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Spectral dependence of absorption sensitivity on concentration of oxygenated hemoglobin: Pulse oximetry implications
Abstract
The sensitivity analysis indicates that the effective absorption coefficient is most sensitive to the concentration of oxygenated hemoglobin in spectral bands centered at 700 and 960 nm. We find that the highest temporal modulation due to heart function for a thick sample, like an arm, is at 940 nm, a significant shift from 710 nm measured for a finger. The most favorable spectral region for a thick transmission sample, such as a forearm, is the domain defined by intervals [900 nm≤λ1≤1000 nm] and [650 nm≤λ2≤720 nm]. We evaluated five near-infrared light-emitting diodes (LEDs) for their potential applications in oximetry. The LED with peak emission at 930 nm emits well in this spectral region. Here the temporal noise is low, and the effective absorption coefficient is strongly dependent on the concentration of the oxygenated hemoglobin. High-quality saturation results are obtained through the forearm during a short measurement (30 s).
... From these 2 × 15 optodes 2 × 22 detector channels were generated, located halfway between emitter and detector. The Hb absorption coefficient is most sensitive to measuring the Hb concentration in spectral bands centered at 700 and 960 nm (Strojnik & Paez, 2013). The applied ETG-4000 system, therefore, measured absorption changes at the omitted light at 695 nm (dominated by deoxy-Hb) and at 830 nm (dominated by oxy-Hb). ...
During neuronal activation, a local decrease of deoxygenated hemoglobin concentration (deoxy‐Hb) occurs which is the basis of functional brain imaging with blood oxygenation level dependent functional magnetic resonance imaging (BOLD‐fMRI). Elevated intracranial pressure (eICP) has been shown to impair functional deoxy‐Hb changes. This study investigated this effect and its relation to the underlying neuronal activity in the human primary somatosensory cortex (SI). Functional near‐infrared spectroscopy (fNIRS) during somatosensory evoked potentials (SEP) monitoring was performed on 75 subjects during conditions of median nerve stimulation (MNS) and resting state, combined with normal breathing (NB) and eICP by escalating breathing maneuvers (breath holding [BH], Valsalva maneuver with 15 mmHg [V15] and 35 mmHg expiratory pressure [V35]). During NB, fNIRS revealed a typical oxygenated hemoglobin concentration (oxy‐Hb) increase with deoxy‐Hb decrease during MNS enabling SI brain mapping. Breathing maneuvers associated eICP produced a known global change of oxy‐Hb and deoxy‐Hb with and without MNS. When subtracting measurements during resting state from measurements during MNS, neither functional oxy‐Hb nor deoxy‐Hb changes could be recovered while SEPs remained unchanged. In conclusion, Valsalva‐induced eICP prevents oxy‐Hb and deoxy‐Hb changes during neuronal activation in SI. This finding raises questions on the validity of oxy‐Hb‐ and deoxy‐Hb‐based brain imaging (e.g., BOLD‐fMRI) during eICP.
... In this study, an mOEPS with at least four wavelength illumination sources was used to multiply illumination on a designated tissue and to extract SpO 2 from same device. Conventionally, pulse oximeters with red and IR illuminations are used to measure SpO 2 non-invasively as haemoglobin is the oxygen-transport metalloprotein in red blood cells [14] and the light absorption of oxy-haemoglobin (HbO 2 ) relative to that of deoxyhaemoglobin (Hb) is dependent on the illumination wavelength. Under red illumination, the absorption of Hb is greater than that of HbO 2 , whereas the relative absorption is reversed under IR light. ...
Photoplethysmography (PPG) based pulse oximetry devices normally use red and infrared illuminations to obtain oxygen saturation (SpO2) readings. In addition, the presence of motion artefacts severely restricts the utility of pulse oximetry physiological measurements. In the current study, a combination of green and orange illuminations from a multi-wavelength optoelectronic patch sensor (mOEPS) was investigated in order to improve robustness to subjects’ movements in the extraction of SpO2 measurement. The experimental protocol with 31 healthy subjects was divided into two sub-protocols, and was designed to determine SpO2 measurement. The datasets for the first sub-protocol were collected from 15 subjects at rest, with the subjects free to move their hands. The datasets for the second sub-protocol with 16 subjects were collected during cycling and walking exercises. The results showed good agreement with SpO2 measurements (r = 0.98) in both sub-protocols. The outcomes promise a robust and cost-effective approach of physiological monitoring with the prospect of providing health monitoring that does not restrict user physical movements.
... In this study, an mOEPS with at least four wavelength illumination sources was used to multiply illumination on a designated tissue and to extract SpO 2 from same device. Conventionally, pulse oximeters with red and IR illuminations are used to measure SpO 2 non-invasively as haemoglobin is the oxygen-transport metalloprotein in red blood cells [14] and the light absorption of oxy-haemoglobin (HbO 2 ) relative to that of deoxyhaemoglobin (Hb) is dependent on the illumination wavelength. Under red illumination, the absorption of Hb is greater than that of HbO 2 , whereas the relative absorption is reversed under IR light. ...
Photoplethysmography (PPG) based pulse oximetry devices normally use red and infrared illuminations to obtain oxygen saturation (SpO2) readings. In addition, the presence of motion artefacts severely restricts the utility of pulse oximetry physiological measurements. In the current study, a combination of green and orange illuminations from a multi-wavelength optoelectronic patch sensor (mOEPS) was investigated in order to improve robustness to subjects’ movements in the extraction of SpO2 measurement. Two experimental protocols with 31 healthy subjects were designed to determine SpO2 measurement. The datasets for the first protocol were collected from 15 subjects at rest, with the subjects free to move their hands. The datasets for the second protocol with 16 subjects were collected during cycling and running exercises. The results showed good agreements with SpO2 measurements (r = 0.98) in the both protocols. The outcomes promise a robust and cost-effective approach of physiological monitoring with the prospect of providing health monitoring that does not restrict user physical movements.
