Design and implementation of an end tidal CO2 simulator for the evaluation of clinical gas analysers

This paper presents a new technique to determine the dynamic and frequency response of capnographs using a custom built 'EtCO2 simulator system'. Capnographs are devices that use CO2 from a patients' expired breath to monitor the cardiopulmonary status during anaesthesia and mechanical ventilation. Capnographs at present are routinely calibrated via a static calibration method only. The dynamic response of the capnographs is not accounted for. The frequency and time response are important as they determine if capnographs can be used in high frequency and pediatric ventilation schemes. Experiments performed using the method described in this paper proved that old capnographs usually do not satisfy the manufacturer quoted specifications for time and frequency response. Therefore, a routine check for capnographs is recommended. The method can also be used to verify manufacturer quoted specifications. The EtCO2 simulator system, designed and constructed simulates human respiration cycle. The gas sources used are 5% CO2 and room air that can be switched alternatively. Both supplies are pressure regulated and connected through non-return valves to electric valves. The valves are microprocessor controlled and the on/off time is user defined allowing a wide range of waveforms to be simulated. The output from the simulator is delivered to capnograph. Capnograms are captured by digital video recording. The captured video in 'avi' file format is then converted into individual frames. These frames are converted into digital data through image processing in Matlab. The data obtained is subjected to extensive analysis to determine the frequency and time response of the respective capnograph.