Validation of near-infrared spectroscopy in humans

Cold water immersion enhances recovery of submaximal muscle function after resistance exercise

We investigated the effect of cold water immersion (CWI) on the recovery of muscle function and physiological responses after high-intensity resistance exercise. Using a randomized, cross-over design, 10 physically active men performed high-intensity resistance exercise followed by one of two recovery interventions: 1) 10 min of CWI at 10°C or 2) 10 min of active recovery (low-intensity cycling). After the recovery interventions, maximal muscle function was assessed after 2 and 4 h by measuring jump height and isometric squat strength. Submaximal muscle function was assessed after 6 h by measuring the average load lifted during 6 sets of 10 squats at 80% of 1 repetition maximum. Intramuscular temperature (1 cm) was also recorded, and venous blood samples were analyzed for markers of metabolism, vasoconstriction, and muscle damage. CWI did not enhance recovery of maximal muscle function. However, during the final three sets of the submaximal muscle function test, participants lifted a greater load (P < 0.05, Cohen's effect size: 1.3, 38%) after CWI compared with active recovery. During CWI, muscle temperature decreased ∼7°C below postexercise values and remained below preexercise values for another 35 min. Venous blood O2 saturation decreased below preexercise values for 1.5 h after CWI. Serum endothelin-1 concentration did not change after CWI, whereas it decreased after active recovery. Plasma myoglobin concentration was lower, whereas plasma IL-6 concentration was higher after CWI compared with active recovery. These results suggest that CWI after resistance exercise allows athletes to complete more work during subsequent training sessions, which could enhance long-term training adaptations.

Electromyographic, cerebral, and muscle hemodynamic responses during intermittent, isometric contractions of the biceps brachii at three submaximal intensities

This study examined the electromyographic, cerebral and muscle hemodynamic responses during intermittent isometric contractions of biceps brachii at 20, 40, and 60% of maximal voluntary contraction (MVC). Eleven volunteers completed 2 min of intermittent isometric contractions (12/min) at an elbow angle of 90° interspersed with 3 min rest between intensities in systematic order. Surface electromyography (EMG) was recorded from the right biceps brachii and near infrared spectroscopy (NIRS) was used to simultaneously measure left prefrontal and right biceps brachii oxyhemoglobin (HbO₂), deoxyhemoglobin (HHb), and total hemoglobin (Hb_tot). Transcranial Doppler ultrasound was used to measure middle cerebral artery velocity (MCAv) bilaterally. Finger photoplethysmography was used to record beat-to-beat blood pressure and heart rate. EMG increased with force output from 20 to 60% MVC (P < 0.05). Cerebral HbO₂ and Hb_tot increased while HHb decreased during contractions with differences observed between 60% vs. 40% and 20% MVC (P < 0.05). Muscle HbO₂ decreased while HHb increased during contractions with differences being observed among intensities (P < 0.05). Muscle Hb_tot increased from rest at 20% MVC (P < 0.05), while no further change was observed at 40 and 60% MVC (P > 0.05). MCAv increased from rest to exercise but was not different among intensities (P > 0.05). Force output correlated with the root mean square EMG and changes in muscle HbO₂ (P < 0.05), but not changes in cerebral HbO₂ (P > 0.05) at all three intensities. Force output declined by 8% from the 1st to the 24th contraction only at 60% MVC and was accompanied by systematic increases in RMS, cerebral HbO₂ and Hb_tot with a leveling off in muscle HbO₂ and Hb_tot. These changes were independent of alterations in mean arterial pressure. Since cerebral blood flow and oxygenation were elevated at 60% MVC, we attribute the development of fatigue to reduced muscle oxygen availability rather than impaired central neuronal activation.

The use of skeletal muscle near infrared spectroscopy and a vascular occlusion test at high altitude

Microcirculatory function, central to tissue regulation of oxygen flux, may be altered by the chronic hypoxemia experienced at high altitude. We hypothesized that at high altitude, adaptations within skeletal muscle would result in reduced oxygen consumption and reduced microcirculatory responsiveness, detectable by near infrared spectroscopy (NIRS) during a vascular occlusion test (VOT). The VOT comprised 3 min of noninvasive arterial occlusion; thenar eminence tissue oxygenation (Sto2) was measured by NIRS during the VOT at sea level, 4900 m and 5600 m (after 7 and 17 days at altitude, respectively) in 12 healthy volunteers. Data were derived from Sto2 time-curves using specifically designed computer software. Mean (±SD) resting Sto2 was reduced at 4900 m and 5600 m (69.3 (± 8.2)% (p=0.001) and 64.2 (± 6.1)% (p<0.001) respectively) when compared to sea level (84.4 (± 6.0)%. The rate of Sto2 recovery after vascular occlusion (Sto2 upslope) was significantly reduced at 4900 m (2.4 (± 0.4)%/sec) and 5600 m (2.4 (± 0.8)%/sec) compared to sea level (3.7 (± 1.3)%/sec) (p=0.021 and p=0.032, respectively). There was no change from sea level in the rate of desaturation during occlusion (Sto2 downslope) at either altitude. The findings suggest that in resting skeletal muscle of acclimatizing healthy volunteers at high altitude, microvascular reactivity is reduced (Sto2 upslope after a short period of ischemia) but that oxygen consumption remains unchanged (Sto2 downslope).

Correlation of near-infrared spectroscopy and direct pressure monitoring in an acute porcine compartmental syndrome model

OBJECTIVE

To correlate near-infrared spectroscopy (NIRS) and the tibial intracompartmental perfusion pressure (TIPP) in an acute limb compartmental syndrome.

METHODS

Landrace swine were subdivided into 2 groups: plasma infusion (n = 16) and blunt trauma plus plasma infusion (n = 15). NIRS sensors were placed over the craniolateral muscle compartment of proximal both tibiae. Albumin infusion elevated tibial intracompartmental pressures (TICP). Time-synchronized measures of systolic, diastolic, and mean arterial pressures, TICP, and percent oxygenation from each leg were collected. For the blunt trauma group, trauma was induced by dropping a 2-kg weight 30 times from 100 cm directly on the muscle compartment. For each group, a repeated-measures analysis of variance model was used to test differences in the TICP, TIPP, and oxygenation values. Pearson correlations were calculated between TICP and oxygenation and between TIPP and oxygenation.

