50 research outputs found
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Photoplethysmography for Quantitative Assessment of Sympathetic Nerve Activity (SNA) During Cold Stress
The differences in the degree of sympathetic nerve activity (SNA) over cutaneous blood vessels, although known to be more prominent in the periphery than the core vasculature, has not been thoroughly investigated quantitatively. Hence, two studies were carried out to investigate the differences in SNA between the periphery and the core during the cold pressor test (CPT) (right-hand immersion in ice water) and cold exposure (whole body exposed to cold air) using photoplethysmography (PPG). Two methods utilizing PPG, namely differential multi-site PTT measurements and low-frequency spectral analysis were explored for quantitative determination of SNA. Each study involved 12 healthy volunteers, and PPG signals were acquired from the right index finger (RIF), left index finger (LIF) (periphery) and the ear canal (core). During CPT, Pulse Transit Time (PTT) was measured to the respective locations and the mean percentage change in PTT during ice immersion at each location was used as an indicator for the extent of SNA. During cold exposure, the low-frequency spectral analysis was performed on the acquired raw PPGs to extract the power of the sympathetic [low-frequency (LF): 0.04–0.15 Hz] and parasympathetic components [high-frequency (HF): 0.15–0.4 Hz]. The ratio of LF/HF components was then used to quantify the differences in the influence of SNA on the peripheral and core circulation. PTT measured from the EC, and the LIF has dropped by 5 and 7%, respectively during ice immersion. The RIF PTT, on the other hand, has dropped significantly (P < 0.05) by 12%. During the cold exposure, the LF/HF power ratio at the finger has increased to 86.4 during the cold exposure from 19.2 at the baseline (statistically significant P = 0.002). While the ear canal LF/HF ratio has decreased to 1.38 during the cold exposure from 1.62 at baseline (P = 0.781). From these observations, it is evident that differential PTT measurements or low-frequency analysis can be used to quantify SNA. The results also demonstrate the effectiveness of the central auto-regulation during both short and long-term stress stimulus as compared to the periphery
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Investigation of photoplethysmography and arterial blood oxygen saturation from the ear-canal and the finger under conditions of artificially induced hypothermia
Pulse oximeters relay on the technique of photoplethysmography (PPG) to estimate arterial oxygen saturation (SpO2). In conditions of poor peripheral perfusion such as hypotension, hypothermia, and vasoconstriction, pulse oximeters become inaccurate or provide no reading. This is due to the poor quality of the PPG signals detected at that instance. In order to overcome this problem, the ear canal has been proposed as a alternative measurement site for measuring reliable SpO2. Hence, an ear canal PPG sensor was developed along with a PPG processing system. The performance of the sensor was evaluated by measuring the red and infrared PPGs and SpO2 from 10 healthy volunteers undergoing artificially induced hypothermia. The results from the ear canal sensor were compared with simultaneously acquired results from the finger. Hypothermia was induced by exposing the volunteers to cold temperatures of 10 ± 1°C. The results acquired suggest that the ear canal pulse oximeter endures more in estimating SpO2 values accurately when compared with the more common finger pulse oximeter
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Investigation of Pulse Transit Times utilizing multisite reflectance photoplethysmography under conditions of artificially induced peripheral vasoconstriction
Pulse Transit Time (PTT) is the time it takes for an arterial pulsation to travel from the heart to a peripheral site. In recent times, PTT has been advocated as a marker for assessing increased vascular resistance. However, the reliability of PTT as a marker for cardiovascular risks and its inverse relation to beat-to-beat blood pressure is still being investigated. In order to validate the technique as a reliable marker of vascular resistance, PTT measurements were made using photoplethysmographic (PPG) signals obtained from multiple measurement sites in 12 healthy volunteers undergoing right hand immersion in ice water for 30 secs. PTT measurements were made from the ear canal (EC), the left (LIF) and right index fingers (RIF) using custom made photoplethysmographic probes. Activation of the sympathetic nervous system during the ice water immersion caused an increase in vascular resistance, which is associated with an increase in mean arterial pressure and a decrease in PTT in all measurement sites. However, the change in PTT was much larger in the RIF when compared to the LIF and the EC. This demonstrates the cerebral flow autoregulation and the profound peripheral vasoconstriction seen in the right hand. After the ice immersion period, the mean PTT measured from the EC returned to baseline, whereas the LIF PTTs exceeded baseline values. This is due to the local vasodilation resulted from the activation of a thermoregulatory mechanism
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Development of an optical probe to investigate the suitability of measuring photoplethysmographs and blood oxygen saturation from the human auditory canal
