Quantification of ischemic muscle deoxygenation by near infrared time-resolved spectroscopy

Acute Effects of a Practical Blood Flow Restriction Device During Swimming Exercise

Purpose: The present study aimed to analyze: 1) the reliability of the tissue saturation index (TSI) and ratings of perceived discomfort (RPD) responses wearing a neoprene practical cuff (PrC), comparing with the responses from traditional (TrC) pneumatic cuffs (study I); 2) the effects of PrC on metabolic (blood lactate concentration, BLC), perceptual (rate of perceived effort, RPE) and kinematic responses at sub-maximal swimming velocities (study II). Methods: Study I; 1) PrC test-retest at rest and during swimming ergometer exercise; 2) BFR at rest with TrC inflated to different percentages of the minimum arterial occlusion pressure (MAOP; 60, 80, 100, 120 and 140%). Test-retest reliability of TSI and RPD was assessed by the intraclass correlation coefficient (ICC) and comparisons among conditions were analyzed by one-way repeated-measures ANOVA. Study II; 1) 50, 200 and 400 m swimming performances; 2) sub-maximal incremental swimming protocol with and without PrC. Two-way repeated measures ANOVA was used to compare all variables during sub-maximal velocities. Results: TSI (ICC = 0.81; 95%CI 0.62-0.91) and RPD (ICC = 0.97; 95%CI 0.94-0.99) were reliable under restricted exercise using PrC. TSI during restricted exercise was lower (p <.001) compared to unrestricted exercise (6.8 ± 6.1% vs. 21.6 ± 8.2% of physiological normalization). PrC showed higher BLC only at or above 91% of critical velocity (p < .03), while stroke rate and RPE were higher (p < .005), and stroke length was lower (p < .03) during all swimming velocities. Conclusion: This easy-to-handle and affordable practical BFR device increased physiological stress at sub-maximal efforts which could be an additional training tool for swimmers.

Nanocrystals for Deep-Tissue In Vivo Luminescence Imaging in the Near-Infrared Region

In vivo imaging technologies have emerged as a powerful tool for both fundamental research and clinical practice. In particular, luminescence imaging in the tissue-transparent near-infrared (NIR, 700-1700 nm) region offers tremendous potential for visualizing biological architectures and pathophysiological events in living subjects with deep tissue penetration and high imaging contrast owing to the reduced light-tissue interactions of absorption, scattering, and autofluorescence. The distinctive quantum effects of nanocrystals have been harnessed to achieve exceptional photophysical properties, establishing them as a promising category of luminescent probes. In this comprehensive review, the interactions between light and biological tissues, as well as the advantages of NIR light for in vivo luminescence imaging, are initially elaborated. Subsequently, we focus on achieving deep tissue penetration and improved imaging contrast by optimizing the performance of nanocrystal fluorophores. The ingenious design strategies of NIR nanocrystal probes are discussed, along with their respective biomedical applications in versatile in vivo luminescence imaging modalities. Finally, thought-provoking reflections on the challenges and prospects for future clinical translation of nanocrystal-based in vivo luminescence imaging in the NIR region are wisely provided.

Effects of Prolonged Sitting with or without Elastic Garments on Limb Volume, Arterial Blood Flow, and Muscle Oxygenation

Supplemental digital content is available in the text.

Purpose

The physiological response induced by acute prolonged sitting is not fully understood. Therefore, we examined the effects of 8-h constant sitting on microcirculation and associated factors in the lower extremity among healthy males. We also evaluated the protective effects of lower-pressure thigh-length elastic compression garments on these parameters.

Methods

Nine healthy males (age, 22.6 ± 1.4 yr; body mass index, 22.4 ± 1.8 kg·m⁻²) completed the 8-h constant sitting experiment. Following baseline measurements, each subject was randomized to wear a lower-pressure elastic garment on the right or left leg from the inguinal region to the ankle joint, with the noncompressed contralateral leg as a control. Circumferences of the calf and malleolus, extracellular water contents, blood flow and shear rate of the dorsal metatarsal artery, and oxygen dynamics in the gastrocnemius muscles were measured in both extremities before and during 8-h constant sitting.

Results

Compared with baseline values, 8-h constant sitting caused enlargement of circumferences (calf, 2.4% ± 0.7%; malleolus, 2.7% ± 1.4%), retention of extracellular water in lower extremity muscles (10.1% ± 1.78%), deterioration of the blood flow (61.4% ± 16.2% of baseline) and shear rate of the dorsal metatarsal artery, and decrease in oxygenated hemoglobin and total hemoglobin levels in the gastrocnemius muscle (P < 0.05, respectively). When subjects wore the lower-pressure thigh-length compression garment, a significant reduction of these effects was observed (P < 0.05, for all).

Conclusions

Prolonged sitting for 8 h induced edema, as well as deterioration of the arterial blood flow, shear rate, and microcirculation in lower limb muscles. Conversely, application of the lower-pressure elastic garment successfully prevented the pathophysiological deterioration associated with prolonged sitting.

The balance of muscle oxygen supply and demand reveals critical metabolic rate and predicts time to exhaustion

We tested the hypothesis that during whole body exercise, the balance between muscle O₂ supply and metabolic demand may elucidate intensity domains, reveal a critical metabolic rate, and predict time to exhaustion. Seventeen active, healthy volunteers (12 males, 5 females; 32 ± 2 yr) participated in two distinct protocols. Study 1 (n = 7) consisted of constant work rate cycling in the moderate, heavy, and severe exercise intensity domains with concurrent measures of pulmonary V̇o₂ and local %SmO₂ [via near-infrared spectroscopy (NIRS)] on quadriceps and forearm sites. Average %SmO₂ at both sites displayed a domain-dependent response (P < 0.05). A negative %SmO₂ slope was evident during severe-domain exercise but was positive during exercise below critical power (CP) at both muscle sites. In study 2 (n = 10), quadriceps and forearm site %SmO₂ was measured during three continuous running trials to exhaustion and three intermittent intensity (ratio = 60 s severe: 30 s lower intensity) trials to exhaustion. Intensity-dependent negative %SmO₂ slopes were observed for all trials (P < 0.05) and predicted zero slope at critical velocity. %SmO₂ accurately predicted depletion and repletion of %D' balance on a second-by-second basis (R² = 0.99, P < 0.05; both sites). Time to exhaustion predictions during continuous and intermittent exercise were either not different or better with %SmO₂ [standard error of the estimate (SEE) < 20.52 s for quad, <44.03 s for forearm] versus running velocity (SEE < 65.76 s). Muscle O₂ balance provides a dynamic physiological delineation between sustainable and unsustainable exercise (consistent with a "critical metabolic rate") and predicts real-time depletion and repletion of finite work capacity and time to exhaustion.NEW & NOTEWORTHY Dynamic muscle O₂ saturation discriminates boundaries between exercise intensity domains, exposes a critical metabolic rate as the highest rate of steady state O₂ supply and demand, describes time series depletion and repletion for work above critical power, and predicts time to exhaustion during severe domain whole body exercise. These results highlight the matching of O₂ supply and demand as a primary determinant for sustainable exercise intensities from those that are unsustainable and lead to exhaustion.

Changes in Optical Path Length Reveal Significant Potential Errors of Muscle Oxygenation Evaluation during Exercise in Humans

PURPOSE

Near-infrared spectroscopy (NIRS), performed with a commonly available noninvasive tissue oxygenation monitoring device, is based on the modified Beer-Lambert law (MBLL). Although NIRS based on MBLL (NIRSMBLL) assumes that the optical path length (PL) is constant, the effects of changes in PL during exercise on muscle oxygenation calculated by MBLL are still incompletely understood. Thus, the purposes of this study were to examine the changes in optical properties during ramp incremental exercise and to compare muscle oxygen dynamics measured by time-resolved NIRS with those calculated based on MBLL.

METHODS

Twenty-two healthy young men performed ramp incremental cycling exercise until exhaustion. Optical properties (reduced scattering coefficient and PL) and absolute oxygenated, deoxygenated, and total hemoglobin and myoglobin concentrations (oxy[Hb + Mb], deoxy[Hb + Mb], and total[Hb + Mb], respectively) at the vastus lateralis were continuously monitored by a three-wavelength (763, 801, and 836) time-resolved NIRS device. The values of oxy-, deoxy-, and total[Hb + Mb] were then recalculated by assuming constant PL.