RESULTS

Both models created reproducible increases in TICP and decreases in TIPP. Trauma did not alter TICP, TIPP, or percent oxygenation in the model. NIRS was able to detect significant changes in tissue oxygenation at all the same time points. NIRS was able to detect decreased oxygenation at every TIPP decrease and subsequent increase after fasciotomies. An increase in percent oxygenation was seen in all cases once fasciotomy was performed and TICP was reduced.

CONCLUSIONS

NIRS provided a sensitive measure correlating to both an increase and decrease in TICP and TIPP, respectively, in this infusion model. The addition of blunt trauma to the model did not alter the correlations of NIRS values with TICP and TIPP. Fasciotomy produced a rebound in oxygenation values.

Reproducibility of near-infrared spectroscopy measurements of oxidative function and postexercise recovery kinetics in the medial gastrocnemius muscle

The purpose of this study was to assess the reproducibility of resting blood flow, resting oxygen consumption, and mitochondrial capacity in skeletal muscle using near-infrared spectroscopy (NIRS). We also determined the influence of 2 exercise modalities (ergometer and rubber exercise bands) on the NIRS measurements. Fifteen young, healthy participants (5 female, 10 male) were tested on 2 nonconsecutive occasions within an 8-day period. The NIRS device was placed on the medial gastrocnemius. Venous and arterial occlusions were performed to obtain blood flow and oxygen consumption. A series of repeated arterial occlusions was used to measure the recovery kinetics of muscle oxygen consumption after ∼7–10 s of voluntary plantar flexion exercise. Resting blood flow had mean coefficients of variation (CV) of 42% and 38% for bands and ergometer, respectively, and resting metabolism had mean CVs of 17% and 12% for bands and ergometer, respectively. The recovery time constant of oxygen consumption (day 1 bands and ergometer: 23.2 ± 3.7 s, 27.6 ± 6.5 s, respectively; day 2 bands and ergometer: 25.5 ± 5.4 s, 25.0 ± 4.9 s, respectively) had mean CVs of 10% and 11% for bands and ergometer, respectively. We conclude that measurements of oxygen consumption and mitochondrial capacity using NIRS can be obtained with good reproducibility.

Oxygenation and hemodynamics do not underlie early muscle fatigue for patients with work-related muscle pain

Patients suffering from work-related muscle pain (WRMP) fatigue earlier during exercise than healthy controls. Inadequate oxygen consumption and/or inadequate blood supply can influence the ability of the muscles to withstand fatigue. However, it remains unknown if oxygenation and hemodynamics are associated with early fatigue in muscles of WRMP patients. In the present study we applied near-infrared spectroscopy (NIRS) on the extensor carpi radialis (ECR) and trapezius (TD) muscles of patients with WRMP (n = 18) and healthy controls (n = 17). Our objective was to determine if there were group differences in endurance times for a low-level contraction of 15% maximal voluntary contraction (MVC) – sustained for 12–13 min, and to see if these differences were associated with differences in muscle oxygenation and hemodynamics. At baseline, oxygen saturation (StO₂%) was similar between groups for the ECR, but StO₂% was significantly lower for TD for the WRMP patients (76%) compared to controls (85%) (P<0.01). Also, baseline ECR blood flow was similar in the two groups. For both muscles there were a larger number of patients, compared to controls, that did not maintain the 15% MVC for the allotted time. Consequently, the endurance times were significantly shorter for the WRMP patients than controls (medians, ECR: 347 s vs. 582 s; TD: 430 s vs. 723 s respectively). Responses in StO₂% during the contractions were not significantly different between groups for either muscle, i.e. no apparent difference in oxygen consumption. Overall, we interpret our findings to indicate that the early fatigue for our WRMP patients was not associated with muscle oxygenation and hemodynamics.

Optical absorption and scattering properties of bulk porcine muscle phantoms from interstitial radiance measurements in 650-900 nm range

We demonstrated the application of relative radiance-based continuous wave (cw) measurements for recovering absorption and scattering properties (the effective attenuation coefficient, the diffusion coefficient, the absorption coefficient and the reduced scattering coefficient) of bulk porcine muscle phantoms in the 650-900 nm spectral range. Both the side-firing fiber (the detector) and the fiber with a spherical diffuser at the end (the source) were inserted interstitially at predetermined locations in the phantom. The porcine phantoms were prostate-shaped with ∼4 cm in diameter and ∼3 cm thickness and made from porcine loin or tenderloin muscles. The described method was previously validated using the diffusion approximation on simulated and experimental radiance data obtained for homogenous Intralipid-1% liquid phantom. The approach required performing measurements in two locations in the tissue with different distances to the source. Measurements were performed on 21 porcine phantoms. Spectral dependences of the effective attenuation and absorption coefficients for the loin phantom deviated from corresponding dependences for the tenderloin phantom for wavelengths <750 nm. The diffusion constant and the reduced scattering coefficient were very close for both phantom types. To quantify chromophore presence, the plot for the absorption coefficient was matched with a synthetic absorption spectrum constructed from deoxyhemoglobin, oxyhemoglobin and water. The closest match for the porcine loin spectrum was obtained with the following concentrations: 15.5 µM (±30% s.d.) Hb, 21 µM (±30% s.d.) HbO2 and 0.3 (±30% s.d.) fractional volume of water. The tenderloin absorption spectrum was best described by 30 µM Hb (±30% s.d), 19 µM (±30% s.d.) HbO2 and 0.3 (±30% s.d.) fractional volume of water. The higher concentration of Hb in tenderloin was consistent with a dark-red appearance of the tenderloin phantom. The method can be applied to a number of biological tissues and organs for interstitial optical interrogation.

Early diagnosis of orthopedic implant failure using macromolecular imaging agents

PURPOSE

To develop and evaluate diagnostic tools for early detection of wear particle-induced orthopaedic implant loosening.

METHODS

N-(2-Hydroxypropyl)methacrylamide (HPMA) copolymer was tagged with a near infrared dye and used to detect the inflammation induced by polymethylmethacrylate (PMMA) particles in a murine peri-implant osteolysis model. It was established by inserting an implant into the distal femur and challenging with routine PMMA particles infusion. The osteolysis was evaluated by micro-CT and histological analysis at different time points.

