Noninvasive evaluation of skeletal muscle mitochondrial capacity with near-infrared spectroscopy: correcting for blood volume changes

Near-infrared spectroscopy (NIRS) is a well-known method used to measure muscle oxygenation and hemodynamics in vivo. The application of arterial occlusions allows for the assessment of muscle oxygen consumption (mVo(2)) using NIRS. The aim of this study was to measure skeletal muscle mitochondrial capacity using blood volume-corrected NIRS signals that represent oxygenated hemoglobin/myoglobin (O(2)Hb) and deoxygenated hemoglobin/myoglobin (HHb). We also assessed the reliability and reproducibility of NIRS measurements of resting oxygen consumption and mitochondrial capacity. Twenty-four subjects, including four with chronic spinal cord injury, were tested using either the vastus lateralis or gastrocnemius muscles. Ten healthy, able-bodied subjects were tested on two occasions within a period of 7 days to assess the reliability and reproducibility. NIRS signals were corrected for blood volume changes using three different methods. Resting oxygen consumption had a mean coefficient of variation (CV) of 2.4% (range 1-32%). The recovery of oxygen consumption (mVo(2)) after electrical stimulation at 4 Hz was fit to an exponential curve, which represents mitochondrial capacity. The time constant for the recovery of mVo(2) was reproducible with a mean CV of 10% (range 1-22%) only when correcting for blood volume changes. We also examined the effects of adipose tissue thickness on measurements of mVo(2). We found the mVo(2) measurements using absolute units to be influenced by adipose tissue thickness (ATT), and this relationship was removed when an ischemic calibration was performed, supporting its use to compare mVo(2) between individuals of varying ATT. In conclusion, in vivo oxidative capacity can be assessed using blood volume-corrected NIRS signals with a high degree of reliability and reproducibility.     

Multispectral optoacoustic tomography of muscle perfusion and oxygenation under arterial and venous occlusion: A human pilot study

Perfusion and oxygenation are critical parameters of muscle metabolism in health and disease. They have been both the target of many studies, in particular using near‐infrared spectroscopy (NIRS). However, difficulties with quantifying NIRS signals have limited a wide dissemination of the method to the clinics. Our aim was to investigate whether clinical multispectral optoacoustic tomography (MSOT) could enable the label‐free imaging of muscle perfusion and oxygenation under clinically relevant challenges: the arterial and venous occlusion. We employed a hybrid clinical MSOT/ultrasound system equipped with a hand‐held scanning probe to visualize hemodynamic and oxygenation changes in skeletal muscle under arterial and venous occlusions. Four (N = 4) healthy volunteers were scanned over the forearm for both 3‐minute occlusion challenges. MSOT‐recorded pathophysiologically expected results during tests of disturbed blood flow with high resolution and without the need for contrast agents. During arterial occlusion, MSOT‐extracted Hb‐values showed an increase, while HbO₂‐ and total blood volume (TBV)‐values remained roughly steady, followed by a discrete increase during the hyperemic period after cuff deflation. During venous occlusion, results showed a clear increase in intramuscular HbO₂, Hb and TBV within the segmented muscle area. MSOT was found to be capable of label‐free non‐invasive imaging of muscle hemodynamics and oxygenation under arterial and venous occlusion. We introduce herein MSOT as a novel modality for the assessment of vascular disorders characterized by disturbed blood flow, such as acute limb ischemia and venous thrombosis.     

Regional transcranial oximetry with near infrared spectroscopy (NIRS) in comparison with measuring oxygen saturation in the jugular bulb in infants and children for monitoring cerebral oxygenation]

