Sources of inaccuracy in photoplethysmography for continuous cardiovascular monitoring

Photoplethysmography (PPG) is a low-cost, noninvasive optical technique that uses change in light transmission with changes in blood volume within tissue to provide information for cardiovascular health and fitness. As remote health and wearable medical devices become more prevalent, PPG devices are being developed as part of wearable systems to monitor parameters such as heart rate (HR) that do not require complex analysis of the PPG waveform. However, complex analyses of the PPG waveform yield valuable clinical information, such as: blood pressure, respiratory information, sympathetic nervous system activity, and heart rate variability. Systems aiming to derive such complex parameters do not always account for realistic sources of noise, as testing is performed within controlled parameter spaces. A wearable monitoring tool to be used beyond fitness and heart rate must account for noise sources originating from individual patient variations (e.g., skin tone, obesity, age, and gender), physiology (e.g., respiration, venous pulsation, body site of measurement, and body temperature), and external perturbations of the device itself (e.g., motion artifact, ambient light, and applied pressure to the skin). Here, we present a comprehensive review of the literature that aims to summarize these noise sources for future PPG device development for use in health monitoring

Measuring Venous Oxygen Saturation Using the Photoplethysmograph Waveform

The pulse oximeter measures the arterial oxygen saturation. It accomplishes this through the use of the photoplethysmograph waveform (PPG) at two or more wavelengths. It has been recognized for some time that the movement of venous blood can be detected using the PPG. We hypothesize that the PPG waveform, obtained non-invasively by modern pulse oximeters, can be analyzed via digital signal processing to infer the venous oxygen saturation. Fundamental to the successful isolation of the venous saturation is the identification of PPG characteristics that are unique to the peripheral venous system. Two such characteristics have been identified. First, the peripheral venous waveform tends to reflect atrial contraction (e.g., a-c-v waveform). Second, ventilation tends to move venous blood preferentially due to the low pressure and high compliance of the venous system. Red (660nm) and IR (940nm) PPG waveforms were collected from 10 cardiac surgery patients using an esophageal PPG probe. These waveforms were analyzed using algorithms written in Mathematica (Wolfram Research). The eight saturation algorithms (ArtSat, VenSat, ArtInstSat, VenInstSat, RespDC, RespAC, Cardiac, and Harmonic) were applied to the data set. Three of the methods (VenSat, VenInstSat, and RespDC) demonstrate significance difference from ArtSat using the Wilcoxon signed-rank test with Bonferroni correction (p\u3c0.0071). This thesis introduces new methods of PPG analysis. Three methods of analysis (VenSat, VenInstSat, and RespDC) succeed in detecting lower saturation blood. The next step is to confirm the accuracy of the measurement by comparing them to a gold standard (i.e., venous blood gas)

PDA based ambulatory pulse oximeter

The main aim of the research was to develop an ambulatory pulse oximeter which can be used to monitor the SpO2 heart rate (HR) and plethsymograph (PPG) waveform of a person. To achieve this, an algorithm was developed in LabVIEW 8.0 to extract the HR, SpO2 and PPG data from a Nonin Xpod device, Nonin Medical, Inc. LabVIEW PDA software was developed to make it compatible with the PDA. LabVIEW software was also developed for sending the data to the PDA via Bluetooth Pulse Oximeter commercialized by Nonin Medical Company. Using this algorithm, data was collected from three different sensors, namely finger, ear and reflectance sensor which can be used in many places. All the three sensors were attached each at a time during different activities and movements which included hand movement, vertical and horizontal head movement, twisting, walking and spot jogging. Comparative study of each sensor was made to conclude which sensor was a preferred choice over the other during each activity. Comparative study among the sensors was also done to observe which sensor worked better with HR and SpO2 monitoring along with motion. Various tests such as supine-stand test and mental activity were performed to observe the changes in the blood flow of a person using the PPG waveform. From the results obtained it was concluded that different sensors were preferred during different movements. For monitoring HR with motion, a reflectance sensor worked better, while the finger sensor for SpO2 monitoring with motion. Results obtained from the supine stand and mental activity tests were as per the expected results

PHOTOPLETHYSMOGRAPHIC WAVEFORM ANALYSIS DURING LOWER BODY NEGATIVE PRESSURE SIMULATED HYPOVOLEMIA AS A TOOL TO DISTINGUISH REGIONAL DIFFERENCES IN MICROVASCULAR BLOOD FLOW REGULATION.