Pulse oximetry has become a standard for patient monitoring in the operating room, and the finger is the most common site used for monitoring blood oxygen saturation (SpO2). However, SpO2 measurements made from extremities such as the finger, ear lobe and toes become susceptible to inaccuracies, when patients become hypothermic, hypovolemic and vasoconstrictive. This is due to the week arterial pulsations detected in these conditions. To overcome this limitation, the external auditory canal has been proposed as an alternative monitoring site for estimating SpO2, on the hypothesis that this central site will be better perfused. A dual wavelength optoelectronic sensor along with a processing system was developed to investigate the suitability of measuring photoplethysmographic (PPG) signals and SpO2 values in the human auditory canal. A pilot study was conducted on 12 healthy volunteers to validate the developed sensor. The red and infrared PPG signals obtained from all the volunteers were of very good quality. The SpO2 values recorded from the ear canal were compared with simultaneously acquired data from a commercial finger pulse oximeter. The results show good correlation between the commercial pulse oximeter and the custom made ear canal sensor (r(2) = 0.825)
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Near infrared spectrometric investigation of lactate in a varying pH buffer
Lactic acidosis is commonly observed in various disease states in critical care and can be adopted as a hemodynamic biomarker, as well as a target for therapy. pH is the main biomarker for the diagnosis of acid–base disorders and is currently measured utilizing invasive blood sampling techniques. Therefore, there is a need for a non-invasive and continuous technology for the measurement of pH and lactate levels. In this work, near infrared spectroscopy is explored as a technique for investigating lactic acidosis. In-vitro studies on 20 isotonic phosphate buffer solutions of varying pH with constant lactate concentration (2 mmol/L) were performed. The whole near infrared spectrum (800–2600 nm) was then divided into four parts for analysis: (a) water absorption peaks, (b) 1000–1250 nm, (c) 1700–1760 nm, and (d) 2200–2400 nm. The water absorption peaks showed a linear variation with the changes in pH in the spectra. The range from 1700–1760 nm showed good correlation with calculated values for lactate ionization, with the changes in pH. However, the region from 2200–2400 nm showed a reverse correlation with respect to the concentration changes of lactate and a distinction could be made from pH 6–7 and 7–8. This study successfully identifies wavelengths (1233 nm, 1710 nm, 1750 nm, 2205 nm, 2319 nm, and 2341 nm) which can be directly correlated to lactic acidosis. Knowledge from this study will contribute toward the development of lactate-based pH monitoring optical sensor for critical care
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Design and Development of a Modular, Multichannel Photoplethysmography System
In this paper, we present the design, development, and validation of a `modular photoplethysmography (PPG) system called ZenPPG. This portable, dual-channel system has the capability to produce ``raw'' PPG signals at two different wavelengths using commercial and/or custom-made PPG sensors. The system consists of five modules, each consisting of circuitry required to perform specific tasks, and are all interconnected by a system bus. The ZenPPG system also facilitates the acquisition of other physiological signals on-demand including electrocardiogram (ECG), respiration, and temperature signals. This report describes the technical details and the evaluation of the ZenPPG along with results from a pilot in vivo study on healthy volunteers. The results from the technical evaluations demonstrate the superiority and flexibility of the system. Also, the systems' compatibility with commercial pulse oximetry sensors such as the Masimo reusable sensors was demonstrated, where good quality raw PPG signals were recorded with the signal-to-noise ratio (SNR) of 50.65 dB. The estimated arterial oxygen saturation (SpO & #x2082;) values from the system were also in close agreement with commercial pulse oximeters, although the accuracy of the reported SpO & #x2082; value is dependent on the calibration function used. Future work is targeted toward the development of variations of each module, including the laser driver and fiber optic module, onboard data acquisition and signal processing modules. The availability of this system will help researchers from a wide range of disciplines to customize and integrate the ZenPPG system to their research needs and will most definitely enhance research in related fields
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Investigation of photoplethysmography, laser doppler flowmetry and near infrared spectroscopy during induced thermal stress
Continuous assessment of blood flow, blood volume, and blood and tissue oxygenation are of vital importance in critically ill patients. Photoplethysmography (PPG), Pulse Oximetry (PO), Laser Doppler Flowmetry (LDF) and Near Infrared Spectroscopy (NIRS) are amongst the most widely used techniques to monitor such perfusion parameters. In this study, we investigated the feasibility of using dual-wavelength PPG signals on providing comparable information as LDF and NIRS, besides arterial oxygen saturation (SpO2) as measured by pulse oximetry. All three techniques were investigated on six healthy volunteers during whole-body cold exposure. PPG and LDF sensors were attached on the finger and hand respectively, while NIRS was positioned above the left forearm. Measurements at room temperature (24°C) were followed and preceded by a cold exposure (10°C). The results showed that changes in pulsatile PPG amplitudes and hemoglobin concentration estimated from finger PPG signals indicate strong similarities with gold-standard LDF and NIRS measurements
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In vitro quantification of lactate in Phosphate Buffer Saline (PBS) samples.
