Reference values for respiratory system impedance using impulse oscillometry in healthy preschool children

Article information

Abstract

Introduction

Pulmonary function tests (PFTs) are important for the evaluation of respiratory problems in children. In particular, these tests allow measurement of respiratory status by monitoring changes in lung function of children with chronic lung diseases, such as asthma. In addition, PFTs can be used to help to diagnosis children with chronic lung diseases1). For the diagnosis and management of respiratory disease, reference data of healthy children are needed to establish baseline PFT results2).

Spirometric determination of forced expiratory volume in 1 second (FEV₁) and forced vital capacity (FVC) are widely used to evaluate airway obstruction, to diagnose and monitor various respiratory diseases3). However, spirometry is difficult to perform in young children due to their short attention span and limited cooperation3-5). Thus, respiratory resistance obtained by the forced oscillation technique (FOT) is commonly used in young children6, 7). The impulse oscillation system (IOS) is a commercial device used to measure oscillatory resistance at multiple frequencies3). This technique requires little patient cooperation because only tidal volume breaths are required and children as young as 2 years can be readily examined8). Thus, measurement of pulmonary resistance in preschool children can be used to diagnose and manage respiratory diseases6, 9).

IOS is widely used by pediatric caregivers for the diagnosis and management of respiratory and allergic diseases. Like other pulmonary function values, IOS parameters have different normal reference levels from different races and age groups2, 10-12). Several reports have identified normal reference levels in Korean school-aged children13, 14). However, there are no reports on reference values for preschool children, thus making it difficult to diagnose and monitor respiratory diseases in these children using IOS.

The aim of the present study was to determine reference values of IOS measurements for Korean preschool children so that clinicians can more readily diagnose and manage respiratory disease in this group of patients.

Materials and methods

Results

1. Characteristics of children

We initially recruited 639 preschool children. The parents of 630 children (98.6%) were screened using the questionnaire. A total of 497 children were excluded based on our exclusion criteria, and 133 healthy children, aged 3-6 years, were enrolled. The success rate of IOS was 89.9%, so our analysis ultimately considered data of 119 children.

Table 1 shows the characteristics of enrolled children stratified by age. A total of 56% of the patients (n=67) were boys, mean age was 4.9±0.86 years, mean height was 109.1±7.11 cm, and mean weight was 18.7±3.34 kg.

Table 1

Demographic Characteristics of Enrolled Children

2. Relationships of IOS and anthropometric data

Within-test repeatability of Rrs₅ indicated that CR was 0.175 and CV was 6.1%. Linear regression analysis of IOS measures indicated that Rrs₅ was significantly associated with height, weight, and age. Log-transformation of the IOS values and height did not improve this relationship. In addition, height had the greatest correlation with Rrs₅ (r=0.72). Table 2 shows the results of linear regression models for all IOS variables with height.

Table 2

Regression Equations for All Variables versus Height

3. IOS predictive equations for boys and girls

Table 3 shows the results of multiple linear regression equations for age, height, and weight. Height was the strongest independent variable for all IOS values. Rrs was negatively correlated with height. For boys, Rrs₅=(1.934-0.009×height), with a coefficient of determination (R²) of 12.1%. For girls, Rrs₅=(2.201-0.012×height), with an R² of 18.2%. Xrs was also positively correlated with height. For boys, Xrs₅=(-0.774+0.006×height-0.002×age), with R² of 20.2%. For girls, Xrs₅=(-0.674+0.004×height), with R² of 10.5%. Fig. 1 shows a significant negative correlation between Rrs₅ values and height for all children.

Table 3

Impulse Oscillation System Predictive Equations for Boys and Girls

Fig. 1

Correlation between height and respiratory resistance at 5 Hz (Rrs₅). The regression equation was Rrs₅ (kPa/L/s) = 1.934-0.009 × Height (cm); the coefficient of determination was 12.1%.

Discussion

In this study, we determined IOS values for healthy preschool Korean children, in whom spirometry is difficult to perform. We obtained valid IOS measurements for 89.9% of the enrolled children, a higher success rate than reported in previous studies12, 21). This discrepancy may be due to differences in inclusion and exclusion criteria, differences in IOS techniques, and/or our use of one experienced operator for all measurements. The European Respiratory Society (ERS) and the American Thoracic Society (ATS) recommend the selection of a normal population for reference values of respiration measurements, with healthy children selected by a questionnaire that screens for history of asthma and allergies and family history of allergies. Our results indicate that it is relatively easy to perform IOS in healthy children less than 6 years of age.

A description of the normal variability of IOS values is required to define clinically relevant alterations. In this study, we reported within-test variability as CR and CV. Our reported CR and CV values were similar to values reported in previous studies9, 22-24). In addition, our calculations of Rrs, Xrs, Fr, and AX using a regression equation indicated that height was the only significant predictor, also consistent with previous studies12, 23). Similar to previous studies, we found that Rrs was negatively correlated with height and Xrs was positively correlated with height6, 15, 23, 25). Interestingly, our regression coefficients of Rrs with height are strikingly similar to those published by Hellinckx et al.26), Frei et al.6) and Klug et al.24), but different from those published by Ducharme et al.12) and Malberg et al.23). In particular, our study indicated lower resistance values than several previous studies. Table 4 shows the comparison of reference equations of Rrs₅ between the present study and previous studies on healthy preschool children. This difference may be caused by differences in age distribution, and/or racial differences. A limitation of our study is that the R² was less than 20%, which is poorer than that reported in several other previous studies. This may be due to our small number of studied subjects. Comparing to previous school-aged children in Korea, Korean school studies13, 14), our regression equations at multiple frequencies were similar to their reference values, especially about the Rrs₅.

Table 4

Overview of the Regression Equations of Rrs at 5 Hz in Healthy Preschool Children

In line with other studies, we confirmed that IOS is an effective and easily performed noninvasive technique that is well suited for lung function measurements in young children. Reference values for IOS are important for the interpretation of pulmonary function test measures, and are needed for the diagnosis and management of respiratory diseases1). The measurement of respiratory input impedance using IOS has significant clinical value in children with a variety of respiratory disorders27). This study is the first to provide IOS data for healthy Korean preschool children and can be used as a basis for the diagnosis and assessment of prognosis of pulmonary function in children with chronic pulmonary disorders and asthma.

In conclusion, we have determined reference values of IOS measurements generated from a group of healthy Korean preschool children. Our IOS reference values are different from those for preschool children of other ethnic groups, and are essential for the diagnosis and treatment of various respiratory diseases. We suggest that future studies be performed with larger populations of Korean preschool children to confirm our findings.

Article information Continued