Pulse oximetry in the oesophagus

A prospective study for an alternative probe site for pulse oximetry measurement in male patients with severe burn trauma: penile shaf

BACKGROUND

Authors widely use pulse oximetry in clinical monitoring of heart rate (HR) and peripheral oxygen saturation (SpO2) by attachment to the fingers; however, there can be a need for an alternative attachment site, especially for burned patients. We investigate the availability of a pulse oximeter probe attached to the penile shaft as an alternative site in pediatric male patients if all extremities became unavailable for pulse oximetry measurement due to severe burn and/or trauma.

METHODS

We designed a prospective comparative study in a training and research hospital. After local ethical committee approval, pediatric male cases eligible for penile and extremity pulse measurements were evaluated during general anesthesia for medical dressing and/or grafting due to severe burns. One probe was attached to the fingers of the unburned extremity, and the other was to the penile shaft. Furthermore, we recorded SpO2and HR values at 5-min intervals; 0th (baseline), 5th, 10th and 15th minutes. We compared HR and SpO2values measured by the finger probe with those measured by the penile probe.

RESULTS

Data of 51 patients (median age, 2.9 years (interquartile range, 2.0-5.0 years)) in whom the duration of dressing was at least 15min were analyzed. There was no significant difference either in comparisons of hemodynamic measurements (HR and SpO2 ) obtainedby finger probe and by a penile probe for each measurement time. The Bland-Altman plot analysis reveals agreement for penile and finger probes with a mean bias value between 0.20 and 0.37 on HR and between 0.43 and -0.20 on SpO2.

DISCUSSION

This clinical trial demonstrated that pulse oximetry measurement under nonhypoxic conditions we could perform confidently using penile probes in pediatric male patients whose extremities are unavailable for measurement.

Oropharyngeal and Tongue Pulse Oximetry in 2 Critically Ill Pediatric Patients: A Case Report

Pulse oximetry has become a standard of care to monitor oxygenation. Absent or inaccurate readings can occur with varied patient states. We present preliminary experience with a modification of a standard pulse oximetry using readily available equipment (oral airway and a tongue blade) to allow for continuous pulse oximetry from the oral cavity and tongue in 2 critically ill pediatric patients when standard application of pulse oximetry was not feasible or nonfunctional. These modifications can assist in the care of critically ill patients, allowing for adaptability in monitoring techniques when other options are unavailable.

Effect of Filtering of Photoplethysmography Signals in Pulse Rate Variability Analysis. ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. ANNUAL INTERNATIONAL CONFERENCE 2021;2021:5500-5503. [PMID: 34892370 DOI: 10.1109/embc46164.2021.9629521]

Due to the widespread use and simplicity of photoplethysmography (PPG) signals, and because this signal contains information related to pulse rate, several studies have started to propose the use of Pulse Rate Variability (PRV) for the assessment of cardiovascular autonomic nervous activity, instead of using Heart Rate Variability (HRV) obtained with the electrocardiogram (ECG). However, there is a lack of standardisation and guidelines for the measurement of PRV from PPG signals, which might hinder comparability among studies and validation of results. The aim of this study was to evaluate different digital filters on PPG signals and their effects on PRV information, compared to HRV obtained from ECG. PPG and ECG signals obtained from healthy volunteers were used to measure HRV and PRV. PPG signals were filtered using different FIR and IIR digital filters, with several cut-off frequencies. The results indicate that filtering PPG signals using IIR filters and lower low-cut-off frequencies allow for the acquisition of more reliable PRV information, with lower Bland-Altman ratios and higher cross-correlations when compared to HRV. This is a first step in establishing guidelines and standards for the analysis of PRV information using PPG signals.Clinical relevance- Pulse rate variability might be a useful tool for the assessment of the cardiovascular autonomic nervous system. This study is the first step for establishing standards of measurement of this signal, which helps in the comparability and validation of the technique.

Non-Contact SpO2 Prediction System Based on a Digital Camera

Patients with the COVID-19 condition require frequent and accurate blood oxygen saturation (SpO2) monitoring. The existing pulse oximeters, however, require contact-based measurement using clips or otherwise fixed sensor units or need dedicated hardware which may cause inconvenience and involve additional appointments with the patient. This study proposes a computer vision-based system using a digital camera to measure SpO2 on the basis of the imaging photoplethysmography (iPPG) signal extracted from the human’s forehead without the need for restricting the subject or physical contact. The proposed camera-based system decomposes the iPPG obtained from the red and green channels into different signals with different frequencies using a signal decomposition technique based on a complete Ensemble Empirical Mode Decomposition (EEMD) technique and Independent Component Analysis (ICA) technique to obtain the optical properties from these wavelengths and frequency channels. The proposed system is convenient, contactless, safe and cost-effective. The preliminary results for 70 videos obtained from 14 subjects of different ages and with different skin tones showed that the red and green wavelengths could be used to estimate SpO2 with good agreement and low error ratio compared to the gold standard of pulse oximetry (SA210) with a fixed measurement position.

A review of wearable and unobtrusive sensing technologies for chronic disease management

With the rapidly increasing number of patients with chronic disease, numerous recent studies have put great efforts into achieving long-term health monitoring and patient management. Specifically, chronic diseases including cardiovascular disease, chronic respiratory disease and brain disease can threaten patients' health conditions over a long period of time, thus effecting their daily lives. Vital health parameters, such as heart rate, respiratory rate, SpO2 and blood pressure, are closely associated with patients’ conditions. Wearable devices and unobtrusive sensing technologies can detect such parameters in a convenient way and provide timely predictions on health condition deterioration by tracking these biomedical signals and health parameters. In this paper, we review current advancements in wearable devices and unobtrusive sensing technologies that can provides possible tools and technological supports for chronic disease management. Current challenges and future directions of related techniques are addressed accordingly.

Photoplethysmography in postoperative monitoring of deep inferior epigastric perforator (DIEP) free flaps

OBJECTIVE

Deep inferior epigastric perforator (DIEP) free flaps are widely used as a reconstruction option following mastectomy in breast cancer. During such cases partial tissue necrosis can occur due to the insufficient blood supply to the transplanted tissue site. Therefore, monitoring of flap perfusion and early detection of flap failure is a prerequisite to flap survival. There is a need to develop a non-invasive, easy to use, reproducible and inexpensive monitoring device to assess flap perfusion postoperatively.

APPROACH

A three-wavelength reflective optical sensor and processing system based on the principle of photoplethysmography (PPG) has been developed to investigate blood volumetric changes and estimate free flap blood oxygen saturation continuously and non-invasively in DIEP free flaps in the postoperative period. The system was evaluated in 15 patients undergoing breast reconstructive surgery using DIEP free flap. Main results and Significance: Good quality red, infrared and green PPG signals were obtained in the postoperative period. Initial estimation of blood oxygen saturation values estimated from the free flap PPGs seem to be in broad agreement with the commercial finger pulse oximeter used in this study. This pilot study has demonstrated that PPG has the potential to be used as a monitoring technique in assessing free flap viability.

