Measurement Of Pulmonary Function Parameters Based On Bioimpedance Method

Severe air pollution and population aging lead the incidence rate of respiratory diseases in China rising year by year, now the mortality of chronic respiratory diseases accounted for16%of the disease mortality in the country.With the rapid improvement of our living standard, we pay more attention to our health conditions and daily monitoring of chronic respiratory disease. Pulmonary function test is one of the necessary examination of respiratory diseases, and it is important for early detection of lung disease and airway disease, assessment of the severity of disease, evaluation of curative effect of drugs or treatments, identification of the cause of respiratory disorders, detection of pathological areas, assessment the tolerance of pulmonary function to operation or labor intensity, as well as investigation of the role of pulmonary function in the care of critically ill patients and so on.In order to maintain our life we take in essential O2, and at the same time discharge CO2produced in metabolism process. This activity is called respiration. Air exchange between alveolar and blood called external respiration or pulmonary respiration, air exchange between blood and bistiocyte called internal respiration or cell respiration. Cell respiration also called tissue respiration, tissue respiration usually means the transport of H+metabolic in the processes of substance metabolism in the cell. In one word, respiration take blood as intermediary, transport O2to tissue and discharge CO2out of our body. When there is less O2than CO2in the lung,exhalation begin to conduct gas exchange with outside air, this process called ventilation, realization of ventilation lies on respiratory movement. The way that use curve to trace the process of respiratory movement called pneumogram. This paper is aimed at studying a new way of measurement of pulmonary respiration (or external respiration).Now the way to measure pulmonary function mainly recur to variable flowmeter: differential pressure type flowmeter, thermal flowmeter, impeller and turbine flowmeter and ultra sound flowmeter. Product based on these methods have developed comparatively mature and the measurement accuracy is comparatively high, but they are expensive. Moreover, it doesn’t meet the requirement of long time monitoring because it conducts measurement by inhaling and exhaling to flowmeter directly. It indicated that traditional pulmonary function instrument is hard to get in community and family, and can’t let us get our pulmonary function state at appropriate time and response as soon as possible. So, it is very important to develop easily control and low cost pulmonary function measurement way and instrument applicable to community and family to prevent and cure chronic respiratory disease. Based on the above requirement, this paper first present a new way of measuring pulmonary function which is based on the principle of bioimpedance, and we further develop the thoracic impedance measuring instrument as the experimental prototype.The experimental prototype takes TI’s MSP430ultra low power MCU as core, and the peripheral circuit consists of high frequency output module, voltage controlled current source, envelope detection, amplification, filtering and voltage conversion circuit to realize the pulmonary function measurement. Based on the principle of bioimpedance we extract the value change and rate change of human thoracic impedance by means of two electrodes placed on human pleural and then calculate the main parameters of lung function. When we exhale contraction of the alveolar and oppression of windpipe make gas run into bronchus, at the same time bronchus’s collapse because of oppression force gas out of our body. Along with the aggravation of bronchus collapse and the pressure of windpipe, the airflow rate is limited. The curve change of bioimpedance indicates that peak flow rate decreases along with the decrease of lung volume. It fits the physiological process of exhalation. The main parameters to be detected are as follow:(1)forced vital capacity(FVC):It means the total expiratory volume exhaled forcibly and swiftly from the maximum suction location;(2)forced expiratory volume in one second/FVC(FEV1/FVC):It means the proportion between the gas volume exhaled forcibly from the maximum inspiratory position by1second and FVC,that is FEV1%=(FEVj/FVC) x100%;(3)peak flow (PEF, peak expiratory flow):It means the fastest instantaneous velocity of expiratory flow in the process of FVC test.The basic method of measurement is as follow:using extreme value and threshold algorithm, we first determined whether the waveform has exceeded a threshold, and the threshold was obtained after many times of tests. We then determined the maxima and minima thresholds from the waveform, extracted the peaks and troughs of respiratory wave, and then calculated the FVC and FEV1/FVC. We used difference method for difference before and after the impedance value at the maximum, since each sampling interval is the same, the maximum difference here is the largest slope of respiratory wave, which can be used to calculate the value of peak flow.There are following advantages of the new system based on bioimpedance to measure lung function parameters compared to that based on traditional flowmeter method:(1)Small, light, easy to carry, and it can be used convenientlyin such limited space as community hospital or family;(2)Cheap, compared to traditional high price commercially available pulmonary function instrument the new instrument is more easily accepted by people;(3)Convenient to use, measurement can be conducted just by placing two electrode on the chest but the traditional instrument’s operating requirement is very strictly,it is hard to finish measure without guidance of professional staff.In this study we conduct an experiment carried out on23healthy people to compare the thoracic impedance instrument and MRI spirolabⅢ lung function test instrument which is measuring pulmonary function parameters based onflowmeter to test the availability of the new method, the pulmonary function parameters measured include:forcedvital capacity (FVC),forced expiratory volume in one second/FVC(FEV1/FVC) and peak expiratory flow(PEF).We did comparison statistical analysis using Bland-Altman method, we found that the differences of these parameters measured by two methods are within positive and negative two times standard deviation range of average value, thus according toBland-Altman statistical analysis method the two methods are comparable.