Using a dual channel near infrared (NIR) in vivo optical spectroscopy (INVOS) system (INVOS 3100A, Somanetics Corp. Troy, MI, USA) we investigated the relationship between jugular venous oxygen saturation (SjvO2) and regional cerebral oxygen saturation (rSO2) in 30 infants and children (mean age 4.5 years) with congenital heart disease undergoing cardiac catheterisation. The NIRS-SomaSensor (emitter and dual receiver probe) was applied at a standardised right fronto-temporal location (over the right frontal cortex) on the infant's head and covered with an adhesive flexible bandage. Using NIR light (730 and 810 nm) and two source-detector spacings (3 and 4 cm from the transmitter), percentage values of rSO2 were calculated from detected haemoglobin saturations. Simultaneously, jugular venous oxygen saturation (SjvO2) monitoring was performed via a catheter placed in the right internal jugular vein with its tip positioned in the jugular bulb, as verified by fluoroscopy. To compare the reliability of NIRS measurement characteristics, jugular venous blood was analysed for SjvO2 as a reference measure of global cerebral oxygenation, by co-oximetry (OSM3-Hemoximeter, Radiometer Copenhagen, Denmark). Other measured variables included pulse oximetry, arterial blood pressure, and venous and arterial oxygen saturations. Over a jugular venous oxygen saturation range of 31-83%, a significant positive linear correlation was found between rSO2 (NIRS measurement) and SjvO2 (jugular bulb oximetry) (r = 0.93, p < 0.001). No significant correlation was observed between rSO2 values and arterial blood saturation or pulse oximetry. The quantitative correlation between rSO2 (haemoglobin oxygenation in a small hemi-elliptical area of the brain) and reference SjvO2 measurement (method for monitoring global cerebral oxygenation) suggests that NIRS measurement with subtraction algorithm should identify predominantly intracranial saturation in the pediatric age group, and will tend to reflect global oxygenation under physiological conditions. Transcranial oximetry using dual receiving channel NIRS offers a noninvasive, real-time, reliable and practicable means of monitoring cerebral haemoglobin oxygenation changes infants and children with cyanotic and noncyanotic congenital heart disease.     

近红外光谱技术在运动脑功能研究中的应用

近红外光谱是一种评估生物组织氧合水平的无创性光学技术,这一技术以血液动力学原理为基础,能实时监测局部脑区的动态变化。近红外光谱作为一种客观的测量工具,在人体运动科学领域广泛运用。近红外光谱技术可用于区别体能水平、监控耐力训练和抗阻训练过程特征以及考察运动中的认知活动变化。本文综述了近红外光谱技术原理及其在抗阻运动、运动中枢疲劳和运动认知领域的运用,并对近红外光谱在运动科学领域的未来研究趋向作了分析。     

Development, set-up and first results for a one-channel near-infrared spectroscopy system.

Abstract Near-infrared spectroscopy (NIRS) is a non-invasive optical technique that can be used to assess functional activity in the human brain. This work describes the set-up of a one-channel NIRS system designed for use as an optical brain-computer interface (BCI) and reports on first measurements of deoxyhemoglobin (Hb) and oxyhemoglobin (HbO(2)) changes during mental arithmetic tasks. We found relatively stable and reproducible hemodynamic responses in a group of 13 healthy subjects. Unexpected observations of a decrease in HbO(2) and increase in Hb concentrations measured over the prefrontal cortex were in contrast to the typical hemodynamic responses (increase in HbO(2), decrease in Hb) during cortical activation previously reported.     

Effects of acute hypoxia on cerebral and muscle oxygenation during incremental exercise.

