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Breathing sound analysis for estimation of airlow rate

This invention relates to an apparatus for use in breathing sound analysis for estimation of airflow rate.

This application is related to a co-pending Application filed on the same day as this application under Attorney Docket No. 84201-1402 and entitled BREATHING SOUND ANALYSIS FOR DETECTION OF SLEEP APNEA/HYPOPNEA EVENTS.

Acoustical respiratory flow estimation has drawn much attention in recent years due to difficulties in airflow measurement. In clinical respiratory and/or swallowing assessment, flow is usually measured by spirometry devices, such as pneumotachograph, nasal cannulae connected to a pressure transducer, heated thermistor or anemometry. Airflow is also measured by indirect means, i.e., detection of chest and/or abdominal movements using respiratory inductance plethysmography (RIP), strain gauges, or magnetometers. The most reliable measurement of airflow is achieved by a mouth piece or facemask connected to a pneumotachograph. However, this device cannot be used during the swallowing assessment. Therefore, when recording sound during a swallow, flow is usually measured by nasal cannulae connected to a pressure transducer. Potentially, this method could be an inaccurate measure of airflow because the air leaks around the nasal cannulae. In addition, if the subject breathes through the mouth, flow is not registered at all.

For these reasons, the combined use of nasal cannulae connected to a pressure transducer and the measurement of respiratory inductance plethomogoraphy to monitor volume changes has been recommended as the best approach in recording flow to assess respiratory and swallowing patterns. However, application of these techniques has some disadvantages, especially when studying young children or patients with neurological impairments, where the study of swallowing is clinically important. Although the application of nasal cannulae may seem a minor intrusion, it can produce agitation in children and patients with neurological impairment. In addition, applying the RIP devices is difficult in children with neurological impairment as their poor postural control and physical deformities can make it challenging to ensure stable positioning.

In one of the first attempts at acoustical flow estimation, researchers attempted to estimate flow from tracheal sound by investigating eight different methods in the two categories of “reference curve” and “hierarchical clustering analysis”. The results showed a mean error between 13-15% of the measured flow for seven of the methods, with 31% for the eighth method. In the works by another group, flow estimation using either tracheal or lung sounds was achieved by investigating different models with about 90% overall accuracy over different flow rates from low to high flow rates. In these studies the exponential model between flow and average power of tracheal sound was found to be superior to other models.

In another study, the tracheal sound envelope was investigated for flow estimation. The tracheal sound was band-pass filtered in the range of 200-1000 Hz and then a Hilbert transform was applied to the filtered signal. The transformed signal was used to calculate the tracheal sound envelope and to estimate the flow from the calculated envelope by a linear model. The estimated flow was then used to measure ventilation, but the flow estimation error was not reported. The flow rate in that study was constant at tidal flow and half of the recorded flow signal was used to calibrate the model.

All of the above mentioned methods assume that at least some samples of breath sound with known flow at each flow rate were available to derive the model coefficients for flow estimation. Capturing respiratory sounds at different flow rates for calibration may not always be possible prior to assessment especially when assessing young children, patients with neurological impairments and/or patients in emergency conditions.

Analysis of breathing sounds from a patient for determination of sleep apnea and/or hypopnea is proposed in a paper entitled “Validation of a New System of Tracheal sound Analysis for the diagnosis of Sleep Apnea-Hypopnea Syndrome” by Nakano et al in “SLEEP” Vol27 No. 5 published in 2004. This constitutes a research paper postulating that sleep apnea can be detected by breathing sound analysis but providing no practical details for a system which may be used in practise. It is believed that no further work has been published and no commercial machine has arisen from this paper.

U.S. Pat. No. 6,241,683 (Macklem) issued Jun. 5th 2001 discloses a method for estimating air flow from breathing sounds where the system determines times when sounds are too low to make an accurate determination and uses an interpolation method to fill in the information in these times. Such an arrangement is of course of no value in detecting apnea or hypopnea since it accepts that the information in these times is inaccurate.

It is one object of the invention to provide an apparatus for use in analysis of breathing of a patient.

According to a first aspect of the invention there is provided an apparatus comprising:

a microphone arranged to be located on the patient for detecting breathing sounds;

a detector module for receiving and analyzing the signals to extract data relating to the breathing;

the detector module being arranged to analyze the signals to generate an estimate of air flow;

and a display for displaying for a clinician the estimated air flow rate relative time;

wherein the detector module is arranged to calculate a function representing the range of the signal or the entropy of the signal providing an estimate of air flow during breathing.

Preferably the detector module is arranged to cancel heart sounds from the function.

In one preferred method, the function is the range of the signal which is defined as the log of the difference between minimum and maximum of the signal within each short window (i.e. 100 ms) of data.

In another preferred method, the function is the entropy of the signal which is defined by the following formula:

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