... Since the absorption spectra of HbO 2 and Hb are different, pulse oximetry uses photoplethysmography (PPG) at two different wavelengths (usually red and near-infrared) to obtain the S p O 2 value [3,4]. The PPG signal is the intensity of the light penetrating or reflected by body tissues, which consists of a small pulsatile component (AC) and a large static component (DC). ...
A novel optical sensor probe combining monitoring of blood oxygen saturation (SpO2) with contact pressure is presented. This is beneficial as contact pressure is known to affect SpO2 measurement. The sensor consists of three plastic optical fibres (POF) used to deliver and collect light for pulse oximetry, and a fibre Bragg grating (FBG) sensor to measure contact pressure. All optical fibres are housed in a biocompatible epoxy patch which serves two purposes: (i) to reduce motion artefacts in the photoplethysmogram (PPG), and (ii) to transduce transverse loading into an axial strain in the FBG. Test results show that using a combination of pressure measuring FBG with a reference FBG, reliable results are possible with low hysteresis which are relatively immune to the effects of temperature. The sensor is used to measure the SpO2 of ten volunteers under different contact pressures with perfusion and skewness indices applied to assess the quality of the PPG. The study revealed that the contact force ranging from 5 to 15 kPa provides errors of <2%. The combined probe has the potential to improve the reliability of reflectance oximeters. In particular, in wearable technology, the probe should find use in optimising the fitting of garments incorporating this technology.
... Pulse oximeters are non-invasive devices designed to measure SpO 2 %. Haemoglobin is the oxygen-transport metalloprotein in the red blood cells [17] and as the light absorption of oxy-haemoglobin (HbO 2 ) is different from that of deoxyhaemoglobin (Hb) at different wavelengths of the spectral window, then, from a ratio of the absorptions at two different wavelengths, SpO 2 % can be determined. Figure 2.1 shows the absorption of haemoglobin under red and IR illumination, with Hb absorbing relatively more red light than HbO 2 and Hb relatively less IR light than HbO 2 . 1 Absorption spectra of HbO 2 and Hb [18] As tissues in the body generally contain water, they generally absorb light more strongly in the longer wavelength IR region [3] , whereas melanin is known to absorb more light at shorter wavelengths [2] . ...
... This concept is illustrated schematically in Figure 2. [5] The obvious advantage of the detection of abnormal tissue in-vivo with IR radiation is its human-friendliness. Additionally, the living tissue is may be probed within the so-called therapeutic spectral interval, encompassing red to near IR [6], permitting transmission of illuminating radiation up to the point where a boundary between two optically/ physically different materials is encountered. Fig. 2. We may model a differentiated tissue as a three-layer setup to perform the pulse thermography. ...
... Finally, it is the number of spectral bands used in the ratioing methods, employed when the absolute calibration is not possible or critical for the validity of measurement. Ratioing methods are often used in fiber optic sensors [38] and always in blood oxymetry [39]. ...
We describe a bispectral, 1000-frames per second imaging instrument working simultaneously in two spectral bands. These bands may be selected for a specific application; however, we implement a pair centered at 4.3 μm and 4.66 μm. Synchronization is accomplished by employing a single focal plane array. To demonstrate the performance of the bispectral imager, we apply it to the methane flame of a Bunsen burner in a near conjugate configuration with flame image length subtending at about 200 pixels. The instrument detects bispectral puffing at 2 Hz, pulsations, and bispectral radiation oscillations, first reported here in two spectral intervals. The period of oscillatory spectral components in two bands is the same, about 3 Hz for this flame, with delay of a quarter period between them, first reported here. With 1-ms integration time, we detect significant formation of turbulence and vortices, especially pronounced in the region where the flame transitions into a plume. We display bispectral ratioed images of flames in near-real time with either the laboratory or the field device.
... In the future, we plan to use a Nd:YAG laser, with nominally 100−W output power at 1.064 μm. Around 1 μm tissue is more transparent to the radiation than in the visible [30]. The output beam of this laser has been characterized for its power density as a function of position and power level [31]. ...
We describe a new method to separate ballistic from the scattered photons for optical tissue characterization. It is based on the hypothesis that the scattered photons acquire a phase delay. The photons passing through the sample without scattering or absorption preserve their coherence so they may participate in interference. We implement a Mach-Zehnder experimental setup where the ballistic photons pass through the sample with the delay caused uniquely by the sample indices of refraction. We incorporate a movable mirror on the piezo-electric actuator in the sample arm to detect the amplitude of the modulation term. We present the theory that predicts the path-integrated (or total) concentration of the scattering and absorption centres. The proposed technique may characterize samples with transmission attenuation of ballistic photons by a factor of 10(-14).
We have demonstrated and evaluated an optical setup to obtain two simultaneous infrared images of the same scene with a single IR camera. The simultaneous images were obtained in two IR spectral bands. We obtained over 1000 bi-spectral images per second of a combustion flame.
The main results obtained at the author’s laboratory over the
recent five years with respect to optical in-vivo
skin assessment are reviewed. The exploited optical
properties of human skin are briefly regarded, with following
description of the newly developed methods and prototype devices. In
particular, six non-invasive diagnostic and monitoring technologies
based on skin autofluorescence photobleaching, diffuse reflectance
spectrometry, multispectral skin imaging, and remission
photoplethysmography have been proposed, experimentally implemented, and
clinically tested.
