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

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

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)

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

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

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

In vivo investigations of photoplethysmograms and arterial oxygen saturation from the auditory canal in conditions of compromised peripheral perfusion

Pulse oximeters rely on the technique of photoplethysmography (PPG) to estimate arterial oxygen saturation (SpO2). In conditions of poor peripheral perfusion such as hypotension, hypothermia, and vasoconstriction, the PPG signals detected are often small and noisy, or in some cases unobtainable. Hence, pulse oximeters produce erroneous SpO2 readings in these circumstances. The problem arises as most commercial pulse oximeter probes are designed to be attached to peripheral sites such as the finger or toes, which are easily affected by vasoconstriction. In order to overcome this problem, the ear canal was investigated as an alternative site for measuring reliable SpO2 on the hypothesis that blood flow to this central site is preferentially preserved. Novel miniature ear canal PPG sensors were developed along with a state of the art PPG processing unit and a data acquisition system to allow for PPG measurements from different depths and surfaces of the ear canal. A preliminary in vivo investigation on seven healthy volunteers has revealed that good quality PPG signals with high amplitude can be obtained from the posterior surface of the outer ear canal. Based on these observations, a second prototype probe suitable for acquisition of PPGs from the posterior surface of the outer ear canal was developed. A pilot study was then carried out on 15 healthy volunteers to validate the feasibility of measuring PPGs and SpO2 from the ear canal in conditions of induced local peripheral vasoconstriction (right hand immersion in ice water). The PPG signals acquired from the ear canal probe were compared with those obtained simultaneously from finger probes attached to the left and the right index fingers. Significant drop (p 45%) and right (> 50%) index fingers during the ice water immersion, while good quality PPG signals with relatively constant amplitude were obtained from the ear canal. Also, the SpO2 values showed that the ear canal pulse oximeter performed better than the two finger pulse oximeters (mean failure rate 30%). A second in vivo investigation was carried out in 15 healthy volunteers, where hypoperfusion was induced more naturally by exposing the volunteer to cold temperatures of 10C for 10min. Normalised Pulse Amplitude (NPA) and SpO2 was calculated from the PPG signals acquired from the ear canal, the finger and the earlobe. By the end of the cold exposure, a mean drop of > 80% was found in the NPA of finger PPGs. The % drop in the NPA of red and infrared earlobe PPG signals was 20% and 26% respectively. Contrarily to both these sites, the NPA of the ear canal PPGs had only dropped by 0.2% and 13% respectively. The SpO2 estimated from the finger sensor was below 90% in 5 volunteers (failure) by the end of the cold exposure. The earlobe pulse oximeter failed in 3 volunteers. The ear canal sensor on the other hand had only failed in 1 volunteer. These results strongly suggest that the ear canal may be used as a suitable alternative site for reliable monitoring of PPGs and SpO2 in cases of compromised peripheral perfusion

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

Design and Development of a novel Multi-channel Photoplethysmographic Research System

Photoplethysmography (PPG) is a non-invasive optical technique used in detecting blood volume changes in vascular tissue. In recent years, photoplethysmography has generated renewed interest among researchers, particularly due to its potential for widespread clinical applications beyond estimating heart rate and arterial oxygen saturation. Progress in these areas of research is dependent on PPG systems with the ability to record raw photoplethysmographic signals with high signal-to-noise ratio, ideally from more than one channel and one wavelength, for real-time or retrospective analysis. In this work we present the design, development and validation of such a modular PPG system. “Zen PPG”- a portable (16.0 x 10.3 x 5.4 cm) battery operated dual channel, dual wavelength system with the capability to operate with commercial sensors was developed and calibrated using a FLUKE Index 2 SpO2 simulator. A brief study has been conducted on a small group of healthy volunteers to demonstrate the functionality of the system. Red and Infrared PPG signals obtained from volunteers were used to estimate SpO2 and heart rate. These results were compared with simultaneously acquired data from a commercial pulse oximeter (Masimo Radical 7). The SpO2 values showed close correlation between commercial and custom made system (Channel 1: r2 = 0.978; channel 2: r2= 0.708)

The human ear canal: investigation of its suitability for monitoring photoplethysmographs and arterial oxygen saturation

For the last two decades, pulse oximetry has been used as a standard procedure for monitoring arterial oxygen saturation (SpO2). However, SpO2 measurements made from extremities such as the finger, ear lobe and toes become susceptible to inaccuracies when peripheral perfusion is compromised. To overcome these limitations, 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. Therefore, a dual wavelength optoelectronic probe along with a processing system was developed to investigate the suitability of measuring photoplethysmographic (PPG) signals and SpO2 in the human auditory canal. A pilot study was carried out in 15 healthy volunteers to validate the feasibility of measuring PPGs and SpO2 from the ear canal (EC), and comparative studies were performed by acquiring the same signals from the left index finger (LIF) and the right index finger (RIF) in conditions of induced peripheral vasoconstriction (right hand immersion in ice water). Good quality baseline PPG signals with high signal-to-noise ratio were obtained from the EC, the LIF and the RIF sensors. During the ice water immersion, significant differences in the amplitude of the red and infrared PPG signals were observed from the RIF and the LIF sensors. The average drop in amplitude of red and infrared PPG signals from the RIF was 52.7% and 58.3%. Similarly, the LIF PPG signal amplitudes have reduced by 47.52% and 46.8% respectively. In contrast, no significant changes were seen in the red and infrared EC PPG amplitude measurements, which changed by +2.5% and -1.2% respectively. The RIF and LIF pulse oximeters have failed to estimate accurate SpO2 in seven and four volunteers respectively, while the EC pulse oximeter has only failed in one volunteer. These results suggest that the EC may be a suitable site for reliable monitoring of PPGs and SpO2s even in the presence of peripheral vasoconstriction