Augmentation of CO 2 elimination during high frequency oscillation by removing the bias tube — an in vitro study

Abstract

In clinical applications of high frequency oscillation (HFO), sufficient CO 2 elimination (V̇ CO 2 ) may represent a problem mainly at higher oscillation frequencies. With the intention of examining how to increase V̇ CO 2 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 V̇ CO 2 was achieved in the present study by removing the bias tube. This impovement 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 CO 2 concentration gradient from the alveoli to the atmosphere. Removing the bias tube results in a sleeper CO 2 concentration gradient and in a correspondingly enhanced V̇ CO 2 . Furthermore, the large oscillatory volume at the exit of the bias flow system in HFO −BT supports V̇ CO 2 as an additional wash-out mechanism. Based upon longitudinal tracer gas concentration measurements between the alveoli and the atmosphere during HFO 16,17, an increase of gas transport up to 20% can be expected for in vivo applications by removing the bias tube.