FREQUENCY-ANALYSIS OF SOUNDS FOR THE IDEN TIFICATION OF AIR EMBOLI

Air emboli occuring during surgery are considered to be life-threateni ng incidents. With the aim of achieving acoustic identification of ven ous air emboli, a frequency analysis of the sounds induced by an air e mbolism (millwheel murmurs) as well as of all other unusual sounds, wa s undertaken during 20 operations, and in experiments with animals. Th e frequency spectra of the sounds induced by air emboli are characteri sed by an increase in the amplitudes in the frequency range 1,100 to 3 ,000 Hz, while the amplitudes of normal heart sounds continuously decr ease with increasing frequency. The frequency spectrum was examined fo r characteristics using an electronic filter system. The sounds induce d by air emboli can be clearly distinguished from normal heart sounds. During operations on patients, suction sounds occur, the frequency pa tterns of which are not easy to distinguish from those of embolus-indu ced sounds, although an acoustic distinction can be made via a stethos cope or a loudspeaker. With optimal adjustment of the filter system, 7 3 out of 81 (90 %) embolism-related sounds were correctly identified in animal experiments. On no occasion were normal heart sounds wrongly identified as due to an embolus. However, an embolus sound was freque ntly mimicked by interfering sounds such as those produced by artifici al respiration, and other ambiend sounds. By modifying the oesophageal catheter to achieve optimal suppression of interfering sounds, this f iter system could be developed into an alarm.