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Differences in the Measurement of Functional Residual Capacity Between Body Plethysmographs of Two Manufacturers
Authors Alter P , Orszag J, Wouters EF, Vogelmeier CF, Jörres RA
Received 21 February 2022
Accepted for publication 9 June 2022
Published 24 June 2022 Volume 2022:17 Pages 1477—1482
Checked for plagiarism Yes
Review by Single anonymous peer review
Peer reviewer comments 4
Editor who approved publication: Dr Richard Russell
Peter Alter,1 Jan Orszag,2,3 Emiel FM Wouters,4,5 Claus F Vogelmeier,1 Rudolf A Jörres2
1Department of Medicine, Pulmonary and Critical Care Medicine, Philipps-University of Marburg (UMR), Germany, Member of the German Center for Lung Research (DZL), Marburg, Germany; 2Institute and Outpatient Clinic for Occupational, Social and Environmental Medicine, University Hospital, LMU Munich, Comprehensive Pneumology Center Munich (CPC-M), Member of the German Center for Lung Research (DZL), Munich, Germany; 3Department of Anaesthesiology, University Hospital, LMU Munich, Munich, Germany; 4Department of Respiratory Medicine, Maastricht University Medical Centre, Maastricht, Netherlands; 5Ludwig Boltzmann Institute for Lung Health, Vienna, Austria
Correspondence: Peter Alter, Department of Medicine, Pulmonary and Critical Care Medicine, Philipps-University of Marburg (UMR), Baldingerstrasse, Marburg, 35033, Germany, Email [email protected]
Abstract: Body plethysmography is a fundamental method for the assessment of static lung volumes. Although equipment is provided by several manufacturers, there are no established cross-vendor tools for quality control. In the process of quality control and analysis of data on hyperinflation from the large COPD cohort COSYCONET, hints appeared that plethysmographs of different vendors might yield different values for static lung volumes. Functional residual capacity (FRC) differed about 0.67 litres between plethysmographs of the manufacturer Vyaire and Ganshorn. Absolute differences of residual lung volume (RV) and total lung capacity (TLC) were similar. It appears undeniable that differences of this magnitude have an impact on clinical interpretation. Thus, device harmonization seems to be required.
Keywords: lung function, body plethysmography, static lung volumes, functional residual capacity
Body plethysmography is a major tool in the inventory of lung function measurements and suited for the assessment of static lung volumes, especially functional residual capacity (FRC), residual volume (RV) and total lung capacity (TLC). Its physiological basis and practical implementation have been described in detail.1 Equipment is provided by a number of manufacturers. There seem to be no independent devices for quality control to be used by all manufacturers. In view of the intricacies of the method, one has to consider the possibility that plethysmographs of different manufacturers show systematic differences in measured values.
In the multicenter COPD cohort COSYCONET (COPD and Systemic Consequences - Comorbidities Network), body plethysmography is part of assessments.2 In the process of quality control and analysis, more and more hints appeared that plethysmographs of different manufacturers located at different participating study centers might yield different values for FRC and derived parameters.3,4 This raised the question, whether suspected differences between devices could be verified, whether it reached relevant magnitude, and which factors might be responsible. If such differences should exist and exceed a certain magnitude, they might have consequences with regard to the applicability of reference values, irrespective of the use of ECSC (European community for coal and steel)5 or more recent equations.6,7
Data from COSYCONET were used. Anthropometric, clinical and functional data including spirometry (forced expiratory volume in 1 s, FEV1; forced vital capacity, FVC) and body plethysmography (FRC, RV, TLC) were obtained following standardized protocols in 30 study centres throughout Germany.2 The study protocol was approved by the Ethical Committees of all centres, and all participants gave their written informed consent. The study was performed in accordance with the declaration of Helsinki. ClinicalTrials.gov: NCT01245933.
The centres using a plethysmograph of the manufacturer Vyaire (CareFusion, Jaeger; Höchberg, Germany) provided data from 1988 COPD patients of GOLD grades 1–48 as described previously (38.1% female, mean (SD) age 65.1±8.4 years, GOLD grades 1/2/3/4: 9.4/42.6/38.5/9.5%).4 Additionally, we included 156 patients (51.9% female, mean (SD) age 64.1±9.2 years, GOLD grades: 1/2/3/4: 8.3/46.8/33.3/11.5%) from one centre using a Ganshorn (Niederlauer, Germany) plethysmograph, applying the same criteria of data validity and completeness.4 Thus, the present analysis included 2144 patients.
Analysis of baseline characteristics and comparison among both groups was performed (Table 1). As we did not measure the same patient in both plethysmographs, we than performed linear regression analyses of static lung volumes (FRC, RV, TLC), either as % predicted9 or as absolute values in litres, as well as of RV/TLC %. As previously,4 predictors were FEV1 and FVC, both as % predicted,10 sex, age, height and body mass index (BMI), and introducing an indicator variable for the type of plethysmograph. The magnitude of the respective regression coefficient would then indicate the difference between devices after adjustment for anthropometric and spirometric characteristics.