Sniegts pārskats par autora
laboratorijā pēdējo piecu gadu laikā
iegūtajiem in-vivo ādas
optiskā novērtējuma svarīgākajiem
rezultātiem. Īsumā aplūkotas dzīvas
ādas optiskās īpašības,
turpinājumā aprakstot jaunradītās metodes un
prototipa ierīces. Piedāvātas, eksperimentāli
realizētas un klīniski pārbaudītas sešas
neinvazīvas diagnostikas un monitoringa tehnoloģijas, kas
balstītas uz ādas autofluorescences
fotoizbalēšanu, difūzās refleksijas
spektrometriju, multispektrālu ādas attēlošanu
un remisijas fotopletizmogrāfiju.
We report a novel noncontact diffuse correlation spectroscopy flow-oximeter for simultaneous quantification of relative changes in tissue blood flow (rBF) and oxygenation (Δ[oxygenation]). The noncontact probe was compared against a contact probe in tissue-like phantoms and forearm muscles (n = 10), and the dynamic trends in both rBF and Δ[oxygenation] were found to be highly correlated. However, the magnitudes of Δ[oxygenation] measured by the two probes were significantly different. Monte Carlo simulations and phantom experiments revealed that the arm curvature resulted in a significant underestimation (~-20%) for the noncontact measurements in Δ[oxygenation], but not in rBF. Other factors that may cause the residual discrepancies between the contact and noncontact measurements were discussed, and further comparisons with other established technologies are needed to identify/quantify these factors. Our research paves the way for noncontact and simultaneous monitoring of blood flow and oxygenation in soft and vulnerable tissues without distorting tissue hemodynamics.
A survey of the literature is presented that provides an analysis of the optical properties of human skin, with particular regard to their applications in medicine. Included is a description of the primary interactions of light with skin and how these are commonly estimated using radiative transfer theory (RTT). This is followed by analysis of measured RTT coefficients available in the literature. Orders of magnitude differences are found within published absorption and reduced-scattering coefficients. Causes for these discrepancies are discussed in detail, including contrasts between data acquired in vitro and in vivo. An analysis of the phase functions applied in skin optics, along with the remaining optical coefficients (anisotropy factors and refractive indices) is also included. The survey concludes that further work in the field is necessary to establish a definitive range of realistic coefficients for clinically normal skin.
The authors describe the development of diffuse optical imaging (DOI) technologies, specifically the use of spatial and temporal modulation to control near infrared light propagation in thick tissues. We present theory and methods of DOI focusing on model-based techniques for quantitative, in vivo measurements of endogenous tissue absorption and scattering properties. We specifically emphasize the common conceptual framework of the scalar photon density wave for both temporal and spatial frequency-domain approaches. After presenting the history, theoretical foundation, and instrumentation related to these methods, we provide a brief review of clinical and preclinical applications from our research as well as our outlook on the future of DOI technology.
Feasibility of pulse oxigraphy is examined using an in-vitro phantom, perfused with human blood by a heart-lung machine. 29 different oxygen saturation levels, measured with our experimental camera and a clinical pulse oximeter, were matched against laboratory blood gas analysis. Discrete transfer functions between measurements & laboratory values were derived for camera (fCAM-LAB) & pulse oximeter (fPULSE-LAB). 87 additional independent pulse oximeter & camera measurements were subjected to these functions to check reproducibility (R2 = 0.99). To demonstrate imaging capacities, a dual reservoir was applied to image venous and arterial samples from 2 patients. Camera-derived vs laboratory arterial values were: 97.0% vs 99.5% (pat. 1, pH=7.41) & 97.5% vs 99.5% (pat. 2, pH=7.37). Venous resulst were: 86.5% vs 74.4% (pat. 1, pH=7.39) & 89.1% vs 86.2% (pat. 2, pH=7.34). In vitro pulse oxigraphy can visualize regions with different blood oxygenation.
We present a new method of formation photoplethysmographic images with high spatial resolution from video recordings of a living body in the reflection geometry. The method (patent pending) is based on lock-in amplification of every pixel of the recorded video frames. A reference function required for synchronous detection of cardiovascular pulse waves is formed from the same frames. The method is featured by ability to visualize dynamic changes in cardiovascular pulse wave during the cardiac (or respiratory) cycle. We demonstrate that the system is capable to detect the minimal irritations of the body such as gentle scratching of the skin by own finger.
Chronic arterial insufficiency (CAI) of lower extremities is important socio-economical and healthcare problem, due to its high incidence of morbidity, disability and mortality.
The aim of our work was to determine the diagnostic importance of pulse oximetry in the early detection of stage of lower extremities CAI based on peripheral arterial oxygen saturation of haemoglobin (SpO2).
Prospectively, we analyzed a group of 50 patients, admitted at the Vascular Department of Surgical Clinic in Nis during the period from September 2006 to October 2007, with evident symptoms and signs of different stages of lower extremities CAI verified by ultrasonography. In patients with lower extremity disorder of tissue arterial capillaries, SpO2 was determined by pulse oximetry.
Using pulse oximetry, depending on the of stage of lower extremities CAI, we revealed a considerable difference in the stages of functional ischemia SpO2: Fontaine I - 95.33 +/- 1.41%, Fontaine IIa - 92.14 +/- 2.27% and Fontaine IIb - 79.67 +/- 2.73%; in stage critical ischemia SpO2: Fontaine III - 62.54 +/- 4.39% and Fontaine IV - 47.67 +/- 6.16%. In 3 patients with gangrenous foot and fingers SpO2 was immeasurable and progressive decrease in SpO2 of arterial capillaries (p < 0.01 between stages).
Due to the reliability and simplicity of pulse oximetry it can be a routinely used diagnostic device for patients with early determined stage of lower extremities CAI.
In this paper, we present a novel photoplethysmographic device that operates remotely, i.e. not in contact with the skin. The device allows for real time measurements of heart rate with motion artifact reduction from a distance of a few centimeters up to several meters. High mobility of users is achieved in assessment of vital body signs, such as heart rate.
