Ballistocardiography and Seismocardiography: A Review of Recent Advances <s> A. Correlation Studies With Healthy Subjects <s> Abstract Ballistocardiograms were made 106 times on 55 patients with coronary heart disease. In the acute period after infarction the circulation may be either normal or below. If normal, it tends to diminish, and usually reaches a minimum below the normal limit between the third and fifth week. Later, recovery sets in. The left ventricular work, as calculated from the cardiac output and the blood pressure, follows a generally similar course. In chronic coronary heart disease, with angina of effort, the circulation is subnormal in the great majority of cases, and the ballistocardiogram provides objective evidence to support the diagnosis. Abnormalities of the form of the ballistocardiogram are frequently encountered in coronary heart disease, usually when the patient has greatly reduced exercise tolerance. <s> BIB001 </s> Ballistocardiography and Seismocardiography: A Review of Recent Advances <s> A. Correlation Studies With Healthy Subjects <s> Seismocardiogram (SCG) is the recording of body vibrations induced by the heart beat. SCG contains information on cardiac mechanics, in particular heart sounds and cardiac output. In this paper we present a new wearable device for SCG recordings during long term monitorings, and the results of a validation test in 4 subjects. The system is based on the integration of the MagIC smart shirt (i.e., a textile-based wearable system for the assessment of ECG and respiratory movements), and an external triaxial MEMS accelerometer positioned on the left clavicle. SCG was estimated as the average of accelerations occurred in each heart beat. The SCG components due to the valve closure and to recoil forces following the heart contraction (ballistocardiogram) were extracted by high-pass (>18 Hz) and band-pass (0.6-20 Hz) filters respectively. Then the difference between the I and J waves of the ballistocardiogram ( lI-Jl index, possibly related to the cardiac output) was identified by an ad-hoc procedure and compared with the model flow indirect estimation of cardiac output. Validation on 4 volunteers showed that: 1) our wearable system provides statistically consistent estimates of both heart- sound related vibrations and recoil movements; 2) reliable estimates of the II-Jl index can be obtained by considering about 1 minute of SCG recording in stationary conditions; and 3) changes of the II-Jl index during exercise correlate well with changes of cardiac output estimated by the model flow. <s> BIB002 </s> Ballistocardiography and Seismocardiography: A Review of Recent Advances <s> A. Correlation Studies With Healthy Subjects <s> 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. <s> BIB003 </s> Ballistocardiography and Seismocardiography: A Review of Recent Advances <s> A. Correlation Studies With Healthy Subjects <s> Cardiac ejection of blood into the aorta generates a reaction force on the body that can be measured externally via the ballistocardiogram (BCG). In this study, a commercial bathroom scale was modified to measure the BCGs of nine healthy subjects recovering from treadmill exercise. During the recovery, Doppler echocardiogram signals were obtained simultaneously from the left ventricular outflow tract of the heart. The percentage changes in root-mean-square (RMS) power of the BCG were strongly correlated with the percentage changes in cardiac output measured by Doppler echocardiography (R2 = 0.85, n = 275 data points). The correlation coefficients for individually analyzed data ranged from 0.79 to 0.96. Using Bland–Altman methods for assessing agreement, the mean bias was found to be −0.5% (±24%) in estimating the percentage changes in cardiac output. In contrast to other non-invasive methods for trending cardiac output, the unobtrusive procedure presented here uses inexpensive equipment and could be performed without the aid of a medical professional. <s> BIB004 </s> Ballistocardiography and Seismocardiography: A Review of Recent Advances <s> A. Correlation Studies With Healthy Subjects <s> This work presents a system which estimates blood pressure variability (BPV) via pulse arrival time (PAT) using two distinct methods. Three heart rate estimates are acquired from beat-to-beat time interval extraction of electrocardiogram (ECG), finger photoplethysmogram (PPG), and ballistocardiogram (BCG). Using both ECG and BCG as reference signals, PAT is computed. Since PAT variability (PATV) is correlated to blood pressure variability, the system obtains an estimation of both heart rate variability (HRV) and BPV, parameters which carry vast information to characterize physiological and pathological states. Since BCG has not been studied yet as an alternative to EGG in PAT calculations, this study focus on the correlation of these two PATV estimates. <s> BIB005 </s> Ballistocardiography and Seismocardiography: A Review of Recent Advances <s> A. Correlation Studies With Healthy Subjects <s> In this study, we developed a novel technique for estimating non-constrained and cuffless blood pressure (BP) that was based on electrocardiogram (ECG) and ballistocardiogram (BCG). The BCG was non-invasively measured using a common electronic weighing scale when a subject was standing on it. The ECG was measured using three different methods: on the chest using Ag/AgCl electrodes, on the hands using dry electrodes and on the feet also using dry electrodes. For a BP correlated parameter, a time