RESULTS

Significant peri-implant osteolysis was found 3-month post PMMA particle challenge by micro-CT and histological analysis. At 1-month post challenge, when there was no significant peri-implant bone loss, the HPMA copolymer-near infrared dye conjugate was found to specifically target the femur with PMMA particles deposition, but not the contralateral control femur with phosphate buffered saline (PBS) infusion.

CONCLUSION

The results from this study demonstrate the feasibility of utilizing the macromolecular diagnostic agent to detect particle-induced peri-implant inflammation prior to the development of detectable osteolysis. Recognition of this early pathological event would provide the window of opportunity for prevention of peri-implant osteolysis and subsequent orthopaedic implant failure.

Determination of maximal lactate steady state in healthy adults: can NIRS help?

PURPOSE

We tested the hypothesis that the maximal lactate steady state (MLSS) can be accurately determined in healthy subjects based on measures of deoxygenated hemoglobin (deoxyHb), an index of oxygen extraction measured noninvasively by near-infrared spectroscopy (NIRS).

METHODS

Thirty-two healthy men (mean ± SD age = 48 ± 17 yr, range = 23-74 yr) performed an incremental cycling test to exhaustion and square wave tests for MLSS determination. Cardiorespiratory variables were measured bbb and deoxyHb was monitored noninvasively on the right vastus lateralis with a quantitative NIRS device. The individual values of V˙O2 and HR corresponding to the MLSS were calculated and compared to the NIRS-derived MLSS (NIRSMLSS) that was, in turn, determined by double linear function fitting of deoxyHb during the incremental exercise.

RESULTS

V˙O2 and HR at MLSS were 2.25 ± 0.54 L·min (76% ± 9% V˙O2max) and 133 ± 14 bpm (81% ± 7% HRmax), respectively. Muscle O2 extraction increased as a function of exercise intensity up to a deflection point, NIRSMLSS, at which V˙O2 and HR were 2.23 ± 0.59 L·min (76% ± 9% V˙O2max) and 136 ± 17 bpm (82% ± 8% HRmax), respectively. For both V˙O2 and HR, the difference of NIRSMLSS from MLSS values was not significant and the measures were highly correlated (r = 0.81 and r = 0.76). The Bland-Altman analysis confirmed a nonsignificant bias for V˙O2 and HR (-0.015 L·min and 3 bpm, respectively) and a small imprecision of 0.26 L·min and 8 bpm.

CONCLUSIONS

A plateau in muscle O2 extraction was demonstrated in coincidence with MLSS during an incremental cycling exercise, confirming the hypothesis that this functional parameter can be accurately estimated with a quantitative NIRS device. The main advantages of NIRSMLSS over lactate-based techniques are the noninvasiveness and the time/cost efficiency.

Comparison of oscillations of skin blood flow and deoxygenation in vastus lateralis in light exercise

The purpose of the present study was to compare oscillation of skin blood flow with that of deoxygenation in muscle during light exercise in order to determine the physiological significance of oscillations in deoxygenation. Prolonged exercise with 50% of peak oxygen uptake was performed for 60 min. Skin blood flow (SBF) was measured using a laser blood flow meter on the right vastus lateralis muscle. Deoxygenated haemoglobin/myoglobin (DHb/Mb) concentration in the left vastus lateralis were measured using a near-infrared spectroscopy system. SBF and DHb/Mb during exercise were analysed by fast Fourier transform. We classified frequency bands according to previous studies (Kvernmo et al. 1999, Kvandal et al. 2006) into phase I (0.005-0.0095 and 0.0095-0.02 Hz), phase II (0.02-0.06 Hz: phase II) and phase III (0.06-0.16 Hz). The first peak of power spectra density (PSD) in SBF appeared at 0.0078 Hz in phase I. The second peak of PSD in SBF appeared at 0.035 Hz. The third peak of PSD in SBF appeared at 0.078 Hz. The first peak of PSD in DHb/Mb appeared at 0.0039 Hz, which was out of phase I. The second peak of PSD in DHb/Mb appeared at 0.016 Hz. The third peak of PSD in DHb/Mb appeared at 0.035 Hz. The coefficient of cross correlation was very low. Cross power spectra density showed peaks of 0.0039, 0.016 and 0.035 Hz. It is concluded that a peak of 0.016 Hz in oscillations of DHb/Mb observed in muscle during exercise is associated with endothelium-dependent vasodilation (phase I) and that a peak of 0.035 Hz in DHb/Mb is associated with sympathetic nerve activity (phase II). It is also confirmed that each peak of SBF oscillations is observed in each phase.

Exploring the vascular smooth muscle receptor landscape in vivo: ultrasound Doppler versus near-infrared spectroscopy assessments

Ultrasound Doppler and near-infrared spectroscopy (NIRS) are routinely used for noninvasive monitoring of peripheral hemodynamics in both clinical and experimental settings. However, the comparative ability of these methodologies to detect changes in microvascular and whole limb hemodynamics during pharmacological manipulation of vascular smooth muscle receptors located at varied locations within the arterial tree is unknown. Thus, in 10 healthy subjects (25 ± 2 yr), changes in resting leg blood flow (ultrasound Doppler; femoral artery) and muscle oxygenation (oxyhemoglobin + oxymyoglobin; vastus lateralis) were simultaneously evaluated in response to intra-arterial infusions of phenylephrine (PE, 0.025-0.8 μg·kg(-1)·min(-1)), BHT-933 (2.5-40 μg·kg(-1)·min(-1)), and angiotensin II (ANG II, 0.5-8 ng·kg(-1)·min(-1)). All drugs elicited significant dose-dependent reductions in leg blood flow and oxyhemoglobin + oxymyoglobin. Significant relationships were found between ultrasound Doppler and NIRS changes across doses of PE (r(2) = 0.37 ± 0.08), BHT-933 (r(2) = 0.74 ± 0.06), and ANG II (r(2) = 0.68 ± 0.13), with the strongest relationships evident with agonists for receptors located preferentially "downstream" in the leg microcirculation (BHT-933 and ANG II). Analyses of drug potency revealed similar EC50 between ultrasound Doppler and NIRS measurements for PE (0.06 ± 0.02 vs. 0.10 ± 0.01), BHT-933 (5.0 ± 0.9 vs. 4.5 ± 1.3), and ANG II (1.4 ± 0.8 vs. 1.3 ± 0.3). These data provide evidence that both ultrasound Doppler and NIRS track pharmacologically induced changes in peripheral hemodynamics and are equally capable of determining drug potency. However, considerable disparity was observed between agonist infusions targeting different levels of the arterial tree, suggesting that receptor landscape is an important consideration for proper interpretation of hemodynamic monitoring with these methodologies.

