Oxygen is the fuel of life and its efficient use is the key to athletic fitness. The respiratory system of the racehorse must work hard to harvest the 20 percent of oxygen present in the air we all breathe. Observing a horse after his work on a cold morning provides a visual reminder of this, as the breath surges from his nostrils.
The respiratory process
Glucose and oxygen interact via a process called aerobic respiration, resulting in the production of energy. This cellular process powers all of our bodily functions, from breathing and digesting food to sprinting for the winning line. Carbon dioxide and water are by-products of this reaction.
Cells can also produce energy in the absence of oxygen
(anaerobic respiration). In this case, glucose is also broken down to
release energy. However, the lactic acid formed by this reaction is a
more noxious compound than the carbon dioxide and water created by
aerobic respiration. Its presence in cells results in muscle cramps and
fatigue. This lactic acid must be broken down into carbon dioxide and
water, but this reaction again requires oxygen. Essentially, anaerobic
respiration allows muscles and other cells to function for a short time
in the absence of oxygen, building up an ‘oxygen debt.' This must be
repaid later to allow the lactic acid to be broken down.
A good measure of a horse's fitness is how quickly breathing and pulse return to normal after exercise. This is because aerobic respiration is more efficient in fit horses, so they build up less of an oxygen debt while exercising. Less oxygen is required to breakdown lactic acid once the work has been completed.
Oxygen is delivered to the body tissues via the respiratory and cardiovascular systems. The respiratory system of the horse includes everything from the nostrils to the lungs. Air is delivered to the lungs, where oxygen diffuses into the bloodstream and is taken up by red blood cells. These cells are the body's carriers for molecules of oxygen and they are pumped around the body by the heart through arteries, capillaries and veins.
Physical causes of inefficient oxygen delivery Trainers and veterinary surgeons must make sure that all of the components of the respiratory and cardiovascular systems are working to their full potential. This ensures that the body delivers oxygen as efficiently as possible to the muscles, in order that they have the energy necessary to win races!
The following may all be considered surgical conditions of the upper respiratory tract, in that a surgical procedure is often necessary to effect a cure:
Some of the main clinical problems that affect the upper respiratory tract (nostrils, larynx and pharynx) are related to physical obstruction of these ‘tubes.' Anything which reduces the diameter of these tubes will reduce the amount of air, and thus oxygen, which is delivered to the lungs. Physical obstructions of the upper respiratory tract are generally diagnosed by endoscopic examination carried out by a vet. Some conditions, e.g. profound cases of laryngeal hemiplegia, can be seen and thus diagnosed when the horses are at rest. Other conditions only occur when the horse is exercising. In these cases dynamic endoscopy, i.e. endoscopy whilst the horse is actually exercising on a treadmill, may be needed to fully elucidate the problem (Figure 1).
Horses are obliged to breathe through their noses, and cannot breathe through their mouths as many other species can. Sometimes the alar folds (skin folds above the nostrils) are incriminated as the cause of abnormal respiratory noise. Nasal strips are marketed as a means of stopping the soft outer nasal passages collapsing during exercise, thus increasing airflow into the lungs. Relatively common causes of reduced airflow through the nasal passages are space-occupying lesions such as cysts within the sinuses, or vascular growths (ethmoid hematomas). Surgical removal is often necessary.
The pharynx and larynx are other areas which may be affected by conditions which physically restrict the amount of air pulled into the lungs. Probably the best known of these is laryngeal hemiplegia. The larynx is a dynamic structure, which closes to prevent food entering the windpipe and opens to allow air to enter the lungs. Provided that the muscles controlling the larynx are functioning properly, the larynx can be opened to form a diamond-shaped structure. However, it is relatively common for the left side of the larynx to become paralyzed (Figure 2). This results in only half of the larynx opening, reducing airflow into the lungs. Horses that suffer from this condition often make a characteristic ‘roaring' sound. Surgery is usually indicated to correct this. A laryngeal tieback, whereby a suture is placed through the left side of the larynx to permanently hold it open is often the treatment of choice for roarers. Normal procedure combines this with a ‘hobday' operation, which removes the vocal folds from the larynx and further opens the airway. Treatment for dorsal displacement of the soft palate, another relatively common condition affecting the upper respiratory tract, is more controversial. Some horses respond to conservative management, including the use of drop-over nosebands or tongue ties. Others need more invasive treatment. This may involve producing scar tissue on the soft palate so that it is ‘less floppy,' and thus less likely to displace. Thermocautery of the soft palate under general anaesthetic is an option, but perhaps a more elegant means of achieving this is the use of a laser. This may be performed in the standing, sedated horse. More recently, an operation which mobilizes the larynx to prevent dorsal displacement of the soft palate (‘Tie Forward') has been developed. Other physical conditions that may impede airflow through the pharynx include entrapment of the epiglottis by the folds of soft tissue (aryepiglottic folds) which lie alongside it, and chondritis or inflammation of the cartilages which form the larynx.
