Discussion
In the principle of forced oscillation, rectangular electric pulse is generated by pulse generator and superimposed on the resting breath of the subject. Through Fourier transform and spectrum analysis, different respiratory impedance values (viscous resistance, elastic resistance, and inertial resistance) at different frequencies can be calculated. The impulse oscillation method (IOS) can detect airway resistance and calculate compliance by using sound waves with frequencies from 5 to 35 Hz. Compared with conventional ventilation, IOS is a simple, no special cooperation lung function test method, easy to use, more suitable for the elderly, critically ill patients and children who are not easy to cooperate with the crowd, especially 3-5 young children[5-7]. IOS can measure the resistance and reactance at different frequencies, so it can provide important information of different areas of the lung. According to the frequency dependence and reactance of resistance, it can determine the location and severity of airway obstruction, and can be used to monitor the pathophysiological changes of the respiratory system and patients with asthma and chronic obstructive pulmonary disease, it can replace the conventional ventilation to assist the diagnosis and follow-up of asthma[8-10].
Reference values are essential for interpreting IOS. However, there are few studies to determine the prediction equation of IOS parameters in the world. Most of the existing prediction equations are based on the data of white children of European origin. The results of lung function are the same as other methods, because of the differences of race, region, geographical location and environment, the parameters of lung function are also different. For example, the relevant research reports in Beijing, Guangzhou, Chengdu, and other regions in China found that some parameters were significantly different from the foreign parameters provided by the instrument, and there were also differences in different regions in China [11-12]. Therefore, it is urgent to establish the reference values of children’s pulse lung function in China, to provide the basis for the rational application of IOS.
In this study, 6270 healthy children aged 4-17 years old and 100-180cm in height from 20 Regions and 24 research centers in China were selected as subjects. The results showed that gender had no significant influence on the measurement of IOS, and age, weight and height were closely related to IOS values, among which height was the most important. The second is age, and weight has the weakest effect. With the increase of age and height, the total respiratory impedance and airway resistance gradually decreased, while the negative value of elastic resistance decreased (increased). During the growth and development of children, with the growth of height and age, vital capacity increases and airway diameter widens. The airway resistance was inversely proportional to the fourth order of airway radius. The airway resistance of older children was significantly lower than that of the younger children, especially the decrease of peripheral resistance. With the increase of vital capacity, the elastic retraction force of lung increases, and X5 increases with age. Fres is more sensitive to assess lung function, and its fluctuation range is also large. It reaches as high as 24Hz at the age of 4 years, and decreases to 14Hz at the age of 17 years, which tends to be about 10Hz in adults. It indicates that Fres decreases dynamically with age, and this result was similar to the result of Zheng Jinping [11,12].
The European lechtenboerger prediction is widely used in China, but it is found that the correlation between the respiratory function parameters and ethnic differences is obvious, so the equation is not suitable for the evaluation of healthy people in China, especially children. The results of this study suggest that height is the most important factor for IOS parameters, followed by age and weight. With the increase of age, height and body mass, airway resistance (Rrs) gradually decreases, Fres, which represents lung compliance, and electrical resistance (Xrs) increases gradually.
There were some limitations in the study. This study is a multi-center study and the results of each center are different. Considering of the difficulty for recruiting the healthy non-smokers aged<3 years, the subjects within such age range were excluded. However, the reference values for Chinese aged 4 to 17 years met the most demands of clinical practice. In addition, IOS measurements did not include area of reactance (AX), which is considered as a useful parameter in the evaluation of children with asthma [13,14]. Therefore, these issues will be concerned in future studies.