Medical Engineering Education: Designing a Product Innovation and Technology Session for First-Year Medical Students

The Carle Illinois College of Medicine created an innovative model for medical education that integrates engineering principles into an active learning curriculum. First-year students were introduced to a medical device in an engaging product innovation and technology session. The goals were to discuss the physiology of oxygen saturation and demonstrate the ability to use observation and research to develop a new product idea. Students hypothetically competed with others to raise money from investors to pursue an efficient medical device and attend the users' needs. Student's feedback reflected a positive impact on their understanding of oximetry measurements and product innovation.

A Textile Sleeve for Monitoring Oxygen Saturation Using Multichannel Optical Fibre Photoplethysmography

Textile-based systems are an attractive prospect for wearable technology as they can provide monitoring of key physiological parameters in a comfortable and unobtrusive form. A novel system based on multichannel optical fibre sensor probes integrated into a textile sleeve is described. The system measures the photoplethysmogram (PPG) at two wavelengths (660 and 830 nm), which is then used to calculate oxygen saturation (SpO₂). In order to achieve reliable measurement without adjusting the position of the garment, four plastic optical fibre (POF) probes are utilised to increase the likelihood that a high-quality PPG is obtained due to at least one of the probes being positioned over a blood vessel. Each probe transmits and receives light into the skin to measure the PPG and SpO₂. All POFs are integrated in a stretchable textile sleeve with a circumference of 15 cm to keep the sensor in contact with the subject’s wrist and to minimise motion artefacts. Tests on healthy volunteers show that the multichannel PPG sensor faithfully provides an SpO₂ reading in at least one of the four sensor channels in all cases with no need for adjusting the position of the sleeve. This could not be achieved using a single sensor alone. The multichannel sensor is used to monitor the SpO₂ of 10 participants with an average wrist circumference of 16.0 ± 0.6 cm. Comparing the developed sensor’s SpO₂ readings to a reference commercial oximeter (reflectance Masimo Radical-7) illustrates that the mean difference between the two sensors’ readings is −0.03%, the upper limit of agreement (LOA) is 0.52% and the lower LOA is −0.58%. This multichannel sensor has the potential to achieve reliable, unobtrusive and comfortable textile-based monitoring of both heart rate and SpO₂ during everyday life.

Hearables: New Perspectives and Pitfalls of In-Ear Devices for Physiological Monitoring. A Scoping Review

Technological advancements are opening the possibility of prolonged monitoring of physiological parameters under daily-life conditions, with potential applications in sport science and medicine, and in extreme environments. Among emerging wearable technologies, in-ear devices or hearables possess technical advantages for long-term monitoring, such as non-invasivity, unobtrusivity, good fixing, and reduced motion artifacts, as well as physiological advantages related to the proximity of the ear to the body trunk and the shared vasculature between the ear and the brain. The present scoping review was aimed at identifying and synthesizing the available evidence on the use and performance of in-ear monitoring of physiological parameters, with focus on applications in sport science, sport medicine, occupational medicine, and extreme environment settings. Pubmed, Scopus, and Web of Science electronic databases were systematically searched to identify studies conducted in the last 10 years and addressing the measurement of three main physiological parameters (temperature, heart rate, and oxygen saturation) in healthy subjects. Thirty-nine studies were identified, 24 performing temperature measurement, 12 studies on heart/pulse rate, and three studies on oxygen saturation. The collected evidence supports the premise of in-ear sensors as an innovative and unobtrusive way for physiological monitoring during daily-life and physical activity, but further research and technological advancement are necessary to ameliorate measurement accuracy especially in more challenging scenarios.

Wearables for the Next Pandemic

This paper reviews the current state of the art in wearable sensors, including current challenges, that can alleviate the loads on hospitals and medical centers. During the COVID-19 Pandemic in 2020, healthcare systems were overwhelmed by people with mild to severe symptoms needing care. A careful study of pandemics and their symptoms in the past 100 years reveals common traits that should be monitored for managing the health and economic costs. Cheap, low power, and portable multi-modal-sensors that detect the common symptoms can be stockpiled and ready for the next pandemic. These sensors include temperature sensors for fever monitoring, pulse oximetry sensors for blood oxygen levels, impedance sensors for thoracic impedance, and other state sensors that can be integrated into a single system and connected to a smartphone or data center. Both research and commercial medically approved devices are reviewed with an emphasis on the electronics required to realize the sensing. The performance characteristics, such as accuracy, power, resolution, and size of each sensor modality are critically examined. A discussion of the characteristics, research challenges, and features of an ideal integrated wearable system is also presented.

The Various Oximetric Techniques Used for the Evaluation of Blood Oxygenation

Adequate oxygen delivery to a tissue depends on sufficient oxygen content in arterial blood and blood flow to the tissue. Oximetry is a technique for the assessment of blood oxygenation by measurements of light transmission through the blood, which is based on the different absorption spectra of oxygenated and deoxygenated hemoglobin. Oxygen saturation in arterial blood provides information on the adequacy of respiration and is routinely measured in clinical settings, utilizing pulse oximetry. Oxygen saturation, in venous blood (SvO₂) and in the entire blood in a tissue (StO₂), is related to the blood supply to the tissue, and several oximetric techniques have been developed for their assessment. SvO₂ can be measured non-invasively in the fingers, making use of modified pulse oximetry, and in the retina, using the modified Beer–Lambert Law. StO₂ is measured in peripheral muscle and cerebral tissue by means of various modes of near infrared spectroscopy (NIRS), utilizing the relative transparency of infrared light in muscle and cerebral tissue. The primary problem of oximetry is the discrimination between absorption by hemoglobin and scattering by tissue elements in the attenuation measurement, and the various techniques developed for isolating the absorption effect are presented in the current review, with their limitations.

Heart Rate Variability (HRV) and Pulse Rate Variability (PRV) for the Assessment of Autonomic Responses

Introduction: Heart Rate Variability (HRV) and Pulse Rate Variability (PRV), are non-invasive techniques for monitoring changes in the cardiac cycle. Both techniques have been used for assessing the autonomic activity. Although highly correlated in healthy subjects, differences in HRV and PRV have been observed under various physiological conditions. The reasons for their disparities in assessing the degree of autonomic activity remains unknown.

Methods: To investigate the differences between HRV and PRV, a whole-body cold exposure (CE) study was conducted on 20 healthy volunteers (11 male and 9 female, 30.3 ± 10.4 years old), where PRV indices were measured from red photoplethysmography signals acquired from central (ear canal, ear lobe) and peripheral sites (finger and toe), and HRV indices from the ECG signal. PRV and HRV indices were used to assess the effects of CE upon the autonomic control in peripheral and core vasculature, and on the relationship between HRV and PRV. The hypotheses underlying the experiment were that PRV from central vasculature is less affected by CE than PRV from the peripheries, and that PRV from peripheral and central vasculature differ with HRV to a different extent, especially during CE.

