Andrew M. Carek

Ballistocardiogram as Proximal Timing Reference for Pulse Transit Time Measurement: Potential for Cuffless Blood Pressure Monitoring

Goal: We tested the hypothesis that the ballistocardiogram (BCG) waveform could yield a viable proximal timing reference for measuring pulse transit time (PTT). Methods: From 15 healthy volunteers, we measured PTT as the time interval between BCG and a noninvasively measured finger blood pressure (BP) waveform. To evaluate the efficacy of the BCG-based PTT in estimating BP, we likewise measured pulse arrival time (PAT) using the electrocardiogram (ECG) as proximal timing reference and compared their correlations to BP. Results: BCG-based PTT was correlated with BP reasonably well: the mean correlation coefficient (r ) was 0.62 for diastolic (DP), 0.65 for mean (MP), and 0.66 for systolic (SP) pressures when the intersecting tangent method was used as distal timing reference. Comparing four distal timing references (intersecting tangent, maximum second derivative, diastolic minimum, and systolic maximum), PTT exhibited the best correlation with BP when the systolic maximum method was used (mean r value was 0.66 for DP, 0.67 for MP, and 0.70 for SP). PTT was more strongly correlated with DP than PAT regardless of the distal timing reference: mean r value was 0.62 versus 0.51 (p = 0.07) for intersecting tangent, 0.54 versus 0.49 (p = 0.17) for maximum second derivative, 0.58 versus 0.52 (p = 0.37) for diastolic minimum, and 0.66 versus 0.60 (p = 0.10) for systolic maximum methods. The difference between PTT and PAT in estimating DP was significant (p = 0.01) when the r values associated with all the distal timing references were compared altogether. However, PAT appeared to outperform PTT in estimating SP ( p = 0.31 when the r values associated with all the distal timing references were compared altogether). Conclusion: We conclude that BCG is an adequate proximal timing reference in deriving PTT, and that BCG-based PTT may be superior to ECG-based PAT in estimating DP. Significance: PTT with BCG as proximal timing reference has potential to enable convenient and ubiquitous cuffless BP monitoring.

Weighing Scale-Based Pulse Transit Time is a Superior Marker of Blood Pressure than Conventional Pulse Arrival Time

Pulse transit time (PTT) is being widely pursued for cuff-less blood pressure (BP) monitoring. Most efforts have employed the time delay between ECG and finger photoplethysmography (PPG) waveforms as a convenient surrogate of PTT. However, these conventional pulse arrival time (PAT) measurements include the pre-ejection period (PEP) and the time delay through small, muscular arteries and may thus be an unreliable marker of BP. We assessed a bathroom weighing scale-like system for convenient measurement of ballistocardiography and foot PPG waveforms – and thus PTT through larger, more elastic arteries – in terms of its ability to improve tracking of BP in individual subjects. We measured “scale PTT”, conventional PAT, and cuff BP in humans during interventions that increased BP but changed PEP and smooth muscle contraction differently. Scale PTT tracked the diastolic BP changes well, with correlation coefficient of −0.80 ± 0.02 (mean ± SE) and root-mean-squared-error of 7.6 ± 0.5 mmHg after a best-case calibration. Conventional PAT was significantly inferior in tracking these changes, with correlation coefficient of −0.60 ± 0.04 and root-mean-squared-error of 14.6 ± 1.5 mmHg (p < 0.05). Scale PTT also tracked the systolic BP changes better than conventional PAT but not to an acceptable level. With further development, scale PTT may permit reliable, convenient measurement of BP.

SeismoWatch: Wearable Cuffless Blood Pressure Monitoring Using Pulse Transit Time

The current norm for measuring blood pressure (BP) at home is using an automated BP cuff based on oscillometry. Despite providing a viable and familiar method of tracking BP at home, oscillometric devices can be both cumbersome and inaccurate with the inconvenience of the hardware typically limiting measurements to once or twice per day. To address these limitations, a wrist-watch BP monitor was developed to measure BP through a simple maneuver: holding the watch against the sternum to detect micro-vibrations of the chest wall associated with the heartbeat. As a pulse wave propagates from the heart to the wrist, an accelerometer and optical sensor on the watch measure the travel time - pulse transit time (PTT) - to estimate BP. In this paper, we conducted a study to test the accuracy and repeatability of our device. After calibration, the diastolic pressure estimations reached a root-mean-square error of 2.9 mmHg. The watch-based system significantly outperformed (p<0.05) conventional pulse arrival time (PAT) based wearable blood pressure estimations - the most commonly used method for wearable BP sensing in the existing literature and commercial devices. Our device can be a convenient means for wearable BP monitoring outside of clinical settings in both health-conscious and hypertensive populations.1.

Robust Sensing of Distal Pulse Waveforms on a Modified Weighing Scale for Ubiquitous Pulse Transit Time Measurement

The measurement of aortic pulse transit time (PTT), the time for the arterial pulse wave to travel from the carotid to the femoral artery, can provide valuable insight into cardiovascular health, specifically regarding arterial stiffness and blood pressure (BP). To measure aortic PTT, both proximal and distal arterial pulse timings are required. Recently, our group has demonstrated that the ballistocardiogram signal measured on a modified weighing scale can provide an unobtrusive, yet accurate, means of obtaining a proximal timing reference; however, there are no convenient, reliable methods to extract the distal timing from a subject standing on the modified weighing scale. It is common to use a photoplethysmograph (PPG) attached to a toe to measure this distal pulse, but we discovered that this signal is greatly deteriorated as the subject stands on the scale. In this paper, we propose a novel method to measure the distal pulse using a custom reflective PPG array attached to the dorsum side of the foot (D-PPG). A total of 12 subjects of varying skin tones were recruited to assess the preliminary validation of this approach. Pulse measurements using the D-PPG were taken from seated and standing subjects, and the commercially available PPG were measured for facilitating comparison of timing measurements. We show that the D-PPG was the only sensor to retain the high detection rate of feasible timing values. To further test and optimize the system, various factors such as applied pressure, measurement location, and LED/photodiode configuration were tested.

