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

LED light can falsify pulse oximetry readings via the stroboscopic effect

Because of its simplicity, pulse oximetry plays a ubiquitous role in neonatology. Its measurements are based on the absorption of light by hemoglobin. Ambient light can affect these values, therefore algorithms are designed to compensate for constant ambient light. Modern light-emitting diodes often flicker at a very high frequency. Such flickering ambient light can lead to significant measurement errors in saturation. To present a novel way in which light-emitting diodes influence the function of pulse oximeters and to demonstrate mathematically that a stroboscopic effect may well be responsible for this disturbance. Using publicly available data, a mathematical model of a pulse oximeter with a calibration curve and a proprietary measurement algorithm was created. This was used to simulate saturation measurements in flickering ambient light. To do this, photopletysmograms for red and infrared light at 98% oxygen saturation were mathematically superimposed on the light emission from an examination lamp used in the intensive care unit. From these results, presumable saturation measurements from a pulse oximeter were extrapolated. The light-emitting diodes in the examination lamp flicker at 207 Hz. The pulsating light from the light-emitting diodes causes superimposition of the photoplethysmogram due to the stroboscopic effect. With increasing brightness, the saturation dropped to 85% and the pulse rate to 108 bpm. The pulsed light of light-emitting diodes can distort pulse oximetry measurements. The stroboscopic effect leads to low saturation values, which can lead to the risk of blindness in premature infants due to excessive oxygenation.

Self-calibrated pulse oximetry algorithm based on photon pathlength change and the application in human freedivers

Significance

Pulse oximetry estimates the arterial oxygen saturation of hemoglobin (SaO 2 ) based on relative changes in light intensity at the cardiac frequency. Commercial pulse oximeters require empirical calibration on healthy volunteers, resulting in limited accuracy at low oxygen levels. An accurate, self-calibrated method for estimating SaO 2 is needed to improve patient monitoring and diagnosis.

Aim

Given the challenges of calibration at low SaO 2 levels, we pursued the creation of a self-calibrated algorithm that can effectively estimate SaO 2 across its full range. Our primary objective was to design and validate our calibration-free method using data collected from human subjects.

Approach

We developed an algorithm based on diffuse optical spectroscopy measurements of cardiac pulses and the modified Beer-Lambert law (mBLL). Recognizing that the photon mean pathlength (⟨ L ⟩ ) varies with SaO 2 related absorption changes, our algorithm aligns/fits the normalized ⟨ L ⟩ (across wavelengths) obtained from optical measurements with its analytical representation. We tested the algorithm with human freedivers performing breath-hold dives. A continuous-wave near-infrared spectroscopy probe was attached to their foreheads, and an arterial cannula was inserted in the radial artery to collect arterial blood samples at different stages of the dive. These samples provided ground-truth SaO 2 via a blood gas analyzer, enabling us to evaluate the accuracy of SaO 2 estimation derived from the NIRS measurement using our self-calibrated algorithm.

Results

The self-calibrated algorithm significantly outperformed the conventional method (mBLL with a constant ⟨ L ⟩ ratio) for SaO 2 estimation through the diving period. Analyzing 23 ground-truth SaO 2 data points ranging from 41% to 100%, the average absolute difference between the estimated SaO 2 and the ground truth SaO 2 is 4.23 % ± 5.16 % for our algorithm, significantly lower than the 11.25 % ± 13.74 % observed with the conventional approach.

Conclusions

By factoring in the variations in the spectral shape of ⟨ L ⟩ relative to SaO 2 , our self-calibrated algorithm enables accurate SaO 2 estimation, even in subjects with low SaO 2 levels.