Results: Most of the PRV time-domain and Poincaré plot indices increased during cold exposure. Frequency-domain parameters also showed differences except for relative-power frequency-domain parameters, which remained unchanged. HRV-derived parameters showed a similar behavior but were less affected than PRV. When PRV and HRV parameters were compared, time-domain, absolute-power frequency-domain, and non-linear indices showed differences among stages from most of the locations. Bland-Altman analysis showed that the relationship between HRV and PRV was affected by CE, and that it recovered faster in the core vasculature after CE.

Conclusion: PRV responds to cold exposure differently to HRV, especially in peripheral sites such as the finger and the toe, and may have different information not available in HRV due to its non-localized nature. Hence, multi-site PRV shows promise for assessing the autonomic activity on different body locations and under different circumstances, which could allow for further understanding of the localized responses of the autonomic nervous system.

In silico and in vivo investigations using an endocavitary photoplethysmography sensor for tissue viability monitoring

SIGNIFICANCE

Colorectal cancer is one of the major causes of cancer-related deaths worldwide. Surgical removal of the cancerous growth is the primary treatment for this disease. A colorectal cancer surgery, however, is often unsuccessful due to the anastomotic failure that may occur following the surgical incision. Prevention of an anastomotic failure requires continuous monitoring of intestinal tissue viability during and after colorectal surgery. To date, no clinical technology exists for the dynamic and continuous monitoring of the intestinal perfusion.

AIM

A dual-wavelength indwelling bowel photoplethysmography (PPG) sensor for the continuous monitoring of intestinal viability was proposed and characterized through a set of in silico and in vivo investigations.

APPROACH

The in silico investigation was based on a Monte Carlo model that was executed to quantify the variables such as penetration depth and detected intensity with respect to the sensor-tissue separations and tissue perfusion. Utilizing the simulated information, an indwelling reflectance PPG sensor was designed and tested on 20 healthy volunteers. Two sets of in vivo studies were performed using the driving current intensities 20 and 40 mA for a comparative analysis, using buccal tissue as a proxy tissue-site.

RESULTS

Both simulated and experimental results showed the efficacy of the sensor to acquire good signals through the "contact" to a "noncontact" separation of 5 mm. A very slow wavelength-dependent variation was shown in the detected intensity at the normal and hypoxic states of the tissue, whereas a decay in the intensity was found with the increasing submucosal-blood volume. The simulated detected-to-incident-photon-ratio and the experimental signal-to-noise ratio exhibited strong positive correlations, with the Pearson product-moment correlation coefficient R ranging between 0.65 and 0.87.

CONCLUSIONS

The detailed feasibility analysis presented will lead to clinical trials utilizing the proposed sensor.

A Pulsatile Optical Tissue Phantom for the Investigation of Light-Tissue Interaction in Reflectance Photoplethysmography

The aim of this study was to investigate the effect of emitter-detector separation distance and arterial depth in reflectance photoplethysmography (PPG), utilizing a homogenous pulsatile phantom that exhibits the broad optical absorbance and scattering properties of human tissue. The developed phantom comprised of embedded silicone arteries (outer diameter = 4 mm) that were arranged parallel to one another at nine increasing depths (3.2 mm to 24.4 mm). A pulsatile pump (Harvard Apparatus, MA, USA) circulated a blood imitating fluid through the vessels at the desired heart rate (60 bpm) and stroke volume (5 Lmin^-1). The PPG sensor's emitter and detector were isolated on a translation bridge to provide a computer-controlled separation distance between them. Recordings were taken at each vessel depth for emitter-detector separation distances from 2 mm to 8 mm in 0.1 mm steps. The optimum separation distance between the emitter and detector for vessels between depths of 3.2 mm and 10.5 mm was between 3.7 and 4.3 mm, suggesting that the maximum penetration of IR (930 nm) light in a homogenous pulsatile phantom is no greater than 10.5 mm.

Sleep-wake stages classification using heart rate signals from pulse oximetry

The most important index of obstructive sleep apnea/hypopnea syndrome (OSAHS) is the apnea/hyponea index (AHI). The AHI is the number of apnea/hypopnea events per hour of sleep. Algorithms for the screening of OSAHS from pulse oximetry estimate an approximation to AHI counting the desaturation events without consider the sleep stage of the patient. This paper presents an automatic system to determine if a patient is awake or asleep using heart rate (HR) signals provided by pulse oximetry. In this study, 70 features are estimated using entropy and complexity measures, frequency domain and time-scale domain methods, and classical statistics. The dimension of feature space is reduced from 70 to 40 using three different schemes based on forward feature selection with support vector machine and feature importance with random forest. The algorithms were designed, trained and tested with 5000 patients from the Sleep Heart Health Study database. In the test stage, 10-fold cross validation method was applied obtaining performances up to 85.2% accuracy, 88.3% specificity, 79.0% sensitivity, 67.0% positive predictive value, and 91.3% negative predictive value. The results are encouraging, showing the possibility of using HR signals obtained from the same oximeter to determine the sleep stage of the patient, and thus potentially improving the estimation of AHI based on only pulse oximetry.

Practicality and importance of selected endothelial dysfunction measurement techniques: review

The measurement of endothelial dysfunction (ED) has importance in that it indicates the presence of coronary artery disease (Kuvin et al. in J Am Coll Cardiol 38(7):1843-1849, 2001) in addition to acting as a predictor of future adverse events (Halcox et al. in Circulation 106:653-658, 2002). Various tools, methods, and metrics exist that can provide an indicator of endothelial dysfunction. Given the significance of ED, it is of utmost importance to find a measurement technique that is reliable, while defining a metric providing a framework for an overall system that is practical, accurate, and repeatable. Success would provide a tool for the early detection of cardiovascular disease not only moving patients that are currently classified as asymptomatic to symptomatic, but also providing a method to monitor the efficacy of treatments.

Pulse Oximetry as a Screening Test for Hemodynamically Significant Lower Extremity Peripheral Artery Disease in Adults with Type 2 Diabetes Mellitus

Objective

The main objective is to determine if digital pulse oximetry is an acceptable screening tool to detect hemodynamically significant lower extremity peripheral artery disease (PAD) in patients 50 years old and above with type 2 diabetes mellitus (T2DM) seen at the University of Santo Tomas Hospital (USTH).

Methodology

A total of 78 subjects (155 limbs) were included. Using duplex ultrasonography as the reference standard for the presence of hemodynamically significant lower extremity PAD, sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were obtained for abnormal percent oxygen saturation (SpO₂) gradients and for ankle-brachial index (ABI).