Preliminary methods for wearable neuro-vascular assessment with non-invasive, active sensing.

In this study, a non-invasive and active sensing scheme that is ultimately aimed to be integrated in a wearable system for neuro-vascular health assessment is presented with preliminary results. With this system, vascular tone is modulated by local heating and cooling of the palm, and the resulting changes in local hemodynamics are monitored via impedance plethysmography (IPG) and photoplethysmography (PPG) sensors interfaced with custom analog electronics. Proof-of-concept measurements were conducted on three subjects using hot packs / ice bags to modulate the palmar skin temperature. From ensemble averaged and smoothed versions of pulsatile IPG and PPG signals, the effects of local changes in skin temperature on a series of parameters associated with neuro-vascular mechanisms (heart rate, blood volume, blood flow rate, blood volume pulse inflection point area ratio, and local pulse transit time) have been observed. The promising experimental results suggest that, with different active temperature modulation schemes (consisting of heating / cooling cycles covering different temperature ranges at different rates), it would be possible to enhance the depth and specificity of the information associated with neuro-vascular health by using biosensors that can fit inside a wearable device (such as a sleeve). This study sets the foundation for future studies on designing and testing such a wearable neuro-vascular health assessment system employing active sensing.

Sternal vibrations during head-out immersion: A preliminary demonstration of underwater wearable ballistocardiography

Ballistocardiography (BCG) measures vibrations of the body caused by ejection of blood from the heart, and the root mean square (RMS) of BCG measured with a weighing scale trends with cardiac output. However, BCG underwater has not been studied. Head-to-foot BCG signals were recorded with an accelerometer on the sternum of three human subjects. The heartbeats were clearly visible in the signals recorded underwater, and the resting change in RMS BCG was +360 μg (+36%) from air to cold water immersion (27.8 °C) while standing. This is within the 32%-62% increase in cardiac output observed in previous head-out immersion studies.

Author Correction: Weighing Scale-Based Pulse Transit Time is a Superior Marker of Blood Pressure than Conventional Pulse Arrival Time

A correction has been published and is appended to both the HTML and PDF versions of this paper. The error has not been fixed in the paper.

A temperature-controlled glove with non-invasive arterial pulse sensing for active neuro-vascular assessment

A non-invasive, active neuro-vascular assessment system was developed using a modified temperature-controlled glove and feedback techniques. The glove incorporates local heating and cooling of the hand using multi-point skin temperature measurements as feedback, and thereby induces local and central mechanisms involved in thermoregulation. Electrocardiogram (ECG), photoplethysmogram (PPG) and non-invasive finger-cuff based blood pressure (BP) measurements were used to monitor electrophysiology and hemodynamic changes in response to temperature modulation. Then, the triggered neuro-vascular mechanisms associated with thermoregulation were quantified by extracting parameters from the measured waveforms, specifically heart rate variability (HRV), vascular tone, and BP. The system was tested on six young, healthy individuals with no history of microvascular diseases. During heating, vasodilation, decrease in systolic BP, and a decrease in parasympathetic tone were observed, while during cooling, vasoconstriction and increased BP were observed. While such changes are expected physiologically using passive experiments, the ability to modulate the physiology non-invasively with a controlled, quantitative, and inexpensive instrument can potentially enable serial assessments of neuro-vascular control outside of clinical settings.

Live demonstration: Pulse transit time measurement on a modified weighing scale for cuffless blood pressure estimation

The demonstration setup will include a modified weighing scale with an analog circuit built into the device, a National Instruments myRIO system for data acquisition, and a laptop for visualization of the data and implementation of the demo. The weighing scale measures the ballistocardiogram (BCG) signal — the reaction forces of the body to ejection of blood from the heart into the vasculature and movement of the blood through the vascular tree [1] — as well as a reflectance mode photoplethysmogram (PPG) from the top of the foot, representing the local blood volume pulse [2]. The BCG is measured from the strain gauges built into the scale, interfaced to a low noise instrumentation amplifier and filter circuit for amplifying the AC component of bodyweight which fluctuates synchronously with the heartbeat. The reflectance PPG is measured using a custom strap that we have developed, including an array of photodiodes and light emitting diodes (LEDs) for illuminating the top of the foot — close to the dorsal artery — and measuring the changes in reflectance at that location associated with the blood volume pulse. The time delay from the BCG I-wave to the PPG waveform foot is pulse transit time (PTT), and we have shown in previous work that this measurement of PTT is correlated to the subjects blood pressure [3].

Sternal vibrations reflect hemodynamic changes during immersion: Underwater ballistocardiography

Ballistocardiography (BCG) is a method for measuring the small vibrations of the body caused by the beating of the human heart. In this study, vibration measurements of the sternum for the purpose of noninvasive hemodynamic monitoring during total body immersion in water are recorded and examined for the first time. Three individuals wore a low-noise accelerometer while immersed in water of varying temperature up to the neck, and Valsalva maneuvers were performed. The resulting waveforms reveal distinct differences in signal morphology between three postures and two water temperatures, suggesting that underwater BCG could be applied in aquatic environments without a need for electrodes.