Richard M. Wiard

Robust ballistocardiogram acquisition for home monitoring

The ballistocardiogram (BCG) measures the reaction of the body to cardiac ejection forces, and is an effective, non-invasive means of evaluating cardiovascular function. A simple, robust method is presented for acquiring high-quality, repeatable BCG signals from a modified, commercially available scale. The measured BCG waveforms for all subjects qualitatively matched values in the existing literature and physiologic expectations in terms of timing and IJ amplitude. Additionally, the BCG IJ amplitude was shown to be correlated with diastolic filling time for a subject with premature atrial contractions, demonstrating the sensitivity of the apparatus to beat-by-beat hemodynamic changes. The signal-to-noise ratio (SNR) of the BCG was estimated using two methods, and the average SNR over all subjects was greater than 12 for both estimates. The BCG measurement was shown to be repeatable over 50 recordings taken from the same subject over a three week period. This approach could allow patients at home to monitor trends in cardiovascular health.

Novel methods for estimating the ballistocardiogram signal using a simultaneously acquired electrocardiogram

The ballistocardiogram (BCG) signal represents the movements of the body in response to cardiac ejection of blood. Recently, many groups have developed low-cost instrumentation for facilitating BCG measurement in the home. The standard method used in the literature for estimating the BCG pulse response has generally been ensemble averaging over several beats. Unfortunately, since the BCG pulse response is likely longer than a typical heartbeat interval, this standard approach does not yield a full-length estimate of the response. This paper describes a simple, novel algorithm for estimating the full-length BCG pulse response using the R-wave timing of a simultaneously acquired electrocardiogram (ECG). With this pulse response, the full signal can be reconstructed, enabling the analysis of slow transient effects in the BCG signal, and of the measurement noise. Additionally, while this paper focuses only on the BCG signal, the same algorithm could be applied to other biomedical signals such as the phonocardiogram or impedance cardiogram, particularly when the heartbeat interval is shorter than the duration of the cpulse response.

Non-invasive measurement of Valsalva-induced hemodynamic changes on a bathroom scale Ballistocardiograph

Unobtrusive and compact methods for monitoring time varying hemodynamic trends can allow physicians to monitor heart failure of outpatients at home. In this paper, the ballistocardiogram (BCG), measured on a modified commercial bathroom scale, is proposed as a viable option for this important need. The BCG measures the reaction force of the body to cardiac ejection of blood and is a non-invasive tool for evaluating cardiovascular function. The Valsalva maneuver was used to modulate the hemodynamics in a well documented manner, and BCG signals were acquired from 15 subjects. The electrocardiogram (ECG) was simultaneously obtained to measure the electrical to mechanical delay in ventricular contraction: the interval from the ECG R-wave peak to the BCG J-wave peak. This interval, called the RJ interval, decreased for all subjects following the release of intrathoracic strain compared to the resting value, suggesting that it is inversely correlated to cardiac contractility. The power spectrum magnitude of the BCGs showed that the high frequency content increased after release, also consistent with increased contractility (faster ejection). Additionally, J-wave amplitudes increased following release, suggesting that it is correlated to stroke volume. Since RJ interval computation required the ECG, BCG J-wave rise time was proposed as an alternative for evaluating cardiac contractility. The correlation between this rise time and RJ interval was high (R2 = 0.78).

Estimation of central aortic forces in the ballistocardiogram under rest and exercise conditions

The ballistocardiogram (BCG) signal represents the movements of the body in response to cardiac ejection of blood. The BCG signal can change considerably under various physiological states; however, little information exists in literature describing how these forces are generated. A physical analysis is presented using a finite element model of thoracic aortic vasculature to quantify forces generated by the blood flow during the cardiac cycle. The traction at the fluid-solid interface of this deformable wall model generates a Central Aortic Force (CAF) which appears of similar magnitude to recorded BCG forces. The increased pulse pressure in an exercise simulation caused a significant increase in CAF, which is consistent with recent BCG measurements in exercise recovery.

Non-invasive assessment of cardiac contractility on a weighing scale

Myocardial contractility, the intrinsic ability of the heart muscle to produce force, has been difficult to quantify non-invasively. Pre-ejection-period (PEP), the time the ventricles spend in isovolumetric contraction, is widely accepted as a way to measure contractility. This work presents a way by which the ballistocardiogram — a readily accessible non-invasive cardiovascular signal — can be used in tandem with the electrocardiogram to obtain a parameter highly correlated to PEP and thus to myocardial contractility. This parameter is the delay from the electrocardiogram R-wave to the peak (the J-wave) of the ballistocardiogram. In this work, we showed that this delay, the RJ interval, was correlated to PEP (r2 = 3D 0.75) for 709 heartbeats across 4 subjects, with a slope of 1.11, and a y-intercept of 151 ms. This suggests that the RJ interval can be used in place of the PEP for a reliable, practical, and non-invasive assessment of myocardial contractility.

