Background: The repeatability of lung function tests and methacholine inhalation tests was evaluated in recurrently wheezy infants over a one month period using the rapid thoracic compression technique.
Methods: Eighty-one wheezy, symptom free infants had pairs of methacholine challenge tests performed one month apart. Maximal flow at functional residual capacity (VmaxFRC) and transcutaneous oxygen tension (Ptco2) were measured at baseline and after methacholine inhalation. Provocative doses of methacholine causing a 15% fall in Ptco2 (PD15 Ptco2) or a 30% fall in VmaxFRC (PD30 VmaxFRC) were determined.
Results: Large changes in VmaxFRC were measured from T1 to T2 with a mean difference between measurements (T2-T1) of 7 (113) ml/s and a 95% range for a single determination for VmaxFRC of 160 ml/s. The mean (SD) difference between pairs of PD30 VmaxFRC measurements was 0.33 (1.89) doubling doses with a 95% range for a single determination of 2.7 doubling doses. Repeatability of PD15Ptco2 was similar. A change of 3.7 doubling doses of methacholine measured on successive occasions represents a significant change.
Conclusions: Baseline VmaxFRC values are highly variable in wheezy, symptom free infants. Using either VmaxFRC or Ptco2 as the outcome measure for methacholine challenges provided similar repeatability. A change of more than 3.7 doubling doses of methacholine is required for clinical significance.