Instrumentation Problems in a High-Frequency Heart Sound Analyzer

It is accepted generally in medical practice that the low-frequency vibrations (up to 600 cps) of heart sounds and murmurs contain virtually all the available information. Newer concepts have been proposed that these signals actually consist of irregularly occurring transients which comprise the important information regarding heart action. These transients can be analyzed by recording the frequencies above 1000 cps. The level of these signals is subaudible and in some cases is below the noise level of the usual electronic amplifier. A preliminary model of such a heart sound analyzer which has been built and tested reveals the existence of vibrations in the range mentioned with amplitudes less than 1 per cent of those in the lower frequency ranges. Instrumentation aspects of this are discussed. Limitations imposed by lack of suitable microphones are mentioned and specifications for such a microphone are suggested. Available variable cutoff frequency filters have the inherent capability of introducing artifactual signals not present in the original heart sound. A specially designed filter, free of such artifact, is described.