Like having asthmatic attack all the time!
The issue of whether a patient with COPD (chronic obstructive pulmonary disease) should dive is very similar to the individual who has asthma. The same theoretical arguments apply to the individual with COPD concerning the increased risk of AGE (arterial gas embolism), except that in the person with COPD airway function never returns to normal. Thus, the diver or diver candidate with COPD may have an increased theoretical risk of burst lung from rupture of obstructed small airways at all times.
How does this affect the individual?
From a practical point of view, by the time individuals with chronic lung diseases become symptomatic they are usually so short of breath that they are incapable of sustaining even the small exercise capacity necessary to dive, and as a result, it is extremely rare to see a diver with significant COPD.
What should you tell the diver with COPD?
COPD is generally a disease that develops after decades of exposure to tobacco smoke, and is a disease of older individuals, which again makes it rare to encounter a diver with COPD in a diving medicine practice. By the time COPD can be detected clinically, the person has usually deteriorated physically to the point that such individuals should be advised against diving merely on the basis of their exercise tolerance.
Abnormal pulmonary function tests
Thus, the question of advising someone with COPD is boils down to a question of advising someone who is asymptomatic but who has abnormal pulmonary function tests.
Reactive airway disease
In addition, there may be a component of reactive airway disease; that is, their pulmonary obstruction acts somewhat like an asthmatic attack, varying with external stimuli, and the obstruction is treated with similar bronchodilating drugs as for asthma. If we are to be consistent and manage individuals with COPD in a similar fashion to the way we manage asthmatics, then individuals with clear-cut laboratory evidence of COPD should be advised not to dive.
In practice, this evidence is defined as pulmonary function tests that are more than two standard deviations from normal. Unfortunately, the exact definition of normal is still unclear, and as a result, individuals with mild disease may have pulmonary function test values that overlap predicted normal values between two standard deviations and the mean.
Borderline cases should undergo more extensive testing
As a result, individuals whose isolated values may be at the low end of the normal range should undergo more extensive testing if their clinical history is suggestive of chronic lung disease. If, however, further studies confirm preliminary observations in submarine escape trainees that pulmonary function tests lack useful predictive value for predicting pulmonary barotrauma, using pulmonary function tests as criteria for diving (other than to assess exercise capability) will have to be reconsidered.