The Prognostic Performance of Lung Diffusing Capacity in Preserved Ratio Impaired Spirometry: An Observational Cohort Study

by | Feb 9, 2023 | 0 comments

Hiroaki Ogata, Kachi Sha, Yasuaki Kotetsu, Aimi Enokizu-Ogawa, Katsuyuki Katahira, Akiko Ishimatsu, Kazuhito Taguchi, Atsushi Moriwaki, Makoto Yoshida

Department of Respiratory Medicine, National Hospital Organization Fukuoka National Hospital, Fukuoka, Japan

Correspondence: Hiroaki Ogata, Department of Respiratory Medicine, National Hospital Organization Fukuoka National Hospital, 4-39-1 Yakatabaru, Minami-ku, Fukuoka, 811-1394, Japan, Tel +81-92-565-5534, Fax +81-92-566-0702, Email [email protected]

Purpose: Similar to chronic obstructive pulmonary disease (COPD), the diffusing capacity of the lung (DLCO) might be decreased and associated with poor prognosis in preserved ratio impaired spirometry (PRISm), a clinical entity as a prodromal phase of COPD. The aims of the present study were to evaluate the distributions of DLCO and to assess the association between DLCO and mortality among subjects with PRISm.
Patients and Methods: We conducted an observational cohort study at the National Hospital Organization Fukuoka National Hospital. We classified the 899 patients ≥ 40 years of age with an assessment of DLCO into five groups based on spirometry: preserved spirometry, PRISm, mild COPD, moderate COPD, and severe/very severe COPD. The prevalence of low DLCO (LCO with all-cause mortality.
Results: The prevalence of low DLCO in the PRISm group (58.8%) was significantly higher than that in the preserved-spirometry group (21.8%), the mild-COPD group (23.5%), and the moderate-COPD group (36.0%) (all P LCO group than in the preserved-DLCO group (P LCO group than in the preserved-DLCO group (HR = 10.10 (95% confidence interval 2.33– 43.89)).
Conclusion: Diffusing capacity was more impaired in PRISm subjects than in those with preserved spirometry or mild to moderate COPD. Regarding PRISm, low DLCO was a significant risk factor for all-cause mortality. Clinicians should assess DLCO in the management of PRISm to predict the future risk of overall death.

Keywords: preserved ratio impaired spirometry, chronic obstructive pulmonary disease, diffusing capacity of the lungs, all-cause mortality

Introduction

Preserved ratio impaired spirometry (PRISm), also referred to as restrictive pattern or unclassified spirometry, is a clinical entity associated with an increased risk of developing chronic obstructive pulmonary disease (COPD).1,2 Epidemiological studies worldwide revealed that 3.7–22.3% of the general population were compatible with PRISm,3–11 indicating that PRISm is a common disease condition. Considering its high prevalence and clinical aspects as a prodromal phase of COPD, PRISm, whose pathophysiology is still largely unknown, is an increasing threat to public health. Since the diffusing capacity of the lung for carbon monoxide (DLCO), a useful biomarker for evaluating the gas transfer properties of the respiratory system,12 is generally deficient in COPD,13 it might be also impaired in cases of PRISm. However, this issue has not yet been investigated.

PRISm has been demonstrated to be associated with an increased risk of premature mortality,2,3,11,14 although little is known about the predictive biomarkers for death in PRISm. Since a deficit in DLCO is a strong risk factor for a poor prognosis in COPD,15 DLCO may also be inversely associated with morbidity and mortality in PRISm. However, there has been no study assessing the influence of DLCO on mortality in subjects with PRISm; therefore, the verification of this hypothesis could be of great benefit for improving the clinical management for such patients.

Based on these considerations, we conducted the present study to evaluate the prevalence of reduced DLCO in subjects with PRISm and to compare it with that in cases of normal spirometry or COPD. We also assessed the clinical implication of DLCO impairment as a biomarker for all-cause mortality among subjects with PRISm.

Materials and Methods

Study Population

The current study was conducted as an observational cohort study through a review of medical records at the National Hospital Organization Fukuoka National Hospital. The entire cohort consisted of 899 patients ≥ 40 years of age who had a DLCO assessment from June 1, 2017, to May 31, 2020, regardless of department, with complete information on all relevant covariates. We classified the subjects into five groups based on the results of spirometry: preserved spirometry, PRISm, mild COPD, moderate COPD, and severe/very severe COPD. PRISm was defined as the coexistence of two major criteria: (i) post-bronchodilator forced expiratory volume in 1 second to forced vital capacity (post-BD FEV1/FVC) ≥ 70%; and (ii) reduction in FEV1, that is, post-BD FEV1 per predicted (FEV1% pred) < 80%.3 According to the Global Initiative for Chronic Obstructive Lung Disease (GOLD) criteria,16 the severity of COPD was defined as follows: mild, FEV1% pred ≥ 80%; moderate, 50% ≤ FEV1% pred < 80%; severe and very severe, FEV1% pred < 50%.

The distribution of DLCO per predicted (DLCO % pred) and the prevalence of low DLCO by lung function category were evaluated as a cross-sectional analysis using the total cohort. With regard to PRISm, we further investigated the association of low DLCO with all-cause mortality using 111 patients (the PRISm cohort), excluding 8 patients with no follow-up data. For each case in the PRISm cohort, mortality data were collected from June 1, 2017, to December 31, 2021.

