Desiccation and hypertonicity of the airway surface fluid and thermally induced asthma

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Abstract
To determine whether drying and hypertonicity of the airway surface fluid (ASF) are involved in thermally induced asthma, nine subjects performed isocapnic hyperventilation (HV) (minute ventilation 62.2 ± 8.3 l/min) of frigid air (−8.9 ± 3.3°C) while periciliary fluid was collected endoscopically from the trachea. Osmolality was measured by freezing-point depression. The baseline 1-s forced expiratory volume was 73 ± 4% of predicted and fell 26.4% 10 min postchallenge ( P > 0.0001). The volume of ASF collected was 11.0 ± 2.2 μl at rest and remained constant during and after HV as the airways narrowed (HV 10.6 ± 1.9, recovery 6.5 ± 1.7 μl; P = 0.18). The osmolality also remained stable throughout (rest 336 ± 16, HV 339 ± 16, and recovery 352 ± 19 mosmol/kgH2O, P = 0.76). These data demonstrate that airway desiccation and hypertonicity of the ASF do not develop during hyperpnea in asthma; therefore, other mechanisms must cause exercise- and hyperventilation-induced airflow limitation.