To determine if fatigue at maximal aerobic power output was associated with a critical decrease in cerebral oxygenation, 13 male cyclists performed incremental maximal exercise tests (25 W/min ramp) under normoxic (Norm: 21% Fi(O2)) and acute hypoxic (Hypox: 12% Fi(O2)) conditions. Near-infrared spectroscopy (NIRS) was used to monitor concentration (microM) changes of oxy- and deoxyhemoglobin (Delta[O2Hb], Delta[HHb]) in the left vastus lateralis muscle and frontal cerebral cortex. Changes in total Hb were calculated (Delta[THb] = Delta[O2Hb] + Delta[HHb]) and used as an index of change in regional blood volume. Repeated-measures ANOVA were performed across treatments and work rates (alpha = 0.05). During Norm, cerebral oxygenation rose between 25 and 75% peak power output {Power(peak); increased (inc) Delta[O2Hb], inc. Delta[HHb], inc. Delta[THb]}, but fell from 75 to 100% Power(peak) {decreased (dec) Delta[O2Hb], inc. Delta[HHb], no change Delta[THb]}. In contrast, during Hypox, cerebral oxygenation dropped progressively across all work rates (dec. Delta[O2Hb], inc. Delta[HHb]), whereas Delta[THb] again rose up to 75% Power(peak) and remained constant thereafter. Changes in cerebral oxygenation during Hypox were larger than Norm. In muscle, oxygenation decreased progressively throughout exercise in both Norm and Hypox (dec. Delta[O2Hb], inc. Delta [HHb], inc. Delta[THb]), although Delta[O2Hb] was unchanged between 75 and 100% Power peak. Changes in muscle oxygenation were also greater in Hypox compared with Norm. On the basis of these findings, it is unlikely that changes in cerebral oxygenation limit incremental exercise performance in normoxia, yet it is possible that such changes play a more pivotal role in hypoxia.     

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 ofthe 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.

     

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 (A(p)), time delay (TD(p)), and time constant (τ(p)) of the primary component kinetics in muscle deoxy(Hb + Mb) were spatially heterogeneous (intersite coefficient of variation range for the subjects: 10-50 for A(p), 16-58 for TD(p), 14-108% for τ(p)). 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 O(2) delivery and O(2) utilization, not an artifact caused by changes in optical properties of the tissue during exercise or variability in the overlying adipose tissue.     

Muscle oxygenation dynamics in response to electrical stimulation as measured with near‐infrared spectroscopy: A pilot study

Neuromuscular electrical stimulation (NMES) is used for preventing muscle atrophy and improving muscle strength in patients and healthy people. However, the current intensity of NMES is usually set at a level that causes the stimulated muscles to contract. This typically causes pain. Quantifying the instantaneous changes in muscle microcirculation and metabolism during NMES before muscle contraction occurs is crucial, because it enables the current intensity to be optimally tuned, thereby reducing the NMES‐induced muscle pain and fatigue. We applied near‐infrared spectroscopy (NIRS) to measure instantaneous tissue oxygenation and deoxygenation changes in 43 healthy young adults during NMES at 10, 15, 20, 25, 30, and 35 mA. Having been stabilized at the NIRS signal baseline, the tissue oxygenation and total hemoglobin concentration increased immediately after stimulation in a dose‐dependent manner (P < 0.05) until stimulation was stopped at the level causing muscle contraction without pain. Tissue deoxygenation appeared relatively unchanged during NMES. We conclude that NIRS can be used to determine the optimal NMES current intensity by monitoring oxygenation changes.      

Noninvasive optical characterization of muscle blood flow,oxygenation, and metabolism in women with fibromyalgia

Introduction

Women with fibromyalgia (FM) have symptoms of increased muscular fatigue and reduced exercise tolerance, which may be associated with alterations in muscle microcirculation and oxygen metabolism. This study used near-infrared diffuse optical spectroscopies to noninvasively evaluate muscle blood flow, blood oxygenation and oxygen metabolism during leg fatiguing exercise and during arm arterial cuff occlusion in post-menopausal women with and without FM.

Methods

Fourteen women with FM and twenty-three well-matched healthy controls participated in this study. For the fatiguing exercise protocol, the subject was instructed to perform 6 sets of 12 isometric contractions of knee extensor muscles with intensity steadily increasing from 20 to 70% maximal voluntary isometric contraction (MVIC). For the cuff occlusion protocol, forearm arterial blood flow was occluded via a tourniquet on the upper arm for 3 minutes. Leg or arm muscle hemodynamics, including relative blood flow (rBF), oxy- and deoxy-hemoglobin concentration ([HbO₂] and [Hb]), total hemoglobin concentration (THC) and blood oxygen saturation (StO₂), were continuously monitored throughout protocols using a custom-built hybrid diffuse optical instrument that combined a commercial near-infrared oximeter for tissue oxygenation measurements and a custom-designed diffuse correlation spectroscopy (DCS) flowmeter for tissue blood flow measurements. Relative oxygen extraction fraction (rOEF) and oxygen consumption rate (rVO₂) were calculated from the measured blood flow and oxygenation data. Post-manipulation (fatiguing exercise or cuff occlusion) recovery in muscle hemodynamics was characterized by the recovery half-time, a time interval from the end of manipulation to the time that tissue hemodynamics reached a half-maximal value.