We have performed non-invasive, real-time optical mapping of the piglet brain during a subcortical injection of autologous blood. The time resolution of the optical maps is 192 ms, thus allowing us to generate a real-time video of the growing subcortical hematoma. The increased absorption at the site of blood injection is accompanied by a decreased absorption at the contralateral brain side. This contralateral decrease in the optical absorption and the corresponding time traces of the cerebral hemoglobin parameters are consistent with a reduced cerebral blood flow caused by the increased intracranial pressure.
Recently, we have introduced a transillumination technique for biomedical diagnosis. The technique, pass-through photon-based transillumination, relies on interferometric measurements to recover the information of interest. In this work, we present the forward-calculated analytical interferograms that describe the behavior of the system. Stochastic modeling of radiation interacting with tissue enables determination of amplitude and phase parameters, indispensable for computation of the interferograms. Sample variability is assessed by studying tissue phantoms similar to those used in the experimental verification of the technique and that are representative of (abnormal) dental tissues. For tissue characterization, perfect recovery of the integrated attenuation ensues by employing spatially compact radiation sources. For tissue imaging, spatially extended sources with broad bandwidth are superior due to the implicit longitudinal coherence filter. For both applications, sample variability issues may be neutralized by permitting spatial divergence of scattered photons.
Plethysmographic signals were measured remotely (>1m) using ambient light and a simple consumer level digital camera in movie mode. Heart and respiration rates could be quantified up to several harmonics. Although the green channel featuring the strongest plethysmographic signal, corresponding to an absorption peak by (oxy-) hemoglobin, the red and blue channels also contained plethysmographic information. The results show that ambient light photo-plethysmography may be useful for medical purposes such as characterization of vascular skin lesions (e.g., port wine stains) and remote sensing of vital signs (e.g., heart and respiration rates) for triage or sports purposes.
Biomedical sensors, integrated into textiles would enable monitoring of many vitally important physiological parameters during our daily life. In this paper we demonstrate the design and performance of a textile based pulse oximeter, operating on the forefinger tip in transmission mode. The sensors consisted of plastic optical fibers integrated into common fabrics. To emit light to the human tissue and to collect transmitted light the fibers were either integrated into a textile substrate by embroidery (producing microbends with a nominal diameter of 0.5 to 2 mm) or the fibers inside woven patterns have been altered mechanically after fabric production. In our experiments we used a two-wavelength approach (690 and 830 nm) for pulse wave acquisition and arterial oxygen saturation calculation. We have fabricated different specimens to study signal yield and quality, and a cotton glove, equipped with textile based light emitter and detector, has been used to examine movement artifacts. Our results show that textile-based oximetry is feasible with sufficient data quality and its potential as a wearable health monitoring device is promising.
In this article, we introduce a simple method to characterize optical properties and blood oxygenation in tissue using spatially resolved, steady-state reflectance. The method considers multiple source-detector separations larger than 2 cm, i.e. 20 times the optical mean free path in tissue, and makes an approximation to linearize the relationship between the separation and reflectance. Simulation results show that errors of the algorithm due to the approximation are less than 10%. Using a calibration sample, we calculate from the slope and intercept the absorption and reduced scattering coefficients, mu a and mu's, of a tissue-like solution, and experimental results confirm the usefulness of the method for quantitation of haemoglobin saturation in tissue.
This study tests a multiwavelength frequency-domain near-infrared oximeter (fdNIRS) in an in vitro model of the human brain. The model is a solid plastic structure containing a vascular network perfused with blood in which haemoglobin oxygen saturation (SO2) was measured by co-oximetry, providing a standard for comparison. Plastic shells of varying thickness (0.5-2 cm), with a vascular network of their own and encircling the brain model, were also added to simulate extracranial tissues of the infant, child and adult. The fdNIRS oximeter utilizes frequency-domain technology to monitor phaseshifts at 754 nm, 785 nm and 816 nm relative to a 780 nm reference to derive SO2 through photon transport and Beer-Lambert equations. We found a linear relationship between fdNIRS SO2 and co-oximetry SO2 with excellent correlation (r2 > or = 0.95) that fitted the line of identity in all experiments (n = 7). The bias of fdNIRS oximetry was -2% and the precision was 6%. Blood temperature and fdNIRS source-detector distance did not affect fdNIRS oximetry. Low haemoglobin concentration (6 g dl(-1)) altered the fdNIRS versus co-oximetry line slope and intercept, producing a 15% error at the extremes of SO2. The infant- and child-like shells overlying the brain model did not alter fdNIRS oximetry, whereas the adult-like shell yielded an error as high as 32%. In conclusion, fdNIRS accurately measures SO2 in an in vitro brain model, although low haemoglobin concentration and extracranial tissue of adult thickness influence accuracy.
A method for the measurement of oxygen saturation in the venous blood, SvO2, based on optical measurements of light absorption in the infrared region is presented. The method consists of applying relatively low external pressure of 25 mm Hg on the forearm, thereby increasing the venous blood volume in the tissue, and comparing the light absorption before and after the external pressure application. SvO2 has been determined from light absorption measurements in two wavelengths, before and after the pressure application, using a formula derived for two adjacent wavelengths. The method has been applied to the hands and fingers of 17 healthy male subjects, using wavelengths of 767 and 811 nm. SaO2, the oxygen saturation for arterial blood, was also obtained from photoplethysmographic measurements in these two wavelengths (pulse oximetry) using the same formula. The mean (+/- SD) value of SaO2 was 94.5% (+/- 3.0). The mean value of SvO2 was 86.2% (+/- 4.1) for the finger and 80.0% (+/- 8.2) for the hand. These SvO2 values are reasonable for the finger and the hand where arterio-venous anastomoses exist. The method enables the measurement of SvO2 in the limbs, a parameter which is related to tissue blood flow and oxygen consumption.
