Phillip Shaltis

Mobile monitoring with wearable photoplethysmographic biosensors

We address both technical and clinical issues of wearable biosensors (WBS). First, design concepts of a WBS are presented, with emphasis on the ring sensor developed by the authors group at MIT. The ring sensor is an ambulatory, telemetric, continuous health-monitoring device. This WBS combines miniaturized data acquisition features with advanced photoplethysmographic (PPG) techniques to acquire data related to the patients cardiovascular state using a method that is far superior to existing fingertip PPG sensors. In particular, the ring sensor is capable of reliably monitoring a patients heart rate, oxygen saturation, and heart rate variability. Technical issues, including motion artifact, interference with blood circulation, and battery power issues, are addressed, and effective engineering solutions to alleviate these problems are presented. Second, based on the ring sensor technology the clinical potentials of WBS monitoring are addressed.

Utility of the Photoplethysmogram in Circulatory Monitoring

The photoplethysmogram is a noninvasive circulatory signal related to the pulsatile volume of blood in tissue and is displayed by many pulse oximeters and bedside monitors, along with the computed arterial oxygen saturation. The photoplethysmogram is similar in appearance to an arterial blood pressure waveform. Because the former is noninvasive and nearly ubiquitous in hospitals whereas the latter requires invasive measurement, the extraction of circulatory information from the photoplethysmogram has been a popular subject of contemporary research. The photoplethysmogram is a function of the underlying circulation, but the relation is complicated by optical, biomechanical, and physiologic covariates that affect the appearance of the photoplethysmogram. Overall, the photoplethysmogram provides a wealth of circulatory information, but its complex etiology may be a limitation in some novel applications.

Implementation and validation of a power-efficient, high-speed modulation design for wireless oxygen saturation measurement systems

This paper describes the implementation and initial validation results of a novel power-efficient, high-speed modulation design for wearable oxygen saturation sensor systems. The research presented is in conjunction with the development of continuous photo plethysmographic health monitoring device known as the Ring Sensor. It is demonstrated that a high LED modulation rate coupled with a low duty cycle significantly reduce LED power consumption. Additionally, initial benchmarking results using a Nellcor N-395 pulse oximeter indicate good agreement with the new design for both heart rate and oxygen saturation measurements.

Wearable, cuff-less PPG-based blood pressure monitor with novel height sensor.

A truly wearable non-invasive blood pressure (NIBP) sensor- light-weight, compact, unobstrusive, and essentially unnoticeable to the patient-could revolutionize healthcare delivered beyond the traditional walls of medical facilities, offering new ways to care for patients in their everyday surroundings. This paper presents results from our work towards the development of a self-contained, wearable blood pressure sensor. A PPG-based approach to blood pressure monitoring is presented. The design enables significant miniaturization of traditional oscillometric devices without the need for occlusive circumferential pressures. It will be shown how natural raising and lowering of the arm replaces the need for bulky actuators. Additionally, a dual-accelerometer height sensor that is tetherless is proposed and supported by experimental results

Cuffless Blood Pressure Monitoring Using Hydrostatic Pressure Changes

This paper presents a new principle for noninvasive blood pressure measurements through a modified volume-oscillometric technique that eliminates an inflatable pressure cuff, and instead takes advantage of natural hydrostatic pressure changes caused by raising and lowering the subjects arm. This new methodology provides the distinct advantage of using an absolute gauge pressure reference for measurements, and does not necessarily require additional actuation.

Adaptive hydrostatic blood pressure calibration: Development of a wearable, autonomous pulse wave velocity blood pressure monitor

A technique for calibrating non-invasive peripheral arterial sensor signals to peripheral arterial blood pressure (BP) is proposed. The adaptive system identification method utilizes a measurable intra-arterial hydrostatic pressure change in the sensor outfitted appendage to identify the transduction dynamics relating the peripheral arterial blood pressure and the measured arterial sensor signal. The proposed algorithm allows identification of the calibration dynamics despite unknown physiologic fluctuations in arterial pressure during the calibration period under certain prescribed conditions. By employing unique wearable sensor architecture to estimate pulse wave velocity (PWV), this technique is used to calibrate peripheral pulse transit time measurements to arterial blood pressure. This sensor architecture is comprised of two inline photoplethysmograph sensors one in the form of a wristwatch measuring the pulse waveform in the ulnar artery and one in the form of a ring measuring the pulse waveform from the digital artery along the base of the little finger. Experimental results using the proposed algorithm to calibrate PTT to BP on human subjects will be presented.