Results

Subjects with FM had similar hemodynamic and metabolic response/recovery patterns as healthy controls during exercise and during arterial occlusion. However, tissue rOEF during exercise in subjects with FM was significantly lower than in healthy controls, and the half-times of oxygenation recovery (Δ[HbO₂] and Δ[Hb]) were significantly longer following fatiguing exercise and cuff occlusion.

Conclusions

Our results suggest an alteration of muscle oxygen utilization in the FM population. This study demonstrates the potential of using combined diffuse optical spectroscopies (i.e., NIRS/DCS) to comprehensively evaluate tissue oxygen and flow kinetics in skeletal muscle.     

Effects of assuming constant optical scattering on measurements of muscle oxygenation by near-infrared spectroscopy during exercise.

The aim of this study was to examine the effects of assuming constant reduced scattering coefficient (mu'(s)) on the muscle oxygenation response to incremental exercise and its recovery kinetics. Fifteen subjects (age: 24 +/- 5 yr) underwent incremental cycling exercise. Frequency domain near-infrared spectroscopy (NIRS) was used to estimate deoxyhemoglobin concentration {[deoxy(Hb+Mb)]} (where Mb is myoglobin), oxyhemoglobin concentration {[oxy(Hb+Mb)]}, total Hb concentration (Total[Hb+Mb]), and tissue O(2) saturation (Sti(O(2))), incorporating both continuous measurements of mu'(s) and assuming constant mu'(s). When measuring mu'(s), we observed significant changes in NIRS variables at peak work rate Delta[deoxy(Hb+Mb)] (15.0 +/- 7.8 microM), Delta[oxy(Hb+Mb)] (-4.8 +/- 5.8 microM), DeltaTotal[Hb+Mb] (10.9 +/- 8.4 microM), and DeltaSti(O(2))(-11.8 +/- 4.1%). Assuming constant mu'(s) resulted in greater (P < 0.01 vs. measured mu'(s)) changes in the NIRS variables at peak work rate, where Delta[deoxy(Hb+Mb)] = 24.5 +/- 15.6 microM, Delta[oxy(Hb+Mb)] = -9.7 +/- 8.2 microM, DeltaTotal[Hb+Mb] = 14.8 +/- 8.7 microM, and DeltaSti(O(2))= -18.7 +/- 8.4%. Regarding the recovery kinetics, the large 95% confidence intervals (CI) for the difference between those determine measuring mu'(s) and assuming constant mu'(s) suggested poor agreement between methods. For the mean response time (MRT), which describes the overall kinetics, the 95% confidence intervals were MRT - [deoxy(Hb+Mb)] = 26.7 s; MRT - [oxy(Hb+Mb)] = 11.8 s, and MRT - Sti(O(2))= 11.8 s. In conclusion, mu'(s) changed from light to peak exercise. Furthermore, assuming a constant mu'(s) led to an overestimation of the changes in NIRS variables during exercise and distortion of the recovery kinetics.     

Muscle oxygenation and pulmonary gas exchange kinetics during cycling exercise on-transitions in humans.

