Bias flow rate and ventilation efficiency during adult high-frequency oscillatory ventilation: a lung model study

Abstract

BackgroundBias flow (BF) is essential to maintain mean airway pressure (MAP) and to washout carbon dioxide (CO2) from the oscillator circuit during high-frequency oscillatory ventilation (HFOV). If the BF rate is inadequate, substantial CO2 rebreathing could occur and ventilation efficiency could worsen. With lower ventilation efficiency, the required stroke volume (SV) would increase in order to obtain the same alveolar ventilation with constant frequency. The aim of this study was to assess the effect of BF rate on ventilation efficiency during adult HFOV.MethodsThe R100 oscillator (Metran, Japan) was connected to an original lung model internally equipped with a simulated bronchial tree. The actual SV was measured with a flow sensor placed at the Y-piece. Carbon dioxide (CO2) was continuously insufflated into the lung model (CO2), and the partial pressure of CO2 (PCO2) in the lung model was monitored. Alveolar ventilation (A) was estimated as CO2 divided by the stabilized value of PCO2. A was evaluated by setting SV from 80 to 180 mL (10 mL increments, n = 5) at a frequency of 8 Hz, a MAP of 25 cmH2O, and a BF of 10, 20, 30, and 40 L/min (study 1). Ventilation efficiency was calculated as A divided by the actual minute volume. The experiment was also performed with an actual SV of 80, 100, and 120 mL and a BF from 10 to 60 L/min (10 L/min increments: study 2).ResultsStudy 1: With the same setting SV, the A with a BF of 20 L/min or more was significantly higher than that with a BF of 10 L/min. Study 2: With the same actual SV, the A and the ventilation efficiency with a BF of 30 L/min or more were significantly higher than those with a BF of 10 or 20 L/min.ConclusionsIncreasing BF up to 30 L/min or more improved ventilation efficiency in the R100 oscillator.Electronic supplementary materialThe online version of this article (10.1186/s40635-018-0176-3) contains supplementary material, which is available to authorized users.

Comparison of the ventilation characteristics in two adult oscillators: a lung model study

“…Therefore, we applied the power approximation curves to the relationships between aSV (the unit is L) and A, and those were well fitted at both frequencies (Fig. 4), similar to the last report [11]. Furthermore, we calculated the exponential for the frequency (i.e., a ) using cycle per minute as the unit of frequency, namely 360 cycle/min for 6 Hz and 480 cycle/min for 8 Hz (Table 1).…”

“…The lung model consisted of a 20-L airtight rigid plastic container internally equipped with a simulated bronchial tree model (KYOTO KAGAKU Co. Ltd., Kyoto, Japan) which had 3 to 7 steps of bifurcations to 20 segmental bronchial branches [ 11 ] (Fig. 1 ).…”

“…However, some studies have reported that the aSV during HFOV could be higher than the anatomical dead space volume at low frequencies [ 8 – 10 ]. Recently, we have examined the effect of bias flow (BF) rate on ventilation efficiency using a lung model and reported that the ventilation efficiency improved with a BF of 30 L/min or more compared to that with a BF of 20 L/min or less in the R100 oscillator (Metran Co. Ltd., Kawaguchi, Saitama, Japan) at 8 Hz [ 11 ]. If the BF rate is inadequate, the required aSV consequently increases in order to obtain the same alveolar ventilation ( A) for a given frequency.…”

High-frequency oscillatory ventilation: A narrative review

“…Although controls relating to oxygenation and ventilation are of vital importance to managing successful outcomes in HFOV, there are other controls of similar importance. Bias flow (BF) controls and indicates the rate of continuous flow of humidified blended gas through the circuit [4, 52]. The control knob is a 15-turn pneumatic valve, which increases flow when turned clockwise.…”

“…BF is typically the first parameter set by the clinician, because it is part of the oscillator peruse calibration. If the BF is set too low, it may result in increased P a CO 2 secondary to inadequate circuit washout [52]. If the BF is set too high, it may inhibit CO 2 elimination.…”