Detecting Aortic Valve Opening and Closing from Distal Body Vibrations
  Objective: Proximal and whole-body vibrations are well studied in seismocardiography and ballistocardiography, yet distal vibrations are still poorly understood. In this paper we develop two methods to measure aortic valve opening (AVO) and closing (AVC) from distal vibrations. Methods: AVO and AVC were detected for each heartbeat with accelerometers on the upper arm (A), wrist (W), and knee (K) of 22 consenting adults following isometric exercise. Exercise-induced changes were recorded with impedance cardiography, and nine-beat ensemble averaging was applied. Our first method, FilterBCG, detects peaks in distal vibrations after filtering with individually-tuned bandpass filters while RidgeBCG uses ridge regression to estimate AVO and AVC without peaks. Pseudocode is provided. Results: In agreement with recent studies, we did not find peaks at AVO and AVC in distal vibrations, and the conventional R-J interval method from the literature also correlated poorly with AVO (r2 = 0.22 A, 0.14 W, 0.12 K). Interestingly, distal vibrations filtered with FilterBCG resembled seismocardiogram signals and yielded reliable peaks at AVO (r2 = 0.95 A, 0.94 W, 0.77 K) and AVC (r2 = 0.92 A, 0.89 W, 0.68 K). Conclusion: FilterBCG measures AVO and AVC accurately from arm, wrist, and knee vibrations, and it outperforms R-J intervals and RidgeBCG. Significance: To our knowledge, this study is the first to measure AVC accurately from distal vibrations. Finally, AVO timing is needed to assess cardiovascular disease risk with pulse wave velocity (PWV) and for cuff-less diastolic blood pressure measurement via aortic pulse-transit time (PTT). 
Andrew D. Wiens, Ann Johnson, Omer T. Inan
physics.med-ph