Everything About Pulse Oximetry-Part 1: History, Principles, Advantages, Limitations, Inaccuracies, Cost Analysis, the Level of Knowledge About Pulse Oximeter Among Clinicians, and Pulse Oximetry Versus Tissue Oximetry

Purpose: Pulse oximetry is a noninvasive medical technique that measures the amount of oxygen in a person's blood by shining light through their skin. It is widely used in medical care and is considered as important as the 4 traditional vital signs. In this article, it was aimed to review all aspects of pulse oximetry in detail. Materials and Methods: The international and national reliable sources were used in the literature review for critical data analysis. A total of 13 articles including 9 reviews, 1 comparative clinical research, 1 cost-saving quality improvement project, 1 cross-sectional and multicenter descriptive study, and 1 questionnaire study were used for the preparation of this part of the review. Results: The history, principles, advantages, limitations inaccuracies, cost analysis, the level of knowledge about pulse oximeter among clinicians, and pulse oximetry versus tissue oximetry were all reviewed in detail. Conclusion: The device has a significant impact on modern medicine, allowing continuous monitoring of hemoglobin oxygen saturation in arterial blood. Oximeters are valuable in managing oxygen levels in respiratory and nonrespiratory diseases and have become an essential tool in hospital settings. Detecting low levels of oxygen saturation early can alert patients to seek medical attention promptly. It is crucial to comprehend the working and limitations of pulse oximetry technology to ensure patient safety.

Performance of Near-Infrared Spectroscopy in Detecting Acute Tourniquet-Induced Upper-Extremity Ischemia Across Different Skin Phenotypes

PURPOSE

Diagnosing acute tissue ischemia is challenging, particularly in patients with higher skin melanin content. We investigated whether near-infrared spectroscopy (NIRS) is effective and consistent in detecting upper extremity ischemia across various skin phenotypes.

METHODS

Volunteers underwent tourniquet-induced upper extremity ischemia. Skin color was evaluated by the Fitzpatrick scale (FP, range: I-VI) and the Von Luschan scale (vL, range: 1-36). A NIRS probe was placed on one finger. The tourniquet was inflated to 250 mmHg and perfusion was restricted for 7 minutes, followed by a 10-minute monitored reperfusion period. The percent tissue oxygenation (StO₂) was recorded.

RESULTS

A total of 55 volunteers were enrolled (22 self-identified as Caucasian, 21 African American, 7 Asian, 2 Latinx, and 2 Biracial). Average starting and ending StO₂ for the cohort was 72.2% and 45.9%, respectively. However, there was variability based on skin melanin content. Increasing vL correlated with lower starting StO₂, smaller StO₂ decrease, and shorter time to reach ischemic steady state. High skin melanin (FP scale IV-VI) was associated with significantly lower starting StO₂ (-7.1%) and shorter time to reach ischemic steady state (-0.3 mins). African Americans had lower starting StO₂ (-8.6%) and 7.8% lesser total StO₂ decrease than other groups.

CONCLUSIONS

NIRS can rapidly detect acute onset tissue ischemia in the upper extremity. However, given the lower starting StO₂ and smaller total StO₂ decrease after tourniquet-induced ischemia for patients with higher skin melanin, using NIRS for clinical detection of acute ischemia may be more challenging in these patients. These inconsistencies may limit use of NIRS clinically for spot identification of ischemia.

CLINICAL RELEVANCE

Although NIRS has utility in tracking tissue oxygenation, variable performance with different skin melanin content raises concerns as to whether different cutoff/threshold levels are needed for different groups, and whether NIRS is reliable for spot checks in acute events.

Wearable Multisensor Ring-Shaped Probe for Assessing Stress and Blood Oxygenation: Design and Preliminary Measurements

The increasing interest in innovative solutions for health and physiological monitoring has recently fostered the development of smaller biomedical devices. These devices are capable of recording an increasingly large number of biosignals simultaneously, while maximizing the user's comfort. In this study, we have designed and realized a novel wearable multisensor ring-shaped probe that enables synchronous, real-time acquisition of photoplethysmographic (PPG) and galvanic skin response (GSR) signals. The device integrates both the PPG and GSR sensors onto a single probe that can be easily placed on the finger, thereby minimizing the device footprint and overall size. The system enables the extraction of various physiological indices, including heart rate (HR) and its variability, oxygen saturation (SpO₂), and GSR levels, as well as their dynamic changes over time, to facilitate the detection of different physiological states, e.g., rest and stress. After a preliminary SpO₂ calibration procedure, measurements have been carried out in laboratory on healthy subjects to demonstrate the feasibility of using our system to detect rapid changes in HR, skin conductance, and SpO₂ across various physiological conditions (i.e., rest, sudden stress-like situation and breath holding). The early findings encourage the use of the device in daily-life conditions for real-time monitoring of different physiological states.