RESULTS

PL at all wavelengths statistically significantly shortened during exercise. In particular, PL at 763 nm was greatly shortened, and the average changes during exercise were a 9.8% ± 3.1% reduction. In addition, significant differences in the kinetics of oxy-, deoxy-, and total[Hb + Mb] between directly measuring PL and assuming constant PL were found. The average changes in measured PL and assuming constant PL-deoxy[Hb + Mb] were increases of 28.8 ± 16.0 μM and increases of 16.4 ± 9.3 μM, respectively.

CONCLUSION

Assuming constant PL in NIRSMBLL significantly underestimated actual muscle oxy/deoxygenation as compared with measurements obtained by real-time PL determination. The percent degree of the underestimated oxy/deoxygenation was greater than the percent degree of the changes in PL.

Diminished muscle oxygen uptake and fatigue in spinal muscular atrophy

OBJECTIVE

To estimate muscle oxygen uptake and quantify fatigue during exercise in ambulatory individuals with spinal muscular atrophy (SMA) and healthy controls.

METHODS

Peak aerobic capacity (VO_2peak ) and workload (W_peak ) were measured by cardiopulmonary exercise test (CPET) in 19 ambulatory SMA patients and 16 healthy controls. Submaximal exercise (SME) at 40% W_peak was performed for 10 minutes. Change in vastus lateralis deoxygenated hemoglobin, measured by near-infrared spectroscopy, determined muscle oxygen uptake (ΔHHb) at rest and during CPET and SME. Dual energy X-ray absorptiometry assessed fat-free mass (FFM%). Fatigue was determined by percent change in workload or distance in the first compared to the last minute of SME (Fatigue_SME ) and six-minute walk test (Fatigue_6MWT ), respectively.

RESULTS

ΔHHb-PEAK, ΔHHb-SME, VO_2peak , W_peak , FFM%, and 6MWT distance were lower (P < 0.001), and Fatigue_6MWT and Fatigue_SME were higher (P < 0.001) in SMA compared to controls. ΔHHb-PEAK correlated with FFM% (r = 0.50) and VO_2peak (r = 0.41) only in controls. Only in SMA, Fatigue_6MWT was inversely correlated with W_peak (r = -0.69), and Fatigue_SME was inversely correlated with FFM% (r = -0.55) and VO_2peak (r = -0.69).

INTERPRETATION

This study provides further support for muscle mitochondrial dysfunction in SMA patients. During exercise, we observed diminished muscle oxygen uptake but no correlation with aerobic capacity or body composition. We also observed increased fatigue which correlated with decreased aerobic capacity, workload, and body composition. Understanding the mechanisms underlying diminished muscle oxygen uptake and increased fatigue during exercise in SMA may identify additional therapeutic targets that rescue symptomatic patients and mitigate their residual disease burden.

Application of Molecular Hydrogen as an Antioxidant in Responses to Ventilatory and Ergogenic Adjustments during Incremental Exercise in Humans

We investigated effects of molecular hydrogen (H₂) supplementation on acid-base status, pulmonary gas exchange responses, and local muscle oxygenation during incremental exercise. Eighteen healthy, trained subjects in a randomized, double-blind, crossover design received H₂-rich calcium powder (HCP) (1500 mg/day, containing 2.544 µg/day of H₂) or H₂-depleted placebo (1500 mg/day) for three consecutive days. They performed cycling incremental exercise starting at 20-watt work rate, increasing by 20 watts/2 min until exhaustion. Breath-by-breath pulmonary ventilation (V˙_E) and CO₂ output (V˙CO₂) were measured and muscle deoxygenation (deoxy[Hb + Mb]) was determined via time-resolved near-infrared spectroscopy in the vastus lateralis (VL) and rectus femoris (RF). Blood gases' pH, lactate, and bicarbonate (HCO₃^-) concentrations were measured at rest and 120-, 200-, and 240-watt work rates. At rest, the HCP group had significantly lower V˙_E, V˙CO₂, and higher HCO₃^-, partial pressures of CO₂ (PCO₂) versus placebo. During exercise, a significant pH decrease and greater HCO₃^- continued until 240-watt workload in HCP. The V˙_E was significantly lower in HCP versus placebo, but HCP did not affect the gas exchange status of V˙CO₂ or oxygen uptake (V˙O₂). HCP increased absolute values of deoxy[Hb + Mb] at the RF but not VL. Thus, HCP-induced hypoventilation would lead to lower pH and secondarily impaired balance between O₂ delivery and utilization in the local RF during exercise, suggesting that HCP supplementation, which increases the at-rest antioxidant potential, affects the lower ventilation and pH status during incremental exercise. HPC induced a significantly lower O₂ delivery/utilization ratio in the RF but not the VL, which may be because these regions possess inherently different vascular/metabolic control properties, perhaps related to fiber-type composition.

Sex differences in fatigability after ischemic preconditioning of non-exercising limbs

Background

Ischemic preconditioning (IPC) is suggested to decrease fatigability in some individuals but not others. Sex differences in response to IPC may account for this variability and few studies systematically investigated the effects of IPC in men and women. The goal of this study was to determine if time to task failure, perception of pain, and neuromuscular mechanisms of fatigability were altered by IPC in men and women.

Methods

Ten women (29 ± 5 years old) and 10 men (28 ± 6 years old) performed isometric contractions with the plantar flexor muscles of the dominant leg at 20% of maximal voluntary contraction until task failure. We used a repeated measures design where each individual performed 3 randomized and counterbalanced test sessions: (A) IPC session, cuff inflation and deflation (5 min each repeated 3 times) performed before the exercise by inflating cuffs to the non-dominant leg and arm; (B) sham session, cuffs were inflated for a short period (1 min); and (C) control session, no cuffs were involved.

Results

Compared with control, IPC increased time to task failure in men (mean difference, 5 min; confidence interval (CI) of mean difference, 2.2; 7.8 min; P = 0.01) but not women (mean difference, − 0.6 min; CI of mean difference, − 3.5; 2.4 min; P = 0.51). In men, but not women, the IPC-induced increase in time to task failure was associated with lower response to pressure pain (r = − 0.79). IPC further exposed sex differences in arterial pressure during fatiguing contractions (session × sex: P < 0.05). Voluntary activation, estimated with the twitch interpolation technique, and presynaptic inhibition of leg Ia afferents were not altered after IPC for men and women. The tested variables were not altered with sham.

Conclusions

The ergogenic effect of IPC on time to task failure was observed only in men and it was associated with reductions in the perception of pain. This pilot data suggest the previously reported inter-individual variability in exercise-induced fatigability after IPC could be a consequence of the sex and individual response to pain.

Effects of Capsinoid Intake on Brown Adipose Tissue Vascular Density and Resting Energy Expenditure in Healthy, Middle-Aged Adults: A Randomized, Double-Blind, Placebo-Controlled Study

Capsinoids are some of the most promising ingredients to increase energy expenditure (EE) due to brown adipose tissue (BAT) activation. However, there is limited information regarding the effect of prolonged capsinoid ingestion (CI) on BAT activity and resting EE (REE) in healthy, middle-aged, normal to overweight subjects (Sub_healthy) with distinct BAT characteristics. We examined the changes in BAT density (BAT-d), using near-infrared time-resolved spectroscopy, and REE/kg induced by daily CI. Forty Sub_healthy [age, 43.8 (mean) years; BMI, 25.4 kg/m²] received either capsinoid (9 mg/day) or a placebo daily for 6 weeks in a double-blind design. Total hemoglobin concentration in the supraclavicular region ([total-Hb]_sup), an indicator of BAT-d, and REE/kg were measured. The changes in post-intervention [total-Hb]_sup were greater in the capsinoid group (CA-G) than in the placebo group (PL-G) [5.8 µM (+12.4%) versus 1.0 µM (+2.1%); p = 0.017]. There was a significant relationship between BAT-d and REE/kg; however, post-supplementation REE/kg was not significantly different between the two groups (p = 0.228). In the overweight subgroup, changes in REE/kg were greater in the CA-G than in the PL-G [0.6 cal/kg/min (+4.3%) versus -0.3 cal/kg/min (-2.1%); p = 0.021]. CI enhanced [total-Hb]_sup, a reflection of BAT-d, showing a good correlation with REE in Sub_healthy.

