title: Lack of tachyphylaxis to methacholine at 24 h.
abstract: OBJECTIVE
  To examine for tachyphylaxis to methacholine at 24 h and to use these data to assess the repeatability of the provocative concentration of a substance causing a 20% fall in FEV1 (PC20) for methacholine and to obtain statistical power calculations.
  
  
  METHODS
  Retrospective review of four double-blind, placebo-controlled studies with two methacholine PC20 values measured at 24-h intervals.
  
  
  METHODS
  Tertiary university hospital bronchoprovocation laboratory.
  
  
  METHODS
  Thirty-two subjects with mild-to-moderate well-controlled asthma.
  
  
  METHODS
  The placebo arms of the four studies were examined.
  
  
  METHODS
  Methacholine PC20 (using 2-min tidal breathing method) initial determination and repeat testing at 24 h.
  
  
  RESULTS
  The geometric mean PC20 values were 1.57 mg/mL (95% confidence interval [CI], 1.0 to 2.4 mg/mL) and 1.62 mg/mL (95% CI, 1.0 to 2.6 mg/mL NSD; p = 0.64). The mean absolute difference between the two measurements was < 0.4 doubling concentrations, and 31 of 32 measurements had both values within one doubling concentration. These data provide a statistical power of 92% for 10 subjects to show a one-half concentration deltaPC20 and a mean power of 99 +/- 1% to show a one-concentration deltaPC20.
  
  
  CONCLUSIONS
  There is no evidence for methacholine tachyphylaxis at 24 h in subjects with asthma. At 24 h, the average repeatability was well within a one-half concentration change, and individually 31 of 32 measurements (97%) were within one doubling concentration.
authors.affiliation: Department of Medicine, Division of Respirology, Critical Care and Sleep Medicine, Royal University Hospital, 103 Hospital Dr, Ellis Hall, Fifth Floor, Saskatoon, SK, S7N 0W8 Canada. cockcroft@sask.usask.ca
authors.forename: Donald W
authors.initials: DW
authors.lastname: Cockcroft
authors.forename: Beth E
authors.initials: BE
authors.lastname: Davis
mesh_terms: D000293:Adolescent; D000328:Adult; D001249:Asthma / Q000150:complications / Q000175:diagnosis; D016535:Bronchial Hyperreactivity / Q000175:diagnosis* / Q000209:etiology; D001985:Bronchial Provocation Tests; D016085:Bronchoconstrictor Agents; D002986:Clinical Trials as Topic; D005260:Female; D006801:Humans; D008297:Male; D016210:Methacholine Chloride; D008875:Middle Aged; D012189:Retrospective Studies; D013618:Tachyphylaxis; D013997:Time Factors
chemical_list: D016085:Bronchoconstrictor Agents; D016210:Methacholine Chloride