Since no objective methods could be used to estimate these respiratory disorders, we were only able to reliably investigate respiratory symptoms, and could only indirectly consider defined respiratory diseases. Nevertheless, any mis-classification based on this limitation could not have explained our positive finding, since such an error would probably be nondifferential and would therefore only dilute the risk estimates. Confounding could never be completely ruled out. Obesity, for instance, is a well-known risk factor for reflux that recently has been recognized as a risk factor for asthma as well. To reduce the risk of confounding, we adjusted statistically for all plausible confounding variables, including obesity represented by BMI. However, residual confounding cannot be excluded. An additional limitation of our study is that we had no data on antireflux therapy. Finally, the risk of chance findings clouding the results is diminished by the large size of the study. this
Our results are consistent with the findings in most earlier, large-scale studies, although a lack of association between self-reported asthma and reflux symptoms has been reported. In a study of the comorbid occurrence of laryngeal and pulmonary disease with esophagitis, comprising > 100,000 military veterans in the United States, the ORs ranged between 1.2 and 1.5 for the respiratory diseases investigated. The possible reason why those ORs are slightly lower than ours is that their study population consisted of patients with esophagitis and other inpatients with a higher rate of respiratory disorders in general. In another study, hospitalization due to hiatal hernia or reflux esophagitis increased the risk of future respiratory disease hospitalization to the same extent as our results.
Several possible mechanisms underlying a relation between reflux and respiratory symptoms have been proposed. In anesthesia literature, it is well known that ventilation rates can increase as a result of pain. Field et al reported increased minute ventilation due to sensations of discomfort during acid perfusion of the esophagus in patients with normal lung function. Increased minute volume could cause breathlessness in otherwise pulmonary healthy individuals.