On the Origin of Cardiovascular Sounds Recorded from the Ear

It has been conjectured that sounds recorded at the ear, using microphones, originate from either heart sounds propagating from the heart to the recording site, through tissue and bone, or vascular activity at the recording site, such as vasodilation. However, prior studies have not been able to verify these conjectures. The aim of this study is to gain a deeper understanding of the signals measured in the ear using so called body-coupled microphones. Method: A study was conducted on 10 subjects, who were instructed to stand up, lie down and exercise with a body-coupled microphone mounted in each ear, using personalised ear-pieces. The subjects were additionally instructed to follow a guided breathing session. Recorded body-coupled microphone signals were evaluated against standard measures, such as electrocardiography, photoplethysmography, and respiration flow and effort. All signal modalities were synchronised to a common trigger signal. For the analysis, the body-coupled microphone and photoplethysmography signals were epoched in accordance with the electrocardiography R-peak. Results: Cluster-based permutation test showed that the body-coupled microphone signals were time-locked to the electrocardiography R-peak. No difference was found between ears. Discussion: Comparing the timing of the electrocardiography and photoplethysmography events suggests that signals recorded using body-coupled microphones in the ears reflect blood pressure waves in the arteries in the proximity of the body-coupled microphones.