Spectral analysis of an acoustic respiratory signal with a view to developing an apnoea monitor.

The movement of air through the air passages during normal respiration generates an acoustic signal which can be detected from the surface of the body at suitable sites with the aid of a microphone applied to the skin. The spectral characteristics of these sounds were analysed to see if they could be differentiated from other sources of sound. During normal respiration the other major source of sound was that transmitted from the heart, and this could be filtered out with a band pass filter. A prototype apnoea monitor was built which utilised these sound signals as an indicator of respiration and underwent preliminary testing. Although such acoustic signals have the potential to indicate respiration, and cardiac sounds can be effectively filtered, further work would be required to recognise and reject other extraneous sources of sound interfering with the respiratory signal.