Use of Fluorescent Dyes in Endoscopy and Diagnostic Investigation

Background

The advancement of innovative endoscopic technology in terms of improving the visualization of the mucosa has been of significant benefit.

Summary

Advancements in image resolution, software processing, and optical filter technology have resulted in several techniques complemental to traditional white light endoscopy. These new techniques provide a real-time optical diagnosis as well as virtual histology of detected lesions. Optical molecular imaging permits a functional assessment within cells.

Key Message

Optical molecular imaging provides an understanding of cellular processes and permits validation of the specificity of fluorescent tracers and the possibility of quantifying the signal.

Effect of differential muscle activation patterns on muscle deoxygenation and microvascular haemoglobin regulation

NEW FINDINGS

What is the central question of this study? Does the presence and extent of heterogeneity in the ratio of O2 delivery to uptake across human muscles relate specifically to different muscle activation patterns? What is the main finding and its importance? During ramp incremental knee-extension and cycling exercise, the profiles of muscle deoxygenation (deoxy[haemoglobin + myoglobin]) and diffusive O2 potential (total[haemoglobin + myoglobin]) in the vastus lateralis corresponded to different muscle activation strategies. However, this was not the case for the rectus femoris, where muscle activation and deoxygenation profiles were dissociated and might therefore be determined by other structural and/or functional attributes (e.g. arteriolar vascular regulation and control of red blood cell flux).

ABSTRACT

Near-infrared spectroscopy has revealed considerable heterogeneity in the ratio of O2 delivery to uptake as identified by disparate deoxygenation {deoxy[haemoglobin + myoglobin] (deoxy[Hb + Mb])} values in the exercising quadriceps. However, whether this represents a recruitment phenomenon or contrasting vascular and metabolic control, as seen among fibre types, has not been established. We used knee-extension (KE) and cycling (CE) incremental exercise protocols to examine whether differential muscle activation profiles could account for the heterogeneity of deoxy[Hb + Mb] and microvascular haemoconcentration (i.e. total[Hb + Mb]). Using time-resolved near-infrared spectroscopy for the quadriceps femoris (vastus lateralis and rectus femoris) during exhaustive ramp exercise in eight participants, we tested the following hypotheses: (i) the deoxy[Hb + Mb] (i.e. fractional O2 extraction) would relate to muscle activation levels across exercise protocols; and (ii) KE would induce greater total[Hb + Mb] (i.e. diffusive O2 potential) at task failure (i.e. peak O2 uptake) than CE irrespective of muscle site. At a given level of muscle activation, as assessed by the relative integrated EMG normalized to maximal voluntary contraction (%iEMGmax ), the vastus lateralis deoxy[Hb + Mb] profile was not different between exercise protocols. However, at peak O2 uptake and until 20% iEMGmax for CE, rectus femoris exhibited a lower deoxy[Hb + Mb] (83.2 ± 15.5 versus 98.2 ± 19.4 μm) for KE than for CE (P < 0.05). The total[Hb + Mb] at peak O2 uptake was not different between exercise protocols for either muscle site. These data support the hypothesis that the contrasting patterns of convective and diffusive O2 transport correspond to different muscle activation patterns in vastus lateralis but not rectus femoris. Thus, the differential deoxygenation profiles for rectus femoris across exercise protocols might be dependent upon specific facets of muscle architecture and functional haemodynamic events.

Near-Infrared Time-Resolved Spectroscopy for Assessing Brown Adipose Tissue Density in Humans: A Review

Brown adipose tissue (BAT) mediates adaptive thermogenesis upon food intake and cold exposure, thus potentially contributing to the prevention of lifestyle-related diseases. ¹⁸F-fluorodeoxyglucose (FDG)-positron emission tomography (PET) with computed tomography (CT) (¹⁸FDG-PET/CT) is a standard method for assessing BAT activity and volume in humans. ¹⁸FDG-PET/CT has several limitations, including high device cost and ionizing radiation and acute cold exposure necessary to maximally stimulate BAT activity. In contrast, near-infrared spectroscopy (NIRS) has been used for measuring changes in O₂-dependent light absorption in the tissue in a non-invasive manner, without using radiation. Among NIRS, time-resolved NIRS (NIR_TRS) can quantify the concentrations of oxygenated and deoxygenated hemoglobin ([oxy-Hb] and [deoxy-Hb], respectively) by emitting ultrashort (100 ps) light pulses and counts photons, which are scattered and absorbed in the tissue. The basis for assessing BAT density (BAT-d) using NIR_TRS is that the vascular density in the supraclavicular region, as estimated using Hb concentration, is higher in BAT than in white adipose tissue. In contrast, relatively low-cost continuous wavelength NIRS (NIR_CWS) is employed for measuring relative changes in oxygenation in tissues. In this review, we provide evidence for the validity of NIR_TRS and NIR_CWS in estimating human BAT characteristics. The indicators (Ind_NIRS) examined were [oxy-Hb]_sup, [deoxy-Hb]_sup, total hemoglobin [total-Hb]_sup, Hb O₂ saturation (StO_2sup), and reduced scattering coefficient ( μs sup' ) in the supraclavicular region, as determined by NIR_TRS, and relative changes in corresponding parameters, as determined by NIR_CWS. The evidence comprises the relationships between the Ind_NIRS investigated and those determined by ¹⁸FDG-PET/CT; the correlation between the Ind_NIRS and cold-induced thermogenesis; the relationship of the Ind_NIRS to parameters measured by ¹⁸FDG-PET/CT, which responded to seasonal temperature fluctuations; the relationship of the Ind_NIRS and plasma lipid metabolites; the analogy of the Ind_NIRS to chronological and anthropometric data; and changes in the Ind_NIRS following thermogenic food supplementation. The [total-Hb]_sup and [oxy-Hb]_sup determined by NIR_TRS, but not parameters determined by NIR_CWS, exhibited significant correlations with cold-induced thermogenesis parameters and plasma androgens in men in winter or analogies to ¹⁸FDG-PET. We conclude that NIR_TRS can provide useful information for assessing BAT-d in a simple, rapid, non-invasive way, although further validation study is still needed.

Review of early development of near-infrared spectroscopy and recent advancement of studies on muscle oxygenation and oxidative metabolism

Near-infrared spectroscopy (NIRS) has become an increasingly valuable tool to monitor tissue oxygenation (Toxy) in vivo. Observations of changes in the absorption of light with Toxy have been recognized as early as 1876, leading to a milestone NIRS paper by Jöbsis in 1977. Changes in the absorption and scatting of light in the 700-850-nm range has been successfully used to evaluate Toxy. The most practical devices use continuous-wave light providing relative values of Toxy. Phase-modulated or pulsed light can monitor both absorption and scattering providing more accurate signals. NIRS provides excellent time resolution (~ 10 Hz), and multiple source-detector pairs can be used to provide low-resolution imaging. NIRS has been applied to a wide range of populations. Continued development of NIRS devices in terms of lower cost, better detection of both absorption and scattering, and smaller size will lead to a promising future for NIRS studies.

Near-infrared spectroscopy-derived muscle oxygen saturation on a 0% to 100% scale: reliability and validity of the Moxy Monitor

Near-infrared spectroscopy (NIRS) to monitor muscle oxygen saturation (SmO₂) is rapidly expanding into applied sports settings. However, the technology is limited due to its inability to convey quantifiable values. A test battery to assess reliability and validity of a 0% to 100% scale modeled by a commercially available NIRS device was established. This test battery applies a commonly used technique, the arterial occlusion method (AOM) to assess repeatability, reproducibility, and face validity. A total of 22 participants completed the test battery to scrutinize the 0% to 100% scale provided by the device. All participants underwent repeated AOM tests in passive and active conditions. The SmO₂ minimum and SmO₂ maximum values were obtained from the AOM and were used in the subsequent analysis. Repeatability and reproducibility were tested for equivalency and Bland-Altman plots were generated. Face validity was assessed by testing SmO₂ values against an a priori; defined threshold for mixed venous blood during AOM response. The device exhibits an appropriately functional 0% to 100% scale that is reliable in terms of repeatability and reproducibility. Under the conditions applied in the test battery design, the device is considered valid for application in sports.

Retrieval of Absorption or Scattering Coefficient Spectrum (RASCS) Program: A Tool to Monitor Optical Properties in Real Time

BACKGROUND AND OBJECTIVES

Optical properties characterize light propagation in turbid media, such as tissue. Recovery of optical properties is of great importance in a wide variety of biomedical applications, including both therapeutic treatments and diagnosis. Most of the available methodologies are well established, however, these are not optimized for real-time measurements.

