Design and implementation of a portable system predicting respiratory gas partial pressures of alveoli

Abstract

The partial pressures of oxygen and carbon dioxide in alveoli, $P_ACO_2, P_AO_2$, are very useful medical indices to monitor respiratory ability of patients. There is no method for direct measurement of $P_ACO_2, P_AO_2$. And existing method for predicting $P_ACO_2, P_AO_2$ has limitations in dynamic input handling and its size. In this study, we fabricated a compact electronic equipment that can predict $P_ACO_2, P_AO_2$ with dynamic input. Design and implementation of system is divided into hardware part and software parts. The Hardware part of this system includes a DSP (Digital Signal Processor) and a NDIR (Non-Dispersive Infra-Red) $CO_2$ gas sensor that can continuously measure the expiratory carbon-dioxide partial pressure from which we extract the end-tidal value, $P_{ET}CO_2$,. The Software parts are composed of system operating routine and computing routines. System operating routine for hardware part is designed by axiomatic approach. Computing routine is based on recently developed CBGA(Computational Blood Gas Analysis) algorithm Developed equipment performs fast floating-point calculation for prediction of $P_ACO_2$ and $P_AO_2$$ from the peripheral input data consisting of $P_IO_2$, $P_{ET}CO_2$, the pair ($P_VCO_2, P_VO_2$ : the carbon dioxide and oxygen partial pressure of venous blood gas) and another pair ($PaCO_2, PaCO_2$ : the carbon dioxide and oxygen partial pressure of arterial blood gas). In order to check the performance, animal experiment has been carried on swine with various $F_IO_2$ and RR(Respiratory Rate) values. The results are compared with the alveolar partial pressures calculated by the clinician`s bedside formula. We conclude that the present portable measurement system has exhibited academic and clinical importance for further development of ICU medical equipments.