RESPIRATORY RESPONSE TO INHALED CO2 DURING POSITIVE INSPIRATORY PRESSURE IN HUMANS

To investigate ventilatory CO2 sensitivity during inspiratory pressure support (IFS), we administered inspiratory CO2 [fractional concn (FIc o2) 0.01, 0.03, or 0.05] in eight normal subjects without (CTRL) or wi th (Pinsp) positive inspiratory airway pressure (5 or 10 cmH(2)O). At CTRL and low IFS, CO2 inhalation led to a significant increase in tida l volume (VT) with nearly identical slopes in the plot of VT vs. end-t idal Pco(2). At the high IFS level, VT at FIco2 of 0 was significantly above the value at lower Pinsp and did not increase with CO2 unless F Ico2 was elevated to >0.03. There was very little effect of either Pin sp or FIco2 on respiratory frequency and respiratory timing. The data suggest that the CO2 sensitivity of ventilation is similar at low leve ls of IFS as during CTRL. However, at high levels of IFS, VT is determ ined largely by the passive inflation and, thus, independent of CO2. C O2 has to be increase the respiratory drive before VT becomes CO2 sens itive.