STUDY DESIGN/MATERIALS AND METHODS

Optical properties are recovered using the Inverse Adding Doubling program from reflectance measurements measured with an integrating sphere and light in the visible range. A user-friendly interface was programmed in Visual Studio and the libraries of a particular spectrophotometer were used. To achieve real-time measurements, a parallel computing routine was implemented, splitting the whole spectra in threads to be computed independently. Several tests using living tissue and inorganic materials were carried out to validate the proposed algorithm.

RESULTS

Recovery of absorption/scattering coefficient spectrum in the visible range with high precision in a couple of seconds was achieved, demonstrating its capabilities for real-time monitoring in biomedical applications. The absorption coefficient spectrum shows the expected characteristics according to the different melanin and blood concentration of various volunteers, also showing the expected changes during a thermoregulation process.

CONCLUSIONS

A real-time monitoring of optical properties algorithm was developed, including parallel computing and a user-friendly interface. The proposed algorithm would be of help in biomedical applications, where real-time monitoring optical properties is required. Lasers Surg. Med. © 2019 Wiley Periodicals, Inc.

Applicability of Supraclavicular Oxygenated and Total Hemoglobin Evaluated by Near-Infrared Time-Resolved Spectroscopy as Indicators of Brown Adipose Tissue Density in Humans

Brown adipose tissue (BAT) may potentially be used in strategies for preventing lifestyle-related diseases. We examine evidence that near-infrared time-resolved spectroscopy (NIR_TRS) is capable of estimating human BAT density (BAT-d). The parameters examined in this study are total hemoglobin [total-Hb]_sup, oxygenated Hb [oxy-Hb]_sup, deoxygenated Hb [deoxy-Hb]_sup, Hb O₂ saturation (StO_2sup), and the reduced scattering coefficient in the supraclavicular region (μ_s’_sup), where BAT deposits can be located; corresponding parameters in the control deltoid region are obtained as controls. Among the NIR_TRS parameters, [total-Hb]_sup and [oxy-Hb]_sup show region-specific increases in winter, compared to summer. Further, [total-Hb]_sup and [oxy-Hb]_sup are correlated with cold-induced thermogenesis in the supraclavicular region. We conclude that NIR_TRS-determined [total-Hb]_sup and [oxy-Hb]_sup are useful parameters for evaluating BAT-d in a simple, rapid, non-invasive manner.

Time-Domain Near-Infrared Spectroscopy and Imaging: A Review

This article reviews the past and current statuses of time-domain near-infrared spectroscopy (TD-NIRS) and imaging. Although time-domain technology is not yet widely employed due to its drawbacks of being cumbersome, bulky, and very expensive compared to commercial continuous wave (CW) and frequency-domain (FD) fNIRS systems, TD-NIRS has great advantages over CW and FD systems because time-resolved data measured by TD systems contain the richest information about optical properties inside measured objects. This article focuses on reviewing the theoretical background, advanced theories and methods, instruments, and studies on clinical applications for TD-NIRS including some clinical studies which used TD-NIRS systems. Major events in the development of TD-NIRS and imaging are identified and summarized in chronological tables and figures. Finally, prospects for TD-NIRS in the near future are briefly described.

Near-infrared spectroscopy muscle oximetry of patients with postural orthostatic tachycardia syndrome

Postural orthostatic tachycardia syndrome (POTS) is a disabling condition characterized by orthostatic intolerance with tachycardia in the absence of drop-in blood pressure. A custom-built near-infrared spectroscopy device (NIRS) is applied to monitor the muscle oxygenation, noninvasively in patients undergoing incremental head-up tilt table (HUT). Subjects (6 POTS patients and 6 healthy controls) underwent 30 mins of 70°on a HUT. The results showed a significant difference in deoxyhemoglobin (Hb), change-in-oxygenation (ΔOxy) and blood volume (ΔBV) between patients and healthy controls. However, oxyhemoglobin (HbO2) showed a significantly faster rate of change in the healthy controls during the first 10 mins of the tilt and during the recovery. This NIRS muscle oximetry tool provides quantitative measurements of blood oxygenation monitoring in diseases such as POTS.

The effect of dietary nitrate supplementation on the spatial heterogeneity of quadriceps deoxygenation during heavy-intensity cycling

This study investigated the influence of dietary inorganic nitrate (NO₃⁻) supplementation on pulmonary O₂ uptake (V˙O₂) and muscle deoxyhemoglobin/myoglobin (i.e. deoxy [Hb + Mb]) kinetics during submaximal cycling exercise. In a randomized, placebo‐controlled, cross‐over study, eight healthy and physically active male subjects completed two step cycle tests at a work rate equivalent to 50% of the difference between the gas exchange threshold and peak V˙O₂ over separate 4‐day supplementation periods with NO₃⁻‐rich (BR; providing 8.4 mmol NO₃⁻∙day⁻¹) and NO₃⁻‐depleted (placebo; PLA) beetroot juice. Pulmonary V˙O₂ was measured breath‐by‐breath and time‐resolved near‐infrared spectroscopy was utilized to quantify absolute deoxy [Hb + Mb] and total [Hb + Mb] within the rectus femoris, vastus lateralis, and vastus medialis. There were no significant differences (P > 0.05) in the primary deoxy [Hb + Mb] mean response time or amplitude between the PLA and BR trials at each muscle site. BR significantly increased the mean (three‐site) end‐exercise deoxy [Hb + Mb] (PLA: 91 ± 9 vs. BR: 95 ± 12 μmol/L, P < 0.05), with a tendency to increase the mean (three‐site) area under the curve for total [Hb + Mb] responses (PLA: 3650 ± 1188 vs. BR: 4467 ± 1315 μmol/L sec⁻¹, P = 0.08). The V˙O₂ slow component reduction after BR supplementation (PLA: 0.27 ± 0.07 vs. BR: 0.23 ± 0.08 L min⁻¹, P = 0.07) correlated inversely with the mean increases in deoxy [Hb + Mb] and total [Hb + Mb] across the three muscle regions (r² = 0.62 and 0.66, P < 0.05). Dietary NO₃⁻ supplementation increased O₂ diffusive conductance across locomotor muscles in association with improved V˙O₂ dynamics during heavy‐intensity cycling transitions.

Ischemic Preconditioning and Repeated Sprint Swimming: A Placebo and Nocebo Study

PURPOSE

Ischemic preconditioning (IPC) has been shown to improve performance of exercises lasting 10-90 s (anaerobic) and more than 90 s (aerobic). However, its effect on repeated sprint performance has been controversial, placebo effect has not been adequately controlled, and nocebo effect has not been avoided. Thus, the IPC effect on repeated sprint performance was investigated using a swimming task and controlling placebo/nocebo effects.

METHODS

Short-distance university swimmers were randomized to two groups. One group (n = 15, 24 ± 1 yr [mean ± SEM]) was exposed to IPC (ischemia cycles lasted 5 min) and control (CT) (no ischemia); another (n = 15, 24 ± 1 yr) to a placebo intervention (SHAM) (ischemia cycles lasted 1 min) and CT. Seven subjects crossed over groups. Subjects were informed IPC and SHAM would improve performance compared with CT and would be harmless despite circulatory occlusion sensations. The swimming task consisted of six 50-m all-out efforts repeated every 3 min.

RESULTS

IPC, in contrast with SHAM, reduced worst sprint time (IPC, 35.21 ± 0.73 vs CT, 36.53 ± 0.72 s; P = 0.04) and total sprints time (IPC, 203.7 ± 4.60 vs CT, 206.03 ± 4.57 s; P = 0.02), moreover augmented swimming velocity (IPC, 1.45 ± 0.03 vs CT, 1.44 ± 0.03 m·s; P = 0.049). Six of seven subjects who crossed over groups reduced total sprints time with IPC versus SHAM (delta = -3.95 ± 1.49 s, P = 0.09). Both IPC and SHAM did not change blood lactate concentration (P = 0.20) and perceived effort (P = 0.22).

CONCLUSION

IPC enhanced repeated sprint swimming performance in university swimmers, whereas a placebo intervention did not.