interval parameter, which was defined as the time difference between the ECG R-peak and BCG J-peak, was employed for evaluating and estimating beat-to-beat BP. Under a BP varying experiment with a Valsalva manoeuvre, the R-J intervals were extracted at every beat cycle and a systolic blood pressure (SBP) estimation equation was established using linear regression analysis for each subject. In the case of feet delivered ECG (F-ECG), an ensemble average technique synchronized at the BCG J-peak point was applied to extract the ECG signal from the feet. The performance of the proposed method was evaluated using Finapres, a non-invasive blood pressure measurement system, as a reference BP signal, and a scatter plot was used to find the regression line between the reference values and estimated BPs. A moving-window averaging technique was applied to remove the high-frequency noise in the R-J intervals and was applied to enhance the accuracy of the SBP estimation. For all individuals, the estimated SBP was similar to the measured SBP with a reliable correlation, which makes the proposed method suitable for use in a home healthcare system to monitor blood pressure on a weighing scale at the same time as measuring weight. <s> BIB006 </s> Ballistocardiography and Seismocardiography: A Review of Recent Advances <s> A. Correlation Studies With Healthy Subjects <s> Continuous and wearable heart monitoring is essential for early detection and diagnosis of cardiovascular diseases. We demonstrate a continuous, wearable, and wireless heart monitor that is worn at the ear. The device has the form factor of a hearing aid and is wirelessly connected to a PC for data recording and analysis. With the ear as an anchoring point, the heart monitor measures the ballistocardiographic (BCG) motion of the head using a MEMS tri-axial accelerometer, which is an electrode-less method to measure heart rate. Additionally, electrocardiogram (ECG) is measured locally near the ear using a single-lead configuration. The peak timing delay between the head ECG and the head BCG, or RJ interval, can be extracted in the presence of noise using cross-correlation. The RJ interval is shown to correlate to the heart's pre-ejection period during both Valsalva and whole-body tilt maneuvers. <s> BIB007 </s> Ballistocardiography and Seismocardiography: A Review of Recent Advances <s> A. Correlation Studies With Healthy Subjects <s> Analyzing systolic time intervals-specifically the preejection-period (PEP)-is widely accepted as one of the few methods for the noninvasive assessment of cardiac contractility. In this paper, we investigated the ballistocardiogram (BCG) as a way to noninvasively measure myocardial contractility when combined with the ECG. Specifically, we derived a parameter from the BCG and ECG that we hypothesized would be highly correlated to PEP. This is the time delay between the J-wave peak of the BCG and the R-wave of the ECG, which we refer to as the RJ interval. The RJ interval was correlated to PEP (r2 = 0.86) for 2126 heart beats across ten subjects, with a y-intercept of 138 ms and slope of 1.05. This suggests that the RJ interval can be reliably used as a noninvasive assessment of cardiac contractility. <s> BIB008 </s> Ballistocardiography and Seismocardiography: A Review of Recent Advances <s> A. Correlation Studies With Healthy Subjects <s> Time intervals measured between the electrocardiogram (ECG), the photoplethysmogram (PPG) or the impedance plethysmogram (IPG), have long been used to noninvasively assess cardiovascular function. Recently, the ballistocardiogram (BCG) has been proposed as an alternative physiological signal to be used in time interval measurements for the same purpose. In this work, we study the behavior of the RJ interval, defined as the time between the R wave of the electrocardiogram (ECG) and the J wave of the BCG, under fast pressure changes induced by paced respiration and tracked by a beat-to-beat blood pressure (SBP and DBP) waveform monitor. The aim of this work is to gain a deeper understanding of these newly proposed time intervals and to further assess their usefulness to determine cardiovascular performance. <s> BIB009 </s> Ballistocardiography and Seismocardiography: A Review of Recent Advances <s> A. Correlation Studies With Healthy Subjects <s> A continuous, wearable and wireless vital signs monitor at the ear is demonstrated. The device has the form factor of a hearing aid and is wirelessly connected to a PC for data recording and analysis. The device monitors the electrocardiogram (ECG) in a single lead configuration, the ballistocardiogram (BCG) with a MEMS triaxial accelerometer, and the photoplethysmograms (PPG) with 660 nm and 940 nm LED sources and a static photocurrent subtraction analog front end. Clinical tests are conducted, including Valsalva and head-up tilt maneuvers. Peak timing intervals between the ECG, BCG and PPG are extracted and are shown to relate to pre-ejection period and mean arterial blood pressure (MAP). Pulse Transit Time (PTT) extracted from cross-correlation between the PPG and BCG shows improved results compared to the pulse arrival time (PAT) method for tracking changes in MAP. <s> BIB010 </s> Ballistocardiography and Seismocardiography: A Review of Recent Advances <s> A. Correlation Studies With Healthy Subjects <s> Abstract Seismocardiogram (SCG) is the measure of the micro-vibrations produced by the heart contraction and blood ejection into the vascular tree. Over time, a large body of evidence