The effects of the inspiratory threshold load (ITL) on breathlessness and ventilatory mechanics during acute bronchoconstriction were studied by comparing responses to continuous positive airway pressure (CPAP) and inspiratory positive airway pressure (IPAP) in 12 asthmatic subjects after methacholine bronchoprovocation to a maximum change (delta) in FEV1 of 50%. At maximum response, "optimal CPAP" (CPAPOPT) was selected as the level of CPAP providing maximum subjective improvement in breathlessness. Spirometry, breathing pattern, esophageal pressure (Pes), and operational lung volumes were monitored. At maximum response, FEV1 decreased by 54 +/- 3% (mean +/- SEM) (p < 0.001), dynamic end-expiratory volume (EELVdyn) increased 66 +/- 8%, by 1.4 +/- 0.2 L (p < 0.001), and subjects reported severe breathlessness (Borg Scale = 5.6 +/- 0.8). CPAPOPT (5.3 +/- 0.6 cm H2O) significantly (p < 0.001) reduced breathlessness (delta Borg Scale = -3.0 +/- 0.5) and did not cause further dynamic hyperinflation. CPAPOPT reduced peak inspiratory Pes by 27% (p < 0.001), the tension-time index (TTI) for the inspiratory muscles by 27% (p < 0.01), and the inspiratory work rate per liter of ventilation by 14% (p < 0.05). During CPAPOPT, the delivered extrinsic positive end-expiratory pressure (PEEPe) (6.4 +/- 0.4 cm H2O) was strongly related (p < 0.001) to the measured ITL (6.9 +/- 1.0 cm H2O) at maximum response. Responses to IPAP of the same magnitude as CPAP OPT at maximum response were similar to those during CPAPOPT, except that IPAP did not counteract ITL or reduce breathlessness.(ABSTRACT TRUNCATED AT 250 WORDS)