The role of perfusion in the oxygen extraction capability of skin and skeletal muscle

Oxygen extraction (OE) by all cells is dependent on an adequate supply of oxygen in proximal blood vessels and the cell's need and ability to uptake that oxygen. Here the role of blood flow in regulating OE in skin and skeletal muscle was investigated in lean and obese men. OE was derived by two optical reflectance spectroscopy techniques: 1) from the rate of fall in mean blood saturation during a 4 min below knee arterial occlusion, and thus no blood flow, in calf skin and skeletal muscle and 2) in perfused, unperturbed skin, using the spontaneous falls in mean blood saturation induced by vasomotion in calf and forearm skin of 24 subjects, 12 lean and 12 obese. OE in perfused skin was significantly higher in lean compared with obese subjects in forearm (Mann-Whitney, P < 0.004) and calf (P < 0.001) and did not correlate with OE in unperfused skin (ρ = -0.01, P = 0.48). With arterial occlusion and thus no blood flow, skin OE in lean and obese subjects no longer differed (P = 0.23, not significant). In contrast in skeletal muscle with arterial occlusion and no blood flow, the difference in OE between lean and obese subjects occurred, with obese subjects exhibiting significantly higher OE (P < 0.012). The classic model of metabolic blood flow regulation to support oxygen extraction is evident in perfused skin; OE is perturbed without blood flow and reduced in obesity. In resting skeletal muscle other mechanism(s), independent of blood flow, are implicated in oxygen extraction.

Near-infrared spectrometry in pregnancy: progress and perspectives, a review of literature

Near-infrared spectroscopy (NIRS) allows continuous noninvasive monitoring of in vivo oxygenation in selected tissues. It has been used primarily as a research tool for several years, but it is seeing wider application in the clinical arena all over the world. It was recently used to monitor brain circulation in cardiac surgery, carotid endarteriectomy, neurosurgery and robotic surgery. According to the few studies used NIRS in pregnancy, it may be helpful to assess the impact of severe forms of preeclampsia on brain circulation, to evaluate the efficacy of different treatments. It may also be used during cesarean section to detect earlier sudden complications. The evaluation of placental function via abdominal maternal approach to detect fetal growth restriction is a new field of application of NIRS.

Possible Influences on the Interpretation of Functional Domain (FD) Near-Infrared Spectroscopy (NIRS): An Explorative Study

The influence of subcutaneous adipose tissue (ATT) and oxygen (O2) delivery has been poorly defined in frequency domain (FD) near-infrared spectroscopy (NIRS). Therefore, the aim of this study was to investigate the possible influence of these variables on all FD NIRS responses using a reliable protocol. Moreover, these influences were also investigated when using relative oxy- and deoxyhemoglobin and -myoglobin (oxy[Hb + Mb] and deoxy[Hb + Mb]) values (in %). A regression analysis was carried out for ATT and maximal-minimum oxy[Hb + Mb], deoxy[Hb + Mb], oxygen saturation (SmO2), and total hemoglobin (totHb) amplitudes during an incremental cyclic contraction protocol (ICCP) in a group of 45 participants. Moreover, the same analysis was carried out between subcutaneous ATT and the relative oxy- and deoxy[Hb + Mb] values (in %). In the second part of this study, a regression analysis was performed for peak forearm blood flow (FBF) during ICCP and the absolute and relative NIRS values in a group of 37 participants. Significant exponential correlation coefficients were found between ATT and deoxy[Hb + Mb] (r = 0.53; P < 0.001), oxy[Hb + Mb] (r = 0.57; P < 0.001), and SmO2 amplitudes (r = 0.57; P < 0.001). No significant relations were found between ATT and relative oxy[Hb + Mb] (r = 0.37; P = 0.07) and deoxy[Hb + Mb] (r = 0.09; P = 0.82). Significant positive correlation coefficients were found between force at exhaustion and maximal FBF (r = 0.66; P < 0.001), maximal differences in deoxy[Hb + Mb] (r = 0.353; P = 0.032) and totHb (r = 0.512; P = 0.002) while no significant correlation coefficients were found between these maximal force values and maximal differences in oxy[Hb + Mb] (r = -0.267; P = 0.111) and SmO2 (r = -0.267; P = 0.111). Significant linear correlation coefficients were found between FBF and deoxy[Hb + Mb] (r = 0.51; P = 0.001), oxy[Hb + Mb] (r = -0.50; P = 0.001), SmO2 (r = -0.54; P = 0.001), and totHb amplitude (r = 0.61; P < 0.001). No significant correlations were found when using relative oxy[Hb + Mb] (r = -0.01; P = 0.957) and deoxy[Hb + Mb] (r = -0.02; P = 0.895). Based on these findings, caution is advised when using NIRS values, as subcutaneous ATT and O2 delivery significantly influence NIRS measurements. To eliminate these influences, use of relative deoxy[Hb + Mb] is advised, especially in clinical settings or in people with a higher subcutaneous ATT layer.

Monitoring hemodynamic changes in stroke-affected muscles using near-infrared spectroscopy

The oxygenation level of a tissue is an important marker of the health of the tissue and has a direct effect on performance. It has been shown that the blood flow to the paretic muscles of hemiparetic post-stroke patients is significantly reduced compared to non-paretic muscles. It is hypothesized that hemodynamic activity in paretic muscles is suppressed as compared to non-paretic muscles, and that oximetry can be used to measure this disparity in real-time. In order to test this hypothesis, a custom-made oximetry device was used to measure hemodynamic activity in the forearm extensor muscles in post-stroke patients’ paretic and non-paretic sides and in a control population during three exercise levels calibrated to the subject’s maximum effort. The change in oxygenation (ΔOxy) and blood volume (ΔBV) were calculated and displayed in real-time. Results show no apparent difference in either ΔOxy or ΔBV between control subjects’ dominant and non-dominant muscles. However, the results show a significant difference in ΔOxy between paretic and non-paretic muscles, as well as a significant difference between normalized post-stroke and control data. Further work will be necessary to determine if the observed difference between the paretic and non-paretic muscles changes over the course of physical therapy and can be correlated with functional improvements.

Muscle deoxygenation in the quadriceps during ramp incremental cycling: Deep vs. superficial heterogeneity

Muscle deoxygenation (i.e., deoxy[Hb + Mb]) during exercise assesses the matching of oxygen delivery (Q̇O2) to oxygen utilization (V̇O2). Until now limitations in near-infrared spectroscopy (NIRS) technology did not permit discrimination of deoxy[Hb + Mb] between superficial and deep muscles. In humans, the deep quadriceps is more highly vascularized and oxidative than the superficial quadriceps. Using high-power time-resolved NIRS, we tested the hypothesis that deoxygenation of the deep quadriceps would be less than in superficial muscle during incremental cycling exercise in eight males. Pulmonary V̇O2 was measured and muscle deoxy[Hb + Mb] was determined in the superficial vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF-s) and the deep rectus femoris (RF-d). deoxy[Hb + Mb] in RF-d was significantly less than VL at 70% (67.2 ± 7.0 vs. 75.5 ± 10.7 μM) and 80% (71.4 ± 11.0 vs. 79.0 ± 15.4 μM) of peak work rate (WR(peak)), but greater than VL and VM at WR(peak) (87.7 ± 32.5 vs. 76.6 ± 17.5 and 75.1 ± 19.9 μM). RF-s was intermediate at WR(peak) (82.6 ± 18.7 μM). Total hemoglobin and myoglobin concentration and tissue oxygen saturation were significantly greater in RF-d than RF-s throughout exercise. The slope of deoxy[Hb + Mb] increase (proportional to Q̇O2/V̇O2) in VL and VM slowed markedly above 70% WR(peak), whereas it became greater in RF-d. This divergent deoxygenation pattern may be due to a greater population of slow-twitch muscle fibers in the RF-d muscle and the differential recruitment profiles and vascular and metabolic control properties of specific fiber populations within superficial and deeper muscle regions.