has been collected on the ability of SCG to reflect cardiac mechanical events such as opening and closure of mitral and aortic valves, atrial filling and point of maximal aortic blood ejection. We recently developed a smart garment, named MagIC-SCG, that allows the monitoring of SCG, electrocardiogram (ECG) and respiration out of the laboratory setting in ambulant subjects. The present pilot study illustrates the results of two different experiments performed to obtain a first evaluation on whether a dynamical assessment of indexes of cardiac mechanics can be obtained from SCG recordings obtained by MagIC-SCG. In the first experiment, we evaluated the consistency of the estimates of two indexes of cardiac contractility, the pre-ejection period, PEP, and the left ventricular ejection time, LVET. This was done in the lab, by reproducing an experimental protocol well known in literature, so that our measures derived from SCG could have been compared with PEP and LVET reference values obtained by traditional techniques. Six healthy subjects worn MagIC-SCG while assuming two different postures (supine and standing); PEP was estimated as the time interval between the Q wave in ECG and the SCG wave corresponding to the opening of aortic valve; LVET was the time interval between the SCG waves corresponding to the opening and closure of the aortic valve. The shift from supine to standing posture produced a significant increase in PEP and PEP/LVET ratio, a reduction in LVET and a concomitant rise in the LF/HF ratio in the RR interval (RRI) power spectrum. These results are in line with data available in literature thus providing a first support to the validity of our estimates. In the second experiment, we evaluated in one subject the feasibility of the beat-by-beat assessment of LVET during spontaneous behavior. The subject was continuously monitored by the smart garment from 8 am to 8 pm during a workday. From the whole recording, three data segments were selected: while the subject was traveling to work (M1), during work in the office (O) and while traveling back home (M2). LVET was estimated on a beat-by-beat basis from SCG and the RRI influence was removed by regression analysis. The LVET series displayed marked beat-by-beat fluctuations at the respiratory frequency. The amplitude of these fluctuations changed in the three periods and was lower when the LF/HF RRI power ratio was higher, at O, thus suggesting a possible influence of the autonomic nervous system on LVET short-term variability. To the best of our knowledge this case report provides for the first time a representation of the beat-by-beat dynamics of a systolic time interval during daily activity. The statistical characterization of these findings remains to be explored on a larger population. <s> BIB011 </s> Ballistocardiography and Seismocardiography: A Review of Recent Advances <s> A. Correlation Studies With Healthy Subjects <s> Seismocardiogram (SCG) is the measure of the minute vibrations produced by the beating heart. We previously demonstrated that SCG, ECG and respiration could be recorded over the 24h during spontaneous behavior by a smart garment, the MagIC-SCG system. In the present case study we explored the feasibility of a beat-to-beat estimation of two indices of heart contractility, the Left Ventricular Ejection Time (LVET) and the electromechanical systole (QS2) from SCG and ECG recordings obtained by the MagIC-SCG device in one subject. We considered data collected during outdoor spontaneous behavior (while sitting in the metro and in the office) and in a laboratory setting (in supine and sitting posture, and during recovery after 100W and 140W cycling). LVET was estimated from SCG as the time interval between the opening and closure of the aortic valve, QS2 as the time interval between the Q wave of the ECG and the closure of the aortic valve. In every condition, LVET and QS2 could be estimated on a beat-to-beat basis from the SCG collected by the smart garment. LVET and QS2 are characterized by important beat-to-beat fluctuations, with standard deviations in the same order of magnitude of RR Interval. In all settings, spectral profiles are different for LVET, QS2 and RR Interval. This suggests that the biological mechanisms impinging on the heart exert a differentiated influence on the variability of each of these three indices. <s> BIB012 </s> Ballistocardiography and Seismocardiography: A Review of Recent Advances <s> A. Correlation Studies With Healthy Subjects <s> Graded lower-body negative pressure was used to create a hemodynamic response similar to hemorrhage. Echocardiogram measurements showed a maximal reduction of 32.4% in stroke volume. Analysis of systolic time intervals, such as pre-ejection period and left ventricular ejection time (LVET), derived from a seismocardiogram (SCG), were demonstrated to be more sensitive in detection of early-stage hemorrhage compared with pulse pressure, heart rate, and the amplitude features extracted from SCG. In particular, the LVET and pre-ejection period/LVET features, extracted from SCG, were significantly different between, and correlated with, the different stages of lower-body negative pressure (r = 0.9 and 0.88, P < 0.05), for 32 subjects. These results suggest a portable, cost-effective solution for identification of mild or moderate hemorrhage using accelerometers. <s> BIB013
