Bronchial challenge, assessed with forced expiratory manoeuvres and airway impedance

The Methacholine concentration at which a 20% decrease of the forced expiratory volume in 1s (PC20_FEV1) or a 40% increase in airway resistance (PC40_Rrs6) occur are accepted indicators for airway hyperresponsiveness. We hypothesised that the level of detection of bronchial hyperresponsiveness will differ between the two methods.

The response to Methacholine was assessed by forced oscillation technique (FOT) and spirometry in 20 stable hyperresponsive asthmatics. The effects of repeated lung function measurements on respiratory muscle fatigue were measured from maximal inspiratory mouth pressure (MIP). After each dose, patients scored their perception of dyspnoea on a BORG scale. Differences in patient's burden were measured by comparing the BORG-score at PC40_Rrs6 (BORG-PC40_Rrs6) and at PC20_FEV1 (BORG-PC20_FEV1). Reproducibility was also evaluated.

The PC20_FEV1-values were 2.2 (0.4) doubling dose higher as compared to the PC40_Rrs6 (P<0.001). The mean BORG-score at PC40_Rrs6 was 1.7 points lower as compared to the BORG-score at PC20_FEV1 (P<0.001). The difference (mean(sd)) between the PC20_FEV1 of measurement 1 and 2 was -0.1 (1.4) doubling dose, and -0.3 (2.7) doubling dose for PC40_Rrs6. The MIP after Methacholine provocation was 1.0(0.2) kPa lower as compared to the MIP before the challenge test (P<0.001), suggesting respiratory muscle fatigue.

Measuring PC40_Rrs6 shortens the challenge test and lowers the concentrations of bronchoconstrictor agents as compared to measurements of PC20_FEV1. The FOT-method was less strenuous for patients. In spite of the fact that the reproducibility is two-fold worse than measuring PC20_FEV1, it still remains quite acceptable at a mean of 0.3 doubling dose. The respiratory muscle strength was deteriorated after the challenge test.