Unobtrusive Estimation of Cardiac Contractility and Stroke Volume Changes Using Ballistocardiogram Measurements on a High Bandwidth Force Plate

Abstract

Unobtrusive and inexpensive technologies for monitoring the cardiovascular health of heart failure (HF) patients outside the clinic can potentially improve their continuity of care by enabling therapies to be adjusted dynamically based on the changing needs of the patients. Specifically, cardiac contractility and stroke volume (SV) are two key aspects of cardiovascular health that change significantly for HF patients as their condition worsens, yet these parameters are typically measured only in hospital/clinical settings, or with implantable sensors. In this work, we demonstrate accurate measurement of cardiac contractility (based on pre-ejection period, PEP, timings) and SV changes in subjects using ballistocardiogram (BCG) signals detected via a high bandwidth force plate. The measurement is unobtrusive, as it simply requires the subject to stand still on the force plate while holding electrodes in the hands for simultaneous electrocardiogram (ECG) detection. Specifically, we aimed to assess whether the high bandwidth force plate can provide accuracy beyond what is achieved using modified weighing scales we have developed in prior studies, based on timing intervals, as well as signal-to-noise ratio (SNR) estimates. Our results indicate that the force plate BCG measurement provides more accurate timing information and allows for better estimation of PEP than the scale BCG (r² = 0.85 vs. r² = 0.81) during resting conditions. This correlation is stronger during recovery after exercise due to more significant changes in PEP (r² = 0.92). The improvement in accuracy can be attributed to the wider bandwidth of the force plate. ∆SV (i.e., changes in stroke volume) estimations from the force plate BCG resulted in an average error percentage of 5.3% with a standard deviation of ±4.2% across all subjects. Finally, SNR calculations showed slightly better SNR in the force plate measurements among all subjects but the small difference confirmed that SNR is limited by motion artifacts rather than instrumentation.

Keywords: ballistocardiography (BCG); cardiac contractility; heart failure; stroke volume; unobtrusive cardiovascular monitoring.

Figure 1

(a) A block diagram of the experimental setup; (b) a 5 s time trace showing ECG, $\frac{\mathrm{d}}{\mathrm{dt}}$ ICG, head-to-foot force plate BCG.

Figure 2

Ensemble averaged traces of ECG, $\frac{\mathit{d}}{\mathit{d}\mathit{t}}$ ICG, scale BCG, and head-to-foot force plate BCG with the characteristic points and features.

Figure 3

Linear regression fit for both scale and force plate head-to-foot BCG RI-interval vs. PEP among all subjects.

Figure 4

Bland Altman plot for scale and force plate linear prediction models of PEP. The blue line is the 95% confidence range of the PEP estimations from the scale BCG RI-interval while the red line is the 95% confidence range of the PEP estimations from the force plate BCG RI-interval.

Figure 5

Linear regression fit for head-to-foot force plate BCG vs. PEP during recovery among all 17 subjects.

Figure 6

Estimated stroke volume percent changes from head-to-foot force plate BCG compared to calculated stroke volume percent changes from reference ICG.