The purpose of this investigation was to explore modulation of the photoplethsymographic (PPG) waveform in the setting of simulated hypovolemia as a tool to distinguish regional differences in regulation of the microvasculature. The primary goal was to glean useful physiological and clinical information as it pertains to these regional differences in regulation of microvascular blood flow. This entailed examining the cardiovascular, autonomic nervous, and respiratory systems interplay in the functional hemodynamics of regulation of microvascular blood flow to both central (ear, forehead) and peripheral (finger) sites. We monitored ten healthy volunteers (both men and women age 24-37 ) non-invasively with central and peripheral photoplethysmographs and laser Doppler flowmeters during Lower Body Negative Pressure (LBNP). Waveform amplitude, width, and oscillatory changes were characterized using waveform analysis software (Chart, ADInstruments). Data were analyzed with the Wilcoxon Signed Ranks Test, paired t-tests, and linear regression. Finger PPG amplitude decreased by 34.6 ± 17.6% (p = 0.009) between baseline and the highest tolerated LBNP. In contrast, forehead amplitude changed by only 2.4 ± 16.0% (p=NS). Forehead and finger PPG width decreased by 48.4% and 32.7%, respectively. Linear regression analysis of the forehead and finger PPG waveform widths as functions of time generated slopes of -1.113 (R = -0.727) and -0.591 (R = -0.666), respectively. A 150% increase in amplitude density of the ear PPG waveform was noted within the range encompassing the respiratory frequency (0.19-0.3Hz) (p=0.021) attributable to changes in stroke volume. We also noted autonomic modulation of the ear PPG signal in a different frequency band (0.12 0.18 Hz). The data indicate that during a hypovolemic challenge, healthy volunteers had a relative sparing of central cutaneous blood flow when compared to a peripheral site as indicated by observable and quantifiable changes in the PPG waveform. These results are the first documentation of a local vasodilatation at the level of the terminal arterioles of the forehead that may be attributable to recently documented cholinergic mechanisms on the microvasculature

Wireless Sensor Platform for Pulse Oximetry

Pulse rate and oxygen saturation are two important clinical measurements that indicate the state of a person’s essential body functions. Oxygen saturation is the measurement of oxygenated hemoglobin in arterial blood i.e. it indicates the level of oxygen in the blood. Pulse oximeters, consisting of LEDs and photodetectors, offer a simple and low cost means of monitoring both pulse rate and blood oxygen saturation non-invasively. The primary objective of this project was to develop a wireless platform for MEMS devices. For this project, a pulse oximeter was also developed as a demonstration vehicle for this wireless platform. A microcontroller and a Bluetooth module was used to transmit the data from the sensor to the smartphone and an Android program was developed as a part of the project to connect with the Bluetooth module and receive, plot and save the data. Once the sensor and Android application were developed, the pulse rate and oxygen saturation measurements were compared to measurements taken by a commercial pulse oximeter to determine the accuracy of the device. The sensor was able to accurately measure with an average error percentage of ±2.86% and ±1.08% for pulse rate and oxygen saturation respectively

Pulse oximetry and photoplethysmographic waveform analysis of the esophagus and bowel

Purpose of review This article reviews the development of novel reflectance pulse oximetry sensors for the esophagus and bowel, and presents some of the techniques used to analyze the waveforms acquired with such devices. Recent findings There has been much research in recent years to expand the utility of pulse oximetry beyond the simple measurement of arterial oxygen saturation from the finger or earlobe. Experimental sensors based on reflectance pulse oximetry have been developed for use in internal sites such as the esophagus and bowel. Analysis of the photoplethysmographic waveforms produced by these sensors is beginning to shed light on some of the potentially useful information hidden in these signals. Summary The use of novel reflectance pulse oximetry sensors has been successfully demonstrated. Such sensors, combined with the application of more advanced signal processing, will hopefully open new avenues of research leading to the development of new types of pulse oximetry-based monitoring techniques

Review of sensors for remote patient monitoring

Remote patient monitoring (RPM) of physiological measurements can provide an efficient method and high quality care to patients. The physiological signals measurement is the initial and the most important factor in RPM. This paper discusses the characteristics of the most popular sensors, which are used to obtain vital clinical signals in prevalent RPM systems. The sensors discussed in this paper are used to measure ECG, heart sound, pulse rate, oxygen saturation, blood pressure and respiration rate, which are treated as the most important vital data in patient monitoring and medical examination