O2 uptake and CO2 elimination during mechanical ventilation with high frequency oscillation.

This study was intended to elucidate gas exchange in a quasi-steady state during high frequency oscillatory ventilation (HFO) in terms of arterial blood gases, tidal volume(VT) and frequency of oscillation (f). Firstly, experiments were performed on anesthetized, paralyzed and tracheostomized dogs using a piston-type oscillator with a fresh air bias flow. The f values employed in the animal experiments were 10 to 30 Hz, and VT values were 1 to 3 ml/kg of body weight. Changes in PaO2 observed during HFO could be expressed by the equation PaO2 = 125.2-60.3/(VT X f), which closely coincided with the alveolar ventilation equation for O2, i.e., PaO2 = 125-78/, VA, Where P(A-a) O2 and O2 consumption were assumed to be 25 Torr and 90 ml/min, respectively. PaCO2 during HFO deviated from the curve of the alveolar ventilation equation, PaCO2 = constant/, VA at a higher VT x f, and was distributed along the hyperbolic curve of PaCO2 = 1/, VA + 14.7. This suggested that HFO shows a certain limitation in CO2 elimination. Secondly, indicator gas transport through straight tube models for two directions, i.e., wash-in and wash-out, were observed. Wash-in of indicator gases (He, N2 and SF6) in terms of indicator appearance time at the other end of the tube changed as a function of VT x f. The effect of increasing f at a fixed VT on the wash-in was much less than that of increasing VT at a fixed f. The heavier gas (SF6) was washed in faster than the lighter gas (He) although wash-in of each indicator gas was closely related to the function VT x f. Washout in terms of the appearance time of indicators in the opposite direction was, however, strongly dependent on VT, and the effect of increasing f at a fixed VT on wash-out reached a limit beyond a certain f. It was concluded from the present study, that both convective dispersion and augmented diffusion play important roles, although they are not clearly distinguished, as gas transport mechanisms during HFO. The difference between inspiratory and expiratory gas transport modes could be explained by differences in flow profiles, relative important of convective dispersion, and/or time required for gas mixing in the airways.