Table 1 Baseline Characteristics
Analysis of the group of 156 patients showed that the relationship between predictors, especially FEV1 and FVC, and RV/TLC % was the same as in the larger group,4 thus patients did not appear to be qualitatively different and the pooled analysis was valid. Regarding RV/TLC %, it revealed a significantly (p<0.001 each) smaller average value in the 156 patients (Δ=−6.9; 95% CI: −7.9; −5.8). The same was true for % predicted values of FRC (Δ=−20.9; 95% CI: −25.4; −16.4), RV (Δ=−34.0; 95% CI: −40.5; −27.6), and TLC (Δ=−10.2; 95% CI: −12.7; −7.7), and for the absolute values in litres of FRC (Δ=−0.67; 95% CI: −0.82; −0.53; Figure 1), RV (Δ=−0.77; 95% CI: −0.92; −0.63), and TLC (Δ=−0.61; 95% CI: −0.77; −0.46). Details are shown in Table 2.
Table 2 Results from Multivariate Regression Analyses of the Dependent Variables Given in the Top Row and the Predictors Given Below
In the present analysis we addressed the question, whether body plethysmographs of two manufacturers showed systematic differences in the measured values of static lung volumes. Indeed, there were such differences. In direct comparison, the Ganshorn device yielded lower and the Vyaire devices higher values of static lung volumes, especially functional residual capacity (FRC). These findings were obtained via regression analysis, adjusting for patient characteristics known to affect lung volumes.3 Notably, they were consistent regarding both absolute values and values in % predicted. The absolute differences of FRC, RV and TLC between plethysmographs were similar, with large overlap of confidence intervals, indicating that the 0.67 L difference in FRC translated into parallel differences in RV and TLC. As usual, these parameters were derived from FRC via consecutive breathing maneuvers involving the expiratory reserve volume (ERV) and the inspiratory vital capacity (VCin). It is clear that % predicted values differed as they depended on the volumes used for reference; they reached average values of about 21% for FRC, 34% for RV and 10% for TLC. It seems difficult to deny that differences of this magnitude are relevant, both as absolute values and as % predicted. Apart from the variability arising from the choice of reference equations, such differences are likely to affect the interpretation of plethysmographic results, potentially leading to marked over- or underestimation of hyperinflation depending on the device used.
It is unlikely that the observed differences are confined to the equipment of just the two manufacturers chosen. Their occurrence and magnitude could depend on factors involving hardware, software and the manner the shutter manoeuvre is performed. There are no obligatory calibration devices to be used by all manufacturers. We consider differences in software to be more likely and better amenable to future standardization. The shutter manoeuvre is usually performed at the end of normal tidal expiration, or shortly thereafter, with correction for the inspired volume. To our knowledge, in most plethysmographs, FRC is taken as that volume, while in others half of the tidal volume is added. While this might be adequate regarding the computation of airway resistance (Raw) from specific airway resistance (sRaw) as a value averaged over the breathing cycle, it implies a difference in FRC, and consequently in the computed Raw.1 According to our experience, actual tidal volume may be as large as 1.0 or 1.5 L, despite the fact that more shallow breathing is recommended, thus a numerical difference of 0.67 L may easily be introduced. We also were informed by manufacturers on differences in the advice given to patients prior to the shutter manoeuvre which could affect FRC by promoting changes in lung volumes. In the present analysis, we cannot resolve these issues but the fact of large differences in FRC and derived volumes is not affected. It also has to be noted that the measurements in all study centers were performed did not involve any unusual procedures and were performed in accordance with both guidelines and what we know from clinical routine procedures. Thus, they have impact for clinical practice not just the cohort study from which the data used in this analysis were taken.
Our findings reveal a source of variation in body plethysmographic results that should ameliorated by harmonization between manufacturers. Variations of the observed magnitude hamper the usefulness of body plethysmography, and we believe that clinicians and researchers should be aware of this in order to exploit the full potential of this powerful lung function tool.
Take Home Message
Body plethysmographs of two manufacturers showed marked differences in the measured values of static lung volumes that likely to have an impact on clinical interpretation. Some harmonization seems needed.