Near-infrared spectroscopy (NIRS) was utilized to gain insights into the kinetics of oxidative metabolism during exercise transitions. Ten untrained young men were tested on a cycle ergometer during transitions from unloaded pedaling to 5 min of constant-load exercise below (VT) the ventilatory threshold. Vastus lateralis oxygenation was determined by NIRS, and pulmonary O2 uptake (Vo --> Vo2) was determined breath-by-breath. Changes in deoxygenated hemoglobin + myoglobin concentration Delta[deoxy(Hb + Mb)] were taken as a muscle oxygenation index. At the transition, [Delta[deoxy(Hb + Mb)]] was unmodified [time delay (TD)] for 8.9 +/- 0.5 s at VT (both significantly different from 0) and then increased, following a monoexponential function [time constant (tau) = 8.5 +/- 0.9 s for VT]. For >VT a slow component of Delta[deoxy(Hb + Mb)] on-kinetics was observed in 9 of 10 subjects after 75.0 +/- 14.0 s of exercise. A significant correlation was described between the mean response time (MRT = TD + tau) of the primary component of Delta[deoxy(Hb + Mb)] on-kinetics and the tau of the primary component of the pulmonary Vo2 on-kinetics. The constant muscle oxygenation during the initial phase of the on-transition indicates a tight coupling between increases in O2 delivery and O2 utilization. The lack of a drop in muscle oxygenation at the transition suggests adequacy of O2 availability in relation to needs.     

Skeletal muscle oxygenation monitoring by near infrared spectroscopy.

The oxygenation of human forearm muscle tissue was studied using an optic fiber fast scanning spectrophotometer. We investigated near infrared (700-1100 nm) hemoglobin (Hb) and myoglobin (Mb) spectral changes in flow and/or oxygen-limited conditions. The superimposition of deoxy Hb and deoxy Mb spectra was confirmed "in vivo" on perfluorocarbon-blood exchanged transfused rats. Oxygenation changes were evaluated in 15 volunteers during 10 min forearm arterial occlusion. Rapid desaturation occurred until a plateau was reached after about 4 min, suggesting rapid anaerobic glycolytic activation.     

Mitochondrial coupling in vivo in mouse skeletal muscle

The coupling of mitochondrial ATP synthesis and oxygen consumption (ratio of ATP and oxygen fluxes, P/O) plays a central role in cellular bioenergetics. Reduced P/O values are associated with mitochondrial pathologies that can lead to reduced capacity for ATP synthesis and tissue degeneration. Previous work found a wide range of values for P/O in normal mitochondria. To measure mitochondrial coupling under physiological conditions, we have developed a procedure for determining the P/O of skeletal muscle in vivo. This technique measures ATPase and oxygen consumption rates during ischemia with ³¹P magnetic resonance and optical spectroscopy, respectively. This novel approach allows the independent quantitative measurement of ATPase and oxygen flux rates in intact tissue. The quantitative measurement of oxygen consumption is made possible by our ability to independently measure the saturations of hemoglobin (Hb) and myoglobin (Mb) from optical spectra. Our results indicate that the P/O in skeletal muscle of the mouse hindlimb measured in vivo is 2.16 ± 0.24. The theoretical P/O for resting muscle is 2.33. Systemic treatment with 2,4-dinitrophenol to partially uncouple mitochondria does not affect the ATPase rate in the mouse hindlimb but nearly doubles the rate of oxygen consumption, reducing in vivo P/O to 1.37 ± 0.22. These results indicate that only a small fraction of the oxygen consumption in resting mouse skeletal muscle is nonphosphorylating under physiological conditions, suggesting that mitochondria are more tightly coupled than previously thought. P/O; oxidative phosphorylation; proton leak; optical spectroscopy     

Spatial heterogeneity of quadriceps muscle deoxygenation kinetics during cycle exercise.

To test the hypothesis that, during exercise, substantial heterogeneity of muscle hemoglobin and myoglobin deoxygenation [deoxy(Hb + Mb)] dynamics exists and to determine whether such heterogeneity is associated with the speed of pulmonary O(2) uptake (pVo(2)) kinetics, we adapted multi-optical fibers near-infrared spectroscopy (NIRS) to characterize the spatial distribution of muscle deoxygenation kinetics at exercise onset. Seven subjects performed cycle exercise transitions from unloaded to moderate [GET) work rates and the relative changes in deoxy(Hb + Mb), at 10 sites in the quadriceps, were sampled by NIRS. At exercise onset, the time delays in muscle deoxy(Hb + Mb) were spatially inhomogeneous [intersite coefficient of variation (CV), 3~56% for GET]. The primary component kinetics (time constant) of muscle deoxy(Hb + Mb) reflecting increased O(2) extraction were also spatially inhomogeneous (intersite CV, 6~48% for GET) and faster (P < 0.05) than those of phase 2 pVo(2). However, the degree of dynamic intersite heterogeneity in muscle deoxygenation did not correlate significantly with phase 2 pVo(2) kinetics. In conclusion, the dynamics of quadriceps microvascular oxygenation demonstrates substantial spatial heterogeneity that must arise from disparities in the relative kinetics of Vo(2) and O(2) delivery increase across the regions sampled.     

