Pendelluft and mixing in a single bifurcation lung model during high-frequency oscillation

A single bifurcation with an adjustable daughter branch compliance ratio (VR) was used to simultaneously study pendelluft and longitudinal mixing during flow oscillations at frequencies (f) of 1-15 Hz and amplitudes (VOp) of 25-150 ml/s. Mixing coefficients (Deff) were determined from the dispersion of a CO2 bolus centered at the bifurcation point, and pendelluft volume was computed as a fraction of mother branch tidal volume (PVF) using measurements of airflow in the daughter branches. Plotted against frequency, PVF was a bell-shaped curve insensitive to the value of VOp. When VR = 2, a PVF peak of 0.25 appeared at f = 3 Hz, and when VR = 5, a PVF peak of 0.75 appeared at f = 4 Hz. After normalization by control values at VR = 1, Deff curves were also bell shaped, insensitive to the value of VOp and with peaks appearing at the same frequencies as the PVF peaks. The normalized Deff peak values were 1.7 when VR = 2 and 4.0 when VR = 5. The similarities in the PVF and Deff curves imply a direct relationship between pendelluft and enhanced mixing.