Overestimation of Oxygen Saturation Measured by Pulse Oximetry in Hypoxemia. Part 1: Effect of Optical Pathlengths-Ratio Increase

On average, arterial oxygen saturation measured by pulse oximetry (SpO₂) is higher in hypoxemia than the true oxygen saturation measured invasively (SaO₂), thereby increasing the risk of occult hypoxemia. In the current article, measurements of SpO₂ on 17 cyanotic newborns were performed by means of a Nellcor pulse oximeter (POx), based on light with two wavelengths in the red and infrared regions (660 and 900 nm), and by means of a novel POx, based on two wavelengths in the infrared region (761 and 820 nm). The SpO₂ readings from the two POxs showed higher values than the invasive SaO₂ readings, and the disparity increased with decreasing SaO₂. SpO₂ measured using the two infrared wavelengths showed better correlation with SaO₂ than SpO₂ measured using the red and infrared wavelengths. After appropriate calibration, the standard deviation of the individual SpO₂-SaO₂ differences for the two-infrared POx was smaller (3.6%) than that for the red and infrared POx (6.5%, p < 0.05). The overestimation of SpO₂ readings in hypoxemia was explained by the increase in hypoxemia of the optical pathlengths-ratio between the two wavelengths. The two-infrared POx can reduce the overestimation of SpO₂ measurement in hypoxemia and the consequent risk of occult hypoxemia, owing to its smaller increase in pathlengths-ratio in hypoxemia.

Oximetría de pulso en enfermedades respiratorias

El pulsioxímetro es un dispositivo que utiliza principios de espectrofotometría y fotopletismografía para la medición de la saturación de oxígeno arterial, así como el ciclo cardiaco y respiratorio, lo que resulta útil para monitorear pacientes con compromisorespiratorio. En este trabajo se realiza una revisión bibliográfica de los principios físicos del pulsioxímetro y sus avances más recientes en pacientes con enfermedad pulmonar obstructiva crónica (EPOC), asma y COVID-19. Se encontró que la oximetría de pulso es una herramienta confiable y eficaz en el diagnóstico y la prevención de complicaciones en pacientes con estas enfermedades respiratorias.

Understanding pulse oximetry in hematology patients: Hemoglobinopathies, racial differences, and beyond

Pulse oximetry (SpO₂ ) is a widely used, non-invasive method of estimating arterial oxygen saturation. Measurement of SpO₂ relies on comparing the relative absorption of light in the red and infrared regions with the expected absorption pattern of oxygenated and deoxygenation adult hemoglobin. As this screening tool has entered common clinical use, test limitations have emerged, including concern about the risk of overestimation of oxygen saturation by pulse oximetry in a disproportionate number of people with dark skin pigment, leading to potential for underdiagnosis of true hypoxemia. In addition, a range of challenges may arise in patients with increased levels of methemoglobin - whether acquired or inherited - carboxyhemoglobin, or in patients with a subset of inherited variant hemoglobins. It is important for Hematologists, and indeed all clinicians who rely on pulse oximetry, to understand the principles and limitations of this ubiquitous test.

Design and Application of a Flexible Blood Oxygen Sensing Array for Wearable Devices

The performance of portable or wearable oximeters is affected by improper movement or wear, which causes an error in the blood oxygen concentration calculation. The error comes from external incident stray light or light leakage caused by the improper fit of the sensor to the skin. This study aimed to develop a flexible blood oxygen sensing system with a 3 × 3 array that uses a reflective-type blood oxygen sensing chip to sequentially measure the blood oxygen levels at nine locations through a time division pulse modulation method. Each sensing chip has light transmission and receiving parts. A flip chip package was used to integrate the sensing chip, and a flexible parylene substrate that could fit the curvature of the wrist and locate the array of photo diodes around the radial artery of the wrist was used. By scanning the sensor array in dynamic behavior, the correct light intensity can be extracted to obtain the blood oxygen concentration and prevent errors due to improper fit or sensor movement during exercise.

