Changes in airway resistance and its correlation with disease severity in children with Mycoplasma pneumoniae pneumonia

Introduction

Impulse oscillometry (IOS) is a non-invasive method that does not require any active effort to detect airway resistance and can provide rich respiratory physiological indicators such as viscous resistance, elastic resistance, and inertial resistance. It is widely used to evaluate airway patency and airway function (1). IOS has the advantages of low patient compliance requirements, non-invasiveness, ease of use, and repeatability. It is particularly suitable for preschool children, elderly patients, and those who cannot perform or have contraindications to lung volume measurement tests. Moreover, in terms of detecting small airway dysfunction, IOS has similar or even higher sensitivity than pulmonary ventilation function (2).

Mycoplasma pneumoniae (MP) is the main pathogen causing upper and lower respiratory tract infections in humans, and is one of the main pathogens of community-acquired pneumonia (CAP), among children aged over 5 years, up to 40% of CAP cases are caused by MP (3). In recent years, the incidence of severe MP pneumonia (SMPP) has gradually increased, due to children’s immature immune systems. The outcomes of such cases vary greatly; critically ill children may suffer from pulmonary infection and even multiple organ dysfunction, which can seriously threaten children’s health (4).

Studies have shown that the degree of lung function impairment caused by SMPP and mild MP pneumonia (MMPP) differs (5,6). MP infection can lead to airway mucosa damage and increased airway reactivity, which in turn can trigger airway inflammatory edema, airway wall structure destruction, and airway secretion obstruction. These changes reduce the compliance of the lungs, thereby causing a decrease in pulmonary ventilation function (7).

The effective and timely control of MP infection can improve the wheezing attacks and airway hyperresponsiveness caused by MP infection, shorten the course of the disease, and prevent damage to lung function caused by MP. Therefore, the early diagnosis of SMPP in children is crucial to avoid potential long-term lung function impairment. Pulmonary function tests may be used to predict the risk of SMPP.

To date, few studies have examined the correlation between airway resistance and the severity of MP pneumonia (MPP). Therefore, this study aimed to investigate airway resistance in children with MMPP and SMPP to explore airway function in children with different MPP severity to provide clinicians with a more accurate and effective diagnostic basis, and thus to enable the early identification and intervention of SMPP, and improve treatment effects and the quality of life of children. We present this article in accordance with the STARD reporting checklist (available at https://tp.amegroups.com/article/view/10.21037/tp-2025-103/rc).

Methods

Patients

The data of 96 children diagnosed with MPP, admitted to Chengdu Women’s and Children’s Center Hospital from December 2022 to December 2023, were retrospectively collected. The children were allocated to the MMPP group (n=39) and the SMPP group (n=57) based on disease severity. All the enrolled children underwent IOS testing in the morning following their admission. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Ethics Committee of Chengdu Women’s and Children’s Central Hospital [No. 2022(40)] and informed consent was obtained from the patients’ legal guardians.

Inclusion and exclusion criteria

Inclusion criteria

According to the expert consensus on the diagnosis and treatment of MPP in children (2015) and the guidelines for the diagnosis and treatment of CAP in children (2019 version) (8,9) the inclusion criteria are as follows: (I) single serum MP antibody ≥1:160 (a four-fold or greater increase in the double serum MP antibody titer during the course of the disease); or (II) positive MP-DNA or RNA or positive MP-RNA in alveolar lavage fluid. MPP can be diagnosed if any one of the above two criteria is met, and there are clinical manifestations and/or imaging changes of pneumonia at the same time.

Exclusion criteria

Children were excluded from the study if they met any of the following exclusion criteria: (I) had a history of chronic inflammation of the respiratory system, a defective or suppressed immune system, or chronic diseases of other organs; (II) had recently been treated with immunomodulators or inhibitors; (III) had underlying diseases, or had suffered from a major disease in the past; and/or (IV) had missing medical records.

Grouping criteria

The children were allocated to the MMPP group if they met the diagnostic criteria for MPP. The children were allocated to the SMPP group if they met the diagnostic criteria for MPP, and also presented with any of the following: (I) a poor general condition; (II) food refusal, or signs of dehydration; (III) impaired consciousness; (IV) dyspnea (e.g., groaning, nasal flaring, and the triple concave sign); (V) finger pulse oxygen saturation ≤92% in a resting state; (VI) lung infiltration of more than two-thirds on one side, multilobar pulmonary infiltration, or moderate to large pleural effusion on chest X-ray or computed tomography; (VII) extrapulmonary complications.

IOS test

Pre-measurement preparation

The following procedure was employed: (I) record each patient’s gender, date of birth, weight, and height; and (II) determine the medication history and basic medical history of each patient; and ensure bronchodilators had been discontinued on the day of the examination.

