Effect of the oronasal breathing route on sulfur dioxide-induced bronchoconstriction in exercising asthmatic subjects

We undertook a study to determine how the oronasal breathing route affects the bronchoconstrictor response to sulfur dioxide (SO2) inhaled by asthmatic subjects during exercise. In 6 subjects, we compared the changes in specific airway resistance (SRaw) caused by breathing humidified air through a mouthpiece during 5 min of exercise on a bicycle ergometer (550 kpm/min) to the changes caused by breathing humidified air plus 0.5 ppm of SO2, (a) through a mouthpiece (oral breathing), (b) by facemask (oronasal breathing), and (c) by facemask with the mouth occluded (nasal breathing) during exercise. Breathing humidified air plus 0.5 ppm of SO2 through a mouthpiece or by facemask during exercise significantly increased SRaw in all 6 subjects, and breathing humidified air plus 0.5 PPM of SO2 by facemask with the mouth occluded significantly increased SRaw in 5 of 6 subjects. The increase in SRaw caused by breathing humidified air plus 0.5 PPM of SO2 through a mouthpiece was not significantly different from the increase caused by breathing SO2 by facemask (p greater than 0.05), but was significantly greater than the increase caused by breathing SO2 by facemask with the mouth occluded (p less than 0.05). These results indicate that although nasal breathing partially protected against SO2-induced bronchoconstriction in our subjects, both oral and oronasal breathing of low concentrations of SO2 during exercise can cause significant bronchoconstriction in people with asthma.