Assessment of the Diffusing Capacity and Transfer Coefficient of the Lungs

DLCO and DLCO per alveolar volume (DLCO/VA) were measured via the single-breath method using a CHESTAC-8900 spirometer (Chest MI, Inc., Tokyo, Japan) in accordance with the American Thoracic Society/European Respiratory Society (ATS/ERS) guidelines.17 DLCO % pred and DLCO/VA % pred were calculated using the predicted values of DLCO and DLCO/VA for a person of the same age, gender, and body surface area.18 In accordance with the clinical review article,13 low DLCO was defined as DLCO % pred < 80%. In the same manner, low DLCO/VA was defined as DLCO/VA % pred < 80%. When dividing the PRISm cohort into three groups based on the tertile distribution of DLCO % pred, the cut-off values were as follows: lowest, < 64.0%; middle, 64.0–85.4%; and highest, ≥ 85.5% for DLCO % pred.

Clinical Evaluations

For each case, respiratory physicians reviewed the patient’s medical records and assessed the demographic and clinical characteristics: age, gender, height, weight, smoking exposure, and spirometry. Body mass index (BMI; kg/m2) was calculated as weight divided by height squared. Taking into consideration the guidelines for diagnosing obesity in Japanese subjects,19 obesity, normal weight, and underweight were defined as BMI ≥ 25.0 kg/m2, 18.5 to < 25.0 kg/m2, and > 18.5 kg/m2, respectively. Spirometry was performed before and 15 minutes after BD administration (ie, 200 µg of salbutamol), in line with the guidelines of the Japanese Respiratory Society,20 using CHESTAC-8900. Bronchial reversibility was defined as ≥ 12% and ≥ 200 mL reversibility in post-BD FEV1. The predicted values of FEV1 and slow vital capacity (SVC) for a person of the same age, gender, and height were estimated with the equation for the Japanese population.21

Statistical Analysis

R software version 4.1.2 (R Foundation for Statistical Computing, Vienna, Austria) was used to perform all statistical analyses. Two-sided P < 0.05 was considered to indicate statistical significance. For baseline characteristics, the heterogeneity in each variable among the lung function categories was evaluated using the analysis of variance (ANOVA), chi-square test, or Kruskal–Wallis test. Tukey’s test, logistic regression analysis, or the Mann–Whitney U-test with Bonferroni correction was used to assess the statistical difference between the PRISm group and any of the other groups. The heterogeneity in the distribution of DLCO % pred and DLCO/VA % pred across the lung-function categories was also analyzed using an ANOVA. Stratified analysis was performed according to BMI levels or smoking status. The prevalences of low DLCO and low DLCO/VA were calculated for each lung-function group and compared using unadjusted and multivariable-adjusted logistic regression models and estimated as odds ratios (ORs) with 95% confidence intervals (95% CIs). Adjustments were made for age, gender, BMI, and smoking exposure. We performed a sensitivity analysis after excluding subjects with bronchial reversibility in order to exclude asthmatic patients. Kaplan–Meier curves were constructed to show the overall survival by the levels of DLCO % pred and DLCO/VA % pred. Log rank testing was performed to study the influence of low DLCO and low DLCO/VA on all-cause mortality. The trend in overall survival according to the tertile groups of DLCO % pred was assessed with a Cox proportional hazards model. The multivariable-adjusted hazard ratios (HRs) with their 95% CIs of each level of DLCO % pred or DLCO/VA % pred for all-cause death were estimated using a Cox proportional hazards model adjusted for all aforementioned potential confounders. The same model was used to assess the linear trends in the risk of all-cause death across the tertile groups of DLCO % pred.

Ethical Considerations

The study was approved by the National Hospital Organization Fukuoka National Hospital Institutional Review Board for Clinical Research (#F4-2). The study was conducted in accordance with the Declaration of Helsinki. Informed consent was waived due to the retrospective nature of the study. All the data was anonymized for covering the patient data confidentiality and participant privacy.

Results

Level of DLCO % Pred and Prevalence of Low DLCO for Each Lung-Function Group

The prevalence of PRISm was 13.2% among the entire cohort. Table 1 lists the demographic and clinical characteristics of the total cohort. Among the five lung function groups, the prevalence of male gender, the mean age, the mean BMI, and the amount of smoking exposure were not prominent in the PRISm group. Meanwhile, the mean values of absolute SVC and SVC % pred were lowest in the PRISm group.

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Table 1 Demographic and Clinical Characteristics of Study Subjects by Lung Function Category

Figure 1 shows the level of DLCO % pred for each lung-function group. The mean values of DLCO % pred were 98.4%, 73.9%, 99.3%, 89.4%, and 71.0% in the preserved-spirometry group, the PRISm group, the mild-COPD group, the moderate-COPD group, and the severe/very severe-COPD group, respectively. In the PRISm group, DLCO % pred was significantly lower than in the other lung function groups except for in the severe/very severe-COPD group. Likewise, the prevalence of low DLCO was significantly higher in subjects with PRISm than in those with preserved spirometry, mild COPD, or moderate COPD (all P < 0.01); it was as high as about 60% in the PRISm group and the severe/very severe-COPD group (Figure 2). The results were substantially similar after adjustments for potential confounders; there was a significant increase in OR in the PRISm group as compared to the preserved-spirometry group, the mild-COPD group, and the moderate-COPD group (all P < 0.01) (Table 2). Broadly similar results were obtained in the analysis stratified by BMI levels (Supplementary Figures S1 and S2) or smoking status (Supplementary Figures S3 and S4). The results were not substantially changed after excluding asthmatic subjects (Supplementary Figures S5 and S6). Further, there was no significant decrease in the level of DLCO/VA % pred or increase in the prevalence of low DLCO/VA in the PRISm group compared to the group with preserved spirometry, mild COPD, or moderate COPD (all P > 0.05) (Supplementary Figures S7 and S8).

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Table 2 Multivariable-Adjusted Regression Analysis for Low DLCO in the Patient Groups Based on FEV1 Value

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Figure 1 Level of DLCO % pred by lung-function group.

Abbreviations: DLCO % pred, diffusing capacity of the lung…

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