Pulse oximeters are widely used for noninvasive monitoring of oxygen saturation in arterial blood hemoglobin. We present a transmittance pulse oximetry system based on near-infrared (NIR) laser diodes (750 and 850 nm) for monitoring oxygen saturation of arterial blood hemoglobin. The pulse oximetry system is made up of the optical sensor, sensor electronics, and processing block. Also, we show experimental results obtained during the development of the whole NIR transmittance pulse oximetry system along with modifications in the sensor configuration, signal processing algorithm, and calibration procedure. Issues concerning wavelength selection and its implications for the improvement of the transmittance pulse oximetry technique are discussed. The results obtained demonstrate the proposed system's usefulness in monitoring a wide range of oxygen saturation levels.
We present three-dimensional diffuse optical tomography of the hemodynamic response to somatosensory stimulation in a rat. These images show the feasibility of volumetrically imaging the functional response to brain activity with diffuse light. A combination of positional optode calibration and contrast-to-noise ratio weighting was found to improve imaging performance.
The basic parameters for physiological measurements provided by near-infrared spectroscopy are the local absorption and scattering coefficients. For the adult human head, they have been difficult to measure noninvasively because of the layered structure of the head. The results of measurements of absorption and reduced scattering coefficients through the forehead on 30 adult volunteers using a multidistance frequency domain method are reported. The optode separation distance ranged from 10 to 80 mm and measurements were recorded at 758 and 830 nm. The measured absorption and reduced scattering coefficients of the forehead were used to evaluate the hemoglobin content in the scalp and brain as well as cerebral oxygen saturation. We found that cerebral oxygenation was relatively narrowly distributed within the subject group (the standard deviation was about 3% for scalp and 6% for brain, respectively), whereas hemoglobin concentrations had a relatively broader distribution. We found that as the optode distance increased, the absorption coefficients increased and the scattering coefficients decreased, retrieving the optical values of scalp and brain for shorter and longer optode distances, respectively. We present the transition curves of the absorption and reduced scattering coefficients as functions of the optode distance. In order to verify the values for each layer, a comparison between the experimental data and a prediction based on the two-layer model of the adult head was carried out. The thicknesses of scalp and skull for the two-layer model were obtained by magnetic resonance imaging of a subject's head. The optical parameters obtained from the two-layer model agreed very well with those measured by the multidistance method.
A new eight-wavelength pulse oximeter is designed to measure methemoglobin and carboxyhemoglobin, in addition to the usual measurements of hemoglobin oxygen saturation and pulse rate. This study examines this device's ability to measure dyshemoglobins in human volunteers in whom controlled levels of methemoglobin and carboxyhemoglobin are induced.
Ten volunteers breathed 500 ppm carbon monoxide until their carboxyhemoglobin levels reached 15%, and 10 different volunteers received intravenous sodium nitrite, 300 mg, to induce methemoglobin. All were instrumented with arterial cannulas and six Masimo Rad-57 (Masimo Inc., Irvine, CA) pulse oximeter sensors. Arterial blood was analyzed by three laboratory CO-oximeters, and the resulting carboxyhemoglobin and methemoglobin measurements were compared with the corresponding pulse oximeter readings.
The Rad-57 measured carboxyhemoglobin with an uncertainty of +/-2% within the range of 0-15%, and it measured methemoglobin with an uncertainty of 0.5% within the range of 0-12%.
The Masimo Rad-57 is the first commercially available pulse oximeter that can measure methemoglobin and carboxyhemoglobin, and it therefore represents an expansion of our oxygenation monitoring capability.
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.
We present a camera-based device capable of capturing two photoplethysmographic (PPG) signals at two different wavelengths simultaneously, in a remote noncontact manner. The system comprises a complementary metal-oxide semiconductor camera and dual wavelength array of light emitting diodes (760 and 880 nm). By alternately illuminating a region of tissue with each wavelength of light, and detecting the backscattered photons with the camera at a rate of 16 frames/wavelength s, two multiplexed PPG wave forms are simultaneously captured. This process is the basis of pulse oximetry, and we describe how, with the inclusion of a calibration procedure, this system could be used as a noncontact pulse oximeter to measure arterial oxygen saturation (S(p)O(2)) remotely. Results from an experiment on ten subjects, exhibiting normal S(p)O(2) readings, that demonstrate the instrument's ability to capture signals from a range of subjects under realistic lighting and environmental conditions are presented. We compare the signals captured by the noncontact system to a conventional PPG signal captured concurrently from a finger, and show by means of a J. Bland and D. Altman [Lancet 327, 307 (1986); Statistician 32, 307 (1983)] test, the noncontact device to be comparable to a contact device as a monitor of heart rate. We highlight some considerations that should be made when using camera-based "integrative" sampling methods and demonstrate through simulation, the suitability of the captured PPG signals for application of existing pulse oximetry calibration procedures.
Several investigators have demonstrated techniques for noninvasive measurement of the oxygen saturation of blood in retinal arteries and veins. These techniques have been based on measuring the optical density of a retinal vessel at multiple wavelengths and on calculating the oxygen saturation on the basis of the known absorption coefficients of hemoglobin and oxyhemoglobin. A technique is presented for determining the optimum wavelengths for retinal oximetry measurements. What is believed to be a novel wavelength combination of 488, 635, and 905 nm is found to provide excellent oxygen sensitivity across a broad range of typical vessel diameters and saturations. The use of this wavelength combination should allow for the most accurate retinal saturation measurements made to date.
