Many of you are familiar with the respiratory condition known as "heaves," also termed recurrent airway obstruction (RAO). Primarily caused by chronic exposure to dusts and molds in hay and bedding, heaves can cripple the function of a horse's lungs, making it difficult for him to breathe at rest, let alone during physical exertion. In horses with severe heaves, the breathing rate is much higher than normal, the nostrils flare with each breath, there is noticeable chest movement during breathing (inspiration and expiration), and there often is a persistent cough. These signs make a diagnosis of heaves fairly straightforward. Fortunately, a number of drugs and changes in management will bring some relief to horses with this condition.
But heaves is the tip of the iceberg of respiratory ailments that can affect performance. For many of these conditions, the clinical signs are much more subtle, and thus diagnosis is considerably more challenging. In this article, we consider some common respiratory ailments that affect athletic horses and the steps a veterinarian might take to diagnose them.
Form and Function
The respiratory system has two main components--upper and lower. The upper respiratory tract includes the nostrils, nasal passages, pharynx and larynx (throat), and the trachea (windpipe). (See "Anatomy of the Upper Respiratory System" on page 71.) The lower respiratory system is everything beyond the windpipe--the trachea splits into left and right bronchi that supply air to the left and right lungs. Each lung contains a massive branching network of smaller airways, each of which terminates in a small air sac. These air sacs, called alveoli, are in close contact with the bloodstream. This alveoli-blood vessel unit is the site of gas exchange--with each breath, oxygen crosses into the blood while carbon dioxide moves from the blood into the alveoli and is expelled from the body during expiration.
This exchange of oxygen and carbon dioxide is fundamental to life and exercise. Specialized areas in the brain sense levels of oxygen and carbon dioxide in the blood. When the supply of oxygen is inadequate or blood carbon dioxide levels are too high, the brain directs the respiratory system to increase the breathing rate in an attempt to correct these problems. Respiratory rate increases during physical exertion in proportion with work rate. The demand for oxygen increases dramatically as muscles contract, and the respiratory system responds by "pulling" more oxygen into the body.
As an owner or trainer, you should be familiar with the normal pattern of respiration in your horses at rest and during and after exercise. In the barn, watch your horse while he stands in his stall and pay particular attention to the flanks. You should see shallow, rhythmic movements of the rib cage during inspiration (breathing in) and expiration (breathing out). The normal respiration rate for a horse at rest is slow--eight to 16 breaths per minute.
Don't panic if your horse's breathing rate is higher than normal for short periods. Factors such as excitement and hot ambient conditions can result in an increased resting respiratory rate. The respiratory system is also used for evaporative cooling, so when it's hot, the respiration rate goes up to try to keep the body cool. However, a persistent elevation in breathing rate can mean respiratory difficulties, perhaps caused by heaves or inflammatory airway disease (IAD).
Respiration is markedly increased right after exercise. Early in the post-exercise period, the respiratory rate can be 80-120 breaths per minute depending on the duration and intensity of exercise. The rate will gradually decline in the 30 minutes after exercise, but can remain elevated longer in hot conditions--again reflecting use of the respiratory system for heat loss.
There also is a pronounced flaring of the nostrils. This is one of the strategies the horse uses to increase the rate of airflow during exercise. In fact, the diameter of the entire upper airway increases, along with the volume of air per breath. These strategies help the horse supply much-needed oxygen to contracting muscles.
On the flip side is something peculiar to four-footed animals. When galloping, respiratory rate is coupled to stride rate--one stride, one breath. While this is an efficient system, limitations to maximum stride rate during very hard exercise might prevent breathing rate (and therefore oxygen supply to the body) from keeping pace with oxygen demands. As a result, the oxygen content in blood decreases during very hard exercise.
Problems in Athletic Horses
Respiratory disorders are classified as upper or lower airway problems. The common upper respiratory disorders include laryngeal hemiplegia (paralysis of one side of the larynx, also termed "roaring"), dorsal displacement of the soft palate (DDSP), epiglottic entrapment, and dynamic collapse of the pharynx (when the airway just in front of the throat collapses during exercise). It has been estimated that up to 15% of horses examined for poor performance problems have one of these upper airway obstructive disorders.