Results

Of the 155 limbs, 38.7% had hemodynamically significant stenosis. Pulse oximetry had 76.7% sensitivity (95% CI, 65.2% to 88.1%), 85.3% specificity (95% CI, 78.0% to 92.6%), 76.7% PPV (95% CI, 66.5% to 84.4%) and 85.3% NPV (95% CI, 78.4% to 90.2%). ABI had 40.7% sensitivity (95% CI, 30.1% to 51.3%), 88.2% specificity (95% CI, 80.0% to 96.3%), 68.6% PPV (95% CI, 53.6% to 80.4%) and 70.1% NPV (95% CI, 65.1% to 74.5%). Combining both produces 88.1% sensitivity (95% CI, 78.5% to 97.8%), 74.2% specificity (95% CI, 65-83.4%), 68.4 PPV (95% CI, 60.3% to 75.6%) and 90.8% NPV (95% CI, 83.0% to 95.2%).

Conclusion

The results of this study suggest that pulse oximetry has a higher sensitivity than ABI as a screening tool for hemodynamically significant lower extremity PAD in T2DM patients 50 years old and above. Combining these two tests may be done to achieve a higher sensitivity.

Pulse Oximetry with Two Infrared Wavelengths without Calibration in Extracted Arterial Blood

Oxygen saturation in arterial blood (SaO₂) provides information about the performance of the respiratory system. Non-invasive measurement of SaO₂ by commercial pulse oximeters (SpO₂) make use of photoplethysmographic pulses in the red and infrared regions and utilizes the different spectra of light absorption by oxygenated and de-oxygenated hemoglobin. Because light scattering and optical path-lengths differ between the two wavelengths, commercial pulse oximeters require empirical calibration which is based on SaO₂ measurement in extracted arterial blood. They are still prone to error, because the path-lengths difference between the two wavelengths varies among different subjects. We have developed modified pulse oximetry, which makes use of two nearby infrared wavelengths that have relatively similar scattering constants and path-lengths and does not require an invasive calibration step. In measurements performed on adults during breath holding, the two-infrared pulse oximeter and a commercial pulse oximeter showed similar changes in SpO₂. The two pulse oximeters showed similar accuracy when compared to SaO₂ measurement in extracted arterial blood (the gold standard) performed in intensive care units on newborns and children with an arterial line. Errors in SpO₂ because of variability in path-lengths difference between the two wavelengths are expected to be smaller in the two-infrared pulse oximeter.

The effects of optical sensor-tissue separation in endocavitary photoplethysmography

OBJECTIVE

Intestinal anastomotic failure that occurs mainly due to ischaemia is a serious risk in colorectal cancer patients undergoing surgery. Surgeons continue to rely on subjective methods such as visual inspection to assess intestinal viability during surgery and there are no clinical tools to directly monitor viability postoperatively. A dual-wavelength reflectance optical sensor has been developed for continuous and dynamic monitoring of intestinal viability via the intestinal lumen. Maintaining direct contact between the sensor and the inner intestinal wall can be difficult in an intraluminal design, therefore impacting on signal acquisition and quality. This paper investigates the effect of direct contact versus variable distances between the sensor and the tissue surface of the buccal mucosa as a surrogate.

APPROACH

The in vivo study involved 20 healthy volunteers to measure the effect of optical sensor-tissue distances on the ability to acquire photoplethysmography signals and their quality. Signals were acquired from the buccal mucosa at five optical sensor-tissue distances.

MAIN RESULTS

Distances between 0 mm (contact) to 5 mm were the most optimal, producing signals of high quality and signal-to-noise ratio, resulting in reliable estimations of the blood oxygen saturation. Distances exceeding 5 mm compromised the acquired signals, and were of poor quality, thereby unreliably estimating the blood oxygen saturation.

SIGNIFICANCE

The developed optical sensor proved to be reliable for acquiring photoplethysmography signals for cases where distances between the optical sensor-tissue may arise during the assessment of intraluminal intestinal viability.

Investigating optical path and differential pathlength factor in reflectance photoplethysmography for the assessment of perfusion

Photoplethysmography (PPG) is an optical noninvasive technique with the potential for assessing tissue perfusion. The relative time-change in the concentration of oxyhemoglobin and deoxyhemoglobin in the blood can be derived from DC part of the PPG signal. However, the absolute concentration cannot be determined due to the inadequate data on PPG optical paths. The optical path and differential pathlength factor (DPF) for PPG at red (660 nm) and infrared (880 nm) wavelengths were investigated using a heterogeneous Monte Carlo model of the human forearm. Using the simulated DPFs, the absolute time-change in concentrations were determined from PPG signals recorded from the same tissue site. Results were compared with three conditions of approximated DPFs. Results showed the variation of the optical-path and DPF with different wavelengths and source-detector separations. Approximations resulted in significant errors, for example, using NIRS DPF in PPG led to "cross talk" of -0.4297 and 0.060 and an error of 15.16% to 25.18%. Results confirmed the feasibility of using the PPG (DC) for the assessment of tissue perfusion. The study also identified the inappropriateness of the assumption that DPF is independent of wavelength or source-detector separations and set the platform for further studies on investigating optical pathlengths and DPF in PPG.

PPG pulse direction determination algorithm for PPG waveform inversion by wrist rotation

This paper describes photoplethysmography (PPG)-based pulse direction determination algorithm on a site of the radial artery using a wrist band. It has been well known that PPG is susceptible to noise and motion artifacts in the mobile environment and many research efforts have been made to focus on rejection of the noise and motion artifacts. However, no research has been performed to find PPG pulses when PPG is inverted by wrist movement. We present an algorithm, which accurately yields which direction PPG pulses face regardless of wrist movement. The algorithm is one step closer to robust real-time PPG pulse direction determination for continuous PPG monitoring regardless of body movements.

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.

Pulse oximetric assessment of anatomical vascular contribution to tissue perfusion in the gastric conduit

BACKGROUND

Tubularized stomach is a common substitute used after oesophageal resection. The risk for gastric conduit ischemia, as well as the mechanisms and dynamics for the occurrence of deficient tissue perfusion during the critical construction of a gastric tube, is poorly understood.

METHODS

Twenty-nine patients that underwent oesophagectomy were studied with transmural pulse oximetry of different parts of the stomach, and at predefined preparatory steps during the construction of the gastric conduit.

RESULTS

After ligation of the left gastric artery (LGA), a reduction to 83.5% in tissue saturation was observed. Three patients (10.3%) had a sustained saturation despite ligation at this point. During final preparation of the gastric tube, and after stapling of the minor curvature, saturation fell to 76.5%. Saturation increased significantly to 80.0% 2 h after the stapling, just before construction of the anastomosis (P = 0.021). There was no association between the level of oxygen saturation and the risk of anastomotic dehiscence.

