A bioacoustic method for timing of the different phases of the breathing cycle and monitoring of breathing frequency

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Abstract

It is well known that the flow of air through the trachea during respiration causes vibrations in the tissue near the trachea, which propagate to the surface of the body and can be picked up by a microphone placed on the throat over the trachea. Since the vibrations are a direct result of the airflow, accurate timing of inspiration and expiration is possible. This paper presents a signal analysis solution for automated monitoring of breathing and calculation of the breathing frequency. The signal analysis approach uses tracheal sound variables in the time and frequency domains, as well as the characteristics of the disturbances that can be used to discriminate tracheal sound from noise. One problem associated with the bioacoustic method is its sensitivity for acoustic disturbances, because the microphone tends to pick up all vibrations, independent of their origin. A signal processing method was developed that makes the bioacoustic method clinically useful in a broad variety of situations, for example in intensive care and during certain heart examinations, where information about both the precise timing and the phases of breathing is crucial.

Introduction

Monitoring of breathing is important in numerous clinical situations. In some cases, it is important to monitor the air volume as well as the breathing frequency. In other situations, it is necessary only to monitor the frequency, e.g. monitoring of critically ill patients in intensive care units and monitoring of neonates. Precise timing of the breathing sound is important in certain situations. One example of when the timing can be of benefit is in studying the flow in the heart, since this is modulated by breathing [2], [8], [12], [15], [20], [23], [39]. Here a timing accuracy of about 300 ms be is required [39]. Another example of using the timing of the breathing sound is during computer tomography, radiation therapy or MRI investigations, where breathing causes movements and breathing gated image acquisition is required [13], [16], [17]. The quality of the breathing sound is another parameter of interest where the breathing sound may contribute information concerning the pulmonary function, [1], [3], [4], [5], [6], [18], [31], [32], [33].

The aim of this study was to develop a bioacoustic method for monitoring of breathing and to investigate how the onset time of the breathing phases can be determined accurately. Our intention was also to characterise the tracheal sound time and frequency domains as well as the characteristics of the disturbances added to the tracheal sounds.

Section snippets

Theory

The breathing cycle is divided into four different successive phases: inspiratory phase, inspiratory pause, expiratory phase, and expiratory pause. The breathing cycle is defined here as starting with the onset of inspiration at the moment when air inflow starts. When the airflow stops, the inspiratory phase ends and the inspiratory pause begins and lasts until air begins to flow out from the lungs and the expiratory phase starts. The expiratory phase is followed by the expiratory pause, which

Equipment

To measure the tracheal sound from breathing, a microphone (Siemens Elema no. 6919328, Solna, Sweden) was used and the reference airflow at the subject's mouth was observed by a pneumotachograph with a Fleisch tube (Hugo Sachs Elektronik KG, Germany). An external microphone (Siemens Elema no. 6919328, Solna, Sweden) was used to detect disturbances. The microphone and the pneumotachograph were connected to a data acquisition board (Computer Boards Inc., Mansfield, USA, CIO DAS OS). For the

Results

The sound signal from breathing exhibits a pattern that repeats itself over subsequent breathing cycles. However, some variation in the pattern with regard to intensity and envelope can be seen, not only between different individuals but also between breathing cycles from the same individual. Fig. 3 illustrates two examples of observations of breathing sound signals m(t) over the trachea from two different subjects. The m(t) is observed during a 1-min period; apart from the data acquisition

Discussion

Measurement of pulmonary and tracheal sounds to study pulmonary diseases is a well established method and the advantages of the bioacoustic technique, a monitor application, have been previously reported by other researchers, e.g. Beckermann and Wegmann [7] and Xiong et al. [39]. This study addresses a different problem and shows that monitoring and precise timing of breathing is possible with the bioacoustic method described. A method which is non-invasive, simple, easy to use, portable and

Acknowledgements

Financial support was given from the Swedish National Board for Industrial and Technical Development project no. 9302772-3, Competence center NIMED sponsored by the Swedish National Board for Industrial and Technical Development and the Swedish Heart and Lung Foundation. We are grateful for the skilful technical and medical assistance provided by Professor Tore Fjällbrant, Gösta Sjöholm, Peter Danielsson, Lam Eriksson, Per Sveider, Harald Mårtensson, Associate Professor Birgitta Larsby,

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