[Effects of turbulence on gas transport during high frequency oscillation]

To clarify the enhancing effect of turbulence on gas transfer in high frequency oscillation (HFO), the role of turbulence during oscillatory flow was studied. Oscillatory flow was generated in a straight circular tube as a trachea model, over the range of Reynolds' number (Re: 2000-16000) and Stokes parameter (lambda : 3-7). This simulates clinical settings. Measurements of the axial velocity were made by means of hot wire anemometer, and turbulence intensity (I) as well as ensemble mean velocity was calculated. At the same time, 1 ml of 100% CO2 was injected into the tube, and CO2 concentration was measured by a CO2 analyzer to yield the effective diffusivity (Deff). As Re increases and lambda decreases to a certain level, "conditional turbulence" appears in the whole cross section to produce an efficient lateral mixing. Deff increases with increasing I as well as Re, and increasing lambda results in an increase in the difference of measured and theoretical Deff (Watson's prediction for laminar oscillatory flow). It is concluded that turbulence during oscillatory flow plays an important role in lateral mixing to facilitate gas transport, and decision of ventilator settings is essential so as to give rise to "conditional turbulence" for the purpose of Deff augmentation during HFO.