Design And Realization Of System For ETCO₂ Monitoring

End-tidal carbon dioxide?ETCO₂?monitoring has become an important tool in anesthesia operation,intensive care and other clinical fields.ETCO₂ has been listed by the American society of anesthesiologists?ASA?as one of the routine intraoperative monitoring project in 2005.Therefore,research of an ETCO₂ detecting system which is simple,economic and practical would be of great significance.In this study,a type of mainstream capnometer based on the principle of non-dispersive infrared is designed.Depending on the sampling method,mainstream technology is applied for carbon dioxide monitoring.With a mainstream capnometer,the concentration can be measured directly at the patient-ventilator interface without drawing any gas from the airway,thereby an immediate and precise measurement can be realized without delay.In the measurement mode,single IR source and dual wavelength measurement method is adopted,which can eliminate the influence of optical interference and unsteadiness of the components.According to the experimental principle and measurement method,the measurement formula is improved,which makes the measurement process simple.By choosing key devices and designing pipe adapter,the integral design of the system is completed.Then the hardware circuit is designed,which includes modules like power,light source,detector,communication,microprocessor and so on.The information of the end expiratory gas carbon dioxide concentration is contained in the modulation signal,so it is necessary to choose appropriate sampling window and sampling interval time to obtain the peaks and valleys of the voltage amplitude of the sampled signal in the modulation system.The sampled signals are filtered by using the limited sliding filter and the peak filter respectively.Due to the non-uniform distribution of the transmittance of the bandpass filter in the whole transmission band,the modulation signals of the initial two channels are compensated.Finally,the system can display the respiratory waveform and the error is small,so the system can be used for continuous real-time monitoring of respiratory carbon dioxide concentration.