Asthma does have potential impacts on your ability to dive, but one of the more common things I've heard (it increases risk of DCS) appears to be a myth. There is a fair bit of medical research out there in regards to asthma and diving. As others have mentioned, your best bet it to talk to a doctor who knows about dive medicine. However, some of the medical sources may be of interest to you.
Here's one of the better overviews I've seen about pulmonary health and diving (you may need a subscription to read it):
http://thorax.bmj.com/cgi/content/full/58/1/3
Here's the asthma section:
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"Asthma is a common cause of concern among those wishing to dive. In a review of referrals to the medical committee of the Scottish Sub Aqua Club for expert advice on fitness to dive, respiratory disorders were responsible for 30% of referrals, the majority being for evaluation of asthma.69 However, the literature concerning asthma and diving is contentious and inconclusive. Theoretically, it might be expected that the diving environment would provoke bronchospasm and that asthma might increase the risk of barotrauma and reduce exercise capability. The latter is a potentially dangerous problem for a diver who may require to swim strongly against currents or to escape danger and thus avoid drowning. In a "state of the art" review in 1979, Strauss suggested that contraindications to diving should include the following: significant obstructive pulmonary disease with minimal values for FEV1, FVC, and maximum voluntary ventilation (MVV) of 75% of predicted; any attack of asthma occurring within 2 years; a requirement for medication; or any episode of bronchospasm associated with exertion or inhalation of cold air.70 Some authors would now regard these guidelines as too restrictive and the issue of asthma risk and diving has been approached from several viewpoints.
Is there epidemiological evidence that asthmatic divers are at excess risk? In 1991 Edmonds reported that, in a series of 100 diving deaths in Australia and New Zealand, nine occurred in asthmatics despite the fact that fewer than 1% of divers reported a history of asthma.71 In contrast, Neuman et al72 found a 5% prevalence of asthma among recreational divers in the USA, similar to that in the general USA adult population, and a fatal accident rate of one asthmatic in 2132 deaths. During the period 198894, 369 cases of arterial gas embolism and 2720 cases of decompression illness were reported to Divers Alert Network in the USA; 23 of those reporting arterial gas embolism, and 123 of those reporting decompression illness had coexistent asthma.73 These figures suggest that the prevalence of asthma among those developing arterial gas embolism and decompression illness is similar to that in the general population. There is evidence that some asthmatics can dive without incident. Among 104 responders to a questionnaire for asthmatic divers published in a diving magazine, 96 reported taking a prophylactic ß2 agonist before diving and only one reported decompression illness. There were no reported cases of pneumothorax or gas embolism. However, more than half of these subjects were unaware of the contemporary advice for asthmatic divers74 and this was a self-selected sample. In a case control study of 196 episodes of arterial gas embolism, investigators calculated a 1.6 fold increase in risk for subjects with any asthma and a 1.98 fold increase for those with current asthma, but confidence limits were broad and neither figure reached conventional statistical significance.75
It is unclear whether pulmonary function testing can predict risk of diving related illness. In a study of 50 unselected experienced SCUBA divers in Australia, five gave a history of current asthma and one of previous childhood asthma. Ten subjects had an FEV1/FVC ratio 0f <75% and five of these had reduced mid expiratory flow rates. Twenty three of the subjects had a fall in FEV1 of more than 10% from baseline after either histamine or saline inhalation challenge. The authors suggest that, since this group was diving regularly without significant problems, function testing is a poor predictor of diving related illness.76
Is there a role for bronchial provocation testing in predicting risk for asthmatic divers? Among 180 divers with a history of asthma studied by Anderson et al, 90 had normal pulmonary function tests and no bronchial hyperresponsiveness.77 Thirty had a fall in FEV1 of more than 15% after inhaling hypertonic saline indicating bronchial hyperresponsiveness, and the authors would recommend their exclusion from diving. For subjects in whom the post saline fall in FEV1 was 1014.9%, the authors speculate that diving may be permissible if tests of static lung volumes and expiratory flows at low lung volumes derived from the flow-volume loop are normal. There has been speculation that diving itself might induce bronchial hyperresponsiveness by affecting small airway function. In a cross sectional study of 28 divers and 31 controls, Tetzlaff et al78 reported a higher prevalence of bronchial hyperresponsiveness to histamine among the divers than in non-diving matched controls (12/28 v 5/31). There was a non-significant trend towards an association between the degree of responsiveness and the number of compressed air dives performed. However, there has been no prospective testing of the relationship between bronchial hyperresponsiveness and risk in divers and current evidence does not support routine use of bronchial provocation testing in assessing fitness to dive.
Despite the inconclusive epidemiology and reservations about pulmonary function testing, there is a consensus among diving experts that asthmatics should be advised not to dive if they have wheeze precipitated by exercise, cold, or emotion.3,79 Jenkins et al1 are more conservative, suggesting that any asthma symptoms in the preceding 5 years should prompt advice against diving. However, UK and US authorities recommend that asthmatic individuals who are currently well controlled and have normal pulmonary function tests may dive if they have a negative exercise test. How should the exercise test be performed? Bronchodilators should be withheld for 24 hours before testing. The UK Sports Diving Medical Committee guidelines suggest using a 43 cm step test for 3 minutes or running outside to raise heart rate to 80% of maximum followed by serial peak flow measurements. A fall in peak flow of more than 15% from baseline occurring 3 minutes after exercise is regarded as an exclusion criterion. Since the asthmatic response to exercise is dependent on ventilation rate, temperature, and humidity of inspired air, an ideal protocol would monitor all of these parameters to ensure a reproducible challenge. In practice, a step or free running test and the above heart rate target is more widely applicable and is acceptable. However, a more appropriate schedule to assess the response is to measure FEV1 at 1,3, 5 10, 15 20, and 30 minutes after exercise. A decrease in FEV1 of 10% or more from baseline is abnormal and a decrease of 15% or more is diagnostic of exercise induced bronchoconstriction80 and would contraindicate diving. This protocol has the advantage of detecting subjects with a later nadir in airway function.
Asthmatic subjects who meet the criteria for diving require specific advice on management of their asthma during the diving season. They should monitor their asthma symptoms and twice daily peak flow measurements and should not dive if they have any of the following: active asthmathat is, symptoms requiring relief medication in the 48 hours preceding the dive; reduced PEF (>10% reduction from best values); or increased PEF variability (>20% diurnal variation). There are subjects in whom allergy is the only precipitating factor for wheeze and they may be permitted to dive if they have normal pulmonary function. However, there is a note of caution here. In one unusual case report an asthmatic who was allergic to Pareteria pollen suffered an episode of bronchospasm at depth which proved to be related to contamination of his gas supply and mouthpiece with pollen particles.81
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Hope that helps!
Bryan