Effect of bias flow rate on gas transport during high-frequency oscillatory ventilation

Ventilatory support with low tidal volume, high-frequency oscillatory ventilation (HFOV) usually uses a bias flow system to provide fresh gas. Although the bias flow rates (Vbf) used previously have varied widely among experimental configurations, the precise role of the bias flow in HFOV-mediated gas transport has not been defined. We assessed the effect of bias flow rate on gas transport during HFOV by measuring CO2 removal rate (MCO2) in anesthetized, paralyzed dogs, using a wide range of bias flow rates (0.7-28.9 L X min-1). When a fixed tidal volume of 40 ml was applied at HFOV frequencies of 2-12 Hz, MCO2 was proportional to the time-averaged alveolar-bias flow CO2 concentration difference. Thus, when Vbf was reduced below a value which resulted in a substantial increase in bias flow CO2 concentration, MCO2 was reduced. These findings are consistent with a simple framework in which the relative magnitudes of the resistances to gas transport of the airways and of the bias flow (1/Vbf) determine the contribution of the bias flow rate to overall gas transport during HFOV. This relationship may be employed to assess the intra-airway contribution to HFOV-mediated gas transport at any bias flow rate, and may therefore allow comparison of results from experiments utilizing various bias flow rates.