CONCLUSION

During gastric tube construction for oesophageal replacement, conduit perfusion, measured as oxygen saturation with pulse oximetry, decreases significantly. The main cause of this reduction seems to be ligation of the LGA and the final stapling of the gastric tube. Future studies are needed to establish the clinical implications of this finding.

Photoplethysmography for blood volumes and oxygenation changes during intermittent vascular occlusions

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A reflectance PPG sensor was attached on the left forearm of nineteen (n = 19) volunteers, along with a reference NIRS sensor positioned on the same forearm, above the left brachioradialis. The investigation protocol consisted of seven intermittent and pressure-increasing vascular occlusions. Relative changes in haemoglobin concentrations were obtained by applying the modified Beer–Lambert law to PPG signals, while oxygenation changes were estimated by the difference between red and infrared attenuations of DC PPGs (A\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{Ox}$$\end{document}Ox = \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta$$\end{document}ΔA\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{IR}$$\end{document}IR − \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta$$\end{document}ΔA\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_R$$\end{document}R) and by the conventional SpO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2. The \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta$$\end{document}ΔHbO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta$$\end{document}ΔHHb, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta$$\end{document}ΔtHb from the PPG signals indicated significant changes in perfusion induced by either partial and complete occlusions (p < 0.05). The trends in the variables extracted from PPG showed good correlation with the same parameters measured by the reference NIRS monitor. Bland and Altman analysis of agreement between PPG and NIRS showed underestimation of the magnitude of changes by the PPG. A\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{Ox}$$\end{document}Ox indicated significant changes for occlusion pressures exceeding 20 mmHg (p < 0.05) and correlation with tissue oxygenation changes measured by NIRS, while SpO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 had significant changes after 40 mmHg (p < 0.05). Relative changes in haemoglobin concentrations can be estimated from PPG signals and they showed a good level of accuracy in the detection of perfusion and oxygenation changes induced by different degrees of intermittent vascular occlusions. These results can open up to new applications of the PPG waveform in the detection of blood volumes and oxygenation changes.

In vivo investigation of ear canal pulse oximetry during hypothermia

Pulse oximeters rely on the technique of photoplethysmography (PPG) to estimate arterial oxygen saturation (SpO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2). In conditions of poor peripheral perfusion such as hypotension, hypothermia, and vasoconstriction, the PPG signals detected are often weak and noisy, or in some cases unobtainable. Hence, pulse oximeters produce erroneous SpO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 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 toe, which are easily affected by vasoconstriction. In order to overcome this problem, the ear canal was investigated as an alternative site for measuring reliable SpO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 on the hypothesis that blood flow to this central site is preferentially preserved. A novel miniature ear canal PPG sensor was developed along with a state of the art PPG processing unit to investigate PPG measurements from the bottom surface of the ear canal. An in vivo study was carried out in 15 healthy volunteers to validate the developed technology. In this comparative study, red and infrared PPGs were acquired from the ear canal and the finger of the volunteers, whilst they were undergoing artificially induced hypothermia by means of cold exposure (10 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^\circ$$\end{document}∘C). Normalised Pulse Amplitude (NPA) and SpO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 was calculated from the PPG signals acquired from the ear canal and the finger. Good quality baseline PPG signals with high signal-to-noise ratio were obtained from both the PPG sensors. During cold exposure, significant differences were observed in the NPA of the finger PPGs. The mean NPA of the red and infrared PPGs from the finger have dropped by >80%. Contrary to the finger, the mean NPA of red and infrared ear canal PPGs had dropped only by 0.2 and 13% respectively. The SpO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2s estimated from the finger sensor have dropped below 90% in five volunteers (failure) by the end of the cold exposure. The ear canal sensor, on the other hand, had only failed in one volunteer. These results strongly suggest that the ear canal may be used as a suitable alternative site for monitoring PPGs and arterial blood oxygen saturation at times were peripheral perfusion is compromised.

Optical oximetry of volume-oscillating vascular compartments: contributions from oscillatory blood flow

We present a quantitative analysis of dynamic diffuse optical measurements to obtain oxygen saturation of hemoglobin in volume oscillating compartments. We used a phasor representation of oscillatory hemodynamics at the heart rate and respiration frequency to separate the oscillations of tissue concentrations of oxyhemoglobin (O) and deoxyhemoglobin (D) into components due to blood volume (subscript V V ) and blood flow (subscript F F ): O=O V +O F O=OV+OF , D=D V +D F D=DV+DF . This is achieved by setting the phase angle Arg(O F )−Arg(O) Arg(OF)−Arg(O) , which can be estimated by a hemodynamic model that we recently developed. We found this angle to be −72  deg −72  deg for the cardiac pulsation at 1 Hz, and −7  deg −7  deg for paced breathing at 0.1 Hz. Setting this angle, we can obtain the oxygen saturation of hemoglobin of the volume-oscillating vascular compartment, S V =|O V |/(|O V |+|D V |) SV=|OV|/(|OV|+|DV|) . We demonstrate this approach with cerebral near-infrared spectroscopy measurements on healthy volunteers at rest (n=4 n=4 ) and during 0.1 Hz paced breathing (n=3 n=3 ) with a 24-channel system. Rest data at the cardiac frequency were used to calculate the arterial saturation, S (a) S(a) ; over all subjects and channels, we found ⟨S V ⟩=⟨S (a) ⟩=0.96±0.02 ⟨SV⟩=⟨S(a)⟩=0.96±0.02 . In the case of paced breathing, we found ⟨S V ⟩=0.66±0.14 ⟨SV⟩=0.66±0.14 , which reflects venous-dominated hemodynamics at the respiratory frequency.

Comparison of NIRS, laser Doppler flowmetry, photoplethysmography, and pulse oximetry during vascular occlusion challenges

Monitoring changes in blood volume, blood flow, and oxygenation in tissues is of vital importance in fields such as reconstructive surgery and trauma medicine. Near infrared spectroscopy (NIRS), laser Doppler (LDF) flowmetry, photoplethysmography (PPG), and pulse oximetry (PO) contribute to such fields due to their safe and noninvasive nature. However, the techniques have been rarely investigated simultaneously or altogether. The aim of this study was to investigate all the techniques simultaneously on healthy subjects during vascular occlusion challenges. Sensors were attached on the forearm (NIRS and LDF) and fingers (PPG and PO) of 19 healthy volunteers. Different degrees of vascular occlusion were induced by inflating a pressure cuff on the upper arm. The responses of tissue oxygenation index (NIRS), tissue haemoglobin index (NIRS), flux (LDF), perfusion index (PPG), and arterial oxygen saturation (PO) have been recorded and analyzed. Moreover, the optical densities were calculated from slow varying dc PPG, in order to distinguish changes in venous blood volumes. The indexes showed significant changes (p  <  0.05) in almost all occlusions, either venous or over-systolic occlusions. However, differentiation between venous and arterial occlusion by LDF may be challenging and the perfusion index (PI) may not be adequate to indicate venous occlusions. Optical densities may be an additional tool to detect venous occlusions by PPG.

