Longitudinal mixing in dog lungs during high-frequency forced flow oscillation

Longitudinal mixing in the conducting airways of eight intubated anesthetized beagles (10.8 +/- 0.9 kg) was studied at functional residual capacity in the presence of forced sinusoidal flow oscillations and in the absence of fresh air bias flow. The ranges of oscillation conditions were: frequencies, f, from 3 to 18 Hz and minute volumes, Vosc, from 50 to 150 ml/sec, corresponding to tidal volumes, Vosc/f, from 0.3 to 4.5 ml/kg body mass. Oscillations were imposed during a breath holding interval incorporated into a modified single-breath nitrogen (N2) washout maneuver. The expired N2 fraction curves were analyzed with a Fickian diffusion model by adjusting the value of a global mixing parameter, (DA2), to achieve an optimal fit of the model to the data. The mixing parameter was an increasing function of minute volume and a decreasing function of frequency, which is well represented by the equation: (DA2) = 2.72 Vosc 1.74 f-1.57 By comparison to available theory and previous measurements in physical systems, this formula implies that Taylor-type dispersion is the dominant mixing mechanism in the conducting airways. Also, the diffusion model predicted, and the data verified, the existence of a mouth-ward 'diffusion flow' during breath holding. This effect, caused by the non-uniform nature of the summed airway cross-section, is directly correlated with the value of (DA2).