A bias flow of at least 30 L/min in the adult oscillator R100, operating at a frequency of 8 Hz, is required to maximize the ventilation efficiency, reports a new study presented at the recently concluded 4th SG-ANZICS Intensive Care Forum (SG-ANZICS 2017) in Singapore.

The investigators used a stimulated bronchial tree housed inside a 20-L rigid plastic container as the base of their lung model. An endotreacheal tube leads out of the model and connects to the R100 adult oscillator via a Y-piece. A MEMS type flow sensor to measure actual stroke volume is located between the tube and the Y-piece.

A known volume of CO2 was continuously pumped into the model from a tank. Subsequently, the model was aspirated by a capnometer, which then measured the partial pressure of CO2. From these, the alveolar ventilation was calculated. Finally, using alveolar ventilation and actual stroke volume values, the ventilation efficiency was determined for four different bias flow rates (10, 20, 30 and 40 L/min).

Almost consistently and across all actual stroke volumes (80, 100 and 120 mL), the alveolar ventilation values obtained from higher bias flow values were significantly greater than those obtained from lower bias flow values (p<0.001 for all). [SG-ANZICS 2017, abstract P-12]

For an actual stroke volume of 80 mL, there was no significant difference between bias flows 30 and 40 L/min in terms of alveolar ventilation.

Ventilation efficiency showed a similar trend. Across all stroke volumes, ventilation efficiency values were significantly greater for higher bias flow values (p<0.001 for all). For actual stroke values 80 and 120 mL, there were no significant differences in ventilation efficiency between bias flows 30 and 40 L/min.

The researchers also investigated the effect of bias flow on setting stroke volume. They found that higher bias flows required significantly higher setting stroke volumes to get actual stroke volumes of 80, 100 and 120 mL (p<0.001).

On the other hand, higher bias flows also allowed defined setting stroke volumes (80, 100, 12, 140, 160 and 180 mL) to yield higher actual stroke volumes.

“This study showed that increasing [bias flow] improved the ventilation efficiency up to at least 30 L/min. Improvement of the ventilation efficiency enables to use the lower [actual stroke volume] to get the constant alveolar ventilation,” the investigators explained.

The findings of this study do not corroborate the results of the recent large randomized controlled trials OSCAR and OSCILLATE, which both reported no advantage of high-frequency oscillatory ventilation in adults with acute respiratory distress syndrome (ARDS), the investigators said.

However, the bias flow for the OSCAR trial was initially set at 20 L/min and was not changed in the protocol. “Therefore, the [bias flow] setting in the OSCAR trial might not be the best for the lung protection and might affect the unsatisfactory result of the OSCAR trial,” the investigators noted.