Wearable Forehead Pulse Oximetry: Minimization of Motion and Pressure Artifacts

Although steady progress has been made towards the development of a wearable pulse oximeter to aid in remote physiological status monitoring (RPSM) and triage operations, the ability to extract accurate physiological data from a forehead pulse oximeter during extended periods of activity and in the presence of pressure disturbances acting on the sensor remains a significant challenge. This research was undertaken to assess whether the attachment method used to secure a pulse oximeter sensor affects arterial oxygen saturation (SpO2) and heart rate (HR) accuracy during motion. Additionally, two sensor housings were prototyped to assess whether isolating the sensor from external pressure disturbances could improve SpO2 and HR accuracy. The research revealed that measurement accuracy during walking is significantly affected by the choice of an attachment method. Specifically, the research indicated that an elastic band providing a contact pressure of 60 mmHg can result in decreased measurement error and improved reliability. Furthermore, the research validated that the two isolating housings we have investigated improve SpO2 and HR errors significantly at pressures as high as 1200 mmHg (160 kPa) compared to current commercial housings. This information may be helpful in the design of a more robust pulse oximeter sensor for use in RPSM

Analysis and validation of an artifact resistant design for oxygen saturation measurement using photo pletyhsmographic ring sensors

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering; and, (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2004.Includes bibliographical references (leaves 99-103).Recent advances in continuous noninvasive health monitoring technologies provide clinicians and researchers with a previously unrealistic opportunity for closely tracking the developments and treatments of various pathologies both within and outside of a clinical setting. At the same time, miniaturized, wireless communication technologies have greatly enhanced the transmission of sensor data while reducing the size requirements for traditional, wearable sensors. The synergism of these innovations has led to the development of the Ring Sensor, a miniaturized, telemetric, photo plethysmograph sensor for continuous health monitoring. Previous work on the Ring Sensor has led to significant power savings in regards to data acquisition and transmission. Additionally, early long-term monitoring tests have indicated that the Ring Sensor is capable of acquiring a reliable waveform nearly 30% of the time. However, the utility of the Ring Sensor has remained somewhat limited. This thesis addresses several of the remaining issues associated with the Ring Sensor. The main design consideration associated with the Ring Sensor is achieving minimal power consumption while maintaining high signal quality. To this end, significant effort has been channeled to the development of an appropriate motion artifact model, representing the complex interplay between internal hemodynamics and external influences. Additionally, an artifact resistant, power-efficient, high-speed modulation scheme has been incorporated into the design of the Ring Sensor. It has been shown that this design significantly reduces the amount of data corrupted by motion while also minimizing the power consumed by the LEDs (one of the single largest power consuming elements).(cont.) This thesis also details the refinement of both the analog signal processing circuit and the redesigning of the sensor band for a more secure device interface. In particular, the order and type of filtering utilized by the Ring Sensor have been optimized for signal quality and stability. An improved sensor unit assembly provides a secure, pressurized contact with the patient's skin while protecting the optical components and wires from the external environment, while additional sensors, incorporated into both the sensor band and the ring unit, provide temperature and light feedback for signal quality assurance. In addition to these advancements, preliminary work towards sensor calibration for oxygen saturation measurements is provided. The thesis concludes with promising results obtained from field testing work conducted in the Massachusetts General Hospital's Pulmonary Function Testing Lab.by Phillip Andrew Shaltis.S.M

Reduction of motion artifact in pulse oximetry by smoothed pseudo Wigner-Ville distribution

BACKGROUND: The pulse oximeter, a medical device capable of measuring blood oxygen saturation (SpO2), has been shown to be a valuable device for monitoring patients in critical conditions. In order to incorporate the technique into a wearable device which can be used in ambulatory settings, the influence of motion artifacts on the estimated SpO2 must be reduced. This study investigates the use of the smoothed psuedo Wigner-Ville distribution (SPWVD) for the reduction of motion artifacts affecting pulse oximetry. METHODS: The SPWVD approach is compared with two techniques currently used in this field, i.e. the weighted moving average (WMA) and the fast Fourier transform (FFT) approaches. SpO2 and pulse rate were estimated from a photoplethysmographic (PPG) signal recorded when subject is in a resting position as well as in the act of performing four types of motions: horizontal and vertical movements of the hand, and bending and pressing motions of the finger. For each condition, 24 sets of PPG signals collected from 6 subjects, each of 30 seconds, were studied with reference to the PPG signal recorded simultaneously from the subject's other hand, which was stationary at all times. RESULTS AND DISCUSSION: The SPWVD approach shows significant improvement (p < 0.05), as compared to traditional approaches, when subjects bend their finger or press their finger against the sensor. In addition, the SPWVD approach also reduces the mean absolute pulse rate error significantly (p < 0.05) from 16.4 bpm and 11.2 bpm for the WMA and FFT approaches, respectively, to 5.62 bpm. CONCLUSION: The results suggested that the SPWVD approach could potentially be used to reduce motion artifact on wearable pulse oximeters

