Validation and optimization of a membrane system for carbon dioxide removal in anesthesia circuits under realistic patient scenarios

The performance of a membrane-based device to remove carbon dioxide from anesthesia circuits was investigated experimentally and through numerical simulation for realistic patient scenarios. A model for a mechanical ventilator circuit was developed and coupled with a previously developed membrane model to facilitate dynamic simulation. For experimental validation, the hollow fiber membrane module was incorporated into a system having all components of a typical anesthesia circuit. The response of the patient was included using a lung simulator and carbon dioxide injection. The investigated patient cases were defined by determining the 25th, 50th, 75th and 95th percentile of minute volume from 25 000 clinical ventilation case datasets. Agreement between the experimental measurements and numerical predictions was assessed by comparing the carbon dioxide concentrations leaving the retentate and permeate sides of the membrane module. Given the uncertainties in the inputs to the system, measurement devices used, and modelling approach, agreement between the measured and predicted concentrations was relatively good. The carbon dioxide concentration in the retentate stream, which would be rebreathed by the patient, was strongly influenced by the minute volume, ventilation frequency, and sweep gas flow rate. The 95th percentile patient scenarios were used to establish a relationship between the combination of membrane area and sweep gas flow rate required to achieve the desired design criterion of maintaining an inspired carbon dioxide concentration of at most 0.5 vol% in these extreme cases. The results suggest that a membrane area between 3.7 and 6.5 m² is necessary to achieve the desired performance with reasonable sweep gas flow rates between 15 and 30 L min⁻¹. However, a surface area of 4.2 m² was determined to provide a reasonable compromise between membrane size, gas flow rate, and flexibility in operation.