P0.1 during exercise in normal subjects breathing mixtures of gas with varying densities

The aim of the study was to re-examine the occlusion pressure measured simultaneously in the mouth (P0.1) and the oesophagus (Poes.1) during exercise in normal subjects submitted to different gas mixtures.

7 healthy men breathing random gas mixtures containing 21% oxygen with either by 79% helium (He-O2) or sulphur hexafluoride (SF6-O2) and room-air, were studied during a steady-state 90w exercise performed on a cyclo-ergometer. Ventilatory parameters were derived from the flow signal recorded by a pneumotachograph calibrated with the different gas mixtures. Three pressure transducers (mouth, eosophageal and gastric) were checked to have iso-time identical responses up to 4Hz. P0.1, Poes.1, deltaPoes (difference in oesophageal pressure between end-inspiratory and end-expiratory levels) and deltaPdi (variation of transdiaphragmatic pressure between end-inspiratory and end-expiratory levels) were measured.

Hyperventilation associated with a similar deltaPoes/deltaPdi but a lower P0.1/deltaPdi ratio was observed in He-O2 breathing compared to SF6-O2 and air. Variable time delays between oesophageal and mouth pressures were observed during air and SF6-O2. Whatever the condition involved, no change was detected in the shape of the inspiratory pressure during the occlusion manoeuvres.

He-O2 breathing probably induced a change in the shape of the pressure wave later on in the inspiratory phase, making P0.1 not representing the total inspiratory drive. On the contrary in air and SF6-O2 conditions, P0.1 seemed to remain a useful tool for looking at the output of the respiratory controller.