Intra-airway gas mixing during high-frequency ventilation

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Abstract
Intra-airway gas transport mediated by high-frequency oscillations (HFO) in 10 nonintubated healthy volunteers was examined using a method based on comparisons of single-breath N2-washout curves obtained after various durations of breath hold or high-frequency oscillations. The local transport parameter and effective diffusivity during oscillations of frequency 2-24 H2, tidal volume of 10-120 ml, and during breath hold alone was computed with a mathematical analysis based on Fick''s law of diffusion. Local effective diffusivity increased with both oscillatory frequency and tidal volume at all levels in the tracheobronchial tree. The enhancing effect of tidal volume on local effective diffusivity was more pronounced than that of frequency so that effective diffusivity was greater with larger tidal volume at fixed frequency-tidal volume product (f .cntdot. VT). The greatest enhancement of gas mixing within the lung during HFO (over breath hold) was seen in the central airways. In previous studies examining CO2 removal rate during HFO, it was found that CO2 output was also greater with larger tidal volume at fixed f .cntdot. VT. This was attributed to an end constraint imposed by a fresh gas bias flow. Evidently, bias flow end constraint does not solely account for the observed dependence of CO2 output on frequency and tidal volume.

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