Medical causes of inefficient oxygen delivery A number of medical conditions may also affect the amount of oxygen available to the horse.
Exercise-induced pulmonary hemorrhage (EIPH, or ‘bleeder') is a relatively common condition in which a blood vessel within the lungs bursts, and bleeds into the airways. The severity and amount of blood associated with such an incident can vary markedly. Sometimes relatively large volumes of blood are seen escaping from the horse's nostrils. In other cases, the volume of blood is relatively small and remains confined to the distal airways. This can only be detected after the event by endoscopy and examination of the cell types within the horse's respiratory secretions. Either way, blood takes the place of air within the airways and less oxygen is available to diffuse into the bloodstream. In addition, blood within the airways is a good medium to support bacterial growth and this may predispose horses to bacterial respiratory infections.
The exact cause of EIPH has yet to be elucidated. It may be due to increased blood pressure within the lungs during exercise, concussive damage to the blood vessels as the horse gallops, or a combination of both. Some horses appear to be more prone to this condition than others, and it appears that concurrent respiratory tract infections may also predispose horses to a pulmonary bleed. A variety of treatments have been tried. Furosemide is a diuretic drug, which essentially makes horses urinate more and this in turn dehydrates the horses slightly. It is postulated that this results in less fluid within their circulation and reduced blood pressure within the lung's small capillaries. Its use is permitted on race days by state racing authorities, but this may not be the case forever. However, as a prohibited substance likely to influence performance, it is banned in European racing. In spite of this it retains its place as a drug for training, as when given on work mornings, the chances of a bleed are said to be reduced. Some studies have also suggested that the use of nasal strips may reduce the chance of a bleed occurring, without any untoward side effects. These too are currently banned in European racing but can be used for training purposes. In England, known bleeders often have their water intake restricted before a race in an attempt to reduce blood pressure and thus the chances of a bleed. Clearly, to strike the correct balance between giving the horse sufficient hydration to perform well on the track and lessening the chances of a bleed, the eye of an experienced horseman is required.
Respiratory tract infections may also interfere with optimal oxygenation of the blood. Firstly, infections are associated with an increase in the amount of mucus within the respiratory tract, which may form a physical barrier to airflow and diffusion of oxygen. It is possible to vaccinate against the well-known respiratory viruses, such as influenza and herpes, and some racing authorities require proof of vaccination before a horse can race. However, every horseman will be aware of the non-specific ‘virus,' which results in coughing horses performing at levels below their potential. Viruses may also sensitize the airways, leaving horses more prone to secondary bacterial infections, or post-viral coughs. In addition, it is not uncommon for horses to become anemic following a viral infection. The reduced numbers of circulating red blood cells will lead to sub-optimal oxygen delivery. A number of supplements are available which aim to enable the horses to manufacture more red blood cells.
The importance of good stable hygiene and maintenance of a dust-free environment should not be understated in hastening recovery from airway infections. Even a healthy racehorse should only be exposed to quality roughage feed (good hay or haylage, etc.) and dust-free bedding material.
The cardiovascular system The heart, which pumps oxygenated blood around the horse's body, must also be in excellent condition to ensure effective oxygen delivery. It should come as no surprise that research has shown that horses which have bigger hearts, as measured by ultrasound, are more likely to perform at the top level. An excellent example of this is the 1973 American Triple Crown winner, Secretariat, whose heart was recorded as weighing 21 lbs. at post mortem. It is clear that horses which have problems with their heart function are likely to perform poorly. The heart must beat in a rhythmic manner to allow coordinated filling of its chambers, followed by a coordinated contraction so that blood is pushed around the body in an efficient manner. Arrhythmias, such as atrial fibrillation, prevent this synchronization and are reflected in disappointing efforts on the racecourse. Diseases of the heart muscle itself (cardiomyopathy), alongside problems with the heart valves and abnormal openings in the walls of the heart are further causes of poor performance. Diagnosis of cardiac conditions may require echocardiography (ultrasonic examination of the heart) to assess the structure of the heart and blood flow within it. Electrocardiographic (ECG) evaluation will identify problems with the heart's rhythm and the electrical activity responsible for setting it.
The act of breathing is fundamental to animal life and so optimal uptake and use of oxygen is the foundation upon which equine training regimes are based. The horse's respiratory system must be fine-tuned to perform to its full potential and the skilled trainer must make himself aware of any problem which may be impeding this.