Evaluating the Foot Electromyogram Signal as a Noise Reference for a Bathroom Scale Ballistocardiogram Recorder

A bathroom scale ballistocardiogram (BCG) recorder has been developed in our group as a potential home monitor for heart failure outpatients. While the signal quality obtained by this device is as high as elaborate table- and bed-based BCG systems discussed previously in the literature, the standing posture required by this system may lead to undesired motion induced noise in the signal, particularly for elderly patients. Electromyogram (EMG) signals from the feet are proposed as a noise reference for the standing BCG measurement. The correlation between these signals and the BCG noise is quantified for a case with low (eyes open) and higher (eyes closed) involuntary movement on the scale. For the six subjects considered in this trial, the foot EMG appears to be a valuable reference for BCG movement noise estimation. Additionally, the fact that many bathroom scales have electrodes on the feet for various body fat percentage estimates makes the measurement highly practical for future implementations.

Preliminary results from standing ballistocardiography measurements in microgravity

We report on the feasibility of standing ballistocardiogram (BCG) measurements recorded in a microgravity environment. A clinically-tested BCG monitoring scale was adapted for parabolic flight for the microgravity measurements. Upon completion of this flight campaign, the BCG scale was shown to make measurements in micro-g and one-g environments-which is a first demonstration for a standing BCG system. This screening experiment demonstrated proof-of-concept attributes of the hardware design necessary for future characterization studies with multiple subjects. This scale-based BCG system is proposed as a practical device for hemodynamic monitoring for astronauts in Earth, Lunar, Martian, orbital, and interplanetary environments.

Standing ballistocardiography measurements in microgravity

The performance and practicality of a scale-based ballistocardiogram (BCG) system for hemodynamic monitoring of astronauts on extended space missions was demonstrated. The system consists of a modified electronic weighing scale fitted with foot bindings to mechanically couple the subject to the scale. This system was tested on a recent series of parabolic flights in which scale-based and accelerometry-based free-floating BCG of 10 subjects was measured in microgravity. The signal-to-noise ratio (SNR) of the scale-based BCG was, on average, a factor of 2.1 (6.3 dB) higher than the free-floating method, suggesting that the tethered scale approach might be more robust in terms of signal quality. Additionally, this approach enables practical BCG-based hemodynamic monitoring in fractional-g environments, and on small space vehicles such as NASAs upcoming Orion capsule. The scale-based results in microgravity were also compared to ground measurements (1g), where there was an average 38.7 ms RJ interval reduction from ground to microgravity environments that is consistent across 9 of 10 subjects. This phenomenon is likely due to the transient increase in venous return, and consequent decrease in pre-ejection period, experienced during the microgravity time intervals.

Unobtrusive Monitoring of Cardiovascular Health at Home Using a Modified Weighing Scale

This paper presents a review of our team’s research efforts over the past eight years focusing on providing innovative solutions for actionable and unobtrusive home health monitoring using a common household device: the weighing scale. By interfacing the mechanical and electrical sensors available in modern digital weighing scales to dedicated analog front end circuits, developing a customized toolkit of algorithms for processing and analyzing measured signals, and conducting targeted human subjects studies to elucidate relationships between underlying physiological changes and features of the signals, we have opened up a combination of new potential solutions to important health monitoring needs; additionally, we have uncovered new scientific questions to address in further studies. In particular, we have focused on better understanding longitudinal measurements of the ballistocardiogram (BCG) signal – representing the cardiogenic reactionary forces of the body – and how the BCG can complement other more well-known measures of cardiovascular performance. The key technological challenges and barriers we have attempted to address in this work, as well as the scientific questions that are currently at the forefront of our research, are outlined and discussed here.

Noninvasive pulse transit time measurement for arterial stiffness monitoring in microgravity

The use of a noninvasive hemodynamic monitor to estimate arterial stiffness, by measurement of pulse transit time (PTT), was demonstrated in microgravity. The monitors utility for space applications was shown by establishing the correlation between ground-based and microgravity-based measurements. The system consists of a scale-based ballistocardiogram (BCG) and a toe-mounted photoplethysmogram (PPG). PTT was measured from the BCG I-wave to the intersecting tangents of the first trough and maximum first derivative of the PPG waveforms of each subject. The system was tested on a recent series of parabolic flights in which the PTT of nine subjects was measured on the ground and in microgravity. An average of 60.2 ms PTT increase from ground to microgravity environments was shown, and was consistent across all test subjects (standard deviation = 32.9 ms). This increase in PTT could be explained by a number of factors associated with microgravity and reported in previous research, including elimination of hydrostatic pressure, reduction of intrathoracic pressure, and reduction of mean arterial pressure induced by vasodilation.