What filter passband should be applied to the ballistocardiogram?

The ballistocardiogram (BCG) is receiving an increasing interest for ubiquitous cardiovascular health and disease monitoring. Yet, prior effort to analyze its harmonic and frequency contents is limited. In-depth understanding of the harmonic and frequency structure of the BCG has the potential to facilitate the development of various BCG-based cardiovascular health monitoring methods. In particular, when filtering the BCG, an optimal passband derived based on the knowledge of the harmonic and frequency structure of the BCG can reconcile signal integrity and noise robustness. Therefore, we conducted a comprehensive harmonic and frequency analysis using the head-to-foot BCG collected from 22 volunteers to quantify the importance of each harmonic component in the BCG and investigate the effect of filtering passband on its signal morphology and waves. Overall, the 2nd to the 5th harmonics accounted for most of the total variance in the BCG (14.5 +/- 13%, 36.3 +/- 18.7%, 21.6 +/- 14%, and 14.6 +/- 11.3% for the 2nd to the 5th harmonics). The frequency of the important harmonics (percentage variance >= 10%) ranged from 2.9 Hz to 7.8 Hz. When filtering the BCG, the widely used <= 1 Hz lower bound of the passband may be increased to 3.5 Hz and 1.5 Hz without causing substantial alterations to the BCG timings and amplitudes, respectively. Likewise, a 15 Hz and 20 Hz upper bound of the passband appears to be relevant, respectively. The insights garnered in this work may guide future development of the BCG-based cardiovascular health monitoring methods by application-specific tailoring of the frequency band associated with the BCG signal filtering.