Development Of Portable End - Tidal Carbon Dioxide Monitor

Objective: Arterial blood pressure carbon dioxide(Pa CO2) can be considered as a direct indication of the physical breathing status, but the indication is invasive and non-continuous. Invasive operation not only increases the pain of patients, the probability of occurrence of related complications and the workload of medical staff, but also can’t satisfy the clinical demand for real-time, continuous and dynamic monitoring of physiological parameters. At present, PETCO2(end-tidal CO2 pressure, PETCO2) monitoring can be a proxy index for Pa CO2 to noninvasive and continuous measurements at the end of the expiratory pressure of carbon dioxide. PETCO2 is one of the important respiratory monitoring parameters. The noninvasive and continuous respiratory monitoring parameter has very extensive clinical application. The capnometry devices are dominated by foreign manufacturers, only a few domestic manufacturers. As the labor costs and the related foreign technology costs are high, the corresponding equipment has a high price. So some relevant equipment and module only be used in the ICU or operating room of the big hospital, far from common. This technology as a medical auxiliary diagnosis ensures medical safety. But the use of this technology in domestic is limited. Unlike ECG, blood oxygen, noninvasive blood pressure and other parameters, PETCO2 does not make the clinical testing as routine surveillance content, so it is not widely used. Therefore, in order to spread out this technology and reduce the cost, the urgent need to research and develop a portable and cheap capnometry monitor, benefit the masses of domestic patients.Methods: Based on a large number of literature research and previous studies, a lightweight and portable set of measurement with high accuracy which can be used in the pre-hospital transfer is developed for carbon dioxide concentrations monitoring. The non-dispersive infrared method(NDIR) is used to measure CO2. The principle of design is based on the Lambert Beer’s law, using the probe to detect the infrared light attenuation to calculate the concentration of CO2. ARM chip ADu C7026 is chosen as the master chip of main control unit, which includes data storage circuit and key and LCD display interface circuit. The infrared light source IRL715 and the detector TP2534 is chosen for gas chamber design. The dual channel probe can realize the design of a single light source but two light paths. This design can effectively reduce the measuring error which is caused by the light source and detector and can improve the accuracy of measurement. Infrared detector receives the light signals and temperature signals and converts them into electrical signals. The electrical signals are amplified by the preamplifier and carried out A/D conversion, then are input into microprocessor for analysis. At last, PETCO2 waveform and the measured values can be shown, and the patient’s respiratory rate can be calculated according to the waveform characteristics.Contents: This paper includes the following work:①. System hardware design and implementation. In order to improve the stability and precision of the whole system, the infrared light source, gas chamber and infrared detector components are selected and designed. The suitable operational amplifier is selected to amplify the weak signal. The SD storage interface circuit is designed to facilitate the storage testing data. The suitable LCD display module shows the test results on the screen. The buzzer is used for overrun alarm. And also a high-efficiency power management circuit is designed.②. System software algorithm design. The signal sampling and the data processing program are designed. Based on threshold judgment combined with calibration results, the method of adaptive tracking is adopted for baseline tracking. The PETCO2 valve is obtained by the data feature extraction program, using the data from the sample after processing. Accurate processing waveform data is used to accurately extract the breathing rate. Carbon dioxide waveform is plotted from sample data. The human-computer interaction interface is designed. The function of SD card storage is achieved.③. System reliability evaluation. The laboratory evaluation and clinical control experiment are conducted, evaluating the range and accuracy of CO2 concentration and respiration rate and system response time of the monitoring system. At last the prototype mold is compared with existing product.Results: This system has carried on the laboratory evaluation and clinical control experiment after many debugging. The results of statistical analysis show that the detection range of the CO2 concentration is 0-99 mm Hg and the error rate is less than 5%. Breathing rate error is not more than 2 BPM, with detection range up to 150 BPM. Response time of the system is 19.5 seconds and response time of the infrared receiving device is 25 microseconds. The prototype experiment shows that the main indicators of the system have reached the requirement of clinical application.Conclusions: This portable equipment can provide patient’s PETCO2 value and respiratory frequency, and continuously display the results by numerical value and waveform. The device can display the expiratory CO2 partial pressure of the human body quantitatively, can rapidly and accurately judge the breathing, circulation and metabolism condition of patients. The equipment is easy to use, light and portable. It’s suitable for the patient’s physical condition monitoring during emergency transport process. Meanwhile, waveform figure can continuously display waveforms and change trends of CO2, which is beneficial to the further analysis of the patient breathing, ventilation and perfusion. There are bright prospects in clinic application for the system.