Non-invasive and quantitative near-infrared haemoglobin spectrometry in the piglet brain during hypoxic stress, using a frequency-domain multidistance instrument

Non-invasive diffuse optical monitoring of cerebral physiology in an adult swine-model of impact traumatic brain injury

In this study, we used diffuse optics to address the need for non-invasive, continuous monitoring of cerebral physiology following traumatic brain injury (TBI). We combined frequency-domain and broadband diffuse optical spectroscopy with diffuse correlation spectroscopy to monitor cerebral oxygen metabolism, cerebral blood volume, and cerebral water content in an established adult swine-model of impact TBI. Cerebral physiology was monitored before and after TBI (up to 14 days post injury). Overall, our results suggest that non-invasive optical monitoring can assess cerebral physiologic impairments post-TBI, including an initial reduction in oxygen metabolism, development of cerebral hemorrhage/hematoma, and brain swelling.

Epinephrine iontophoresis attenuates changes in skin blood flow and abolishes cutaneous contamination of near-infrared diffuse correlation spectroscopy estimations of muscle perfusion

Near-infrared diffuse correlation spectroscopy (NIR-DCS) is an optical imaging technique for measuring relative changes in skeletal muscle microvascular perfusion (i.e., fold change above baseline) during reactive hyperemia testing and exercise and is reported as a blood flow index (BFI). Although it is generally accepted that changes in BFI are primarily driven by changes in muscle perfusion, it is well known that large, hyperthermia-induced changes in cutaneous blood flow can uncouple this relationship. What remains unknown, is how much of an impact that changes in cutaneous perfusion have on NIR-DCS BFI and estimates of skeletal muscle perfusion under thermoneutral conditions, where changes in cutaneous blood flow are assumed to be relatively low. We therefore used epinephrine iontophoresis to pharmacologically block changes in cutaneous perfusion throughout a battery of experimental procedures. The data show that 1) epinephrine iontophoresis attenuates changes in cutaneous perfusion for up to 4-h posttreatment, even in the face of significant neural and local stimuli, 2) under thermoneutral conditions, cutaneous perfusion does not significantly impact NIR-DCS BFI during reactive hyperemia testing or moderate-intensity exercise, and 3) during passive whole body heat stress, when cutaneous vasodilation is pronounced, epinephrine iontophoresis preserves NIR-DCS measures of skeletal muscle BFI during moderate-intensity exercise. Collectively, these data suggest that cutaneous perfusion is unlikely to have a major impact on NIR-DCS estimates of skeletal muscle BFI under thermoneutral conditions, but that epinephrine iontophoresis can be used to abolish cutaneous contamination of the NIR-DCS BFI signal during studies where skin blood flow may be elevated but skeletal muscle perfusion is of specific interest.

Neurophysiology tools to lower the stroke onset to treatment time during the golden hour: microwaves, bioelectrical impedance and near infrared spectroscopy

Reperfusion therapy administration timing in acute ischaemic stroke is the main determinant of patients' mortality and long-term disability. Indeed, the first hour from the stroke onset is defined the "golden hour", in which the treatment has the highest efficacy and lowest side effects. Delayed ambulance transport, inappropriate triage and difficulty in accessing CT scans lead to delayed onset to treatment time (OTT) in clinical practice. To date brain CT scan is needed to rule out intracranial haemorrhage, which is a major contraindication to thrombolytic therapy. The availability, dimension and portability make CT suitable mainly for intrahospital use, determining further delays in the therapies administration. This review aims at evaluating portable neurophysiology technologies developed with the scope of speeding up the diagnostic phase of acute stroke and, therefore, the initiation of intravenous thrombolysis. Medline databases were explored for studies concerning near infrared spectroscopy (NIRS), bioelectrical impedance spectroscopy (BIS) and Microwave imaging (MWI) as methods for stroke diagnosis. A total of 1368 articles were found, and 12 of these fit with our criteria and were included in the review. For each technology, the following parameters were evaluated: diagnostic accuracy, ability to differentiate ischaemic and haemorrhagic stroke, diagnosis time from stroke onset, portability and technology readiness level (TRL). All the described methods seem to be able to identify acute stroke even though the number of studies is very limited. Low cost and portability make them potentially usable during ambulance transport, possibly leading to a reduction of stroke OTT along with the related huge benefits in terms of patients outcome and health care costs. In addition, unlike standard imaging techniques, neurophysiological techniques could allow continuous monitoring of patients for timely intrahospital stroke diagnosis.KEY MESSAGESFirst hour from the stroke onset is defined the "golden hour", in which the treatment has the highest efficacy and lowest side effects.The delay for stroke onset to brain imaging time is one of the major reasons why only a minority of patients with acute ischaemic stroke are eligible to reperfusion therapies.Neurophysiology techniques (NIRS, BIS and MWI) could have a potential high impact in reducing the time to treatment in stroke patients.