In spite of the usefulness of continuous-wave near-infrared spectroscopy (CW-NIRS) for noninvasive measurement of oxygenation of the brain and muscle, no theoretically derived algorithms for this technique have yet been presented, although an algorithm can be theoretically determined on the basis of diffusion. We derive the theoretical mean optical pathlength based on diffusion theory, and we develop linear and nonlinear algorithms for CW-NIRS. Both algorithms are validated by the results of experiments using a phantom consisting of Intralipid and hemoglobin solutions. An extra absorber is also used to examine the effects of background absorption in tissues, except for blood. The changes in concentrations of oxy- and deoxyhemoglobin, calculated using both algorithms, agree well with those obtained by experiments around an operating point determined by the optical pathlength. The results of the phantom experiments and theoretical analyses show that the nonlinear algorithm can be used in a wider range of variation in absorption than the linear algorithm. The results also suggest that if the operating point is inappropriately given, the accuracy of both algorithms is greatly reduced. The errors in actual measurements of muscle oxygenation using the linear and nonlinear algorithms are also estimated, assuming a variation of +/- 50% in blood volume, and are found to be less than 12 and 6%, respectively, if the operating point is set appropriately.
The transillumination window in the tissue spectrum, which extends from about 600 to about 1200 nm, is very attractive in diagnosis and therapy because the penetration of light into tissues ranges from several micrometers to several millimeters. Problems with the evaluation of red and near-IR light-tissue interaction are of great importance in the development of noninvasive blood oximetry. It is shown that it is possible to use a significant 1D representation of such an interaction with a reliability acceptance in practice. The state of the art, taken into account here as the background, refers to the reported optical parameters of tissues when exposed to light of wavelengths included in the optical window considered. Against that background, using the arterial blood oxygen saturation as a measurement quantity, a reasonably complicated model of noninvasive processes occurring in a living object as a pulsatile inhomogeneous optical medium is presented. During the calculations and experiments, a novel use of the known transmission variant of the pulse oximetry concept is considered. At two measuring wavelengths (i.e., 660 and 940 nm), the biophysical and optical properties of living tissues are involved in relationships that include the quantities to be directly measured with known accuracy. Finally, the results of calculations referring to the transilluminated representative object (i.e., a theoretical fingertip) are compared to the appropriate results obtained during a series of measurements performed on real human subjects using the designed measuring system.
A dual-wavelength fibre-optic pulse oximetry system is described for the purposes of estimating oxygen saturation (SpO2) from the oesophagus. A probe containing miniature right-angled glass prisms was used to record photoplethysmographic (PPG) signals from the oesophageal wall. Signals were recorded successfully in 19 of 20 patients, demonstrating that PPG signals could be reliably obtained from an internal vascularised tissue site such as the oesophageal epithelium. The value of the mean oxygen saturation recorded from the oesophagus was 94.0 ± 4.0%. These results demonstrate that SpO2 may be estimated in the oesophagus using a fibre-optic probe.
Since virtually all the oxygen carried by blood at normal hematocrit is reversibly bound to red blood cell hemoglobin, the
distribution of oxygen within the microcirculation can be determined from measurements of hemoglobin concentration and hemoglobin
oxygen saturation in vessels of the network. Photometric methods that rely on light absorption and scattering properties of
blood are described. Criteria for selecting the wavelengths needed to analyze hemoglobin in the microcirculation are specified.
Two theoretical descriptions of light absorption and scattering, multiple scattering theory and photon diffusion theory, are
applied to the problem. Practical approaches to the determination of hemoglobin concentration and oxygen saturation in the
microcirculation follow from these theoretical formulations. Technical aspects of microscope photometry including light sources,
microscopy, and detection systems are described with special emphasis on the problem of glare. The importance of in vitro
as well as in vivo calibrations is stressed, and several recent applications of a working system are discussed. Current problems
as well as future developments of this methodology are delineated as a guide to future work in this area.
The inhomogeneity of tissue structure greatly affects the sensitivity of tissue oxygenation measurement by reflectance near-infrared spectroscopy. In this study, we investigated the influence of a fat layer on muscle oxygenation measurement by in vivo tests and Monte Carlo simulation, and we propose a method for correcting the influence. In the simulation, a three-dimensional model consisting of the epidermis, dermis, fat, and muscle layers was used. In in vivo tests, measurement sensitivity was examined by measuring oxygen consumption of the forearm muscle and the peak-to-peak variation of oxygenation in periodic exercise tests on the vastus lateralis using a newly developed multisensor type of tissue oximeter. Fat layer thickness was also measured by ultrasonography. The correction curve of measurement sensitivity against fat layer thickness was obtained from the results of simulation and in vivo tests. The values of corrected oxygen consumption were almost the same and had less variation between individuals (0.13±0.02 ml 100 g -1 min -1 ) than did the uncorrected values (0.08±0.04 ml 100 g -1 min -1 ). © 2000 American Institute of Physics.
Diffuse optical imaging (DOI) is a noninvasive optical technique that employs near-infrared (NIR) light to quantitatively characterize the optical properties of thick tissues. Although NIR methods were first applied to breast transillumination (also called diaphanography) nearly 80 years ago, quantitative DOI methods employing time- or frequency-domain photon migration technologies have only recently been used for breast imaging (i.e., since the mid-1990s). In this review, the state of the art in DOI for breast cancer is outlined and a multi-institutional Network for Translational Research in Optical Imaging (NTROI) is described, which has been formed by the National Cancer Institute to advance diffuse optical spectroscopy and imaging (DOSI) for the purpose of improving breast cancer detection and clinical management. DOSI employs broadband technology both in near-infrared spectral and temporal signal domains in order to separate absorption from scattering and quantify uptake of multiple molecular probes based on absorption or fluorescence contrast. Additional dimensionality in the data is provided by integrating and co-registering the functional information of DOSI with x-ray mammography and magnetic resonance imaging (MRI), which provide structural information or vascular flow information, respectively. Factors affecting DOSI performance, such as intrinsic and extrinsic contrast mechanisms, quantitation of biochemical components, image formation/visualization, and multimodality co-registration are under investigation in the ongoing research NTROI sites. One of the goals is to develop standardized DOSI platforms that can be used as stand-alone devices or in conjunction with MRI, mammography, or ultrasound. This broad-based, multidisciplinary effort is expected to provide new insight regarding the origins of breast disease and practical approaches for addressing several key challenges in breast cancer, including: Detecting disease in mammographically dense tissue, distinguishing between malignant and benign lesions, and understanding the impact of neoadjuvant chemotherapies.