Originally, BCG and SCG were proposed as diagnostic tools for the clinic-for example, a patient would lie on a Starr BCG table, the recording would be printed on a strip chart, and the physician would read the recording to make a diagnosis regarding the patient's cardiovascular health  , BIB001 . However, the large intersubject variability in the signals hampered this approach, particularly given the limited tools available at that time for signal analysis. On the contrary, studies have shown that the intrasubject variability in the signals over serial measurements is actually low BIB003 -except in the presence of changing cardiovascular health. For this reason, in the past decade the BCG and SCG have been proposed as tools for monitoring changes in the same patient's health overtime. Then, the subject is his/her own control, and intersubject variability is no longer an obstacle. To uncover the clinical relevance of BCG and SCG signal features, and to pave the way for future studies with clinical populations, several researchers conducted human subjects studies with a healthy population using modern instrumentation and analysis tools. These studies were mainly designed with a noninvasive protocol for altering the hemodynamics and timing intervals of the heart-such as exercise, Valsalva maneuver, whole-body tilt testing, or lower body negative pressure (LBNP)-then, comparing the changes in the BCG or SCG waveform to changes in a reference standard measurement, such as impedance cardiography (ICG) or Doppler ultrasound. For both BCG and SCG signals the amplitude (or rms power) components have been shown to modulate with changes in left ventricular function-in particular, changes in stroke volume (SV) or cardiac output (CO). Castiglioni et al. measured clavicular SCG signals before and immediately after exercise and compared the percent changes in the peak-to-peak amplitude of the SCG to changes in CO as measured by the finometer model flow method, finding a strong correlation for four data points taken from four subjects BIB002 . Inan et al. further demonstrated that the changes in rms power resulting from exercise, measured during 10 min of recovery time, were strongly correlated to changes in CO measured by Doppler ultrasound for 275 data points taken from nine subjects BIB004 . Tavakolian etal. trained a neural network to estimate SV from SCG parameters and tested this classifier on a separate testing dataset, finding an average correlation coefficient of 0.61, and Bland-Altman agreement limits (95% confidence) of +7.4mL, −7.6mL for 4900 heartbeats analyzed from eight participants  . It is important to note that these error bands are larger than what would be needed for absolute volume estimation using the SCG; however, this may be of interest for future research. Many researchers have also examined the time intervals both within the signals themselves, and between BCG / SCG signal features and other physiological measurements (e.g., ECG or PPG), to form a relationship between these timing intervals to more well-known parameters [e.g., preejection period (PEP), pulse transit time (PTT), or left ventricular ejection time (LVET)]. The time interval between the ECG R-wave peak and the BCG J-wave peak has been proposed as a surrogate for the PEP-a measure of the IVC period of the heart and an index of cardiac contractility BIB007 , BIB008 . These authors used the Valsalva maneuver and/or whole body tilt testing to modulate the PEP by changing the autonomic balance between parasympathetic and sympathetic drive, and compared the R-J interval to the PEP measured using ICG. Etemadi et al. demonstrated a strong correlation (R 2 = 0.86) between the R-J interval and the PEP for 2126 heartbeats across ten subjects performing the Valsalva maneuver BIB008 . He et al. showed similar results for one example subject with both the Valsalva maneuver and whole-body tilt testing BIB007 . Tavakolian et al. proposed the interval between the ECG Q-wave and the SCG AO-point as a surrogate for PEP, and found strong correlation between this interval and PEP measurement using ICG and Doppler ultrasound in 25 subjects  . Researchers have also attempted to extract data from the BCG relating to blood pressure (BP), leveraging the known relationship between pulse wave velocity estimated using PTT, and Pinheiro et al. suggested the use of BCG and PPG for PTT estimation BIB005 . Shin et al. compared the R-J interval of the BCG, modulated using the Valsalva maneuver, to beat-by-beat systolic BP (SBP) measurements taken using the Finapres system, finding a strong correlation BIB006 . Nevertheless, Casanella et al. found that, in case of hemodynamic changes induced by paced respiration, this correlation between R-J interval and SBP was dependent on the subject and was not always observed BIB009 . Winokur et al. found, for one example subject, that the time interval between the BCG and the PPG signal, both measured at the ear, were correlated to PTT, and could thus be used to estimate BP BIB010 . Another important interval is the duration of systolic ejection, the LVET, as it provides an indication of what percentage of the cardiac cycle is being devoted to ejection compared to filling. Tavakolian et al. used LBNP to simulate hemorrhage, and found that LVET measurements taken using SCG were significantly different at various stages of LBNP, and correlates with the LBNP levels (R = 0.90) for 32 subjects BIB013 . Di Rienzo et al. found that with exercise LVET changes measured using wearable SCG are in line with the changes reported in the literature and obtained by traditional laboratory techniques BIB011 , BIB012 .