Accuracy enhancement in reflective pulse oximetry by considering wavelength-dependent pathlengths

Objective. Noninvasive measurement of oxygen saturation (SpO₂) using pulse oximetry based on transmissive photoplethysmography (tPPG) is clinically accepted and widely employed. However, reflective photoplethysmography (rPPG) - present in smartwatches - has not become equally accepted, partially because the pathlengths of the red and infrared PPGs are patient-dependent. Thus, even the most popular "Ratio of Modulation" (R) method requires patient-dependent calibration to reduce the errors in the measurement of SpO2 using rPPGs.Approach. In this paper, a correction factor or "pathlength ratio" β is introduced in an existing calibration-free algorithm that compensates the patient-dependent pathlength variations, and improved accuracy is obtained in the measurement of SpO2 using rPPGs. The proposed β is derived through the analytical model of a rPPG signal. Using the new expression and data obtained from a human hypoxia study wherein arterial oxygen saturation values acquired through Blood Gas Analysis were employed as a reference, β is determined.Main results. The results of the analysis show that a specific combination of the β and the measurements on the pulsating part of the natural logarithm of the red and infrared PPG signals yields a reduced root-mean-square error (RMSE). It is shown that the average RMSE in measuring SpO2 values reduces to 1 %.Significance. The human hypoxia study data used for this work, obtained in a previous study, coversSpO₂values in the range from 70 % to 100 %, and thus shows that the pathlength ratio β proposed here works well in the range of clinical interest. This work demonstrates that the calibration-free method applicable for transmission type PPGs can be extended to determineSpO₂using reflective PPGs with the incorporation of the correction factor β. Our algorithm significantly reduces the number of parameters needed for the estimation, while keeping the RMSE below the clinically accepted 2 %.

Noninvasive hemoglobin sensing and imaging: optical tools for disease diagnosis

SIGNIFICANCE

Measurement and imaging of hemoglobin oxygenation are used extensively in the detection and diagnosis of disease; however, the applied instruments vary widely in their depth of imaging, spatiotemporal resolution, sensitivity, accuracy, complexity, physical size, and cost. The wide variation in available instrumentation can make it challenging for end users to select the appropriate tools for their application and to understand the relative limitations of different methods.

AIM

We aim to provide a systematic overview of the field of hemoglobin imaging and sensing.

APPROACH

We reviewed the sensing and imaging methods used to analyze hemoglobin oxygenation, including pulse oximetry, spectral reflectance imaging, diffuse optical imaging, spectroscopic optical coherence tomography, photoacoustic imaging, and diffuse correlation spectroscopy.

RESULTS

We compared and contrasted the ability of different methods to determine hemoglobin biomarkers such as oxygenation while considering factors that influence their practical application.

CONCLUSIONS

We highlight key limitations in the current state-of-the-art and make suggestions for routes to advance the clinical use and interpretation of hemoglobin oxygenation information.

Is pulse oximeter a reliable tool for non-critically ill patients with COVID-19?

INTRODUCTION

Guidelines recommend using a pulse oximeter rather than arterial blood gas (ABG) for COVID-19 patients. However, significant differences can be observed between oxygen saturation measured by pulse oximetry (SpO₂ ) and arterial oxygen saturation (SaO₂ ) in some clinical conditions. We aimed to assess the reliability of the pulse oximeter in patients with COVID-19.

METHODS

We retrospectively reviewed ABG analyses and SpO₂ levels measured simultaneously with ABG in patients hospitalised in COVID-19 wards.

RESULTS

We categorised total 117 patients into two groups, in whom the difference between SpO₂ and SaO₂ was ≤4% (acceptable difference) and >4% (large difference). A large difference group exhibited higher neutrophil count, C-reactive protein, ferritin, fibrinogen, D-dimer and lower lymphocyte count. Multivariate analyses revealed that increased fibrinogen, increased ferritin and decreased lymphocyte count were independent risk factors for a large difference between SpO₂ and SaO₂ . The total study group demonstrated the negative bias of 4.02% with the limits of agreement of -9.22% to 1.17%. The bias became significantly higher in patients with higher ferritin, fibrinogen levels and lower lymphocyte count.

CONCLUSION

Pulse oximeters may not be sufficient to assess actual oxygen saturation, especially in COVID-19 patients with high ferritin and fibrinogen levels and low lymphocyte count with low SpO₂ measurements.