Influence of cutaneous and muscular circulation on spatially resolved versus standard Beer-Lambert near-infrared spectroscopy

The potential interference of cutaneous circulation on muscle blood volume and oxygenation monitoring by near‐infrared spectroscopy (NIRS) remains an important limitation of this technique. Spatially resolved spectroscopy (SRS) was reported to minimize the contribution of superficial tissue layers in cerebral monitoring but this characteristic has never been documented in muscle tissue monitoring. This study aims to compare SRS with the standard Beer–Lambert (BL) technique in detecting blood volume changes selectively induced in muscle and skin. In 16 healthy subjects, the biceps brachii was investigated during isometric elbow flexion at 70% of the maximum voluntary contractions lasting 10 sec, performed before and after exposure of the upper arm to warm air flow. From probes applied over the muscle belly the following variables were recorded: total hemoglobin index (THI, SRS‐based), total hemoglobin concentration (tHb, BL‐based), tissue oxygenation index (TOI, SRS‐based), and skin blood flow (SBF), using laser Doppler flowmetry. Blood volume indices exhibited similar changes during muscle contraction but only tHb significantly increased during warming (+5.2 ± 0.7 μmol/L·cm, an effect comparable to the increase occurring in postcontraction hyperemia), accompanying a 10‐fold increase in SBF. Contraction‐induced changes in tHb and THI were not substantially affected by warming, although the tHb tracing was shifted upward by (5.2 ± 3.5 μmol/L·cm, P < 0.01). TOI was not affected by cutaneous warming. In conclusion, SRS appears to effectively reject interference by SBF in both muscle blood volume and oxygenation monitoring. Instead, BL‐based parameters should be interpreted with caution, whenever changes in cutaneous perfusion cannot be excluded.

The influence of cutaneous circulation on muscle NIRS monitoring has been seldom investigated and is often overlooked. This study shows that cutaneous dilatation induced by superficial warming produces consistent changes in standard Beer–Lambert (BL) parameters while leaving spatially resolved parameters unaffected. Thus, Beer–Lambert parameters should be interpreted with caution whenever changes in cutaneous perfusion are expected to occur.

Oxygen saturation and electromyographic changes in masseter muscle during experimental chewing of gum with harder texture

OBJECTIVE

The purpose of this study was to clarify the relationship between changes in masseter muscle oxygenation measured by near-infrared spectroscopy (NIRS) and changes in the electromyographic (EMG) power spectrum during experimental chewing of gum with harder texture, to improve the understanding of the use of NIRS in assessing masseter muscle fatigue.

MATERIAL AND METHODS

Ten female volunteers with normal occlusion were examined. Mean age (standard deviation) was 28.4 (3.8) years. Mean fracture stress of gum was 12.5 × 10(4) N/m(2). Subjects were instructed to chew gum for 60 s (75 strokes) on the voluntary chewing side at a pace of 1.25 strokes/s. Simultaneous recordings of NIRS and EMG signals from masseter muscle were performed during gum chewing.

RESULTS

Oxygen saturation levels decreased from the start of chewing, then stabilized with a break point between the two phases. The normalized EMG amplitude increased and the mean frequency of the EMG power spectrum decreased during gum chewing. The timing of break point appearance was related to the timing of a significant decrease in median frequency, but no clear relationships were found between break point appearance and increased EMG amplitude.

CONCLUSIONS

These results suggest that the break point of the oxygen saturation curve, as obtained from NIRS measurements, could be used as an indicator of masseter muscle fatigue as assessed by a shift in the EMG power spectrum to lower frequencies.

Differential effect of sympathetic activation on tissue oxygenation in gastrocnemius and soleus muscles during exercise in humans

NEW FINDINGS

What is the central question of this study? The normal ability of sympathetic nerves to cause vasoconstriction is blunted in exercising skeletal muscle, a phenomenon termed 'functional sympatholysis'. Animal studies suggest that functional sympatholysis appears to occur preferentially in fast-twitch type II glycolytic compared with slow-twitch type I oxidative skeletal muscle. We asked whether these findings can be extended to humans. What is the main finding and its importance? We show that skeletal muscles composed largely of fast-twitch type II fibres may also be more sensitive to functional sympatholysis in humans, particularly at lower exercise intensities. Additionally, independent of muscle fibre type composition, the magnitude of sympatholysis is strongly related to exercise-induced increases in metabolic demand. Animal studies suggest that functional sympatholysis appears to occur preferentially in glycolytic (largely type II) compared with oxidative (largely type I) skeletal muscle. Whether these findings can be extended to humans currently remains unclear. In 12 healthy male subjects, vasoconstrictor responses in gastrocnemius (i.e. primarily type II) and soleus muscles (i.e. primarily type I) were measured using near-infrared spectroscopy to detect decreases in muscle oxygenation (HbO(2)) in response to sympathetic activation evoked by a cold pressor test (CPT). The HbO(2) responses to a CPT at rest were compared with responses during steady-state plantar flexion exercise (30 repetitions min(-1)) performed at 10, 20 and 40% maximal voluntary contraction (MVC) for 6 min. In resting conditions, HbO(2) at the gastrocnemius (-14 ± 1%) and soleus muscles (-16 ± 1%) decreased significantly during CPT, with no differences between muscles. During planter flexion at 20% MVC, the change in HbO(2) in response to the CPT was blunted in gastrocnemius but not soleus, whereas during 40% MVC both muscles exhibited a significant attenuation to sympathetic activation. The decreases in HbO(2) in response to the CPT during exercise were significantly correlated with the metabolic demands of exercise (the decreases in HbO(2) in response to steady-state plantar flexion) in both gastrocnemius and soleus muscles. Collectively, these results suggest that skeletal muscles composed mainly of glycolytic type II fibres are more sensitive to functional sympatholysis, particularly at lower intensities of exercise. Moreover, the blunting of sympathetic vasoconstriction during exercise is strongly related to metabolic demand; an effect that appears independent of fibre type composition.

