Dynamic response measurement of clinical gas analysers

Capnographs are devices that use CO₂ from a patients´ expired breath to monitor the cardiopulmonary status during anaesthesia and mechanical ventilation. The waveforms are called capnograms. Capnographs at present are routinely calibrated via a static calibration method only. There is no specific method of dynamically testing capnographs. Since capnograms are time varying, there is a definite requirement to evaluate the dynamic response of these systems. This paper presents a technique to determine the time and frequency response of capnographs using a custom built EtCO₂ simulator system. This system is computer controlled via a C program. A portable version of the simulator system has also been developed using a microprocessor. The waveforms obtained using the simulator system are recorded and image analysis is performed using Matlab to obtain the time and frequency responses of the capnographs. This paper explains the image analysis part of the technique in detail. Measurements of several types of capnograph systems proved that older capnographs usually do not satisfy the manufacturer quoted specifications for time and frequency response. Monitoring capnographs on a routine basis is thus important and is recommended. This new technique of time and frequency response measurement can be used as part of a quality control protocol for capnographs; for commissioning and routine monitoring as well as to verify manufacturer quoted specifications in new units.