Design of an oximeter based on LED-LED configuration and FPGA technology

sensor

Investigation of Photodetector Optimization in Reducing Power Consumption by a Noninvasive Pulse Oximeter Sensor

Noninvasive pulse oximetry represents an area of potential interest to the army, because it could provide cost-effective, safe, fast and real-time physiological assessment in a combat injured soldier. Consequently, there is a need to develop a reliable, battery-powered, wearable pulse oximeter to acquire and process photoplethysmographic (PPG) signals using an optimized sensor configuration. A key requirement in the optimal design of a wearable wireless pulse oximeter is low power management without compromising signal quality. This research investigated the advantage gained by increasing the area of the photodetector and decreasing the light emitting diode (LED) driving currents to reduce the overall power requirement of a reflectance mode pulse oximeter sensor. In vitro and preliminary in vivo experiments were conducted to evaluate a multiple photodetector reflectance sensor setup to simulate a varying detection area. It was concluded that a reflection pulse oximeter sensor employing a large area photodetector is preferred over a similar transmission type sensor for extending the battery life of a wireless pulse oximeter intended for future telemedicine applications

Optimal fiducial points for pulse rate variability analysis from forehead and finger PPG signals

Objective: The aim of this work is to evaluate and compare five fiducialpoints for the temporal location of each pulse wave from forehead and fingerphotoplethysmographic pulse waves signals (PPG) to perform pulse rate variability(PRV) analysis as a surrogate of heart rate variability (HRV) analysis. Approach: Forehead and finger PPG signals were recorded during tilt-table testsimultaneously to the ECG. Artifacts were detected and removed and, five fiducialpoints were computed: apex, middle-amplitude and foot points of the PPG signal,apex point of the first derivative signal and, the intersection point of the tangent tothe PPG waveform at the apex of the derivative PPG signal and the tangent to thefoot of the PPG pulse defined as intersecting tangents method. Pulse period (PP)time intervals series were obtained from both PPG signals and compared to the RRintervals obtained from the ECG. Heart and pulse rate variability signals (HRV andPRV) were estimated and, classical time and frequency domain indices were computed. Main Results: The middle-amplitude point of the PPG signal (nM), the apexpoint of the first derivative (n*A), and the tangents intersection point (nT) are themost suitable fiducial points for PRV analysis, which result in the lowest relativeerrors estimated between PRV and HRV indices, higher correlation coefficients and reliability indexes. Statistically significant differences according to the Wilcoxon testbetween PRV and HRV signals were found for the apex and foot fiducial points ofthe PPG, as well as the lowest agreement between RR and PP series according toBland-Altman analysis. Hence, they have been considered less accurate for variabilityanalysis. In addition, the relative errors are significantly lower fornMandn*Afeaturesby using Friedman statistics with Bonferroni multiple-comparison test and, we proposenMas the most accurate fiducial point. Based on our results, forehead PPG seems toprovide more reliable information for a PRV assessment than finger PPG. Significance: The accuracy of the pulse wave detections depends on the morphologyof the PPG. There is therefore a need to widely define the most accurate fiducial pointto perform a PRV analysis under non-stationary conditions based on different PPGsensor locations and signal acquisition techniques

Design and evaluation of iCalm : a novel, wrist-worn, low-power, low-cost, wireless physiological sensor module

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references (p. 151-156).The impracticality of the ambulatory electrocardiogram for long-term physiological monitoring has lead to the development of many new, compact sensors that have been designed with form factor and user comfort in mind. Nevertheless, there currently is no single sensor module that would be ideal to use for continuous, long-term monitoring. The sensors tend to either lack wireless capabilities, have a short battery life, or are financially unfeasible. After conducting a quick survey of recently developed sensors, we propose the design of iCalm: a novel, wrist-worn, low-power, low-cost, and wireless physiological sensor module. Its performance is compared against an FDA-approved platform through numerous experiments, including a few user studies. The iCalm skin conductance sensor greatly reduced noise due to motion and pressure artifacts; the iCalm heart rate sensor performed similar to the FDA-approved sensor. In addition, all of the participants in the experiments preferred the iCalm to the FDA-approved comparison sensors we tested. With iCalm, we hope to enable comfortable, long-term monitoring of the autonomic nervous system physiology and improve upon the current commercial sensors on the market.by Hoda Eydgahi.S.M