Factors contributing to increased airway resistance with over humidification of inspired air - abstract

ABSTRACT

Inhalation of air containing increased water content has been shown to cause increased specific airway resistance (sRaw). It has been postulated that these changes might be related to reflex broncho-constriction and/or mucosal swelling. We attempted to elucidate the exact mechanism of action by a series of studies in rats, designed to demonstrate whether changes in specific airway resistance (as measured by whole body plethysmography), due to inhalation of air saturated with distilled water or tap water were preventable or reversible by either sympathomimetic (adrenaline), parasympatholytic (atropine) or mucolytic (N-acetyl cysteine) drugs. The results obtained showed that inhalation of air containing either tap or distilled water above the "safe level" (15.02gm/m(to the 3rd power) of humidification caused a significant increase in specific airway resistance above the control (p<0.01), with a slightly greater increase with tap water. Administration of adrenaline in aerosol to inspired air containing distilled or tap water vapour for six minutes caused no significant change in sRaw. A significant decrease in sRaw was obtained by injecting intramuscularly atropine (p0.01) and administration of N-acetylcysteine with inspired air containing distilled or tap water vapour caused an increase in sRaw above inhalation containing only tap or distilled water vapour, (p<0.05) which decreased with suction (p<0.05). Likewise N-acetyl cystine with distilled water or tap water vapour caused a significant increase in sRaw above that of inspired air with tap or distilled water alone (p<0.05) which significantly decreased with administration of atropine. These results suggest that mucosal swelling, reversible by parasympatholytic therapy, is a major factor in the increased specific airway resistance with over humidification of inspired air. There is no convincing evidence that broncho-constriction is of major importance. Mucolytic agents may aid in decomposing mucus but their effect on sRaw appears to be negative unless the decomposed products are moved by coughing or suction, and may be dangerous in patients who have no cough reflex (AU)