Accuracy of reflectance photoplethysmography on detecting cuff-induced vascular occlusions

Photoplethysmography (PPG) is a noninvasive optical technique, which can also be used to derive important parameters other than arterial oxygen saturation (SpO2). In this work, the accuracy of the technique on detecting changes in blood perfusion during different levels of vascular occlusions has been explored. A dual-wavelength, reflectance PPG probe was applied on the left forearm of 10 healthy volunteers and raw PPG signals were acquired by a research PPG processing system. The raw PPG signals were separated into pulsatile AC and continuous DC PPG components. The signals were used to estimate SpO2 and changes in concentration of oxygenated, deoxygenated, and total haemoglobin. Different levels of occlusions, from 20 mmHg to total occlusion were induced by a pressure-cuff on the left arm. The system was able to indicate all the occlusions. In particular, the haemoglobin concentration changes estimated from PPG were in high agreement with Near Infrared Spectroscopy measurements.

A novel method based on two cameras for accurate estimation of arterial oxygen saturation

BACKGROUND

Photoplethysmographic imaging (PPGi) that is based on camera allows acquiring photoplethysmogram and measuring physiological parameters such as pulse rate, respiration rate and perfusion level. It has also shown potential for estimation of arterial oxygen saturation (SaO2). However, there are some technical limitations such as optical shunting, different camera sensitivity to different light spectra, different AC-to-DC ratios (the peak-to-peak amplitude to baseline ratio) of the PPGi signal for different portions of the sensor surface area, the low sampling rate and the inconsistency of contact force between the fingertip and camera lens.

METHODS

In this paper, we take full account of the above-mentioned design challenges and present an accurate SaO2 estimation method based on two cameras. The hardware system we used consisted of an FPGA development board (XC6SLX150T-3FGG676 from Xilinx), with connected to it two commercial cameras and an SD card. The two cameras were placed back to back, one camera acquired PPGi signal from the right index fingertip under 660 nm light illumination while the other camera acquired PPGi signal from the thumb fingertip using an 800 nm light illumination. The both PPGi signals were captured simultaneously, recorded in a text file on the SD card and processed offline using MATLAB®. The calculation of SaO2 was based on the principle of pulse oximetry. The AC-to-DC ratio was acquired by the ratio of powers of AC and DC components of the PPGi signal in the time-frequency domain using the smoothed pseudo Wigner-Ville distribution. The calibration curve required for SaO2 measurement was obtained by linear regression analysis.

RESULTS

The results of our estimation method from 12 subjects showed a high correlation and accuracy with those of conventional pulse oximetry for the range from 90 to 100%.

CONCLUSIONS

Our method is suitable for mobile applications implemented in smartphones, which could allow SaO2 measurement in a pervasive environment.

Reflectance Photoplethysmography as Noninvasive Monitoring of Tissue Blood Perfusion

In the last decades, photoplethysmography (PPG) has been used as a noninvasive technique for monitoring arterial oxygen saturation by pulse oximetry (PO), whereas near-infrared spectroscopy (NIRS) has been employed for monitoring tissue blood perfusion. While NIRS offers more parameters to evaluate oxygen delivery and consumption in deep tissues, PO only assesses the state of oxygen delivery. For a broader assessment of blood perfusion, this paper explores the utilization of dual-wavelength PPG by using the pulsatile (ac) and continuous (dc) PPG for the estimation of arterial oxygen saturation (SpO2) by conventional PO. Additionally, the Beer-Lambert law is applied to the dc components only for the estimation of changes in deoxyhemoglobin (HHb), oxyhemoglobin (HbO2), and total hemoglobin (tHb) as in NIRS. The system was evaluated on the forearm of 21 healthy volunteers during induction of venous occlusion (VO) and total occlusion (TO). A reflectance PPG probe and NIRS sensor were applied above the brachioradialis, PO sensors were applied on the fingers, and all the signals were acquired simultaneously. While NIRS and forearm SpO2 indicated VO, SpO2 from the finger did not exhibit any significant drop from baseline. During TO, all the indexes indicated the change in blood perfusion. HHb, HbO2, and tHb changes estimated by PPG presented high correlation with the same parameters obtained by NIRS during VO (r(2) = 0.960, r(2) = 0.821, and r(2) = 0.974, respectively) and during TO (r(2) = 0.988, r(2) = 0.940, and r(2) = 0.938, respectively). The system demonstrated the ability to extract valuable information from PPG signals for a broader assessment of tissue blood perfusion.

Pulse oximetry: fundamentals and technology update

Oxygen saturation in the arterial blood (SaO₂) provides information on the adequacy of respiratory function. SaO₂ can be assessed noninvasively by pulse oximetry, which is based on photoplethysmographic pulses in two wavelengths, generally in the red and infrared regions. The calibration of the measured photoplethysmographic signals is performed empirically for each type of commercial pulse-oximeter sensor, utilizing in vitro measurement of SaO₂ in extracted arterial blood by means of co-oximetry. Due to the discrepancy between the measurement of SaO₂ by pulse oximetry and the invasive technique, the former is denoted as SpO₂. Manufacturers of pulse oximeters generally claim an accuracy of 2%, evaluated by the standard deviation (SD) of the differences between SpO₂ and SaO₂, measured simultaneously in healthy subjects. However, an SD of 2% reflects an expected error of 4% (two SDs) or more in 5% of the examinations, which is in accordance with an error of 3%–4%, reported in clinical studies. This level of accuracy is sufficient for the detection of a significant decline in respiratory function in patients, and pulse oximetry has been accepted as a reliable technique for that purpose. The accuracy of SpO₂ measurement is insufficient in several situations, such as critically ill patients receiving supplemental oxygen, and can be hazardous if it leads to elevated values of oxygen partial pressure in blood. In particular, preterm newborns are vulnerable to retinopathy of prematurity induced by high oxygen concentration in the blood. The low accuracy of SpO₂ measurement in critically ill patients and newborns can be attributed to the empirical calibration process, which is performed on healthy volunteers. Other limitations of pulse oximetry include the presence of dyshemoglobins, which has been addressed by multiwavelength pulse oximetry, as well as low perfusion and motion artifacts that are partially rectified by sophisticated algorithms and also by reflection pulse oximetry.