Beetroot juice supplementation speeds O2 uptake kinetics and improves exercise tolerance during severe-intensity exercise initiated from an elevated metabolic rate

Recent research has suggested that dietary nitrate (NO3(-)) supplementation might alter the physiological responses to exercise via specific effects on type II muscle. Severe-intensity exercise initiated from an elevated metabolic rate would be expected to enhance the proportional activation of higher-order (type II) muscle fibers. The purpose of this study was, therefore, to test the hypothesis that, compared with placebo (PL), NO3(-)-rich beetroot juice (BR) supplementation would speed the phase II VO2 kinetics (τ(p)) and enhance exercise tolerance during severe-intensity exercise initiated from a baseline of moderate-intensity exercise. Nine healthy, physically active subjects were assigned in a randomized, double-blind, crossover design to receive BR (140 ml/day, containing ~8 mmol of NO3(-)) and PL (140 ml/day, containing ~0.003 mmol of NO3(-)) for 6 days. On days 4, 5, and 6 of the supplementation periods, subjects completed a double-step exercise protocol that included transitions from unloaded to moderate-intensity exercise (U→M) followed immediately by moderate to severe-intensity exercise (M→S). Compared with PL, BR elevated resting plasma nitrite concentration (PL: 65 ± 32 vs. BR: 348 ± 170 nM, P < 0.01) and reduced the VO2 τ(p) in M→S (PL: 46 ± 13 vs. BR: 36 ± 10 s, P < 0.05) but not U→M (PL: 25 ± 4 vs. BR: 27 ± 6 s, P > 0.05). During M→S exercise, the faster VO2 kinetics coincided with faster near-infrared spectroscopy-derived muscle [deoxyhemoglobin] kinetics (τ; PL: 20 ± 9 vs. BR: 10 ± 3 s, P < 0.05) and a 22% greater time-to-task failure (PL: 521 ± 158 vs. BR: 635 ± 258 s, P < 0.05). Dietary supplementation with NO3(-)-rich BR juice speeds VO2 kinetics and enhances exercise tolerance during severe-intensity exercise when initiated from an elevated metabolic rate.

Skeletal muscle oxygen saturation (StO2) measured by near-infrared spectroscopy in the critically ill patients

According to current critical care management guidelines, the overall hemodynamic optimization process seeks to restore macrocirculatory oxygenation, pressure, and flow variables. However, there is increasing evidence demonstrating that, despite normalization of these global parameters, microcirculatory and regional perfusion alterations might occur, and persistence of these alterations has been associated with worse prognosis. Such observations have led to great interest in testing new technologies capable of evaluating the microcirculation. Near-infrared spectroscopy (NIRS) measures tissue oxygen saturation (StO₂) and has been proposed as a noninvasive system for monitoring regional circulation. The present review aims to summarize the existing evidence on NIRS and its potential clinical utility in different scenarios of critically ill patients.

Validation of a spectroscopic sensor for the continuous, noninvasive measurement of muscle oxygen saturation and pH

New patient monitoring technologies can noninvasively and directly provide an assessment of the adequacy of tissue perfusion through the simultaneous determination of muscle oxygen saturation (SmO2) and muscle pH (pHm). Non-pulsatile near infrared spectroscopy is used to determine these microvascular parameters. Two separate studies were conducted using an isolated perfused swine limb preparation to widely vary venous blood oxygen saturation (SviO2) and pH (pHvi) to assess the accuracy of a noninvasive sensor with the capability to simultaneously measure both parameters. The isolated limb model is necessary to establish equilibrium between the venous output of the perfusion circuit and the venule measurement of the spectroscopic sensor. The average absolute difference between SmO2 and SviO2 determined over 50 conditions of SviO2 between 13% and 83% on 3 pig limbs was 3.8% and the coefficient of determination (R(2)) was 0.95. The average absolute difference between pHm and pHvi determined over 69 conditions of pHvi between pHvi 6.9 and pHvi 7.5 on 3 pig limbs was 0.045 pH units with an R(2) of 0.92. Measured accuracy was acceptable to support clinically relevant decision making for the assessment of impaired tissue perfusion and acidosis. Sensors were also evaluated on human subjects. There was no statistical difference in SmO2 by gender or location when multiple sensors were evaluated on the right and left calf, deltoid, and thigh of resting men and women (N = 33). SmO2 precision for subjects at rest was 5.6% over the six locations with four different sensors.

Near-infrared spectroscopy of the urinary bladder during voiding in men with lower urinary tract symptoms: a preliminary study

Objectives. To determine the difference in response of NIRS of the bladder during voiding between men with and without BOO.LUTS. Methods. A prospective, case series, study included 36 men with LUTS. Patients completed the IPSS questionnaire; prostate volumes were measured sonographically. Patients underwent pressure flow study (PFS) with simultaneous NIRS of the bladder. Amplitudes of HHb, O₂Hb, and Hb_sum were calculated at Q_max, relative to baseline. Patients were urodynamically classified as obstructed and unobstructed. Recursive partition analysis (RPA) was performed to reclassify patients using NIRS amplitudes, followed by combined data of NIRS amplitudes, prostate volume, IPSS, and Q_max to determine the best predictor(s) of BOO. Results. PFS classified 28 patients as obstructed and 8 as unobstructed. The median HHb amplitude was significantly higher in obstructed group. RPA of NIRS amplitudes correctly reclassified 89% of patients [AUC: 0.91]. RPA of the combined IPSS, prostate volume, PVR, and Q_max correctly reclassified 72% of patients [AUC: 0.84]. When NIRS amplitudes were added to this combination, RPA revealed a significantly (P < 0.01) higher rate of correct reclassification in 89% of patients with 89.3% sensitivity and 88% specificity for obstruction [AUC: 0.96]. Conclusion. NIRS data can be of diagnostic value for BOO in men with LUTS.

Estimating soft tissue thickness from light-tissue interactions--a simulation study

Immobilization and marker-based motion tracking in radiation therapy often cause decreased patient comfort. However, the more comfortable alternative of optical surface tracking is highly inaccurate due to missing point-to-point correspondences between subsequent point clouds as well as elastic deformation of soft tissue. In this study, we present a proof of concept for measuring subcutaneous features with a laser scanner setup focusing on the skin thickness as additional input for high accuracy optical surface tracking. Using Monte-Carlo simulations for multi-layered tissue, we show that informative features can be extracted from the simulated tissue reflection by integrating intensities within concentric ROIs around the laser spot center. Training a regression model with a simulated data set identifies patterns that allow for predicting skin thickness with a root mean square error of down to 18 µm. Different approaches to compensate for varying observation angles were shown to yield errors still below 90 µm. Finally, this initial study provides a very promising proof of concept and encourages research towards a practical prototype.

