Novel approach to computerized breath detection in lung function diagnostics
Introduction
Breath detection (i.e. finding the spot where expiration ends and the consecutive inspiration starts) is a crucial step in pulmonary function testing (PFT). It is a starting point for the computing of various clinically significant indices, performing regression analyses or making predictions. With the increasing importance of PFT as a diagnostic tool, new methods of PFT and approaches to data analysis are required especially in infants and toddlers (i.e., uncooperative children). In this age category, precise raw data analysis is of utmost importance, as the infant PFT is very prone to technical errors. Based on our clinical experience, the current PFT algorithms suffer from severe inaccuracy, which may lead to difficult and time-consuming interpretation of results or even raw data rejection.
Although breath detection is a relatively easy task for a physician, the automated detection by a computer remains a challenge, especially in cases of severely distorted data (e.g., as a result of young patients not cooperating well, severe drift etc.). An approach to the breath detection analysis is primarily determined by the signals being measured. Usually, a time-flow signal is captured. In this situation, there exist two basic algorithms for breath detection, that have already been proposed – threshold and smoothing approach, each with numerous modifications and extensions in an attempt to achieve greater reliability and accuracy [1]. The threshold approach rejects any breath having parameters below pre-set threshold. On the other hand, the smoothing approach smooths the signal to eliminate spurious breath endings. Despite the significant progress done in this field, clinicians are still facing situations in which the measured signal is too distorted to be automatically analysed.
Multiple breath washout test (MBW) is an example of a highly sensitive method recently introduced into clinical practice [2] [3], or [4]. It offers an important insight into early stages of several chronic lung diseases [5], [6]. Moreover, it does not require active breath manoeuvres and can be performed on infants during tidal breathing. Consequently, this method may yield clinically extremely relevant information. However, it requires a special approach to raw data analysis which the current algorithms may not offer. In comparison with the conventional methods which are based solely on flow, volume and pressure measurements and estimate primarily airway resistance (e.g. bodypletysmography, tidal breath analysis etc.), MBW brings a new dimension to raw data – the gas concentration signal (, , inert gas). A current commercial software (Spiroware, Ecomedics, Duernten, Switzerland) uses concentrations only for constructing washout curves. However, this information may be also used for breath detection. The aim of our study was to design and justify a new and robust algorithm for breath detection using not only time-flow data but also the gas concentration signal. Such a breath detection algorithm can significantly outperform the current threshold-based algorithms. Moreover, its key ideas have the potential to contribute to the general design of the medical algorithms.
Section snippets
Materials and methods
Raw data from nitrogen multiple breath washout test were used as an input for the breath end analysis. Data were captured by the machine Exhalyzer D, Ecomedics, Duernten, Switzerland with software Spiroware 3.2.0, following the relevant recommendations by European cystic fibrosis society (ECFS) [7] and European Respiratory Society (ERS) [8]. The raw data were stored in. txt files with a specific structure containing time, flow, oxygen (), carbon dioxide () and molar mass (MM) signals, all
Results
All the A-files included in our testing could be successfully analysed by all the implemented algorithms. The analysis time was longer for Alg-OUR than for the threshold algorithms (1.35 0.23s vs. 0.12 0.01s, ). The manual analysis took much longer; the average analysis time was roughly estimated to be between 100 and 180s.
The two specialists in PFT working independently detected the same number of breaths in 35 out of 47 A-files (74%). In the remaining cases, differences were not
Discussion
We propose an innovative algorithm for breath detection that has similar accuracy to that of human experts. In comparison with the existing threshold based algorithms and commercial software algorithm, it exhibits a significantly higher success rate in recognition of true breaths, especially in severely distorted data. The algorithm addresses both the problems of false negative and false positive breath detection. We are convinced that its high performance is because it simultaneously uses more
Conclusion
We developed and tested a new and robust algorithm which extends the possibility to reliably detect breaths during lung function testing. It deals only with presumptions based on general human physiology with no need of arbitrarily pre-set thresholds or limitations. It can be used without modifications throughout the whole age spectrum and in different disease groups. Alg-OUR also shows higher accuracy in breath detection than the previously mentioned algorithms, even in the cases of severely
Specification and availability
Our software is written for GNU Octave. It was tested under Windows and Linux operating systems. There are no specific hardware requirements. However, processing large MBW files may take a while. The current version (with user manual and demo data files) demonstrating the aforementioned ideas is available at: http://kam.mff.cuni.cz/∼horacek/lungo.
Author's contributions
Václav Koucký contributed to the manuscript in following areas: literature search, design of the algorithm and its clinical validation - performing the study on real patients including data collection, analysis and interpretation. Jaroslav Horáček contributed to the manuscript in following areas: literature search, design of the algorithm and its software implementation. He also participated in the data interpretation. He created all the Figures. Milan Hladík contributed to the literature
Conflicts of interest
Jaroslav Horáček, Václav Koucký and Milan Hladík have nothing relevant to disclose.
Acknowledgements
We thank to all the patients and their legal representatives for the participation in the study. We thank to prof. Petr Pohunek, MD., Ph.D. from Department of Paediatrics, 2nd Faculty of Medicine, Charles University for medical consultations and Petra Vančurová from Department of Paediatrics, 2nd Faculty of Medicine, Charles University for the support during measurement. We are grateful to John Wilson and James Cragg from the Department of Languages, 2nd Faculty of Medicine, Charles University
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