title: Bronchoconstriction triggered by breathing hot humid air in patients with asthma: role of cholinergic reflex.
abstract: BACKGROUND
  Hyperventilation of hot humid air induces transient bronchoconstriction in patients with asthma; the underlying mechanism is not known. Recent studies showed that an increase in temperature activates vagal bronchopulmonary C-fiber sensory nerves, which upon activation can elicit reflex bronchoconstriction.
  
  
  OBJECTIVE
  This study was designed to test the hypothesis that the bronchoconstriction induced by increasing airway temperature in patients with asthma is mediated through cholinergic reflex resulting from activation of these airway sensory nerves.
  
  
  METHODS
  Specific airway resistance (SR(aw)) and pulmonary function were measured to determine the airway responses to isocapnic hyperventilation of humidified air at hot (49°C; HA) and room temperature (20-22°C; RA) for 4 minutes in six patients with mild asthma and six healthy subjects. A double-blind design was used to compare the effects between pretreatments with ipratropium bromide and placebo aerosols on the airway responses to HA challenge in these patients.
  
  
  RESULTS
  SR(aw) increased by 112% immediately after hyperventilation of HA and by only 38% after RA in patients with asthma. Breathing HA, but not RA, triggered coughs in these patients. In contrast, hyperventilation of HA did not cause cough and increased SR(aw) by only 22% in healthy subjects; there was no difference between their SR(aw) responses to HA and RA challenges. More importantly, pretreatment with ipratropium completely prevented the HA-induced bronchoconstriction in patients with asthma.
  
  
  CONCLUSIONS
  Bronchoconstriction induced by increasing airway temperature in patients with asthma is mediated through the cholinergic reflex pathway. The concomitant increase in cough response further indicates an involvement of airway sensory nerves, presumably the thermosensitive C-fiber afferents.
authors.affiliation: Department of Physiology, University of Kentucky Medical Center, 800 Rose Street, Lexington, KY 40536, USA.
authors.forename: Don
authors.initials: D
authors.lastname: Hayes
authors.forename: Paul B
authors.initials: PB
authors.lastname: Collins
authors.forename: Mehdi
authors.initials: M
authors.lastname: Khosravi
authors.forename: Ruei-Lung
authors.initials: RL
authors.lastname: Lin
authors.forename: Lu-Yuan
authors.initials: LY
authors.lastname: Lee
mesh_terms: D000280:Administration, Inhalation; D000328:Adult; D000403:Airway Resistance / Q000187:drug effects; D000704:Analysis of Variance; D001249:Asthma / Q000175:diagnosis* / Q000503:physiopathology; D016084:Bronchoconstriction / Q000187:drug effects*; D018680:Cholinergic Antagonists / Q000008:administration & dosage; D002799:Cholinergic Fibers / Q000502:physiology*; D004311:Double-Blind Method; D005260:Female; D005500:Follow-Up Studies; D005541:Forced Expiratory Volume; D006358:Hot Temperature / Q000009:adverse effects*; D006801:Humans; D006813:Humidity / Q000009:adverse effects; D006985:Hyperventilation / Q000503:physiopathology*; D009241:Ipratropium / Q000008:administration & dosage*; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D012018:Reflex; D012021:Reflex, Abnormal / Q000502:physiology*; D012129:Respiratory Function Tests; D018570:Risk Assessment; D016896:Treatment Outcome; D055815:Young Adult
chemical_list: D018680:Cholinergic Antagonists; D009241:Ipratropium