[Sound frequency analysis for identification of venous air embolism]

Air emboli occurring during surgery are considered to be life-threatening incidents. With the aim of achieving acoustic identification of venous air emboli, a frequency analysis of the sounds induced by an air embolism (millwheel murmurs) as well as of all other unusual sounds, was undertaken during 20 operations, and in experiments with animals. The frequency spectra of the sounds induced by air emboli are characterised by an increase in the amplitudes in the frequency range 1,100 to 3,000 Hz, while the amplitudes of normal heart sounds continuously decrease with increasing frequency. The frequency spectrum was examined for characteristics using an electronic filter system. The sounds induced by air emboli can be clearly distinguished from normal heart sounds. During operations on patients, suction sounds occur, the frequency patterns of which are not easy to distinguish from those of embolus-induced sounds, although an acoustic distinction can be made via a stethoscope or a loudspeaker. With optimal adjustment of the filter system, 73 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 frequently mimicked by interfering sounds such as those produced by artificial respiration, and other ambiend sounds. By modifying the oesophageal catheter to achieve optimal suppression of interfering sounds, this filter system could be developed into an alarm.