Measuring instrument

The lung function meter from Jaeger, Germany was used to perform the IOS test.

Measurement posture and process

The patient was instructed to sit in a straight position, straighten their body, lift up their chest, keep their head horizontal, clamp the nose clip, and bite the mouthpiece firmly with their lips to avoid air leakage. The patient was also instructed to press their palms or four fingers of each hand to their cheeks, and place their thumbs under their jaws to support their mouth and reduce the vibration of the cheeks during the measurement process. The technical operator ensures that children aged <12 years maintain a normal respiratory frequency, steady breathing, and stable baseline. Each measurement lasts ≥16 seconds, is conducted ≥3 times, and the average of three valid measurements is taken as the result.

Data collection

The following data were recorded: hospital length of stay, intensive care unit (ICU) admission rates, utilization rate of mechanical ventilation, respiratory resistance at 5 Hz (R5), respiratory resistance at 20 Hz (R20), reactance at 5 Hz (X5), and resonant frequency (Fres).

Statistical analysis

The data were analyzed using SPSS 27.0. The count data are described as the frequency and percentage, and differences between groups were compared using the Chi-squared tests. The normally distributed data are described as the mean and standard deviation, and differences between groups were compared using the independent samples t-test. Receiver operating characteristic (ROC) curves were plotted, and the area of the lower part of the area under the curve (AUC) was used to assess predictive accuracy. The best cut-off value, positive predictive value (PPV) and negative predictive value (NPV) were calculated. A P value <0.05 was considered statistically significant.

Results

Demographics

A total of 96 children with MPP completed the IOS pulmonary function test. The MMPP group comprised 39 (19 males and 20 female) children with an average age of 4.88±1.59 years. The SMPP group comprised 57 children (26 male and 31 females) with an average age of 5.35±0.89 years. There were no significant differences between the two groups in terms of the average age, gender, weight, and height (P>0.05). The children in the SMPP group had a significantly longer hospital length of stay than those in the MMPP group, and a significantly higher proportion of children were admitted to the ICU and used mechanical ventilation in the SMPP group than the MMPP group (P<0.05) (Table 1).

IOS indicators

The IOS indicators were compared between the MMPP and SMPP groups. The average levels of R5, R20, X5 and Fres in the SMPP group were 150.73%±33.51%, 99.37%±17.08%, 134.59%±48.77%, and 24.92±3.52 Hz, respectively, significantly higher than those in the MMPP group (105.48%±17.30%, 80.61%±11.92%, 104.40%±33.74%, and 23.15±3.18 Hz, respectively; P<0.05) (Table 2).

Correlation with severity

The results showed that R5, R20, X5, Fres, and the length of hospital length of stay were positively correlated with the severity of the disease (r=0.62, 0.52, 0.33, 0.25, and 0.36, respectively), and the differences were statistically significant (P<0.05, Table 3).

ROC curve analysis

The ROC curve analysis showed that R5 had the highest predictive value for SMPP (AUC: 0.89, sensitivity: 82.46%, specificity: 87.18%, PPV: 90.38%, and NPV: 77.27%), followed by R20 (AUC: 0.81, sensitivity: 73.68%, specificity: 84.62%, PPV: 87.50%, and NPV: 68.75%), X5 (AUC: 0.70, sensitivity: 61.40%, specificity: 71.79%, PPV: 76.09%, and NPV: 56.00%), Fres (AUC: 0.66, sensitivity: 78.95%, specificity: 58.98%, PPV: 73.77%, and NPV: 65.71%), and the length of hospital length of stay (AUC: 0.72, sensitivity: 78.95%, specificity: 61.54%, PPV: 70.00%, and NPV: 66.67%) (Table 4, Figure 1).

Figure 1 ROC curve of the IOS-related indicators for predicting SMPP. AUC, area under the curve; IOS, impulse oscillometry; R5, respiratory resistance at 5 Hz; R20, respiratory resistance at 20 Hz; ROC, receiver operating characteristic; SMPP, severe Mycoplasma pneumoniae pneumonia; X5, reactance at 5 Hz.

Discussion

MP is one of the most common pathogens in children with CAP aged over 5 years. Most of MPP are self-limited, but some children require intensive care (10). The pathological manifestations of MP infection include alveolar wall thickening, inflammatory cell infiltration around the airways and blood vessels, alveolar damage with fibrinous exudation, and moderate to large amounts of macrophages and polypoid tissue in the alveolar cavity and bronchial lumen, etc. These pathological changes suggest that MMPP and SMPP can involve the large and small airways, pulmonary interstitium and alveolar cavity, which can lead to airway injury and remodeling (7,11,12).

