Flow limitation and dyspnoea in healthy supine subjects during methacholine challenge

The purpose of this study was to assess whether during standard methacholin e (Mch) challenge (concentration up to 128 mg.mL(-1)) healthy supine subjec ts a) develop tidal expiratory flow limitation (FL) and hyperinflation, and b) whether the onset of tidal PL is associated with dyspnoea. Eight healthy subjects were studied. Dyspnoea was assessed using the Borg s cale, FL by the negative expiratory pressure (NEP) method and hyperinflatio n in terms of decrease in inspiratory capacity (IC). Seven patients became flow limited at Mch doses ranging 4-64 mg.mL(-1), wit h PL encompassing 34-84% of the control tidal volume. In six of them the on set of tidal FL was associated,vith little or no dyspnoea and a modest degr ee of hyperinflation (Delta IC <-0.4 L). In one subject, however, onset of FL was associated with a substantial reduction in IC (0.58 L) and moderatel y severe dyspnoea. In all of these seven subjects PL was transiently revers ed after an IC manoeuvre. In conclusion, the results show that a) most healthy subjects may develop n ow limitation and hyperinflation during methacholine challenge in supine po sition, and b) at onset of flow limitation there is lime or no dyspnoea, su ggesting that onset of dynamic airway compression per se does not elicit si gnificant dyspnoea. Significant dyspnoea probably only occurs with marked d ynamic hyperinflation.