In clinical applications of high frequency oscillation (HFO), sufficient CO2 elimination (VCO2) may represent a problem mainly at higher oscillation frequencies. With the intention of examining how to increase VCO2 a modified bias flow system was investigated in vitro with wash-out experiments. In bias flow systems, long tubes have been used in order to minimize the loss of oscillatory volume; however, a distinct increase of VCO2 was achieved in the present study by removing the bias tube. This improvement occurred over the whole frequency range of 2-60 Hz, although the oscillatory volume, effectively delivered to the lungs was smaller with the HFO circuit without bias tube (HFO-BT) as compared to the arrangement with bias tube (HFO + BT). A long bias tube flattens the CO2 concentration gradient from the alveoli to the atmosphere. Removing the bias tube results in a steeper CO2 concentration gradient and in a correspondingly enhanced VCO2. Furthermore, the large oscillatory volume at the exit of the bias flow system in HFO-BT supports VCO2 as an additional wash-out mechanism. Based upon longitudinal tracer gas concentration measurements between the alveoli and the atmosphere during HFO16,17, an increase of gas transport up to 20% can be expected for in vivo applications by removing the bias tube.