The Role of Conducting Airways in Gas Exchange during High-Frequency Ventilation???A Clinical and Theoretical Analysis

Abstract

Observed values for lung clearance index and mixing ratio appeared to be better than the calculated ideal values in seven of nine patients during high-frequency positive-pressure ventilation (HFPPV). The presence of substantial amounts of N2 (9.2% to 22.5%) in the initial expired gas suggests that these findings are explained by amplified mixing of tidal gas with residual gas in the conducting airways secondary to the high gas flow velocity during inspiration. There was a moderate positive correlation (r = 0.59) between the amount of residual gas (N2) present in the initial part of the expirate and the efficiency of nitrogen washout. A "functional" dead space for N2 can be calculated. During HFPPV this was 125 +/- 71 ml and during spontaneous breathing 354 +/- 121 ml (p less than 0.001), giving a VD/VT for N2 of 0.38 +/- 0.06 as compared with 0.84 +/- 0.21 (p less than 0.001) during spontaneous breathing. It also implies a more efficient washout of N2 than of CO2 (VD/VT approximately 0.75) during HFPPV.