Nerve Growth Factor-Targeted Molecular Theranostics Based on Molybdenum Disulfide Nanosheet-Coated Gold Nanorods (MoS2-AuNR) for Osteoarthritis Pain

Osteoarthritis (OA) is a leading cause of chronic pain in the elderly worldwide. Yet current diagnosis and therapy for OA pain are subjective and nonspecific with significant adverse effects. Here, we introduced a theranostic nanoprobe based on molybdenum disulfide nanosheet-coated gold nanorods (MoS₂-AuNR) targeting never growth factor (NGF), a key player in pain sensation, for photoacoustic pain imaging and near-infrared (NIR) imaging-guided photothermal analgesic therapy. MoS₂ coating significantly improved the photoacoustic and photothermal performance of AuNR. Functionalization of MoS₂-AuNR nanoprobes by conjugating with NGF antibody enabled active targeting on painful OA knees in a surgical OA murine model. We observed that our functional nanoprobes accumulated in the OA knee rather than the contralateral intact one, and the amount was correlated with the severity of mechanical allodynia in our mouse model. Under imaging guidance, NIR-excited photothermal therapy could mitigate mechanical allodynia and walking imbalance behavior for both subacute and chronic stages of OA in a preclinical setting. This molecular theranostic approach enabled us to specifically localize the source of OA pain and efficiently block peripheral pain transmission.

Cerebral oxygen saturation and cerebrovascular instability in newborn infants with congenital heart disease compared to healthy controls

Objective

Infants with Congenital Heart Disease (CHD) are at risk for developmental delays, though the mechanisms of brain injury that impair development are unknown. Potential causes could include cerebral hypoxia and cerebrovascular instability. We hypothesized that we would detect significantly reduced cerebral oxygen saturation and greater cerebrovascular instability in CHD infants compared to the healthy controls.

Methods

We performed a secondary analysis on a sample of 43 term infants (28 CHD, 15 healthy controls) that assessed prospectively in temporal cross-section before or at 12 days of age. CHD infants were assessed prior to open-heart surgery. Cerebral oxygen saturation levels were estimated using Near-Infrared Spectroscopy, and cerebrovascular stability was assessed with the response of cerebral oxygen saturation after a postural change (supine to sitting).

Results

Cerebral oxygen saturation was 9 points lower in CHD than control infants in both postures (β = -9.3; 95%CI = -17.68, -1.00; p = 0.028), even after controlling for differences in peripheral oxygen saturation. Cerebrovascular stability was significantly impaired in CHD compared to healthy infants (β = -2.4; 95%CI = -4.12, -.61; p = 0.008), and in CHD infants with single ventricle compared with biventricular defects (β = -1.5; 95%CI = -2.95, -0.05; p = 0.04).

Conclusion

CHD infants had cerebral hypoxia and decreased cerebral oxygen saturation values following a postural change, suggesting cerebrovascular instability. Future longitudinal studies should assess the associations of cerebral hypoxia and cerebrovascular instability with long-term neurodevelopmental outcomes in CHD infants.

Near-Infrared Spectroscopy: Clinical Use in High-Risk Neonates

In this review, we describe the near-infrared spectroscopy (NIRS) technology and its clinical use in high-risk neonates in critical care settings. We searched databases (e.g., PubMed, Google Scholar, EBSCOhost) to find studies describing the use of NIRS on critically ill and high-risk neonates. Near-infrared spectroscopy provides continuous noninvasive monitoring of venous oxygen saturation. It uses technology similar to pulse oximetry to measure the oxygen saturation of hemoglobin in a tissue bed to describe the relative delivery and extraction of oxygen. Near-infrared spectroscopy can be a valuable bedside tool to provide clinicians indirect evidence of perfusion. It may prompt early interventions that promote oxygen delivery, which can improve high-risk neonatal outcomes.

