Analysis of pressure profile in the occluded airway obtained at the beginning of inspiration in steady state hypercapnia.
Using the steady state CO2 response test, we measured inspiratory mouth occlusion pressure and ventilatory volume in 38 normal subjects. At each degree of hypercapnia, 5 occlusion trials were performed. Occlusion pressure (Pt) and the timed differentials (dp/dtt) measured at 0.025-s intervals from the onset of inspiration until 0.2 s, dp/dt at 1 cmH2O pressure (dp/dt1 cmH2O), and peak dp/dt (dp/dtpeak) were determined. To evaluate the usefulness of these indexes, coefficients of variation and correlation coefficients with VT, normalized VE (VN), PACO, and inspiratory flow without occlusion were calculated. Coefficients of variation in Pt decreased with time from the onset of inspiration, being lowest at P.2. In contrast, those for dp/dtt were lowest at 0.075 sec, and thereafter rapidly increased. Correlation coefficients between Pt and respiratory parameters were high (0.94 to 0.98) for most times except at the very beginning of inspiration. Those between dp/dt and respiratory parameters were somewhat lower (0.91 to 0.97). Correlation of tidal volume with occlusion pressure was poor, thus the occlusion analysis was considered inappropriate to evaluate tidal volume output.