Acute effects of inspiratory pressure threshold loading upon airway resistance in people with asthma

Large inspiratory pressures may impart stretch to airway smooth muscle and modify the response to deep inspiration (DI) in asthmatics. Respiratory system resistance (Rrs) was assessed in response to 5 inspiratory manoeuvres using the forced oscillation technique: (a) single unloaded DI; (b) single DI at 25 cmH₂O; (c) single DI at 50% maximum inspiratory mouth pressure [MIP]; (d) 30 DIs at 50% MIP; and (e) 30 DIs at 50% MIP with maintenance of normocapnia. Rrs increased after the unloaded DI and the DI at 25 cmH₂O but not after a DI at 50% MIP (3.6 ± 1.6 hPa L s⁻¹ vs. 3.6 ± 1.5 hPa L s⁻¹; p = 0.95), 30 DIs at 50% MIP (3.9 ± 1.5 hPa L s⁻¹ vs. 4.2 ± 2.0 hPa L s⁻¹; p = 0.16) or 30 DIs at 50% MIP under normocapnic conditions (3.9 ± 1.5 hPa L s⁻¹ vs. 3.9 ± 1.5 hPa L s⁻¹; p = 0.55). Increases in Rrs in response to DI were attenuated after single and multiple loaded breaths at 50% MIP.