A Simple Method to Estimate Flow Restriction for Dual Ventilation of Dissimilar Patients: The BathRC Model

Abstract

With large numbers of COVID-19 patients requiring mechanical ventilation and ventilators being in short supply, in extremis two patients are having to share one ventilator. This possibility has been discussed for at least two decades, and careful matching of patient ventilation requirements is advised. However, with a large range of lung compliance and other characteristics, which may also vary with time, good matching is difficult to achieve. Adjusting the impedance of the flow path between ventilator and patient gives the opportunity to control the airway pressure and hence flow and volume individually for each patient. Several groups are now investigating this, in particular the addition of a flow restrictor in the inspiration tube for the patient who is more compliant, or requires a lower tidal volume. In this paper, we show that a simple linear resistance-compliance model, termed the BathRC model, of the ventilator tubing system and lung allows direct calculation of the relationships between pressures, volumes, and required flow restriction. The BathRC model is experimentally validated using a GE Aisys CS2 ventilator connected to two test lungs. The pressureflow relationships for two restrictors are experimentally determined, and despite the need to approximate them with a linear resistance characteristic, their effect in the breathing circuit is accurately predicted by the simple model. The BathRC model is freely available for download; we do not condone dual ventilation, but this tool is provided to demonstrate that flow restriction can be readily estimated. This research is part of a larger test, simulation and design investigation on dual ventilation being undertaken at the UoB and RUH.

Individualized mechanical ventilation in a shared ventilator setting: limits, safety and technical details.

“…The tidal volumes per patient can then be measured or calculated, and the pressure on the ventilator set so that the desired tidal volume is achieved for the patient with the least compliant lungs [12]. Subsequently, by restricting the flow to the patient with the more compliant lungs, the tidal volume is limited for that person not to exceed a set level.…”

Individualized mechanical ventilation in a shared ventilator setting: limits, safety and technical details.

“…The tidal volumes per patient can then be measured or calculated, and the pressure on the ventilator set so that the desired tidal volume is achieved for the patient with the least compliant lungs. [12] Subsequently, by restricting the ow to the patient with the more compliant lungs, the tidal volume is limited for that person not to exceed a set level.…”

Simulink Modelling For Simulating Intensive Care Mechanical Ventilators

“…In Galileo [9], when this additional function is activated, the directional on/off valve (21) is opened. Afterwards, the compressor (22) increases the pressure of the gas taken from the tank (up to 900 mbar gauge), which can enter a nebulizer jar (23) to blend with the medicine to be supplied to the patient. This method has the advantage of not disrupting the air/oxygen ratio of the gas that Galileo delivers to the patient [9].…”

“…The values of the lung compliance of patients affected with COVID 19 were recently investigated in [22], resulting in an average respiratory system compliance of 50.2 ± 14.3 ml/cmH2O, associated with a shunt fraction of 0.50 ± 0.11 [22]. The possibility of using one ventilator to feed two patients simultaneously was recently investigated in [23]; the authors of [23] came up with a new method, based on flow restriction, to achieve that task. Another very useful study was performed in [24], where the trachea of intubated patients was simulated using three-dimensional computational fluid dynamics (CFD) analysis, allowing a very accurate prediction of the pressure losses in the trachea.…”