Separate resistive loading of the respiratory phases during mild hypercapnia in man

Eleven human subjects were studied during steady state, controlled mild hypercapnia with resistive loading of either inspiration (RI) or expiration (RE). Minute ventilation and frequency were significantly reduced by RI (P = less than 0.01) and even more so by RE (P = less than 0.001). Tidal volume was unchanged. Both RI and RE reduced mean flow in the loaded phase - an effect relatively greater with RE. Neither RI nor RE altered mean flow in the unloaded phase. Although mean inspiratory flow was unchanged with RE, mouth occlusion pressure (P0.1) was increased (P = less than 0.01). Functional residual capacity (seven subjects) was increased with RE, but not with RI (P = less than 0.05). Five additional subjects were similarly studied with and without RE in whom transdiaphragmatic pressure (PDi) and peak diaphragmatic EMG (EMGDi) were examined. Changes in ventilation, breathing pattern and P0.1 were similar to those described above. Neither PDi nor EMGDi were significantly altered by RE, but with RE, diaphragmatic EMG activity began 50-190 ms before inspiratory flow. In conclusion, ventilation is reduced more by RE than by RI due to greater respiratory phase time. Moderately heavy RE does not augment inspiratory drive as reflected by mean flow, PDi or EMGDi. With RE and increased FRC, P0.1 does not accurately reflect inspiratory drive because of dissociation between EMG and flow.