Exercise Induced Bronchoconstriction: Diagnosis
On the surface, making a diagnosis of exercise-induced bronchoconstriction (EIB) should not to be too hard. After all, having an asthma attack while exercising would make the diagnosis fairly apparent. However, as I mentioned earlier, self reported symptoms associated with exercise such as shortness of breath and chest tightness can be associated with a number of causes, both respiratory and non-respiratory, including just being out of shape. In fact, in one study, approximately half of elite athletes with symptoms suggestive of EIB were found to have normal lung function. Even more interesting is the fact that nearly half of the athletes who did not feel that they had EIB in fact tested positive for it.
Because symptoms alone are not sufficient, an objective test is required to make an accurate diagnosis. The best tests for EIB either directly measure airway changes during exercise or produce the inflammatory changes in the airways that occur in patients with EIB and therefore serve as a surrogate for an exercise challenge.
An exercise challenge starts with a baseline pulmonary function determination with serial lung function measurements taken following a period of prescribed exercise. A typical protocol might include running on a treadmill for 6-8 min at 80-95% of an athlete’s calculated max heart rate. This level of exercise requires significant effort but is required to produce changes associated with EIB in highly trained athletes. Following the effort, lung function values are recorded at 5, 10, 15, and 30 minutes and compared with the pre-challenge numbers. A 10% or greater drop in lung function persisting for at least 15 minutes after exercise has stopped is diagnostic of EIB.
Because EIB is triggered in some athletes by inhalation of cold, dry air, a weakness of an exercise challenge on an indoor treadmill is indoor humidity that may be significantly different form the sport environment. For this reason, sport and environment specific challenges (field challenges) have been found to be more sensitive than tests run in controlled indoor environments. This is particularly true for those involved in winter sports such as ice skating and skiing and may very well be important for athletes who train in a desert environment such as Phoenix. In some, the level of intense exertion present in competition may not be reproduced exercising in a calm, indoor setting, producing falsely negative results.
Several alternatives to an exercise challenge have been developed including what is called Eucapnic Voluntary Hyperpnoea or EVH. In this test an athlete breathes a specially formulated, dry, gas mixture at a rapid rate to replicate the conditions of a hard exercise cession. This is currently the only test recommended by the International Olympic Commission to identify EIB in Olympic athletes. Unfortunately, the test requires specialized equipment and is not without some risk of precipitating a severe asthma attack. For this reason the test is not widely available, in spite of the IOC recommendations.
Although an objective diagnostic test for EIB is recommended when possible, another approach to is to treat the athlete with an asthma regimen and see if symptoms improve. I’ll discuss more about this in the next post when I review treatment of EIB.