The BathRC model: a method to estimate flow restrictor size for dual ventilation of dissimilar patients

Andrew R Plummer; Jonathan l du Bois; Siu Man Lee; Patrick Magee; Jens Roesner; Harinderjit S Gill

The University of Bath and the Royal United Hospital in Bath, UK, are investigating dual patient ventilation, where one ventilator is used to ventilate two patients simultaneously. To succeed, we believe it is essential to independently control the gas flow (tidal volume) that each patient receives. A possible solution is to use a flow restrictor in the line of a patient who would otherwise receive too much pressure or flow, resulting in barotrauma or volutrauma. Such a restrictor should ideally be adjustable. We have presented experimental results using a pair of test lungs showing that restricting the flow in one inspiration line does indeed reduce the tidal volume in the corresponding lung. Moreover, using a linear resistance-compliance network model, we have shown that the change of tidal volume can be predicted. In the five tests presented, the largest prediction error was 31 mL of tidal volume. In the form used, the model just needs an airway resistance and lung compliance estimate for each patient, and a resistance and compliance value for the ventilator tubing system. Likewise, an added flow restrictor should be characterised by a linear resistance i.e. a pressure drop proportional to flowrate. The good performance of the model is despite clear linearization errors for the two flow restrictors used in this study. All the parameter values used have been informed by individual component testing.