Lower airway disorders include recurrent airway obstruction (RAO, or heaves), inflammatory airway disease, and exercise-induced pulmonary hemorrhage (EIPH). The significance of EIPH to exercise performance continues to be debated. Some view it as an inevitable consequence of intense exertion and believe that it doesn't alter performance. Others view severe EIPH as a definite limitation of performance in racehorses. Similarly, the contribution of inflammatory airway disease to poor performance is debated among veterinarians and researchers. In part, uncertainty regarding how EIPH and inflammatory airway disease affect performance reflects the lack of good tests for assessment of lung function during exercise (more on this later).
Although these upper and lower airway conditions differ greatly in terms of their location and underlying cause, there is one common feature--they result in some degree of airflow obstruction during exercise. As a result, less air reaches the lungs and the normal exchange of oxygen and carbon dioxide is impaired. In short, the horse's body becomes starved for oxygen during exercise and he tires easily.
The impact of this obstruction is particularly dramatic for upper airway problems. The horse is an obligate nasal breather--little, if any, air can reach the lungs through his mouth. This is very different from humans--we can enhance airflow by mouth breathing. With conditions such as DDSP or laryngeal hemiplegia, more than half of the horse's airway can be blocked, with a devastating effect on performance.
With inflammatory airway disease and heaves, bronchoconstriction (narrowing of the airway due to muscle contraction) and inflammation narrow the small airways in the lung. Because these conditions can involve much of the lung, the obstruction can be as dramatic as that associated with an upper airway problem, with the same devastating effect on performance.
Diagnosing Breathing Problems
When a veterinarian is presented with a horse with a history of poor performance (or a sudden decrease in performance), problems involving the musculoskeletal and/or respiratory systems are usually high on the list of potential diagnoses. Concerning the respiratory system, three major questions arise. First, does the horse have a respiratory problem? Second, can the problem be localized to the upper or lower airway? And third, is the respiratory ailment a likely explanation for the horse's poor exercise performance?
The first step is to get a medical history. Your observations as the owner can be helpful in localizing the problem. For example, you might notice coughing during warm-up exercise or head tossing during collection, both of which could be associated with lower airway problems. Alternatively, an abnormal noise from the upper airway might be audible during galloping, which usually is a sign that there is something amiss with the upper airway.
The diagnostic approach will vary with the suspected location of the problem (upper vs. lower airway), its severity, and the availability of specialized testing facilities. Beyond a thorough physical examination, diagnostic tests might include airway endoscopy, blood gas analysis (to check blood oxygen and carbon dioxide levels), treadmill exercise testing (with endoscopy of the upper airway), bronchoalveolar lavage (BAL), and tests of lung function.
Endoscopic examination of the upper airway is routinely used in equine practice. This procedure allows direct visualization of the throat and windpipe. However, a "resting" examination of the upper airway can be misleading. Low-grade laryngeal hemiplegia is often evident at rest. But in 50% or more of these horses, laryngeal function is normal during exercise, and surgical treatment is not warranted. Conversely, the absence of an abnormality at rest does not rule out the possibility of an obstruction during exercise.
For this reason, using an endoscope to view the throat during treadmill exercise has greatly improved the veterinarian's ability to diagnose upper airway abnormalities. For this examination, the horse must run at or near his maximum exercise capacity. This is because many of the abnormalities of the upper airway, such as laryngeal hemiplegia and displacement of the soft palate, only occur when the ventilation (movement of air in and out of the lung) of the horse is at maximum. The running speeds necessary to achieve maximum effort on the treadmill will vary among horses. Typically, the horse will undergo a thorough warm-up, after which the endoscope will be positioned in one nostril. Then, the treadmill speed is increased in a stepwise fashion--the highest speed (often 27-31 mph or 12-14 meters per second) will be maintained for a distance of about one mile. Problems with upper airway function often occur when the horse tires, and the endoscopic examination will be continued as the treadmill speed is gradually reduced at the end of the test.
Treadmill endoscopy isn't a fail-safe way to diagnose upper airway problems. One of the concerns is that treadmill exercise stress is different from that experienced by a horse exercising in his normal environment. As a result, it can be hard to "induce" the upper airway problem when the horse is tested on a treadmill. Another limitation of treadmills is that they are very expensive, thus limiting the availability of this testing procedure. For these reasons, researchers continue to look for better means of diagnosing obstructive upper airway problems in athletic horses--in particular, tests that do not require the horse to run on a treadmill.