Blood lactate accumulation and muscle deoxygenation during incremental exercise.

Near-infrared spectroscopy (NIRS) could allow insights into controversial issues related to blood lactate concentration ([La](b)) increases at submaximal workloads (). We combined, on five well-trained subjects [mountain climbers; peak O(2) consumption (VO(2peak)), 51.0 +/- 4.2 (SD) ml. kg(-1). min(-1)] performing incremental exercise on a cycle ergometer (30 W added every 4 min up to voluntary exhaustion), measurements of pulmonary gas exchange and earlobe [La](b) with determinations of concentration changes of oxygenated Hb (Delta[O(2)Hb]) and deoxygenated Hb (Delta[HHb]) in the vastus lateralis muscle, by continuous-wave NIRS. A "point of inflection" of [La](b) vs. was arbitrarily identified at the lowest [La](b) value which was >0.5 mM lower than that obtained at the following. Total Hb volume (Delta[O(2)Hb + HHb]) in the muscle region of interest increased as a function of up to 60-65% of VO(2 peak), after which it remained unchanged. The oxygenation index (Delta[O(2)Hb - HHb]) showed an accelerated decrease from 60- 65% of VO(2 peak). In the presence of a constant total Hb volume, the observed Delta[O(2)Hb - HHb] decrease indicates muscle deoxygenation (i.e., mainly capillary-venular Hb desaturation). The onset of muscle deoxygenation was significantly correlated (r(2) = 0.95; P < 0.01) with the point of inflection of [La](b) vs., i.e., with the onset of blood lactate accumulation. Previous studies showed relatively constant femoral venous PO(2) levels at higher than approximately 60% of maximal O(2) consumption. Thus muscle deoxygenation observed in the present study from 60-65% of VO(2 peak) could be attributed to capillary-venular Hb desaturation in the presence of relatively constant capillary-venular PO(2) levels, as a consequence of a rightward shift of the O(2)Hb dissociation curve determined by the onset of lactic acidosis.     

Noninvasive assessment of sympathetic vasoconstriction in human and rodent skeletal muscle using near-infrared spectroscopy and Doppler ultrasound.

The precise role of the sympathetic nervous system in the regulation of skeletal muscle blood flow during exercise has been challenging to define in humans, partly because of the limited techniques available for measuring blood flow in active muscle. Recent studies using near-infrared (NIR) spectroscopy to measure changes in tissue oxygenation have provided an alternative method to evaluate vasomotor responses in exercising muscle, but this approach has not been fully validated. In this study, we tested the hypothesis that sympathetic activation would evoke parallel changes in tissue oxygenation and blood flow in resting and exercising muscle. We simultaneously measured tissue oxygenation with NIR spectroscopy and blood flow with Doppler ultrasound in skeletal muscle of conscious humans (n = 13) and anesthetized rats (n = 9). In resting forearm of humans, reflex activation of sympathetic nerves with the use of lower body negative pressure produced graded decreases in tissue oxygenation and blood flow that were highly correlated (r = 0.80, P < 0.0001). Similarly, in resting hindlimb of rats, electrical stimulation of sympathetic nerves produced graded decreases in tissue oxygenation and blood flow velocity that were highly correlated (r = 0.93, P < 0.0001). During rhythmic muscle contraction, the decreases in tissue oxygenation and blood flow evoked by sympathetic activation were significantly attenuated (P < 0.05 vs. rest) but remained highly correlated in both humans (r = 0.80, P < 0.006) and rats (r = 0.92, P < 0.0001). These data indicate that, during steady-state metabolic conditions, changes in tissue oxygenation can be used to reliably assess sympathetic vasoconstriction in both resting and exercising skeletal muscle.     