Effect of acute hyperthyroidism on blood flow, muscle oxygenation, and sympathetic nerve activity during dynamic handgrip

Hyperthyroidism induces marked changes in hemodynamics. Although considerable research has been done to study the effect of hyperthyroidism on the cardiovascular system, few studies have isolated the short-term, nongenomic effects of thyroid hormone on cardiovascular responses to exercise. We used near-infrared spectroscopy to measure muscle oxygenation, Doppler ultrasound to measure skeletal muscle blood flow, and microneurography to measure muscle sympathetic nerve activity (MSNA) during fatiguing dynamic handgrip in twelve healthy males (26 ± 1 years). Subjects were measured separately in both the euthyroid state, and acute hyperthyroid state (approximately ten times the normal levels of T₃), induced by oral dosage of 300 μg of triiodothyronine (T₃). Forearm blood flow was increased as a function of exercise time in the euthyroid and hyperthyroid state (Δ161.8 ± 45.0 mL/min and Δ140.7 ± 16.3 mL/min, respectively) but there was no significant difference between trials. Forearm vascular conductance (FVC) also increased as a function of exercise time with no significant difference between treatments at submaximal exercise but was significantly less with T₃ treatment. MSNA was not different at rest or during submaximal exercise; however, MSNA was significantly greater at fatigue during the hyperthyroid state. Muscle oxyhemoglobin concentration was decreased during exercise in both euthyroid and hyperthyroid states (Δ19.7 ± 10.8% and Δ14.8 ± 9.6%, respectively); whereas deoxyhemoglobin concentration was increased (Δ50.0 ± 4.1% and Δ50.0 ± 6.2%, respectively). These results indicate that T₃ had no direct effect on skeletal muscle oxygenation or blood flow during dynamic exercise, but elicited greater MSNA and lower FVC during fatiguing exercise.

Muscle oxygen consumption by NIRS and mobility in multiple sclerosis patients

BACKGROUND

The study of muscle metabolism by near-infrared spectroscopy (NIRS) has been poorly implemented in multiple sclerosis (MS). Aims of the study were to compare resting muscle oxygen consumption (rmVO2) at gastrocnemius in MS patients and in age-matched healthy controls (HC) measured using NIRS, and to evaluate its possible relationship with patients' mobility.

METHODS

Twenty-eight consecutively enrolled MS patients (male, n = 16; age = 42.7 ± 14.0 y, Relapsing-Remitting, n = 19; Primary-Progressive, n = 9) and 22 HC (male, n = 13; age = 36.0 ± 8.2 y) were studied during rest applying the NIRS probes at gastrocnemius, producing a venous occlusion at the thigh using a cuff, and analyzing the slope of the total hemoglobin to calculate rmVO2. Mobility was assessed by a 6-Minute Walking Test and 6-Minute Walking Distance (6MWD) was recorded.

RESULTS

rmVO2 was higher in MS compared to HC (0.059 ± 0.038 vs 0.039 ± 0.016 mlO2/min/100 g, P < 0.003), not different in clinical subtypes, not correlated to patients' characteristics (age, disease duration, Expanded Disability Status Scale, resting heart rate, skinfold thickness), and significantly higher in patients with lower walking ability (6MWD < 450 m, n = 12) compared to those at better performance (respectively, 0.072 ± 0.043 vs 0.049 ± 0.032 mlO2/min/100 g, P = 0.03).

CONCLUSION

rmVO2 values, significantly higher in MS patients compared to HC, and in low versus high performing patients, might represent a marker of peripheral adaptations occurred to sustain mobility, as observed in other chronic diseases.

Differential effects of nebivolol versus metoprolol on functional sympatholysis in hypertensive humans

In young healthy humans, sympathetic vasoconstriction is markedly blunted during exercise to optimize blood flow to the metabolically active muscle. This phenomenon known as functional sympatholysis is impaired in hypertensive humans and rats by angiotensin II-dependent mechanisms, involving oxidative stress and inactivation of nitric oxide (NO). Nebivolol is a β1-adrenergic receptor blocker that has NO-dependent vasodilatory and antioxidant properties. We therefore asked whether nebivolol would restore functional sympatholysis in hypertensive humans. In 21 subjects with stage 1 hypertension, we measured muscle oxygenation and forearm blood flow responses to reflex increases in sympathetic nerve activity evoked by lower body negative pressure at rest, and during rhythmic handgrip exercise at baseline, after 12 weeks of nebivolol (5-20 mg/d) or metoprolol (100-300 mg/d), using a double-blind crossover design. We found that nebivolol had no effect on lower body negative pressure-induced decreases in oxygenation and forearm blood flow in resting forearm (from -29±5% to -30±5% and from -29±3% to -29±3%, respectively; P=NS). However, nebivolol attenuated the lower body negative pressure-induced reduction in oxygenation and forearm blood flow in exercising forearm (from -14±4% to -1±5% and from -15±2% to -6±2%, respectively; both P<0.05). This effect of nebivolol on oxygenation and forearm blood flow in exercising forearm was not observed with metoprolol in the same subjects, despite a similar reduction in blood pressure. Nebivolol had no effect on sympathetic nerve activity at rest or during handgrip, suggesting a direct effect on vascular function. Thus, our data demonstrate that nebivolol restored functional sympatholysis in hypertensive humans by a mechanism that does not involve β1-adrenergic receptors. Clinical Trial Registration- URL: http://www.clinicaltrials.gov. Unique identifier: NCT01502787.

Near infrared spectroscopy: clinical and research uses

BACKGROUND

Pulse oximetry is routinely used to measure hemoglobin saturation and is currently the gold standard to assess oxygenation in patients. Due to attenuation of infrared light by skin, bone, and other organs, pulse oximetry cannot assess end-organ tissue oxygenation (StO(2)). Near infrared spectroscopy (NIS) penetrates a broad range of tissues and utilizes reflection rather than direct transmission between an emitter and receiver pair. NIS is able to measure StO(2) and assess end-organ perfusion in a variety of applications.