Work at Michigan State University1 has focused on developing a device for recording respiratory sounds in exercising horses. The researchers first characterized respiratory sounds in normal horses during exercise, then recorded sounds in horses with experimentally induced roaring and DDSP. This test is easy to perform and well tolerated by horses. They also found that most horses with laryngeal hemiplegia and DDSP produced unique sound patterns. More work is needed before use of this technique becomes widespread, but it could be a useful addition to the diagnostic arsenal.
When it comes to lower airway problems such as inflammatory airway disease, diagnosis is heavily reliant on examination of airway secretions collected by BAL or trans-tracheal wash. As with upper airway endoscopy, these procedures are often done in horses with a history of poor performance and/or when a lower airway condition is suspected. For BAL, an endoscope or a specially designed flexible rubber tube is positioned in one of the lower airways. Then, a small volume of fluid is instilled and aspirated through the endoscope or tube. In essence, a portion of the lung is "washed." This sample is analyzed in a manner similar to that for a blood sample--the number of red and white blood cells is counted and the sample is examined under a microscope.
Evidence of inflammation in the BAL sample points to a diagnosis of inflammatory airway disease (or heaves), and it is often assumed that this inflammation is the cause of poor exercise performance. Logically, horses with a chronic cough and evidence of severe lower airway inflammation will have some impairment to gas exchange and performance during exercise.
A more common scenario is the horse with a history of poor performance and evidence of mild airway inflammation on BAL, but no obvious clinical signs of a respiratory problem. Currently, it is not known whether this low degree of inflammation adversely affects lung function and performance during exercise. So, although the BAL is a useful tool for assessing lung health, we need other tests to measure actual function of the respiratory system.
Lung function testing is widespread in human medicine and is considered indispensable in the diagnosis and monitoring of humans with respiratory disease. Unfortunately, equine medicine hasn't reached this level of diagnostic sophistication. One of the major stumbling blocks is that it is not possible to ask the horse to perform the various breathing maneuvers used in humans during lung function testing.
Currently, only a few research laboratories and referral veterinary (university) hospitals do any pulmonary function testing in horses. With these tests, a horse wears a tight-fitting face mask and several measures of lung function are recorded. Lung function can also be measured during a histamine provocation test. This test, also used in human asthmatics, is a means for assessing airway reactivity--the tendency of the airways to constrict when exposed to aerosolized histamine. The histamine provocation test can be a valuable tool in assessing respiratory health in horses with a history of poor exercise performance.2
Measuring lung function during exercise is even more challenging. Again, several researchers have used air flow and lung function to evaluate exercising horses. However, none of these procedures have been widely used for clinical evaluation of horses. Academic and industry researchers are continuing to work in this area, and in the future it is likely that the veterinarian will have easy-to-use systems for the evaluation of lung function in horses, both at rest and during exercise.
1 Derksen, F.J.; Holcombe, S.J.; Hartmann, W.; et al. Spectrum analysis of respiratory sounds in exercising horses with experimentally induced laryngeal hemiplegia or dorsal displacement of the soft palate. American Journal of Veterinary Research, 62, 659-664, 2001.
2 Hoffman, A.M.; Mazan, M.R.; Ellenberg, S. Association between bronchoalveolar lavage cytologic features and airway reactivity in horses with a history of exercise intolerance. American Journal of Veterinary Research, 59, 176-181, 1998.
5 TIPS: Lung Function
- The normal respiration rate for a resting horse is eight to 16 breaths per minute.
- When blood oxygen levels are low or carbon dioxide levels are high, the brain directs the respiratory system to increase the breathing rate in an attempt to correct these problems.
- A persistent elevation in breathing rate can be an indication of respiratory difficulties, perhaps caused by heaves or inflammatory airway disease (IAD).
- Many upper airway abnormalities, such as laryngeal hemiplegia and displacement of the soft palate, only occur when ventilation (air movement through the lungs) is at maximum.
- For lower airway problems such as inflammatory airway disease, diagnosis is heavily reliant on examination of airway secretions collected by bronchoalveolar lavage or trans-tracheal wash.
About the Author
Ray Geor, BVSc, PhD, Dipl. ACVIM, is professor and chairperson of Large Animal Clinical Sciences at the College of Veterinary Medicine at Michigan State University
POLL: University Equine Hospitals