Near-infrared spectroscopy provides useful biological information after the radiation has penetrated through the tissue, within the therapeutic window. One of the significant shortcomings of the current applications of spectroscopic techniques to a live subject is that the subject may be uncooperative and the sample undergoes significant temporal variations, due to his health status that, from radiometric point of view, introduce measurement noise. We describe a novel wavelength selection method for monitoring, based on a standard deviation map, that allows low-noise sensitivity. It may be used with spectral transillumination, transmission, or reflection signals, including those corrupted by noise and unavoidable temporal effects. We apply it to the selection of two wavelengths for the case of pulse oximetry. Using spectroscopic data, we generate a map of standard deviation that we propose as a figure-of-merit in the presence of the noise introduced by the living subject. Even in the presence of diverse sources of noise, we identify four wavelength domains with standard deviation, minimally sensitive to temporal noise, and two wavelengths domains with low sensitivity to temporal noise.
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.
We present the mathematical foundation and the experimental validation of a technique that utilizes pass-through (ballistic) photons in a partial coherence interferometric transillumination setup for biomedical analyses. We demonstrate that the implementation depends closely on tissue under test, incident power, spatial and spectral characteristics of the radiation source, and detection electronics. With the aid of the complex material coherence function concept, we foresee tissue characterization and diagnostic imaging as potential applications for the technique. We propose a normalization procedure for in vitro and in vivo measurements, where nontissue-related quantities are canceled out. The validation of the proposal is achieved by obtaining the sample coherence function of a tissue phantom. The expected exponential attenuation is confirmed, and the corresponding scattering coefficients are determined. A good agreement between theory and experiment, for the initial set of samples, serves to establish that pass-through photon-based transillumination is feasible for selected biomedical applications.
Hypovolemia is one of the most frequent causes of arterial hypotension in the operating room. Pulse oximeter plethysmographic waveform, obtained using a noninvasive and widely available device, has recently shown its potential interest in predicting fluid responsiveness in mechanically ventilated patients under mechanical ventilation. This review highlights new applications of this routine monitoring.
Respiratory variations in the plethysmographic waveform amplitude have been correlated with respiratory variations in arterial pulse pressure and can predict fluid responsiveness in mechanically ventilated patients under general anesthesia. Until recently, pulse oximeter plethysmographic waveform had to be recorded and analyzed off line using software algorithms. Bringing this new index into the clinical field would require devices allowing for automated and continuous real time calculation. Such devices will have potential to guide fluid optimization in the operating room.
Automatic detection of respiratory variations in pulse oximetry plethysmographic waveform amplitude can predict fluid responsiveness in the operating room in patients under mechanical ventilation and has potential for fluid optimization in this setting.
Near infrared (IR) spectroscopy can give continuous, direct information about cerebral oxygenation in vivo by providing signals from oxygenated and deoxygenated haemoglobin and cytochrome aa3. Due to a lack of precise spectral information and uncertainties about optical path length it has previously been impossible to quantify the data. We have therefore obtained the cytochrome aa3 spectrum in vivo from the brains of rats after replacing the blood with a fluorocarbon substitute. Near infrared haemoglobin spectra were also obtained, at various oxygenation levels, from cuvette studies of lysed human red blood cells. Estimates of optical path length have been obtained. The data were used to construct an algorithm for calculating the changes in oxygenated and deoxygenated haemoglobin and oxygenated cytochrome aa3 in tissue from changes in near IR absorption.
Conventional pulse oximeters are accurate at high oxygen saturation under a variety of physiological conditions but show worsening accuracy at lower saturation (below 70%). Numerical modeling suggests that sensors fabricated with 735 and 890 nm emitters should read more accurately at low saturation under a variety of conditions than sensors made with conventionally used 660 and 900 nm band emitters. Recent animal testing confirms this expectation. It is postulated that the most repeatable and stable accuracy of the pulse oximeter occurs when the fractional change in photon path lengths due to perturbations in the tissue (relative to the conditions present during system calibration) is equivalent at the two wavelengths. Additionally, the penetration depth (and/or breadth) of the probing light needs to be well matched at the two wavelengths in order to minimize the effects of tissue heterogeneity. At high saturation these conditions are optimally met with 660 and 900 nm band emitters, while at low saturation 735 and 890 nm provide better performance.
A new method for pulse oximetry is presented that possesses an inherent insensitivity to corruption by motion artifact, a primary limitation in the practical accuracy and clinical applicability of current technology. Artifact corruption of the underlying photoplethysmographic signals is reduced in real time, using an electronic processing methodology that is based upon inversion of a physical artifact model. This fundamental approach has the potential to provide uninterrupted output and superior accuracy under conditions of sustained subject motion, therefore, widening the clinical scope of this useful measurement. A new calibration technique for oxygen saturation is developed for use with these processed signals, which is shown to be a generalization of the classical interpretation. The detailed theoretical and practical issues of implementation are then explored, highlighting important engineering simplifications implicit in this new approach. A quantitative investigation of the degree of insensitivity to artifact is also undertaken, with the aid of a custom electronic system and commercial pulse oximeter probes, which is compared and contrasted with the performance of a conventional implementation. It is demonstrated that this new methodology results in a reduced sensitivity to common classes of motion artifact, while retaining the generality to be combined with conventional signal processing techniques.
