Enforced mouth breathing decreases lung function in mild asthmatics

Background and objective: Nasal breathing provides a protective influence against exercise‐induced asthma. We hypothesized that enforced oral breathing in resting mild asthmatic subjects may lead to a reduction in lung function. Methods: Asymptomatic resting mild asthmatic volunteers (n = 8) were instructed to breathe either nasally only (N; tape over lips) or orally only (O; nose clip) for 1 h each, on separate days. Lung function (% predicted FEV₁) was measured using standard spirometry at baseline and every 10 min for 1 h. ‘Difficulty in breathing’ was rated using a Borg scale at the conclusion of the N and O periods. Results: Baseline FEV₁ on the N (101.2 ± 3.8% predicted) and O (102.7 ± 3.9% predicted) days was not significantly different (P > 0.3). At 60 min, FEV₁ on the O day (96.5 ± 4.1% predicted) was significantly less than on the N day (101.0 ± 3.5% predicted; P < 0.009). On the N day, FEV₁ did not change with time (P > 0.3), whereas on the O day, FEV₁ fell progressively (slope = ?0.06 ± 0.01% FEV₁/min, P < 0.0001; linear mixed effects modelling). Three subjects experienced coughing/wheezing at the end of the O day but none experienced symptoms at the end of the N day. Subjects perceived more ‘difficulty breathing in’ at the end of the O day (1.5 ± 0.4 arbitrary units) than on the N day (0.4 ± 0.3 arbitrary unit; P < 0.05). Conclusions: Enforced oral breathing causes a decrease in lung function in mild asthmatic subjects at rest, initiating asthma symptoms in some. Oral breathing may play a role in the pathogenesis of acute asthma exacerbations.