachea post exercise upon endoscopic examination.  Such examination must have been performed by or in the presence of the official veterinarian or the racing veterinarian,” and the confirmation must be certified by the official veterinarian.  According to RCI, furosimide is to be administered intravenously, with the dosage not less than 150 milligrams nor more than 250 milligrams.

            If the track uses a detention bard, the administration is to take place at that barn no less than four hours prior to post time for the race for which the horse in entered, and is to be under the direction of the official veterinarian or designee.  If the racetrack does not have a detention bard, the “four hours prior” rule still applies, but the horse is to be brought to the official veterinarian no later that one out prior to post time, and, on a form provided by the state racing commission, the trainer is to enter under oath various information, including time and dosage of administration, and the printed name and signature of the attending veterinarian who administered the medication.

            Some states follow RCI closely, others don’t.  Following are the general rules and regulations concerning the administration of therapeutic medication in each of the states in which Quarter Horses last year ran for purses of at least $1 million.  Bear in mind, however, that rules are made to be changed: Louisiana issued new regulations this spring, Kansas is revising its polices, and California in March announce that it is developing a plan that by mid-summer could allow traceable levels of certain therapeutic medications in post-race urine samples.

            Arizona: With permission of the stewards, furosemide or “conjugated estrogens” may be administered not less than four hours prior to post time on race day.  Arizona rules do not specify dosage.  Permission requires a diagnosis of exercise-induced pulmonary hemorrhage by a veterinarian licensed by the Arizona Department of Racing, or the horse must have been on a bleeder list at a track outside of Arizona.  Test levels of phenylbutazone and oxyphenylbutazone are not to exceed five micrograms per milliliter of blood plasma of the horse.

            California: Permitted nonsteroidal anti-inflammatory substances are phenylbutazone, oxyphenylbutazone, and naproxen, with test levels of not more than five micrograms per milliliter of plasma; flunixin and meclofenamic acid are also permitted, with levels of not more than one microgram.  Not more than one such form of medication may be present in any one horse.

            Bleeders must be certified by the official veterinarian.  Once on the bleeder list, the horse must be assigned to a pre-race security stall before going to the receiving bard or saddling paddock.  While there is no minimum dosage of furosemide, up to 250 milligrams may be administered no later than four hours prior to post, and the official veterinarian has the discretion to order, prior to the administration, the taking of a blood sample for comparison testing.

            Kansas: With the written permission of the animal health officer, phenylbutazone may be administered to horses three years old and older.  The administration must be no less than 24 hours prior to post time, and the test sample cannot exceed five micrograms per milliliter of blood plasma.

            Furosemide, up to a maximum dosage of 250 milligrams, may be administered only to horses three years old or older that are on the animal officer’s bleeder list.  The administration must take place at a location to be determined by the animal health officer.  The trainer can designate the veterinarian that administers furosemide, but the administration must be done under the supervision of the health officer.

            The first time a horse is observed bleeding, its name is put on the bleeder list, and remains for 10 days or until the animal health officer removes it.  A second occurrence of bleeding puts the horse on the list for 28 days, or until removed by the officer; a third occurrence bars the horse from pari-mutuel racing in Kansas.

            Louisiana: Phenylbutazone and oxyphenylbutazone are permitted, with urine test levels no to exceed 165 micrograms/milliliter, nor more than five micrograms/milliliter of blood.  Though regulations stipulate no specific drug, dosage or test levels, bleeder medication may be administered no later than four hours prior to post.  A positive test results when a horse not on the bleeder list is shown to have bleeder medication in its system.

            After a recorder workout, a first-time bleeder is eligible to enter and run after the expiration of the 14th day he is placed on the bleeder list.  After a second occurrence of bleeding, the horse must await the expiration of the 90th day, and a third bout entails an absence of 365 days.  A fourth occurrence results in lifetime suspension, “provided hemorrhage from the nostrils is documented by the state veterinarian.”  Louisiana rules also stipulate that, “should a horse which is on the bleeder list race three times within 265 days without bleeding, it shall be considered a first-time bleeder when next it is observed bleeding…”  For counting purposes, the time period commences on the day after the bleeding occurs.

            New Mexico: Phenylbutazone and oxyphenylbutazone are permitted, with test levels not to exceed five micrograms per milliliter of blood plasma, or 165 micrograms/milliliter of urine.  Administration of furosemide or conjugated estrogen must be by intravenous injection, not less than three hours prior to post time.  A positive test is at least 100 nanograms per milliliter of plasma.

            Oklahoma: Phenylbutazone and furosemide are permitted, as per the RCI model rule.  After the first occurrence of bleeding, a horse is not eligible to be entered in a race for at least 10 days, unless excepted by the official veterinarian, and until the horse has been approved by the official veterinarian or racing veterinarian after a satisfactory workout.  A horse that has a second occurrence of bleeding is not eligible to enter a race for 90 days, and must have a satisfactory workout approved by the official veterinarian prior to entry.  A horse that bleeds a third time is no longer eligible to race in Oklahoma.

            To race on Lasix is Oklahoma, a horse must be on the Oklahoma Horse Racing Commission Lasix list.  To be on the Lasix list, the horse must first be on the OHRC bleeder list.  The Lasix list is not carried over from race meeting to race meeting.  The bleeder list is permanent.

            Texas: Furosemide and pheylbutazone are permitted, with dosages, administration and test levels as per RCI guidelines.  A first-time bleeder is prohibited from racing for 11 days; after a second occurrence, 30 days; third, six months; and after four occurrences, the bleeder is barred from racing in Texas.

            So there it is – controversial though it might be – the way things are today, as far as the rules and regs go: To bute of not to bute? What about Lasix?  And how much of either? Those are question facing horsemen.

            The answers are still under debate.