To determine whether blood hemoglobin concentration ([Hb]) could be measured noninvasively as the ratio of pulsatile changes in attenuation (absorbance plus scatter) of light (D) across a body part to changes in light path length (l), we measured transmission of near-infrared light (905±10 nm) through a finger, using a modified pulse oximeter, and simultaneously monitored fingertip diameter, using a sonomicrometer. In 25 subjects with [Hb] ranging from 3.1 to 18.2 gm/dl, and with normal oxygenation, average D/l ratio over 30–60 s correlated strongly with [Hb] measured by Coulter counter (r=0.84, p≪0.001), though with considerable scatter, with absolute value of differences averaging 17% of the mean. Using 12 gm/dl and 0.75 mm-1 as the lower limits of normal for [Hb] and D/l, respectively, two of nine normals had low (D/l) (78% specificity), and only one of 16 anemic subjects had borderline normal (D/l) (94%–100% sensitivity). The positive predictive value of a low (D/l) was 88% and the negative predictive value was 87.5%. With further development, this technique may reduce the need for phlebotomy, thereby reducing risks and costs and improving the experience of being a patient. © 2002 Biomedical Engineering Society.
PAC2002: 8763Lk, 8714Ee
The pulse oximeter has become a vital instrument in the care of infants and children with cardiopulmonary disease. Recent advances in pulse oximetry technology have improved some aspects of pulse oximeter performance. However, the reliability, accuracy, and clinical utility of pulse oximetry remain problematic in some types of patients under certain conditions. Improved signal processing technology has substantially improved the ability of certain oximeters to work reliably under conditions of poor perfusion and motion artifact. There is a growing body of evidence describing the effect of pulse oximeter utilization on processes and outcomes. This article describes the principles, limitations, current state of oximetry technology, and the impact of oximetry data and alarms on diagnosis and clinical decision-making.
Detection of cerebral hypoxia-ischemia in infants remains problematic, as current monitors in clinical practice are impractical, insensitive, or nonspecific. Our study develops a multiwavelength spatial domain construct for near-infrared spectroscopy (NIRS) to detect cerebral hypoxia-ischemia and evaluates the construct in several models. The NIRS probe contains photodiode detectors 2, 3, and 4 cm from a three-wavelength, light-emitting diode. A construct determines cerebral O(2) saturation based on spatial domain principles. Device performance and construct validity are examined in in-vitro models simulating the brain, and in piglets subjected to hypoxia, hypoxia-ischemia, and hyperoxic conditions using a weighted average of arterial and cerebral venous O(2) saturation measured by CO-oximetry. The results in the brain models verify key equations in the construct and demonstrate reliable performance of the device. In piglets, the device measures cerebral O(2) saturation with bias +/-4% and precision +/-8%. In conclusion, this NIRS device accurately detects cerebral hypoxia-ischemia and is of a design that is practical for clinical application.
Pulse oximetry is an important diagnostic and patient monitoring tool. However, motion can induce considerable error into pulse oximetry accuracy, resulting in loss of data, inaccurate readings, and false alarms. We will discuss how motion artifact affects pulse oximetry accuracy, the clinical consequences of motion artifact, and the methods used by various technologies to minimize the impact of the motion noise.
As the use of pulse oximeters increases, the needs for higher performance and wider applicability of pulse oximetry have increased. To realize the full potential of pulse oximetry, it is indispensable to increase the number of optical wavelengths. To develop a multiwavelength oximetry system, a physical theory of pulse oximetry must be constructed. In addition, a theory for quantitative measurement of optical absorption in an optical scatterer, such as in living tissue, remains a difficult theoretical and practical aspect of this problem.
We adopted Schuster's theory of radiation through a foggy atmosphere for a basis of theory of pulse oximetry. We considered three factors affecting pulse oximetry: the optics, the tissue, and the venous blood.
We derived a physical theoretical formula of pulse oximetry. The theory was confirmed with a full SO2 range experiment. Based on the theory, the three-wavelength method eliminated the effect of tissue and improved the accuracy of Spo2. The five-wavelength method eliminated the effect of venous blood and improved motion artifact elimination.
Our theory of multiwavelength pulse oximetry can be expected to be useful for solving almost all problems in pulse oximetry such as accuracy, motion artifact, low-pulse amplitude, response delay, and errors using reflection oximetry which will expand the application of pulse oximetry. Our theory is probably a rare case of success in solving the difficult problem of quantifying optical density of a substance embedded in an optically scattering medium.
In this article, I examine the source of the photoplethysmograph (PPG), as well as methods of investigation, with an emphasize on amplitude, rhythm, and pulse analysis. The PPG waveform was first described in the 1930s. Although considered an interesting ancillary monitor, the "pulse waveform" never underwent intensive investigation. Its importance in clinical medicine was greatly increased with the introduction of the pulse oximeter into routine clinical care in the 1980s. Its waveform is now commonly displayed in the clinical setting. Active research efforts are beginning to demonstrate a utility beyond oxygen saturation and heart rate determination. Future trends are being heavily influenced by modern digital signal processing, which is allowing a re-examination of this ubiquitous waveform. Key to unlocking the potential of this waveform is an unfettered access to the raw signal, combined with standardization of its presentation, and methods of analysis. In the long run, we need to learn how to consistently quantify the characteristics of the PPG in such a way as to allow the results from research efforts be translated into clinically useful devices.