A hydrostatic pressure approach to cuffless blood pressure monitoring

This paper presents the underlying principle and accompanying initial validation results towards the development of an optically-based, cuffless blood pressure monitoring method. As opposed to traditional oscillometric techniques, the optical sensor is calibrated with a known patient-controlled hydrostatic perturbation. In particular, the hydrostatic pressure challenge is utilized to parameterize the characteristic sigmoidal vascular compliance curve that links transmural pressure to the measured PPG output. Formulation of the compliance model will be accompanied by experimental results demonstrating the utility of the method.

Novel Design for a Wearable, Rapidly Deployable, Wireless Noninvasive Triage Sensor

This paper presents a unique design for a low-power, continuous non-invasive sensor capable of remotely monitoring the five major vital signs of a patient. In particular, the sensor is designed for rapid attachment to the fingerbase of a patient by utilizing a clip-type mechanism and is comprised of a photoplethysmograph (PPG), a MEMS accelerometer, a temperature sensor, and a wireless node. Although hastily placed by a medic, the finger sensor will automatically find the location of a digital artery and acquire a clear, pulse signal: a micro-sensor array accommodates the location of the sensor attachment. Additionally, the PPG signal, although corrupted with the patients motion in chaotic environment, will be recovered by using the MEMS accelerometer and an active noise cancellation algorithm

Towards the Development of Wearable Blood Pressure Sensors: A Photo-Plethysmograph Approach Using Conducting Polymer Actuators

lood pressure is among the most critical measures that physicians use for broad clinical applications. Yet, continuous blood pressure measurement is available only for hospital settings and other limited cases. As of today, there is no wearable sensor that can measure blood pressure continuously and reliably as well as for a long period of time. Bulky wrist-band type sensors and armcuff type sensors are practically infeasible to wear for a long time, due to interference with patients’ daily activities. It is expected that development of truly wearable blood pressure (BP) sensors will have a significant impact upon broad health monitoring applications. The authors’ group has been working on the development of BP sensors based on photoplethysmogram (PPG). PPG is a non-invasive circulatory signal related to the pulsatile volume of blood in tissue, displayed by most pulse-oximeters along with the computed arterial oxygen saturation. PPG is a desirable sensor modality for wearable health monitoring, since it is miniaturizable and of low power consumption [1]. Previous developments of PPG ring sensors and others have demonstrated that those devices have the potential for long-term, continuous monitoring of pulse, oxygen saturation, and pulse rate variability. It is known that PPG has a strong correlation with arterial blood pressure (ABP) waveform. The relationship between PPG and ABP, however, varies depending on a number of factors. Caution must be taken when estimating ABP from PPG signals. In this paper, first we will discuss how PPG signals are related to ABP, and which factors influence the PPG-ABP relationship and to what extent. Based on the literature of PPG as well as on our own experiments, we will investigate the relationship between the two. An effective calibration procedure will then be developed to estimate ABP continuously from wearable PPG sensors. Finally, new designs of wearable ABP sensors using conducting polymer actuators will briefly be discussed at the end.

A critical appraisal of opportunities for wearable medical sensors

This paper provides an appraisal of the sensor requirements and prospects available for the growing field of wearable medical sensors. The results of a literature survey for various sensor use-models indicate that the design goals for each intended sensor application must focus on task specific criteria for ultimate sensor acceptance. Provided use-models include the examination of the relevant medical problems, the diagnostic utility of the available physiologic signals, and the impact of false alarms on the specific implementation area.