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

Abstract

Background: Bias 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. Methods: The 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 ( _VCO2), and the partial pressure of CO2 (PCO2) in the lung model was monitored. Alveolar ventilation ( _VA) was estimated as _VCO2 divided by the stabilized value of PCO2 . _VA 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 cmH2 O, and a BF of 10, 20, 30, and 40 L/min (study 1). Ventilation efficiency was calculated as _VA 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). Results: Study 1: With the same setting SV, the _VA 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 _VA 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. Conclusions: Increasing BF up to 30 L/min or more improved ventilation efficiency in the R100 oscillator.