Carbon dioxide diffusion coefficient in noninvasive high-frequency oscillatory ventilation.

OBJECTIVES

The carbon dioxide (CO ) diffusion coefficient (DCO ) reflects CO removal during high-frequency oscillatory ventilation (HFOV). We hypothesized that despite leak flow during noninvasive HFOV (nHFOV) DCO continues to indicate ventilation efficacy.

METHODS

A neonatal airway model including CO production and an adjustable oropharyngeal leak was connected to a ventilator via bi-nasal prongs. Pressures and gas flows were measured at prongs, trachea, and leak. Oscillatory tidal volumes below (V ) and above the leak (V ) were calculated from tracheal and leak flows. DCO was calculated using V (DCO ) and V (DCO ) and compared with CO partial pressure (pCO ). Effects of leak flow (0, 5, or 10 L/min) on DCO were assessed at fixed pressure amplitudes or predefined oscillatory volumes under steady-state pCO conditions in the modeled lung.

RESULTS

DCO correlated strongly with pCO , independent of the leak flow level (P < 0.0001). DCO correlated with pCO without and with moderate leak (P < 0.0001) but not with maximum leak (P = 0.1432). V correlated with the quotient of tracheal pressure amplitude and frequency irrespective of the leak (P < 0.0001). Based on the pressure amplitude at prong level (A ) V continued to follow a linear model of which the slopes decreased with increasing leak flow. V correlated with the quotient of A and frequency, irrespective of the leak (P < 0.0001).

CONCLUSIONS

DCO obtained at the airway opening at prong level reflects ventilation efficacy during nHFOV even in the presence of moderate oropharyngeal leak.