The influence on the breathing pattern in man of moderate levels of continuous positive and negative airway pressure and of positive end-expiratory pressure during air and CO2 inhalation.

The purpose of this study was to determine in man the effect on the breathing pattern of continuous positive (CPAP), continuous negative (CNAP) and positive end-expiratory (PEEP) airway pressure during air breathing and CO2 inhalation. Six subjects were exposed to CPAP, CNAP and PEEP 0.5 kPa, while five subjects were exposed to CPAP and CNAP 0.8 kPa. End-expiratory lung volume increased during CPAP 0.8 kPa and decreased during CNAP 0.8 kPa. CPAP induced more extensive changes in the ventilatory pattern, and the changes in each parameter were larger than observed during CNAP and PEEP at the same pressure level. In contrast to previous reports we found the effect of CO2 inhalation combined with the effect of pressure breathing to be not stronger than additive. Even moderate CPAP induced alveolar hyperventilation with marked reduction in arterial PCO2 (PaCO2) when breathing air. With increasing fraction of CO2 in the inspiratory gas, the difference in PaCO2 between CPAP and no CPAP disappeared. PEEP also affected the breathing pattern in that it induced an increase in mean inspiratory flow and mean expiratory flow and a reduction in inspiratory duration. Occurrence of ventilatory pauses depended on whether mouthpiece or facemask was used. CPAP and CNAP did not influence the occurrence of pauses, while PEEP prolonged post-expiratory pauses. We conclude that CPAP, CNAP and PEEP induce active ventilatory responses in man and that strong mechanisms are involved during CPAP since PaCO2 is markedly reduced.