Noninvasive monitoring of deterioration in skeletal muscle function with forearm cast immobilization and the prevention of deterioration

BACKGROUND: In this research inactivity was simulated by immobilizing the forearm region in a plaster cast. Changes in skeletal muscle oxidative function were measured using near-infrared spectroscopy (NIRS), and the preventative effect of the training protocol on deterioration of skeletal muscle and the clinical utility of NIRS were examined. METHODS: Fourteen healthy adult men underwent immobilization of the forearm of the non-dominant arm by plaster cast for 21 days. Eight healthy adult subjects were designated as the immobilization group (IMM) and six were designated as the immobilization + training group (IMM+TRN). Grip strength, forearm circumference and dynamic handgrip exercise endurance were measured before and after the 21-day immobilization period. Using NIRS, changes in oxidative function of skeletal muscles were also evaluated. Muscle oxygen consumption recovery was recorded after the completion of 60 seconds of 40% maximum voluntary contraction (MVC) dynamic handgrip exercise 1 repetition per 4 seconds and the recovery time constant (TcVO2mus) was calculated. RESULTS: TcVO2mus for the IMM was 59.7 +/- 5.5 seconds (average +/- standard error) before immobilization and lengthened significantly to 70.4 +/- 5.4 seconds after immobilization (p < 0.05). For the IMM+TRN, TcVO2mus was 78.3 +/- 6.2 seconds before immobilization and training and shortened significantly to 63.1 +/- 5.6 seconds after immobilization and training (p < 0.05). CONCLUSIONS: The training program used in this experiment was effective in preventing declines in muscle oxidative function and endurance due to immobilization. The experimental results suggest that non-invasive monitoring of skeletal muscle function by NIRS would be possible in a clinical setting.     

The utility of far‐infrared illumination in oxygenation dynamics as measured with near‐infrared spectroscopy

Near‐infrared spectroscopy (NIRS) is a noninvasive method for measuring the oxygenation in muscle and other tissues in vivo. For quantitative NIRS measurement of oxygenation dynamics, the vessel‐occlusion test was usually applied as physiological intervention. There are several drawbacks of the vessel‐occlusion method that include skin contact, uncomfortable and microcirculation block of patients. Thus, we propose the far‐infrared (FIR) illumination as a new physiological intervention method in this paper. Our preliminary result shows a linear correlation of oxygenation dynamic signals between FIR illumination and arterial‐occlusion test (AOT) that implies the FIR illumination could be applied for hemodynamic response measurement in clinical diagnosis. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Monitoring blood loss with near infrared spectroscopy

Experimental research has shown correlation between near infrared spectroscopy (NIRS) and blood loss, but these findings have not been validated in man. Ten blood donors were monitored before, during and for 10 min after blood collection (470 ml) with NIRS. A Somanetics INVOS 4100 oximeter monitored regional haemoglobin saturation in the cerebral cortex (cSO(2)-left frontal area) and from the left calf (pSO(2)). A Critikon 2001 Cerebral Redox Model monitored total (tHb), oxygenated (O(2)Hb) and deoxygenated (HHb) haemoglobin from the right calf. The oxygenation index [HbD]=[O(2)Hb]-[HHb] was derived from the data. cSO(2) (P<0.001), pSO(2) (P<0.001) and HbD (P=0.001) decreased during blood collection. Maximum changes occurred 10 minutes after collection for cSO(2), with a mean fall (95% C.I.) of 2.5 (-0.06-4.86)%, at the end of blood collection for pSO(2), with a mean fall (95% C.I.) of 3 (0.74-5.26)% and after 8% of blood volume loss for HbD, with a mean fall (95% C.I.) of 7.2 (2.25-12.16). Cerebral and peripheral oxygenation did not recover after blood collection. There was good correlation between NIRS parameters and blood loss. NIRS is a potentially useful technique for monitoring blood loss in humans. Further research is needed to define its role in clinical practice.