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Date: [__date__]
Dear [__name__];
In response to a great topic suggestion from a valued client, we have decided to use the February issue of our newsletter to discuss exercise induced pulmonary haemorrhage in the thoroughbred racehorse. What causes racehorses to bleed during strenuous exercise? What is happening at the level of the lungs in the exercising horse? Why does this condition continue to progress? How can we, as veterinarians help trainers to manage this condition and thus extend the competitive lifespan of their horses? For this and more information on the topic, please read on............
Exercise-induced pulmonary haemorrhage (EIPH)
EIPH has been recognised as a problem in the thoroughbred for over 200 years. For example, Bartlet's Childers who sired the famous horse Eclipse in the 1700's was also known as 'Bleeding Childers'! Today, EIPH remains a major cause of poor performance in not just thoroughbred racehorse but also in other athletic horses such as polo ponies, event horses, barrel racers, reining, cutting and steeplechase horses.
Racehorses with EIPH often have poor performance and sometimes, blood may be noticed in one or both nostrils after exercise. More subtle clinical signs of EIPH include coughing and/or swallowing after exercise. Very occasionally, horses present with respiratory distress secondary to massive bleeding into the lungs, however, respiratory distress may be secondary to a myriad of other conditions and as such always requires immediate veterinary attention.
Repeated episodes of bleeding into the lungs causes a chronic inflammatory response that then contributes to a cycle of gradual worsening of EIPH in horses that continue to race and train. Eventually the lungs of these horses become permanently damaged and necessitate their retirement from racing. The clinical progression of this condition illustrates the importance of early diagnosis and effective management in order to minimise further damage to the lungs during training.
To understand how EIPH actually happens, it is important to understand the concept of the blood-gas barrier. EIPH involves stress failure and rupture of the tiny capillaries that perfuse the small airways in the terminal portion of the lung. These tiny capillaries are called alveolar vessels because they carry blood to the tiny alveoli that fill with air as the horse takes a deep breath. These alveolar vessels are very thin, so thin in fact that oxygen can move from the alveoli into the blood stream and carbon dioxide can move from the blood stream into the alveoli. These tiny, fragile vessels are exposed almost directly to the pressure changes that occur in the alveoli during breathing!
Imagine now, the strength and power of the galloping racehorse. Imagine the shear force that passes through the chest as the foreleg hits the ground with the horse taking one breath per stride as it continues to gallop, it's respiratory rate increasing with stride frequency. Take a moment to consider the enormous volume of blood that passes through these tiny vessels as the heart rate increases and blood is pumped through the lungs, perfusing the alveoli, allowing gas exchange and facilitating the delivery of oxygen to the exercising muscles.
The current thinking, is that increased capillary transmural pressure results in rupture of the alveolar vessels and haemorrhage into the lungs. Transmural pressure is the pressure difference between the inside and the outside of a blood vessel. When the lung inflates, transmural pressure in the capillaries increases as the pressure surrounding them decreases (negative pressure). Combine this with the increase in pressure within the capillary as it receives the enormous cardiac output generated by the heart during exercise and it is not hard to imagine how these fragile capillaries rupture during exercise.
The next concept is difficult, but important because it forms the basis for the invention of the FLAIR nasal dilator strips. When the horse breathes in, pressure within the lung falls. The greater the inspiratory resistance, the greater the magnitude of the fall in pressure within the lung. For example, inspiratory obstruction by left laryngeal hemiplegia results in a greater pressure drop within the trachea and lungs and a subsequent increase in capillary transmural pressure. It is widely accepted that 50% of total inspiratory resistance originates from the nasal passages and it has been suggested that collapse of the nostrils during inspiration is a significant contributing factor to inspiratory resistance. The FLAIR nasal strips are external nasal strips that are stuck across the bridge of the nostrils to provide mechanical support to the nostrils to decrease the degree of nostril collapse during inspiration and thus decrease inspiratory resistance. The overall aim of the FLAIR nasal strips is to mitigate the increase in capillary transmural pressure and thus decrease the risk of alveolar rupture.
The drug frusemide is another commonly used therapeutic for management of EIPH. Frusemide is a diuretic that is administered intravenously before strenuous exercise. Frusemide works in more than one way, firstly it acts at the level of the kidneys and results in fluid loss that translates to a decrease in body weight as well as a decrease in the pressures generated in the alveolar capillaries during strenuous exercise. Frusemide also acts as a bronchodilator. Whilst frusemide administration will not prevent EIPH, it is widely accepted that it will reduce the severity of EIPH by up to 50%. It is important to note, that frusemide has a withhold period and cannot be used close to or in competition.
EIPH is diagnosed by endoscopic examination within 2 hours of galloping to detect the presence of blood in the trachea. Our practice uses a 0 to 4 grading system when describing the severity of EIPH. 0 being clinically normal with no blood in the pharynx, larynx or trachea and grade 4 being the worst with streams of blood covering 90% of the tracheal surface. Click on this link to view a video of a grade 3 bleed: http://www.youtube.com/watch?v=HAe9Q5Kzb9Q
EIPH can also be diagnosed using brochoalveolar lavage (BAL). To perform a BAL, the horse is sedated and a sterile, flexible tube is passed up one nostril, through the larynx and down the trachea where it lodges in one of the bronchi. Sterile saline is then infused into the lung and then aspirated into clean syringes. The recovered fluid is later examined by a pathologist and is described in terms of red blood cell count, white blood cell count and cell appearance. Of most interest is the presence of hemosiderophages. Macrophages are the scavenger cells of the body. When a horse bleeds into its lungs, macrophages come along to 'mop up' the red blood cells. Macrophages that have eaten red blood cells have a characteristic appearance under the microscope and are called hemosiderophages. More than 10% hemosiderophages in a BAL sample is considered diagnostic of a previous episode of EIPH. At the present time a brochoalveolar lavage is the best means by which we can diagnose a subclinical episode of EIPH.
An interesting study was undertaken in 2001 that used BAL results to compare the effects of a control (saline), a nasal strip, frusemide and a combination of a nasal strip and frusemide on EIPH. The results showed that there was a significant decrease in the number of red blood cells recovered in BAL fluid from horses that galloped wearing a nasal strip on its own compared with control horses. The study also showed a significant decrease in the number of red blood cells recovered in BAL fluid of horses that galloped after administration of frusemide compared with control horses. There was an even greater decrease in the number of red blood cells recovered in BAL fluid from horses that were administered frusemide and wore a nasal strip compared with control horses and horses that only wore a nasal strip. However, the decrease in red blood cells recovered in BAL fluid from horses that were administered frusemide and wore a nasal strip was not significantly different from horses that were only administered frusemide. The results of this study suggest that application of an external nasal strip has the potential reduce the severity of EIPH.
It is very difficult to say how great a contribution application of an external nasal strip makes to decreasing the severity of EIPH especially when used in conjunction with frusemide. However, given the fact that there are no side effects associated with the use of nasal strips, in our opinion it is entirely justifiable that they be used on the basis that they have demonstrated the potential to reduce the severity of EIPH.
We hope you enjoyed the February edition of our newsletter. EIPH is a very important and common cause of poor performance. However, it is not the only cause of poor performance! Please do not hesitate to contact the practice if you would like your horse evaluated for poor performance.
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