Potential Applications of Mobile and Wearable Devices for Psychological Support During the COVID-19 Pandemic: A Review

The coronavirus disease 19 (COVID-19) pandemic that has been raging in 2020 does affect not only the physical state but also the mental health of the general population, particularly, that of the healthcare workers. Given the unprecedented large-scale impacts of the COVID-19 pandemic, digital technology has gained momentum as invaluable social interaction and health tracking tools in this time of great turmoil, in part due to the imposed state-wide mobilization limitations to mitigate the risk of infection that might arise from in-person socialization or hospitalization. Over the last five years, there has been a notable increase in the demand and usage of mobile and wearable devices as well as their adoption in studies of mental fitness. The purposes of this scoping review are to summarize evidence on the sweeping impact of COVID-19 on mental health as well as to evaluate the merits of the devices for remote psychological support. We conclude that the COVID-19 pandemic has inflicted a significant toll on the mental health of the population, leading to an upsurge in reports of pathological stress, depression, anxiety, and insomnia. It is also clear that mobile and wearable devices (e.g., smartwatches and fitness trackers) are well placed for identifying and targeting individuals with these psychological burdens in need of intervention. However, we found that most of the previous studies used research-grade wearable devices that are difficult to afford for the normal consumer due to their high cost. Thus, the possibility of replacing the research-grade wearable devices with the current smartwatch is also discussed.

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.

Source-Detector Spectral Pairing-Related Inaccuracies in Pulse Oximetry: Evaluation of the Wavelength Shift

Pulse oximetry enables oxygen saturation estimation (SpO2) non-invasively in real time with few components and modest processing power. With the advent of affordable development kits dedicated to the monitoring of biosignals, capabilities once reserved to hospitals and high-end research laboratories are becoming accessible for rapid prototyping. While one may think that medical-grade equipment differs greatly in quality, surprisingly, we found that the performance requirements are not widely different from available consumer-grade components, especially regarding the photodetection module in pulse oximetry. This study investigates how the use of candidate light sources and photodetectors for the development of a custom SpO2 monitoring system can lead to inaccuracies when using the standard computational model for oxygen saturation without calibration. Following the optical characterization of selected light sources, we compare the extracted parameters to the key features in their respective datasheet. We then quantify the wavelength shift caused by spectral pairing of light sources in association with photodetectors. Finally, using the widely used approximation, we report the resulting absolute error in SpO2 estimation and show that it can lead up to 8% of the critical 90–100% saturation window.

Recent Progress and Perspectives of Thermally Drawn Multimaterial Fiber Electronics

Fibers are the building blocks of a broad spectrum of products from textiles to composites, and waveguides to wound dressings. While ubiquitous, the capabilities of fibers have not rapidly increased compared to semiconductor chip technology, for example. Recognizing that fibers lack the composition, geometry, and feature sizes for more functions, exploration of the boundaries of fiber functionality began some years ago. The approach focuses on a particular form of fiber production, thermal-drawing from a preform. This process has been used for producing single material fibers, but by combining metals, insulators, and semiconductors all within a single strand of fiber, an entire world of functionality in fibers has emerged. Fibers with optical, electrical, acoustic, or optoelectronic functionalities can be produced at scale from relatively easy-to-assemble macroscopic preforms. Two significant opportunities now present themselves. First, can one expect that fiber functions escalate in a predictable manner, creating the context for a "Moore's Law" analog in fibers? Second, as fabrics occupy an enormous surface around the body, could fabrics offer a valuable service to augment the human body? Toward answering these questions, the materials, performance, and limitations of thermally drawn fibers in different electronic applications are detailed and their potential in new fields is envisioned.

Possible Error in Reflection Pulse Oximeter Readings as a Result of Applied Pressure

Pulse oximetry is one of the most widely used techniques in modern medicine. In pulse oximetry, photoplethysmography (PPG) signals are measured at two different wavelengths and converted into the parameter Gamma, which is used to calculate the oxygen saturation of arterial blood. Although most pulse oximetry sensors are based on transmission geometry, the reflection mode is required for different form factors such as the forehead or wrists. In reflection oximetry, local pressure is applied to the measurement surface. We investigated the relationship between applied pressure and Gamma and found that for the reflection mode, Gamma tends to increase with increasing applied pressure. To explain this, we described the PPG signal in terms of two alternative models: a volumetric model and a Scattering-Driven Model (SDM). We assumed that the application of external pressure results in a decrease in local blood flow. We showed that only SDM correctly qualitatively describes Gamma as a function of the decrease in blood flow. We concluded that both described models coexist and that the relative influence of each depends on the measurement geometry and blood perfusion in the skin.