COSYCONET Study Group
Andreas, Stefan (Lungenfachklinik, Immenhausen); Bals, Robert Universitätsklinikum des Saarlandes); Behr, Jürgen and Kahnert, Kathrin (Klinikum der Ludwig-Maximilians-Universität München); Bahmer, Thomas (Universitätsklinikum Schleswig Holstein) and Bewig, Burkhard (Städtisches Krankenhaus Kiel); Ewert, Ralf and Stubbe, Beate (Universitätsmedizin Greifswald); Ficker, Joachim H. (Klinikum Nürnberg, Paracelsus Medizinische Privatuniversität Nürnberg); Grohé, Christian (Ev. Lungenklinik Berlin); Held, Matthias (Klinikum Würzburg Mitte gGmbH, Standort Missioklinik); Behr, Jürgen and Henke, Markus (Asklepios Fachkliniken München-Gauting); Herth, Felix (Thoraxklinik Heidelberg gGmbH); Kirsten, Anne-Marie and Watz, Henrik (Pneumologisches Forschungsinstitut an der Lungenclinic Grosshansdorf GmbH); Koczulla, Rembert (Schön Klinik Berchtesgadener Land); Kronsbein, Juliane (Berufsgenossenschaftliches Universitätsklinikum Bergmannsheil, Bochum); Kropf-Sanchen, Cornelia (Universitätsklinikum Ulm); Herzmann, Christian (Forschungszentrum Borstel); Pfeifer, Michael (Klinik Donaustauf); Randerath, Winfried J. (Wissenschaftliches Institut Bethanien e. V., Solingen); Seeger, Werner (Justus-Liebig-Universität Gießen); Studnicka, Michael (Uniklinikum Salzburg); Taube, Christian (Ruhrlandklinik gGmbH Essen); Timmermann, Hartmut (Hamburger Institut für Therapieforschung GmbH); Alter, Peter; Schmeck, Bernd and Vogelmeier, Claus (Universitätsklinikum Gießen und Marburg GmbH, Standort Marburg); Welte, Tobias (Medizinische Hochschule Hannover); Wirtz, Hubert (Universitätsklinikum Leipzig).
The study was based on 2741 patients recruited within the COSYCONET framework (ClinicalTrials.gov, Identifier: NCT01245933). For further information see Karch A, Vogelmeier C, Welte T, Bals R, Kauczor HU, Biederer J, Heinrich J, Schulz H, Glaser S, Holle R et al.: The German COPD cohort COSYCONET: Aims, methods and descriptive analysis of the study population at baseline. Respir Med 2016, 114:27–37.
Data Sharing Statement
The basic data are part of the German COPD cohort COSYCONET (www.asconet.net) and available upon request. The website of the network provides a detailed procedure for respective applications. The data can be obtained after submission of a proposal that is evaluated by the steering committee. Qualified requests should be addressed to: Mrs Inge Kokot, Competence Network Asthma and COPD, Philipps-University of Marburg, Email: [email protected]
All results to which the manuscript refers are documented appropriately in the text, figures or tables.
Ethics Approval and Consent to Participate
COSYCONET was approved by the ethical committees of all study centres and all patients gave written informed consent. The study was performed in accordance with the declaration of Helsinki.
We are grateful to all COSYCONET study centres, especially to all study nurses, for their excellent and enduring work in data collection, as well as to all patients who were willing to participate in this study.
All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis and interpretation, or in all these areas; took part in drafting, revising or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work.
COSYCONET is supported by the German Federal Ministry of Education and Research (BMBF) Competence Network Asthma and COPD (ASCONET) and performed in collaboration with the German Center for Lung Research (DZL). The project is funded by the BMBF with grant number 01 GI 0881, and is supported by unrestricted grants from AstraZeneca GmbH, Bayer Schering Pharma AG, Boehringer Ingelheim Pharma GmbH & Co. KG, Chiesi GmbH, GlaxoSmithKline, Grifols Deutschland GmbH, MSD Sharp & Dohme GmbH, Mundipharma GmbH, Novartis Deutschland GmbH, Pfizer Pharma GmbH, Takeda Pharma Vertrieb GmbH & Co. KG, Teva GmbH for patient investigations and laboratory measurements. J.V. is supported by the NIHR Manchester Biomedical Research Centre. The funding body had no involvement in the design of the study, or the collection, analysis or interpretation of the data.
Peter Alter declares no relevant competing interests. Jan Orszag declares no relevant competing interests. Emiel F. M. Wouters declares no relevant competing interests. Claus F. Vogelmeier declares grants or contracts to his institution from the German Ministry of Education and Science (BMBF), AstraZeneca, Boehringer Ingelheim, GlaxoSmithKline, Grifols, and Novartis, consulting fees from AstraZeneca, Boehringer Ingelheim, CSL Behring, Chiesi, GlaxoSmithKline, Menarini, Novartis, and Nuvaira, MedUpdate, payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events from Aerogen, AstraZeneca, Boehringer Ingelheim, CSL Behring, Chiesi, GlaxoSmithKline, Menarini, and Novartis, all of which are outside the scope of the submitted work. Rudolf A. Jörres declares no relevant competing interests.
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