Reduction of V̇O2 slow component by priming exercise: novel mechanistic insights from time-resolved near-infrared spectroscopy

Novel time-resolved near-infrared spectroscopy (TR-NIRS), with adipose tissue thickness correction, was used to test the hypotheses that heavy priming exercise reduces the V̇_O2 slow component (V̇_O2SC) (1) by elevating microvascular [Hb] volume at multiple sites within the quadriceps femoris (2) rather than reducing the heterogeneity of muscle deoxygenation kinetics. Twelve subjects completed two 6-min bouts of heavy work rate exercise, separated by 6 min of unloaded cycling. Priming exercise induced faster overall V̇_O2 kinetics consequent to a substantial reduction in the V̇_O2SC (0.27 ± 0.12 vs. 0.11 ± 0.09 L·min⁻¹, P < 0.05) with an unchanged primary V̇_O2 time constant. An increased baseline for the primed bout [total (Hb + Mb)] (197.5 ± 21.6 vs. 210.7 ± 22.5 μmol L⁻¹, P < 0.01), reflecting increased microvascular [Hb] volume, correlated significantly with the V̇_O2SC reduction. At multiple sites within the quadriceps femoris, priming exercise reduced the baseline and slowed the increase in [deoxy (Hb + Mb)]. Changes in the intersite coefficient of variation in the time delay and time constant of [deoxy (Hb + Mb)] during the second bout were not correlated with the V̇_O2SC reduction. These results support a mechanistic link between priming exercise-induced increase in muscle [Hb] volume and the reduced V̇_O2SC that serves to speed overall V̇_O2 kinetics. However, reduction in the heterogeneity of muscle deoxygenation kinetics does not appear to be an obligatory feature of the priming response.

Validation of a high-power, time-resolved, near-infrared spectroscopy system for measurement of superficial and deep muscle deoxygenation during exercise

Near-infrared assessment of skeletal muscle is restricted to superficial tissues due to power limitations of spectroscopic systems. We reasoned that understanding of muscle deoxygenation may be improved by simultaneously interrogating deeper tissues. To achieve this, we modified a high-power (∼8 mW), time-resolved, near-infrared spectroscopy system to increase depth penetration. Precision was first validated using a homogenous optical phantom over a range of inter-optode spacings (OS). Coefficients of variation from 10 measurements were minimal (0.5-1.9%) for absorption (μa), reduced scattering, simulated total hemoglobin, and simulated O2 saturation. Second, a dual-layer phantom was constructed to assess depth sensitivity, and the thickness of the superficial layer was varied. With a superficial layer thickness of 1, 2, 3, and 4 cm (μa = 0.149 cm(-1)), the proportional contribution of the deep layer (μa = 0.250 cm(-1)) to total μa was 80.1, 26.9, 3.7, and 0.0%, respectively (at 6-cm OS), validating penetration to ∼3 cm. Implementation of an additional superficial phantom to simulate adipose tissue further reduced depth sensitivity. Finally, superficial and deep muscle spectroscopy was performed in six participants during heavy-intensity cycle exercise. Compared with the superficial rectus femoris, peak deoxygenation of the deep rectus femoris (including the superficial intermedius in some) was not significantly different (deoxyhemoglobin and deoxymyoglobin concentration: 81.3 ± 20.8 vs. 78.3 ± 13.6 μM, P > 0.05), but deoxygenation kinetics were significantly slower (mean response time: 37 ± 10 vs. 65 ± 9 s, P ≤ 0.05). These data validate a high-power, time-resolved, near-infrared spectroscopy system with large OS for measuring the deoxygenation of deep tissues and reveal temporal and spatial disparities in muscle deoxygenation responses to exercise.

Multispectral imaging in the extended near-infrared window based on endogenous chromophores

To minimize the problem with scattering in deep tissues while increasing the penetration depth, we explored the feasibility of imaging in the relatively unexplored extended near infrared (exNIR) spectral region at 900 to 1400 nm with endogenous chromophores. This region, also known as the second NIR window, is weakly dominated by absorption from water and lipids and is free from other endogenous chromophores with virtually no autofluorescence. To demonstrate the applicability of the exNIR for bioimaging, we analyzed the optical properties of individual components and biological tissues using an InGaAs spectrophotometer and a multispectral InGaAs scanning imager featuring transmission geometry. Based on the differences in spectral properties of tissues, we utilized ratiometric approaches to extract spectral characteristics from the acquired three-dimensional "datacube". The obtained images of an exNIR transmission through a mouse head revealed sufficient details consistent with anatomical structures.

The use of muscle near-infrared spectroscopy in sport, health and medical sciences: recent developments

Near-infrared spectroscopy (NIRS) has been shown to be one of the tools that can measure oxygenation in muscle and other tissues in vivo. This review paper highlights the progress, specifically in this decade, that has been made for evaluating skeletal muscle oxygenation and oxidative energy metabolism in sport, health and clinical sciences. Development of NIRS technologies has focused on improving quantification of the signal using multiple wavelengths to solve for absorption and scattering coefficients, multiple pathlengths to correct for the influence of superficial skin and fat, and time-resolved and phase-modulated light sources to determine optical pathlengths. In addition, advances in optical imaging with multiple source and detector pairs as well as portability using small wireless detectors have expanded the usefulness of the devices. NIRS measurements have provided information on oxidative metabolism in various athletes during localized exercise and whole-body exercise, as well as training-induced adaptations. Furthermore, NIRS technology has been used in the study of a number of chronic health conditions. Future developments of NIRS technology will include enhancing signal quantification. In addition, advances in NIRS imaging and portability promise to transform how measurements of oxygen utilization are obtained in the future.

Methodological validation of the dynamic heterogeneity of muscle deoxygenation within the quadriceps during cycle exercise

The conventional continuous wave near-infrared spectroscopy (CW-NIRS) has enabled identification of regional differences in muscle deoxygenation following onset of exercise. However, assumptions of constant optical factors (e.g., path length) used to convert the relative changes in CW-NIRS signal intensity to values of relative concentration, bring the validity of such measurements into question. Furthermore, to justify comparisons among sites and subjects, it is essential to correct the amplitude of deoxygenated hemoglobin plus myoglobin [deoxy(Hb+Mb)] for the adipose tissue thickness (ATT). We used two time-resolved NIRS systems to measure the distribution of the optical factors directly, thereby enabling the determination of the absolute concentrations of deoxy(Hb+Mb) simultaneously at the distal and proximal sites within the vastus lateralis (VL) and the rectus femoris muscles. Eight subjects performed cycle exercise transitions from unloaded to heavy work rates (>gas exchange threshold). Following exercise onset, the ATT-corrected amplitudes (Ap), time delay (TDp), and time constant (τp) of the primary component kinetics in muscle deoxy(Hb + Mb) were spatially heterogeneous (intersite coefficient of variation range for the subjects: 10–50 for Ap, 16–58 for TDp, 14–108% for τp). The absolute and relative amplitudes of the deoxy(Hb+Mb) responses were highly dependent on ATT, both within subjects and between measurement sites. The present results suggest that regional heterogeneity in the magnitude and temporal profile of muscle deoxygenation is a consequence of differential matching of O2 delivery and O2 utilization, not an artifact caused by changes in optical properties of the tissue during exercise or variability in the overlying adipose tissue.

Noninvasive optical measures of CBV, StO(2), CBF index, and rCMRO(2) in human premature neonates' brains in the first six weeks of life

With the causes of perinatal brain injuries still unclear and the probable role of hemodynamic instability in their etiology, bedside monitoring of neonatal cerebral hemodynamics with standard values as a function of age are needed. In this study, we combined quantitative frequency domain near infrared spectroscopy (FD-NIRS) measures of cerebral tissue oxygenation (StO(2)) and cerebral blood volume (CBV) with diffusion correlation spectroscopy (DCS) measures of a cerebral blood flow index (CBF(ix)) to test the validity of the CBV-CBF relationship in premature neonates and to estimate cerebral metabolic rate of oxygen (rCMRO(2)) with or without the CBF(ix) measurement. We measured 11 premature neonates (28-34 weeks gestational age) without known neurological issues, once a week from one to six weeks of age. In nine patients, cerebral blood velocities from the middle cerebral artery were collected by transcranial Doppler (TCD) and compared with DCS values. Results show a steady decrease in StO(2) during the first six weeks of life while CBV remains stable, and a steady increase in CBF(ix). rCMRO(2) estimated from FD-NIRS remains constant but shows wide interindividual variability. rCMRO(2) calculated from FD-NIRS and DCS combined increased by 40% during the first six weeks of life with reduced interindividual variability. TCD and DCS values are positively correlated. In conclusion, FD-NIRS combined with DCS offers a safe and quantitative bedside method to assess CBV, StO(2), CBF, and rCMRO(2) in the premature brain, facilitating individual follow-up and comparison among patients. A stable CBV-CBF relationship may not be valid for premature neonates.