Studies have shown that pulmonary ventilation function and IOS testing have comparable ability in assessing lung function in children, especially in those who cannot undergo pulmonary ventilation function testing (13,14). The advantage of IOS is that it can detect small airway obstruction earlier and is sensitive to the identification of small airway dysfunction (15). Both Carr et al. and Shi et al. found that IOS can sensitively detect small airway damage before wheezing symptoms and changes in pulmonary ventilation function (16,17).

In the present study, the IOS measurements revealed that the IOS-related indexes R5, R20, X5, and Fres were significantly higher in the SMPP group than the MMPP group, as was airway resistance. The main manifestation was elevated small airway resistance, and was accompanied by elevated central airway resistance. The increase in small airway resistance may be related to the weakening of the force of releasing inflammatory mediators and maintaining small airway opening after MP infection, and the lack of cartilage support in small airways, small airways are easily blocked by secretions, leading to small airway occlusion and airflow limitation, the cause of the increase in small airway resistance is consistent with the findings of Leng et al. (18). Our study found that children with SMPP had an increase in central airway resistance. Increased central airway resistance may be related to exudative inflammation, multilobar infiltration, lung solidity, and lung atelectasis in the lungs of SMPP patients. Wu et al. compared the lung function of MPP children with multilobar infiltration to those with unilobar segmental infiltration, and found that the children with multilobar infiltration had extensive lesions, more pronounced lung function impairment, and large airway damage, which is in line with the findings of the present study (19).

In the present study, we found that the X5 levels were significantly higher in the children with SMPP than those with MMPP, and lung compliance was reduced in the children with SMPP. This suggests that in addition to an increase in respiratory resistance, there is also a change in the elastic resistance of the lungs of children with SMPP. This may be related to the fact that MP infection leads to airway mucosal damage, increased airway reactivity, the release of inflammatory mediators, induced bronchial smooth muscle spasm, airway inflammatory edema, and the disruption of the wall structure, resulting in a reduction of airway diameter, a shortening of airway length, and increased airway stiffness, which collectively lead to decreased lung compliance (20).

In this study, the predictive value of IOS for MPP severity was assessed by drawing ROC curves, and the results showed that R5, R20, X5, and Fres had diagnostic value for SMPP. Among these, R5, R20, and X5 had higher diagnostic values (AUC values: 0.89, 0.81, and 0.70; maximal Youden’s indices: 0.70, 0.58, and 0.33; critical values: 123.80%, 89.60%, and 119.79%; sensitivity: 82.46%, 73.68%, and 61.40%, specificity: 87.18%, 84.62%, 71.79%; PPVs: 90.38%, 87.50%, and 76.09%, and NPVs: 77.27%, 68.75%, and 56.00%, respectively). This study confirmed that IOS is of great significance in determining the severity of MPP, and the higher the levels of R5, R20, and X5, the greater the likelihood of SMPP. Thus, IOS provides more reliable reference values for determining the severity of MPP. For children and patients who cannot cooperate with the routine pulmonary ventilation examination, the indicators of IOS can serve as a reference for assessing the severity of the disease.

This study had a number of limitations. Due to the lack of normal predictive values for R5, R20, and X5 in Chinese children, the criteria used in this study were based on European and American children; however, the lung volumes of European and American children are generally larger than those of Chinese children; thus, the normal predictive values for R5, R20, and X5 might not be fully applicable to Chinese children.

Conclusions

Increased airway resistance was more pronounced in the children with SMPP than those with MMPP. Further, the main manifestation was increased small airway resistance, and it might be accompanied by increased large airway resistance, and decreased lung compliance. R5, R20, and X5 were found to be well correlated with the severity of MPP. Clinicians should dynamically observe the pulmonary function of children with MPP, effectively and promptly control MP infection, and try to avoid long-term damage to pulmonary function caused by MPP.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STARD reporting checklist. Available at https://tp.amegroups.com/article/view/10.21037/tp-2025-103/rc

Data Sharing Statement: Available at https://tp.amegroups.com/article/view/10.21037/tp-2025-103/dss

Peer Review File: Available at https://tp.amegroups.com/article/view/10.21037/tp-2025-103/prf

Funding: This study was supported by Chengdu Women’s and Children’s Central Hospital, School of Medicine, University of Electronic Science and Technology of China (High-level Clinical Research Projects) (No. 2022LC02).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tp.amegroups.com/article/view/10.21037/tp-2025-103/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Ethics Committee of Chengdu Women’s and Children’s Central Hospital [No. 2022(40)] and informed consent was obtained from the patients’ legal guardians.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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