Calibration-free pulse oximetry based on two wavelengths in the infrared - a preliminary study

The assessment of oxygen saturation in arterial blood by pulse oximetry (SpO₂) is based on the different light absorption spectra for oxygenated and deoxygenated hemoglobin and the analysis of photoplethysmographic (PPG) signals acquired at two wavelengths. Commercial pulse oximeters use two wavelengths in the red and infrared regions which have different pathlengths and the relationship between the PPG-derived parameters and oxygen saturation in arterial blood is determined by means of an empirical calibration. This calibration results in an inherent error, and pulse oximetry thus has an error of about 4%, which is too high for some clinical problems. We present calibration-free pulse oximetry for measurement of SpO₂, based on PPG pulses of two nearby wavelengths in the infrared. By neglecting the difference between the path-lengths of the two nearby wavelengths, SpO₂ can be derived from the PPG parameters with no need for calibration. In the current study we used three laser diodes of wavelengths 780, 785 and 808 nm, with narrow spectral line-width. SaO₂ was calculated by using each pair of PPG signals selected from the three wavelengths. In measurements on healthy subjects, SpO₂ values, obtained by the 780–808 nm wavelength pair were found to be in the normal range. The measurement of SpO₂ by two nearby wavelengths in the infrared with narrow line-width enables the assessment of SpO₂ without calibration.

Pulse oximetry of body cavities and organs

The focus of this paper will be in the development and in vivo applications of new custom made photoplethysmographic (PPG) and pulse oximetry optical and fiber optic sensors and instrumentation in an effort to investigate their suitability in the estimation of blood oxygen saturation and their contribution in the assessment of organ/tissue perfusion and viability. The paper describes the development of optical and fiber optic PPG and blood oxygen saturation (SpO2) sensors and covers examples of application areas including real-time PPG monitoring from body cavities (esophagus) and solid or hollow organs (bowel, liver, stomach, brain, etc). The clinical studies presented successfully demonstrated the feasibility in acquiring PPGs and estimating blood oxygen saturation values from a variety of organs and tissues. The technological developments and the measurements presented in this work pave the way in a new era of pulse oximetry where direct and continuous monitoring of blood oxygen saturation of internal organs and tissues could be made possible.

Clinical applications and issues of oxygen saturation level measurements obtained from peripheral sites

Studies have suggested that the saturated oxygen level within one's arterial blood can provide crucial information about the status of one's cardiopulmonary system. Currently, a popular and convenient approach is to obtain this vital physiological sign through non-invasive measurement on suitable peripheral sites such as a finger, toe or ear lobe. This measurement is known as the SpO2 parameter. It has been increasingly adopted in not only clinical settings but also remote monitoring purposes. In order to measure this optical based parameter, light sources of both the red and infrared wavelengths are required. The most recognized waveform feature is the peripheral pulse or its AC component which is synchronized to each heartbeat. The AC component is superimposed on a constant DC baseline attributed to breathing efforts, sympathetic nervous system activities and thermoregulation. The popularity of the SpO2 parameter may be due to its viable cost, simplicity to build and portability. Moreover, the basic building blocks of a SpO2 based measurement consists of control, filtering and amplification functions that can easily be incorporated with an embedded system. In this review, a brief description of the SpO2 measurement, its normative values and technical issues in its application as a clinical monitor are discussed.

Esophageal SpO2 measurements from a pediatric burns-patient: a case study

Pulse oximetry is being used in everyday clinical practice in anesthesia utilizing peripheral saturation sensors. However, it may be unreliable in certain clinical situations such as peripheral hypoperfusion. Similar situations occur in burns patients and more importantly burns to extremities which limit the sites available for measurement of peripheral oxygen saturation (SpO2). To overcome these limitations, the esophagus has been investigated as an alternative measurement site, as perfusion may be preferentially preserved centrally. A miniaturized reflectance esophageal saturation (SpO2 probe has been constructed utilizing infrared and red photodiodes and a photodetector. Our case study was aimed at evaluating the reliability of esophageal pulse oximetry in a major burns infant. Measurable photoplethysmographic (PPG) traces and SpO2 values were obtained in the neonatal esophagus. It was found that the esophageal pulse oximeter results were in good agreement with oxygen saturation measurements obtained by a commercial ear lobe pulse oximeter. This study suggests that the esophagus can be used as an alternative site for monitoring arterial blood oxygen saturation by pulse oximetry in burned infants.

Assessment of bilateral photoplethysmography for lower limb peripheral vascular occlusive disease using color relation analysis classifier

This paper proposes the assessment of bilateral photoplethysmography (PPG) for lower limb peripheral vascular occlusive disease (PVOD) using a color relation analysis (CRA) classifier. PPG signals are non-invasively recorded from the right and left sides at the big toe sites. With the time-domain technique, the right-to-left side difference is studied by comparing the subject's PPG data. The absolute bilateral differences construct various diminishing and damping patterns. These difference patterns in amplitude and shape distortion relate to the grades of PVOD, including the normal condition, lower-grade disease, and higher-grade disease. A CRA classifier is used to recognize the various patterns for PVOD assessment. Its concept is derived from the HSV color model and uses the hue, saturation, and value to depict the disease grades using the natural primary colors of red, green, and blue. PPG signals are obtained from 21 subjects aged 24-65 years using an optical measurement technique. The proposed CRA classifier is tested using the physiological measurements, and the tests reveal its practicality for monitoring PPG signals.

Measuring venous oxygenation using the photoplethysmograph waveform

OBJECTIVE

We investigate the hypothesis that the photoplethysmograph (PPG) waveform can be analyzed to infer regional venous oxygen saturation.

METHODS

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. Second, ventilation tends to move venous blood preferentially due to the low pressure and high compliance of the venous system. Red (660 nm) and IR (940 nm) PPG waveforms were collected from 10 cardiac surgery patients using an esophageal PPG probe. These waveforms were analyzed using algorithms written in Mathematica. Four time-domain saturation algorithms (ArtSat, VenSat, ArtInstSat, VenInstSat) and four frequency-domain saturation algorithms (RespDC, RespAC, Cardiac, and Harmonic) were applied to the data set.

RESULTS

Three of the algorithms for calculating venous saturation (VenSat, VenInstSat, and RespDC) demonstrate significant difference from ArtSat (the conventional time-domain algorithm for measuring arterial saturation) using the Wilcoxon signed-rank test with Bonferroni correction (p < 0.0071).

CONCLUSIONS

This work introduces new algorithms for PPG analysis. Three algorithms (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).

Anesthetic considerations for major burn injury in pediatric patients

Major burn injury remains a significant cause of morbidity and mortality in pediatric patients. With advances in burn care and with the development of experienced multi-disciplinary teams at regionalized burn centers, many children are surviving severe burn injury. As members of the multi-disciplinary care team, anesthesia providers are called upon to care for these critically ill children. These children provide several anesthetic challenges, such as difficult airways, difficult vascular access, fluid and electrolyte imbalances, altered temperature regulation, sepsis, cardiovascular instability, and increased requirements of muscle relaxants and opioids. The anesthesia provider must understand the physiologic derangements that occur with severe burn injury as well as the subsequent anesthetic implications.