Spectral filtering modulation method for estimation of hemoglobin concentration and oxygenation based on a single fluorescence emission spectrum in tissue phantoms

PURPOSE

Hemoglobin concentration and oxygenation in tissue are important biomarkers that are useful in both research and clinical diagnostics of a wide variety of diseases such as cancer. The authors aim to develop simple ratiometric method based on the spectral filtering modulation (SFM) of fluorescence spectra to estimate the total hemoglobin concentration and oxygenation in tissue using only a single fluorescence emission spectrum, which will eliminate the need of diffuse reflectance measurements and prolonged data processing as required by most current methods, thus enabling rapid clinical measurements.

METHODS

The proposed method consists of two steps. In the first step, the total hemoglobin concentration is determined by comparing a ratio of fluorescence intensities at two emission wavelengths to a calibration curve. The second step is to estimate oxygen saturation by comparing a double ratio that involves three emission wavelengths to another calibration curve that is a function of oxygen saturation for known total hemoglobin concentration. Theoretical derivation shows that the ratio in the first step is linearly proportional to the total hemoglobin concentrations and the double ratio in the second step is related to both total hemoglobin concentration and hemoglobin oxygenation for the chosen fiber-optic probe geometry. Experiments on synthetic fluorescent tissue phantoms, which included hemoglobin with both constant and varying oxygenation as the absorber, polystyrene spheres as scatterers, and flavin adenine dinucleotide as the fluorophore, were carried out to validate the theoretical prediction.

RESULTS

Tissue phantom experiments confirm that the ratio in the first step is linearly proportional to the total hemoglobin concentration and the double ratio in the second step is related to both total hemoglobin concentrations and hemoglobin oxygenation. Furthermore, the relations between the two ratios and the total hemoglobin concentration and hemoglobin oxygenation are insensitive to the scattering property of the tissue model for the chosen probe geometry.

CONCLUSIONS

A simple two-step ratiometric method based on the SFM of fluorescence spectra is proposed to estimate the total hemoglobin concentration and oxygenation in a tissue model using only a single fluorescence emission spectrum. This method is immune to the variation in system throughput caused by inconsistent optical coupling because of its ratiometric nature. Calibration curves are insensitive to the scattering coefficient for the chosen probe geometry. Moreover, since only fluorescence intensities at a few wavelengths in a single fluorescence emission spectrum are needed in this method, the SFM method minimizes the amount of required data and reduces the data acquisition time. Finally, since this method does not use nonlinear regression, it can dramatically save computation time in data processing. The high sensitivity of the proposed method to superficial tissue volumes makes it ideal for fluorescence based oximetry and medical diagnostics in applications such as early epithelial cancer diagnosis or wherever the measured tissue volume is exposed to the outside such as in open surgery.

Comparisons of muscle oxygenation changes between arm and leg muscles during incremental rowing exercise with near-infrared spectroscopy

Our purpose is to compare the changes in muscle oxygenation in the vastus lateralis (VL) and biceps brachii (BB) muscles simultaneously using near-infrared spectroscopy (NIRS) during incremental rowing exercise in eight rowers. Based on the BB and VL muscle oxygenation patterns, two points are used to characterize the muscle oxygenation kinetics in both the arm and the leg muscles. The first point is the breaking point (Bp), which refers to an accelerated fall in muscle oxygenation that correlates with the gas exchange threshold (GET). The second point is the leveling-off point (Lo), which suggests the upper limit of O(2) extraction. The GET occurred at 63.3+/-2.4% of maximal oxygen uptake (VO(2 max)). The Bp appeared at 45.0+/-3.8% and 55.6+/-2.4% VO(2 max) in the BB and VL, respectively. The Lo appeared at 63.6+/-4.1% and 86.6+/-1.0% VO(2 max) in these two muscles, respectively. Both the Bp and the Lo occurred earlier in BB compared with VL. These results suggest that arm muscles have lower oxidative capacity than leg muscles during rowing exercise. The rowers with higher exercise performances showed heavier workloads, as evaluated by Bp and Lo. The monitoring of muscle oxygenation by NIRS in arm and leg muscles during rowing could be a useful guide for evaluation and training.

Local increase in trapezius muscle oxygenation during and after acupuncture

PURPOSE

This study aimed to compare the trapezius muscle blood volume and oxygenation in the stimulation region and in a distant region in the same muscle during acupuncture stimulation (AS). We hypothesized that AS provokes a localized increase in muscle blood volume and oxygenation in the stimulation region.

METHODS

Two sets of near-infrared spectrometer (NIRS) probes, with 40-mm light-source detector spacing, were placed on the right trapezius muscle, with a 50-mm distance between the probes. Changes in muscle oxygenation (oxy-Hb) and blood volume (t-Hb) in stimulation and distant regions (50 mm away from the stimulation point) were measured using NIRS. Nine healthy acupuncture-experienced subjects were chosen as the experimental (AS) group, and 10 healthy acupuncture-experienced subjects were chosen for the control (no AS) group. Measurements began with a 3-min rest period, followed by "Jakutaku" (AS) for 2 min, and recovery after stimulation.

RESULTS

There was a significant increase in oxy-Hb (60.7 muM at maximum) and t-Hb (48.1 muM at maximum) in the stimulation region compared to the distant region. In the stimulation region, a significant increase in oxy-Hb and t-Hb compared with the pre-stimulation level was first noted at 58.5 s and 13.5 s, respectively, after the onset of stimulation.

CONCLUSION

In conclusion, oxygenation and blood volume increased, indicating elevated blood flow to the small vessels, not in the distant region used in this study, but in the stimulation region of the trapezius muscle during and after a 2-min AS.

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.

Crosstalk and error analysis of fat layer on continuous wave near-infrared spectroscopy measurements

Accurate estimation of concentration changes in muscles by continuous wave near-IR spectroscopy for muscle measurements suffers from underestimation and crosstalk problems due to the presence of superficial skin and fat layers. Underestimation error is basically caused by a homogeneous medium assumption in the calculations leading to the partial volume effect. The homogeneous medium assumption and wavelength dependence of mean partial path length in the muscle layer cause the crosstalk. We investigate underestimation errors and crosstalk by Monte Carlo simulations with a three layered (skin-fat-muscle) tissue model for a two-wavelength system where the choice of first wavelength is in the 675- to 775-nm range and the second wavelength is in the 825- to 900-nm range. Means of absolute underestimation errors and crosstalk over the considered wavelength pairs are found to be higher for greater fat thicknesses. Estimation errors of concentration changes for Hb and HbO(2) are calculated to be close for an ischemia type protocol where both Hb and HbO(2) are assumed to have equal magnitude but opposite concentration changes. The minimum estimation errors are found for the 700825- and 725825-nm pairs for this protocol.

Optical imaging instrument for muscle oxygenation based on spatially resolved spectroscopy

An imaging instrument based on spatially resolved spectroscopy that enables temporal and spatial analyses of muscle oxygenation was designed. The instrument is portable and can be connected to 32 compact and separate-type optical probes. Its measurement accuracy of O(2) saturation and hemoglobin concentration was evaluated using a tissue-equivalent phantom. Imaging and multi-point measurements of tissue oxygen saturation (S(t)O(2)) in the quadriceps muscle were also performed, and dynamic changes in S(t)O(2) in response to increase in exercise intensity (within the rectus femoris region) and variation in exercise protocol (among the rectus femoris, vastus lateralis and vastus medialis) were clearly shown.

Tutorial on diffuse light transport

A tutorial introduction to diffuse light transport is presented. The basic analytic equations of time-resolved, steady-state and modulated light transport are introduced. The perturbation method for handling slight heterogeneities in optical properties is outlined. The treatment of boundary conditions such as an air/tissue surface is described. Finite mesh-based numerical methods are introduced to calculate the diffuse light field in complex tissues with arbitrary boundaries. Applications in tissue spectroscopy and imaging illustrate these theoretical and computational tools.

Oxygen saturation determined from deep muscle, not thenar tissue, is an early indicator of central hypovolemia in humans

OBJECTIVE

To compare the responses of noninvasively measured tissue oxygen saturation (StO2) and calculated muscle oxygen tension (PmO2) to standard hemodynamic variables for early detection of imminent hemodynamic instability during progressive central hypovolemia in humans.

