Breathing pattern during fog challenges in humans

Inhalation of ultrasonically nebulised distilled water (fog) has been shown repeatedly to induce cough and bronchoconstriction in asthmatic patients, but only cough in normal subjects; whether fog inhalation also influences the pattern of breathing is unclear. In 10 normal volunteers, changes in the pattern of breathing during inhalation of the threshold fog concentration for the cough reflex were evaluated. The appearance of cough was detected by recordings of the electromyographic activity of the abdominal muscles. Tidal volume (VT), the total duration of the respiratory cycle (TT), as well as the duration of the inspiratory (T1) and expiratory times were measured by respiratory inductive plethysmography. Mean inspiratory flow (VT/T1), the duty cycle (T1/TT), respiratory frequency (=60/TT) and minute ventilation (V'E) were subsequently calculated. The median (interquartile range) cough threshold value was 0.73 (0.90) mL· min-1. Compared with the corresponding mean control values, inhalation of the threshold fog concentration caused significant increases in V'E that were achieved mainly by an increase in VT. VT/T1 also increased. No consistent changes were observed in the timing components of the breathing pattern. The authors conclude that fog inhalation in normal subjects induces coughing, associated with increases in minute ventilation and mean inspiratory flow resulting from selective increases in tidal volume. Fog-induced ventilatory and cough responses are probably due to the activation of airway rapidly adapting "irritant" receptors. However, a role for bronchial C-fibre endings cannot be ruled out completely.