Continuous measurement of lactate levels in the blood is a prerequisite in intensive care patients who are susceptible to sepsis due to their suppressed immune system and increased metabolic demand. Currently, there exists no noninvasive tool for continuous measurement of lactate in clinical practice. The current mode of measurement is based on arterial blood gas analyzers which require sampling of arterial blood. In this work, we propose the use of Near Infra-Red (NIR) spectroscopy together with multivariate models as a means to non-invasively predict the concentration of lactate in the blood. As the first step towards this objective, we examined the possibility of accurately predicting concentrations of sodium lactate (NaLac) from the NIR spectra of 37 isotonic phosphate buffer saline (PBS) samples containing NaLac ranging from 0 to 20 mmol/L. NIR spectra of PBS samples were collected using the Lambda 1050 dual beam spectrometer over a spectral range of 800 - 2600 nm with a quartz cell of 1 mm optical path. Estimates and calibration of the lactate concentration with the NIR spectra were made using Partial Least-Squares (PLS) regression analysis and leave-one-out cross-validation on filtered spectra. The regression analysis showed a correlation coefficient of 0.977 and a standard error of 0.89 mmol/L between the predicted and prepared samples. The results suggest that NIR spectroscopy together with multivariate models can be a valuable tool for non-invasive assessment of blood lactate concentrations
Monte Carlo Analysis of Optical Interactions in Reflectance and Transmittance Finger Photoplethysmography
Photoplethysmography (PPG) is a non-invasive photometric technique that measures the volume changes in arterial blood. Recent studies have reported limitations in developing and optimising PPG-based sensing technologies due to unavailability of the fundamental information such as PPG-pathlength and penetration depth in a certain region of interest (ROI) in the human body. In this paper, a robust computational model of a dual wavelength PPG system was developed using Monte Carlo technique. A three-dimensional heterogeneous volume of a specific ROI (i.e., human finger) was exposed at the red (660 nm) and infrared (940 nm) wavelengths in the reflectance and transmittance modalities of PPG. The optical interactions with the individual pulsatile and non-pulsatile tissue-components were demonstrated and the optical parameters (e.g., pathlength, penetration depth, absorbance, reflectance and transmittance) were investigated. Results optimised the source-detector separation for a reflectance finger-PPG sensor. The analysis with the recorded absorbance, reflectance and transmittance confirmed the maximum and minimum impact of the dermis and bone tissue-layers, respectively, in the formation of a PPG signal. The results presented in the paper provide the necessary information to develop PPG-based transcutaneous sensors and to understand the origin of the ac and dc components of the PPG signal
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Design and development of a novel multi-channel photoplethysmographic research system
Photoplethysmography (PPG) is a technique that uses light to non-invasively obtain a volumetric measurement of an organ with each cardiac cycle. Pulse Oximetry (PO) is an empirical technique which allows the arterial blood oxygen saturation (SpO2) evaluation from the PPG signals. There have been many reports in the literature suggesting that other arterial blood chemical components can be evaluated from the PPG signals. Most attempts to perform such evaluation on empirical bases have failed, especially for components concentrations. This paper introduces a non-empirical rational theory called Dynamic Pulsatile Spectroscopy (DPS) which can be used to analytically investigate the phenomena of PPG. The DPS theory provides the mathematically rigid method of how PPG signals can be used for arterial blood analysis to evaluate its chemical component concentrations and molar fractions spectroscopically and transcutaneously. It also highlights what other signals might be required for such evaluation. DPS opens the possibility of extending PPG application for blood analysis beyond conventional PO. The DPS basic principles are introduced in this paper