Three-wavelength technique for the measurement of oxygen saturation in arterial blood and in venous blood

Pulse oximetry is an optical technique for the assessment of oxygen saturation in arterial blood and is based on the different light absorption spectra for oxygenated and deoxygenated hemoglobin and on two-wavelength photoplethysmographic (PPG) measurement of arterial blood volume increase during systole. The technique requires experimental calibration for the determination of the relationship between PPG-derived parameters and arterial oxygen saturation, and this calibration is a source of error in the method. We suggest a three-wavelength PPG technique for the measurement of arterial oxygen saturation that has no need for calibration if the three wavelengths are properly selected in the near-infrared region. The suggested technique can also be implemented for the assessment of venous oxygen saturation by measuring the decrease in transmission of light through a tissue after increasing its blood volume by venous occlusion. The oxygen saturation in venous blood is a parameter that is related to oxygen consumption in tissue and to tissue blood flow. The three-wavelength method has the potential to provide accurate oxygen saturation measurements in arterial and venous blood, but experimental validation of the theory is still required to confirm this claim.

A pilot study of neonatal and pediatric esophageal pulse oximetry

BACKGROUND

In this pilot study we explored the suitability of the esophagus as a new measuring site for blood oxygen saturation (Spo(2)) in neonates.

METHODS

A new miniaturized esophageal pulse oximeter has been developed. Five patients (one child and four neonates) were studied.

RESULTS

Spo(2) values were obtained in the esophagus of all patients. A Bland and Altman plot of the difference between Spo(2) values from the esophageal pulse oximeter and a commercial toe pulse oximeter against their mean showed that the bias and the limits of agreement between the two pulse oximeters were +0.3% and +1.7% to -1.0%, respectively.

CONCLUSIONS

This study suggests that the esophagus can be used as an alternative site for monitoring blood oxygen saturation in children and neonates.

In vivo quantification of gingival inflammation using spectral imaging

Erythema is a reaction of the skin and oral soft tissues commonly associated with inflammation and an increase in blood flow. Diffuse reflection spectroscopy is a powerful tool for the assessment of skin inflammation where erythema has been linked to the relative concentration of oxygenated hemoglobin and blood perfusion. Here we demonstrate the applicability of a spectral imaging method for the quantification of gingival inflammation by looking at the gingival margin and papillary tip erythema. We present a longitudinal study on 22 healthy volunteers divided in two groups. The first was allowed to have normal oral hygiene and the second was subjected to an induced gingivitis for two weeks by cessation of oral hygiene. The spectral reflectance ratio at 615 and 460 nm, R(615)R(460), was proposed as a method to quantify and map the erythema spatial distribution. These wavelengths represent spectral absorption crossovers observed between oxygenated and deoxygenated hemoglobin. The spectral method presented shows a significant separation (p<0.01) between the groups when gingivitis was induced and correlates significantly (p<0.05) with the clinical gingival index scores. We believe that these investigations could contribute to the development of functional imaging methods for periodontal disease detection and monitoring.

Multichannel Reflective PPG Earpiece Sensor With Passive Motion Cancellation

This paper addresses the design considerations of a novel earpiece photoplethymograph (PPG) sensor and its in-situ evaluation results. The device is encapsulated with multiple LEDs and photodiodes based on a reflective PPG design. A compact and low power circuitry was developed for signal control and conditioning. PPG signals with an averaged ac/dc ratio of 0.001-0.01 and 10% relative strength (compared to finger-based approach) were recorded from the superior and posterior auricular skins. The integrity of PPG signal and accuracy of heart rate detection were evaluated and the results showed that with adequate optical shielding and the proposed passive motion cancellation, the device was able to reliably detect heart rate both during rest and moderate exercise. The proposed sensor design is low power, easy to wear compared to conventional earlobe PPG devices.

Combined photoplethysmographic monitoring of respiration rate and pulse: a comparison between different measurement sites in spontaneously breathing subjects

BACKGROUND

The non-invasive photoplethysmographic (PPG) signal reflects blood flow and volume in a tissue. The PPG signal shows variation synchronous with heartbeat (PPGc), as used in pulse oximetry, and variations synchronous with breathing (PPGr). PPGr has been used for non-invasive monitoring of respiration with promising results. Our aim was to investigate PPG signals recorded from different skin sites in order to find suitable locations for parallel monitoring of variations synchronous with heartbeat and breathing.

METHODS

PPG sensors were applied to the forearm, finger, forehead, wrist and shoulder on 48 awake healthy volunteers. From these sites, seven PPG signals were simultaneously recorded during normal spontaneous breathing over 10 min. Capnometry served as respiration and electrocardiogram (ECG) as pulse reference signals. PPG signals were compared with respect to power spectral content and squared coherence.

RESULTS

Forearm PPG measurement showed significantly higher power within the respiratory region of the power spectrum [median (quartile range) 42 (26)%], but significantly lower power within the cardiac region [9 (10)%] compared with the other skin sites. PPG finger measurement showed the opposite; in transmission mode, the power within the respiratory region was significantly lower [4 (10)%] and within the cardiac region significantly higher [45 (25)%] than the other sites. PPGc coherence values were generally high [>0.96 (0.08)], and PPGr coherence values lower [0.83 (0.35)-0.94 (0.17)].

CONCLUSION

Combined PPG respiration and pulse monitoring is possible, but there are significant differences between the respiratory and cardiac components of the PPG signal at different sites.

Photoplethysmography and its application in clinical physiological measurement

Photoplethysmography (PPG) is a simple and low-cost optical technique that can be used to detect blood volume changes in the microvascular bed of tissue. It is often used non-invasively to make measurements at the skin surface. The PPG waveform comprises a pulsatile ('AC') physiological waveform attributed to cardiac synchronous changes in the blood volume with each heart beat, and is superimposed on a slowly varying ('DC') baseline with various lower frequency components attributed to respiration, sympathetic nervous system activity and thermoregulation. Although the origins of the components of the PPG signal are not fully understood, it is generally accepted that they can provide valuable information about the cardiovascular system. There has been a resurgence of interest in the technique in recent years, driven by the demand for low cost, simple and portable technology for the primary care and community based clinical settings, the wide availability of low cost and small semiconductor components, and the advancement of computer-based pulse wave analysis techniques. The PPG technology has been used in a wide range of commercially available medical devices for measuring oxygen saturation, blood pressure and cardiac output, assessing autonomic function and also detecting peripheral vascular disease. The introductory sections of the topical review describe the basic principle of operation and interaction of light with tissue, early and recent history of PPG, instrumentation, measurement protocol, and pulse wave analysis. The review then focuses on the applications of PPG in clinical physiological measurements, including clinical physiological monitoring, vascular assessment and autonomic function.