Abstract
The monitoring of human breathing activity during a long period has multiple fundamental applications in medicine. In breathing sleep disorders such as apnea, the diagnosis is based on events during which the person stops breathing for several periods during sleep. In polysomnography, the standard for sleep disordered breathing analysis, chest movement and airflow are used to monitor the respiratory activity. However, this method has serious drawbacks. Indeed, as the subject should sleep overnight in a laboratory and because of sensors being in direct contact with him, artifacts modifying sleep quality are often observed. This work investigates an analysis of the viability of an ultrasonic device to quantify the breathing activity, without contact and without any perception by the subject. Based on a low power ultrasonic active source and transducer, the device measures the frequency shift produced by the velocity difference between the exhaled air flow and the ambient environment, i.e., the Doppler effect. After acquisition and digitization, a specific signal processing is applied to separate the effects of breath from those due to subject movements from the Doppler signal. The distance between the source and the sensor, about 50 cm, and the use of ultrasound frequency well above audible frequencies, 40 kHz, allow monitoring the breathing activity without any perception by the subject, and therefore without any modification of the sleep quality which is very important for sleep disorders diagnostic applications. This work is patented (patent pending 2013-7-31 number FR.13/57569).
Highlights
Breathing is one of the essential functions for the survival of most living beings
Electromagnetic waves can sense chest movement by Doppler effect [7,8,9,10] or by analyzing the signal backscattered by breathing movements [11] and ultrasound waves telemeters permits detection of small body displacements during respiration [12,13]
We can verify the perfect correspondence between the spectrogram and the breathing activity during normal breathing, simulated apnea or movements
Summary
Breathing is one of the essential functions for the survival of most living beings. Many processes to measure the respiration rate have been proposed: using a stretch sensor or impedance meter to detect chest expansion [1,2,3], a pulse oximeter and extracting the respiration rate from the raw data [4], an accelerometer to detect chest expansion and contraction [5,6], measuring airflow pressure by oral or nasal cannula, and many others. Electromagnetic waves can sense chest movement by Doppler effect [7,8,9,10] or by analyzing the signal backscattered by breathing movements [11] and ultrasound waves telemeters permits detection of small body displacements during respiration [12,13] These solutions give good results but remain indirect because they do not analyze the true air flow and cannot detect obstructive sleep apnea (OSA). The physical principle of this measure is to “illuminate” the subject’s head with an acoustic wave emitted by a transducer, recovering and analyzing the reflected wave Under these conditions, any movement, both in the subject himself and in the exhaled airflow, induce a frequency shift in the signal which is the Doppler effect. By frequency filtering and averaging, it becomes possible to obtain and discriminate air flow signal and head and shoulder movement signal
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