STUDY DESIGN AND METHODS

A retrospective review of recent animal and human StO(2) studies was undertaken. StO(2) measurements and outcomes were assessed.

RESULTS

StO(2) measurements identified visceral organ ischemia in animal hemorrhage models. These measurements were also able to guide optimization of resuscitation and end-organ oxygenation. Human studies demonstrated StO(2) changes preceded those seen in traditionally measured parameters such as blood pressure, heart rate, base deficit, serum lactate, and mental status. Additionally, StO(2) thresholds identified trauma patients who required massive transfusions, developed multiple organ dysfunction syndrome, or experienced lower extremity compartment syndrome. StO(2) measurements also demonstrated a benefit in selecting resuscitation fluids, assessing end-organ oxygenation during blood transfusion, and quantifying the oxygen-carrying deficit secondary to the blood storage lesion.

CONCLUSION

StO(2) measurements have been used to guide resuscitation efforts in trauma patients. This technology and its applications continue to evolve and represent a novel change in patient care.

Textile integrated sensors and actuators for near-infrared spectroscopy

Being the closest layer to our body, textiles provide an ideal platform for integrating sensors and actuators to monitor physiological signals. We used a woven textile to integrate photodiodes and light emitting diodes. LEDs and photodiodes enable near-infrared spectroscopy (NIRS) systems to monitor arterial oxygen saturation and oxygenated and deoxygenated hemoglobin in human tissue. Photodiodes and LEDs are mounted on flexible plastic strips with widths of 4 mm and 2 mm, respectively. The strips are woven during the textile fabrication process in weft direction and interconnected with copper wires with a diameter of 71 μm in warp direction. The sensor textile is applied to measure the pulse waves in the fingertip and the changes in oxygenated and deoxygenated hemoglobin during a venous occlusion at the calf. The system has a signal-to-noise ratio of more than 70 dB and a system drift of 0.37% ± 0.48%. The presented work demonstrates the feasibility of integrating photodiodes and LEDs into woven textiles, a step towards wearable health monitoring devices.

Inspiratory loading and limb locomotor and respiratory muscle deoxygenation during cycling exercise

The aim of this study was to determine the effect of inspiratory loading on limb locomotor (LM) and respiratory muscle (RM) deoxygenation ([deoxy (Hb+Mb)]) using NIRS during constant-power cycling exercise. Sixteen, male cyclists completed three, 6-min trials. The intensity of the first 3-min of each trial was equivalent to ~80% V(O(2max)) (EX(80%)); during the final 3-min, subjects received an intervention consisting of either moderate inspiratory loading (Load(mod)), heavy inspiratory loading (Load(heavy)), or maximal exercise (Load(EX)). Load(heavy) significantly increased LM [deoxy(Hb+Mb)] from 12.2±9.0 μm during EX(80%) to 15.3±11.7 μm, and RM [deoxy(Hb+Mb)] from 5.9±3.6 μm to 9.5±6.6 μm. LM and RM [deoxy(Hb+Mb)] were significantly increased from EX(80%) to Load(EX); 12.8±9.1 μm to 16.4±10.3 μm and 5.9±2.9 μm to 11.0±6.4 μm, respectively. These data suggest an increase in respiratory muscle load increases muscle deoxy(Hb+Mb) and thus may indicate a reduction in oxygen delivery and/or increased oxygen extraction by the active muscles.

What's new in acute compartment syndrome?

Acute compartment syndrome (ACS) after trauma is often the result of increased size of the damaged tissues after acute crush injury or from reperfusion of ischemic areas. It usually is not solely caused by accumulation of free blood or fluid in the compartment, although that can contribute in some cases. There is no reliable and reproducible test that confirms the diagnosis of ACS. A missed diagnosis or failure to cut the fascia to release pressure within a few hours can result in severe intractable pain, paralysis, and sensory deficits. Reduced blood circulation leads to oxygen and nutrient deprivation, muscle necrosis, and permanent disability. Currently, the diagnosis of ACS is made on the basis of physical examination and repeated needle sticks over a short time frame to measure intracompartmental pressures. Missed compartment syndromes continue to be one of most common causes of malpractice lawsuits. Existing technology for continuous pressure measurements are insensitive, particularly in the deep tissues and compartments, and their use is restricted to highly trained personnel. Newer concepts of the pathophysiology accompanied by new diagnostic and therapeutic modalities have recently been advanced. Among these are the concept of inflammatory mediators as markers and anti-inflammatories as medical adjunct therapy. New diagnostic modalities include near-infrared spectroscopy, ultrafiltration catheters, and radio-frequency identification implants. These all address current shortcomings in the diagnostic armamentarium that trauma surgeons can use. The strengths and weaknesses of these new concepts are discussed to allow the trauma surgeon to follow current evolution of the field.

Patients with sickle cell anemia on simple chronic transfusion protocol show sex differences for hemodynamic and hematologic responses to transfusion

BACKGROUND

Chronic transfusion therapy (CTT) is a mainstay for stroke prophylaxis in sickle cell anemia, but its effects on hemodynamics are poorly characterized. Transfusion improves oxygen-carrying capacity, reducing demands for high cardiac output, while decreasing hemoglobin (Hb)S%, reticulocyte count, and hemolysis. We hypothesized that transfusion would improve oxygen-carrying capacity, but that would be counteracted by a decrease in cardiac output due to increased hematocrit (Hct) and vascular resistance, leaving oxygen delivery unchanged.

STUDY DESIGN AND METHODS

To test this hypothesis, we examined patients on CTT immediately before transfusion and again 12 to 120 hours after transfusion, using echocardiography and near infrared spectroscopy.

RESULTS

Comparable increases in Hb and Hct and decreases in reticulocyte count and HbS with transfusion were observed in all patients, but males had a larger rebound of HbS%, reticulocyte count, and free Hb levels between transfusions. In males, transfusion decreased heart rate by 12%, stroke volume by 15%, and cardiac index by 24% while estimates for pulmonary and systemic vascular resistance increased, culminating in 6% decrease in oxygen delivery. In contrast, stroke volume and cardiac index and systemic and pulmonary vascular resistance did not change in women after transfusion, such that oxygen delivery improved 17%.

CONCLUSION

In our sample population, males exhibit a paradoxical reduction in oxygen delivery in response to transfusion because the increase in vascular resistance is larger than the increase in oxygen capacity. This may result from an inability to adequately suppress their HbS% between transfusion cycles.