DESIGN

Prospective study.

SETTING

Research laboratory.

SUBJECTS

Sixteen healthy human volunteers.

INTERVENTIONS

Progressive lower body negative pressure (LBNP) to onset of cardiovascular collapse.

MEASUREMENTS AND MAIN RESULTS

Noninvasive measurements of blood pressures, heart rate, and stroke volume were obtained during progressive LBNP with simultaneous assessments of StO2, PmO2, and muscle oxygen saturation (SmO2). Forearm SmO2 and PmO2 were determined with a novel near infrared spectroscopic measurement device (UMMS) and compared with thenar StO2 measured by a commercial device (HT). All values were normalized to the duration of LBNP exposure required for cardiovascular collapse in each subject (i.e., LBNP maximum). Stroke volume was significantly decreased at 25% of LBNP maximum, whereas blood pressure was a late indicator of imminent cardiovascular collapse. PmO2 (UMMS) was significantly decreased at 50% of maximum LBNP while SmO2 (UMMS) decreased at 75% of maximum LBNP. Thenar StO2 (HT) showed no statistical change throughout the entire LBNP protocol.

CONCLUSIONS

Spectroscopic assessment of forearm muscle PO2 and SmO2 provides noninvasive and continuous measures that are early indicators of impending cardiovascular collapse resulting from progressive reductions in central blood volume.

Near-infrared spectroscopy/imaging for monitoring muscle oxygenation and oxidative metabolism in healthy and diseased humans

Near-infrared spectroscopy (NIRS) was initiated in 1977 by Jobsis as a simple, noninvasive method for measuring the presence of oxygen in muscle and other tissues in vivo. This review honoring Jobsis highlights the progress that has been made in developing and adapting NIRS and NIR imaging (NIRI) technologies for evaluating skeletal muscle O(2) dynamics and oxidative energy metabolism. Development of NIRS/NIRI technologies has included novel approaches to quantification of the signal, as well as the addition of multiple source detector pairs for imaging. Adaptation of NIRS technology has focused on the validity and reliability of NIRS measurements. NIRS measurements have been extended to resting, ischemic, localized exercise, and whole body exercise conditions. In addition, NIRS technology has been applied to the study of a number of chronic health conditions, including patients with chronic heart failure, peripheral vascular disease, chronic obstructive pulmonary disease, varying muscle diseases, spinal cord injury, and renal failure. As NIRS technology continues to evolve, the study of skeletal muscle function with NIRS first illuminated by Jobsis continues to be bright.

Wavelength shift analysis: a simple method to determine the contribution of hemoglobin and myoglobin to in vivo optical spectra

The ability to quantify the contributions of hemoglobin (Hb) and myoglobin (Mb) to in vivo optical spectra has many applications for clinical and research use such as noninvasive measurement of local tissue O(2) uptake rates and regional blood content. Recent work has demonstrated an approach to independently measure oxygen saturations of Hb and Mb in optical spectra collected in vivo. However, the utility of this approach is limited without information on tissue concentrations of these species. Here we describe a strategy to quantify the contributions of Hb and Mb to in vivo optical spectra. We have found that the peak position of the deoxy-heme peak around 760 nm in the optical spectra of the deoxygenated tissue is a linear function of the relative contributions of Hb and Mb to the optical spectra. Therefore, analysis of this peak position, hereafter referred to as wavelength shift analysis, reveals the relative concentration of Hb to Mb in solutions and intact tissue. Biochemical analysis of muscle homogenates confirmed that the wavelength shift of the combined Hb/Mb peak in in vivo spectra reflects the ratio of concentrations (Hb/Mb) in muscle. The importance of quantifying the Hb contribution is illustrated by our data demonstrating that Hb accounts for approximately 80% of the optical signal in mouse skeletal muscle but only approximately 20% in human skeletal muscle. This advance will facilitate comparison of the metabolic properties between individual muscles and provides a fully noninvasive approach to measuring local respiration that can be adapted for clinical use.

A novel method to measure regional muscle blood flow continuously using NIRS kinetics information

BACKGROUND

This article introduces a novel method to continuously monitor regional muscle blood flow by using Near Infrared Spectroscopy (NIRS). We demonstrate the feasibility of the new method in two ways: (1) by applying this new method of determining blood flow to experimental NIRS data during exercise and ischemia; and, (2) by simulating muscle oxygenation and blood flow values using these newly developed equations during recovery from exercise and ischemia.

METHODS

Deoxy (Hb) and oxyhemoglobin (HbO2), located in the blood of the skeletal muscle, carry two internal relationships between blood flow and oxygen consumption. One is a mass transfer principle and the other describes a relationship between oxygen consumption and Hb kinetics in a two-compartment model. To monitor blood flow continuously, we transfer these two relationships into two equations and calculate the blood flow with the differential information of HbO2 and Hb. In addition, these equations are used to simulate the relationship between blood flow and reoxygenation kinetics after cuff ischemia and a light exercise. Nine healthy subjects volunteered for the cuff ischemia, light arm exercise and arm exercise with cuff ischemia for the experimental study.

RESULTS

Analysis of experimental data of both cuff ischemia and light exercise using the new equations show greater blood flow (four to six times more than resting values) during recovery, agreeing with previous findings. Further, the simulation and experimental studies of cuff ischemia and light exercise agree with each other.

CONCLUSION

We demonstrate the accuracy of this new method by showing that the blood flow obtained from the method agrees with previous data as well as with simulated data. We conclude that this novel continuous blood flow monitoring method can provide blood flow information non-invasively with NIRS.

Association between regional quadriceps oxygenation and blood oxygen saturation during normoxic one-legged dynamic knee extension

It is not clear whether muscle oxygenation (O(2-NIRS)) measured by near-infrared spectroscopy (NIRS) correlates with femoral venous SO2 (S(fv)o2) during normoxic exercise. Therefore, the purpose of this study was to compare physiologically calibrated O(2-NIRS) with S(fv)o2 in subjects performing one-legged dynamic knee extension exercise (1L-KEE). Five healthy male subjects (age 25+/-2 year, height 177.8+/-4.8 cm, body weight 67.1 +/- 5.0 kg; mean +/- SD) performed 1L-KEE at 20, 40, and 60% of peak work rate (WR-peak) each for 4 min. S(fv)o2 was measured at rest and during the 3rd minute of each work rate. O(2-NIRS) was continuously monitored in a proximal region of the vastus lateralis (VL-p), a distal region of VL (VL-d), and a proximal region of the rectus femoris (RF-p). S(fv)o2 was 56.0% at rest and decreased to 36.6 at 20% WR-peak, 35.8 at 40% WR-peak, and 31.1 at 60% WR-peak. There was a significant correlation between O(2-NIRS) and S(fv)o2(VL-p: r (2) = 0.62, VL-d: r2 = 0.35, RF-p: r2 = 0.62, with a moderate variation among individuals at each site; residual values = 4.83 - 11.75). These data indicate that NIRS measurement provides a reflection of S(fv)o2 during 20-60% WR-peak of normoxic 1L-KEE.

Blood Oxygen Level–Dependent MRI of Tissue Oxygenation

OBJECTIVES

The contribution of endothelial function to tissue oxygenation is not well understood. Muscle blood oxygen level-dependent MRI (BOLD MRI) provides data largely dependent on hemoglobin (Hb) oxygenation. We used BOLD MRI to assess endothelium-dependent signal intensity (SI) changes.

METHODS AND RESULTS

We investigated mean BOLD SI changes in the forearm musculature using a gradient-echo technique at 1.5 T in 9 healthy subjects who underwent a protocol of repeated acetylcholine infusions at 2 different doses (16 and 64 microg/min) and N(G)-monomethyl-L-arginine (L-NMMA; 5 mg/min) into the brachial artery. Sodium nitroprusside was used as a control substance. For additional correlation with standard methods, the same protocol was repeated, and forearm blood flow was measured by strain gauge plethysmography. We obtained a significant increase in BOLD SI during acetylcholine infusion (64 microg/min) and a significant decrease for L-NMMA infusion (P<0.005 for both). BOLD SI showed a different kinetic signal than did blood flow, particularly after intermittent ischemia and at high flow rates.

CONCLUSIONS

In standard endothelial function tests, BOLD MRI detects a dissociation of tissue Hb oxygenation from blood flow. BOLD MRI may be a useful adjunct in assessing endothelial function.