Impaired skeletal muscle microvascular function and increased skeletal muscle oxygen consumption in severe falciparum malaria

BACKGROUND

Organ dysfunction and tissue hypoxia in severe falciparum malaria result from an imbalance between oxygen delivery and demand. In severe malaria, microvascular obstruction from parasite sequestration decreases oxygen delivery. However, host microvascular function (defined as the capacity to increase oxygen delivery in response to ischemia) and oxygen consumption have not been assessed.

METHODS

We used near-infrared resonance spectroscopy to measure thenar muscle microvascular function (StO(2)recov) and oxygen consumption (VO(2)) in 36 adults in Papua, Indonesia, with severe malaria, 33 with moderately severe malaria (MSM), 24 with severe sepsis, and 36 healthy controls.

RESULTS

In the severe malaria group, the StO(2)recov of 2.7%/second was 16% and 22% lower than that in the MSM group (3.1%/second) and control group (3.5%/second), respectively (P < .001), and comparable to that in the severe sepsis group (2.5%/second). In the severe malaria group, StO(2)recov was inversely correlated with lactate level (r = -0.63; P < .001) and predicted death (area under the receiver operating characteristic curve, 0.71 [95% confidence interval {CI}, .51-.92]), with each percentage decrease associated with an increased odds of mortality (odds ratio, 2.49 [95% CI, 1.05-6.2]). Conversely, VO(2) increased in the severe malaria group by 18%, compared with levels in the control and severe sepsis groups (P < .001), and was associated with parasite biomass (r = 0.49; P = .04).

CONCLUSIONS

Impaired microvascular function is associated with increased mortality among individuals with severe malaria, while oxygen consumption is increased. Tissue hypoxia may result not only from microvascular obstruction, but also from impaired ability of the microvasculature to match oxygen delivery to increased oxygen demand.

Shoulder and forearm oxygenation and myoelectric activity in patients with work-related muscle pain and healthy subjects

We tested hypotheses of (a) reduced oxygen usage, oxygen recovery, blood flow and oxygen consumption; and (b) increased muscle activity for patients diagnosed with work-related muscle pain (WRMP) in comparison to healthy controls. Oxygenation was measured with near infrared spectroscopy (NIRS), and muscle activity with EMG for the extensor carpi radialis (ECR) and trapezius descendens (TD) muscles. Eighteen patients with diffuse neck-shoulder-arm pain and 17 controls (matched in age and sex) were equipped with NIRS and EMG probes. After determining an individual's maximum voluntary contraction (MVC) force, short-term (20 s) isometric contractions for the ECR and TD of 10, 30, 50 and 70 % MVC generated ∆StO₂ and StO₂% recovery (Rslope) from NIRS, and RMS%max from EMG signals. In addition, upper arm venous (VO) and arterial (AO) occlusions generated slopes of total hemoglobin (HbTslope) and deoxyhemoglobin (HHbslope) for the resting ECR as surrogates of blood flow and oxygen consumption, respectively. Mixed model analyses, t tests, and Mann-Whitney test were used to assess differences between groups. There was no significant difference in MVC between groups for either muscle. Also, ∆StO₂%, Rslope for either muscle, and ECR-HbTslope were not different between groups, thus our hypotheses of reduced oxygen use, recovery, and blood flow for patients were not confirmed. However, patients had a significantly lower ECR-HHbslope confirming our hypothesis of reduced consumption. Further, there was no difference in RMS%max during contractions meaning that the hypothesis of increased activity for patients was not confirmed. When taking into account the number of NIRS variables studied, differences we found between our patient group and healthy controls (i.e., in forearm oxygen consumption and shoulder oxygen saturation level) may be considered modest. Overall our findings may have been impacted by the fact that our patients and controls were similar in muscle strength, which is in contrast to previous studies.

Comparison of two different generations of "NIRS" devices and transducers in healthy volunteers and ICU patients

The purpose of this study is to compare Near Infrared Spectroscopy (NIRS) thenar eminence parameters obtained with 2 different devices from the same manufacturer (InSpectra Models 325 and 650, Hutchinson Tech, Min USA), and 2 different probes (15 vs. 25 mm spacing), in healthy volunteers (HV) and ICU patients. Prospective, observational study in ICU setting. Simultaneous, cross over NIRS inter-device comparison and comparison between different probes (25 vs. 15 mm spacing) were done at baseline and during vascular occlusion tests (VOTs). Forty patients (19 septic shock, 21 trauma), and 29 HV were included. NIRS inter-device comparison showed similar baseline StO(2) values in HV and patients. The VOT result were significantly different for minimal StO(2) value reached during VOT (StO(2min)) (intraclass concordance coefficient (ICC) = 0.18), the occlusion slope (ICC = 0.16) and the reperfusion slope (StO(2reperf)) (ICC = 0.26). The probe comparison was also significantly different for VOT parameters (StO(2min) (ICC = 0.43), occlusion (ICC = 0.50) and StO(2reperf) (ICC = 0.48). The low concordance, poor agreement and large bias (ICC and Bland & Altman) observed, were related both to the device used and the probe spacing. StO(2) data obtained with NIRS model 650 and 15 mm probe differ from values obtained with the previous device (325 and probe spacing 25 or 15 mm). This difference is not related to the population tested, but to the device and probe spacing. As a consequence, despite similar trends for variations between HV and patients during VOT, threshold and predictive values for outcome should be revisited with the new device before the acceptance for routine clinical use.

Acute compartment syndrome: obtaining diagnosis, providing treatment, and minimizing medicolegal risk

Acute compartment syndrome (ACS) is a surgical emergency. Diagnosis depends on a high clinical suspicion and an understanding of risk factors, pathophysiology and subtle physical exam findings. The typical high risk scenario for ACS is a male patient younger than 35 years of age, involved in a high energy sport or roadway collision, resulting in a tibial shaft fracture. He will go on to develop acute compartment syndrome of the leg in less than 10 hours and require emergent fasciotomy. Diagnosis of ACS in this patient is primarily a clinical one but can be confirmed with invasive intracompartmental pressure monitoring or non-invasive near infrared spectroscopy (NIRS). Delaying the diagnosis will likely result in some degree of permanent disability and places the surgeon at high risk for litigation. This article reviews the salient features of acute compartment syndrome that should be understood by all orthopaedic residents and surgeons.