Correlation of Cerebral Microdialysis with Non-Invasive Diffuse Optical Cerebral Hemodynamic Monitoring during Deep Hypothermic Cardiopulmonary Bypass

Neonates undergoing cardiac surgery involving aortic arch reconstruction are at an increased risk for hypoxic-ischemic brain injury. Deep hypothermia is utilized to help mitigate this risk when periods of circulatory arrest are needed for surgical repair. Here, we investigate correlations between non-invasive optical neuromonitoring of cerebral hemodynamics, which has recently shown promise for the prediction of postoperative white matter injury in this patient population, and invasive cerebral microdialysis biomarkers. We compared cerebral tissue oxygen saturation (StO₂), relative total hemoglobin concentration (rTHC), and relative cerebral blood flow (rCBF) measured by optics against the microdialysis biomarkers of metabolic stress and injury (lactate–pyruvate ratio (LPR) and glycerol) in neonatal swine models of deep hypothermic cardiopulmonary bypass (DHCPB), selective antegrade cerebral perfusion (SACP), and deep hypothermic circulatory arrest (DHCA). All three optical parameters were negatively correlated with LPR and glycerol in DHCA animals. Elevation of LPR was found to precede the elevation of glycerol by 30–60 min. From these data, thresholds for the detection of hypoxic-ischemia-associated cerebral metabolic distress and neurological injury are suggested. In total, this work provides insight into the timing and mechanisms of neurological injury following hypoxic-ischemia and reports a quantitative relationship between hypoxic-ischemia severity and neurological injury that may inform DHCA management.

Non-Invasive Spectroscopy for Measuring Cerebral Tissue Oxygenation and Metabolism as a Function of Cerebral Perfusion Pressure

Near-infrared spectroscopy (NIRS) and diffuse correlation spectroscopy (DCS) measure cerebral hemodynamics, which in turn can be used to assess the cerebral metabolic rate of oxygen (CMRO₂) and cerebral autoregulation (CA). However, current mathematical models for CMRO₂ estimation make assumptions that break down for cerebral perfusion pressure (CPP)-induced changes in CA. Here, we performed preclinical experiments with controlled changes in CPP while simultaneously measuring NIRS and DCS at rest. We observed changes in arterial oxygen saturation (~10%) and arterial blood volume (~50%) with CPP, two variables often assumed to be constant in CMRO₂ estimations. Hence, we propose a general mathematical model that accounts for these variations when estimating CMRO₂ and validate its use for CA monitoring on our experimental data. We observed significant changes in the various oxygenation parameters, including the coupling ratio (CMRO₂/blood flow) between regions of autoregulation and dysregulation. Our work provides an appropriate model and preliminary experimental evidence for the use of NIRS- and DCS-based tissue oxygenation and metabolism metrics for non-invasive diagnosis of CA health in CPP-altering neuropathologies.

The Role of Systemic Physiology in Individual Hemodynamic Responses Measured on the Head Due to Long-Term Stimulation Involving Colored Light Exposure and a Cognitive Task: An SPA-fNIRS Study

In our previous investigations using systemic physiology augmented functional near-infrared spectroscopy (SPA-fNIRS) neuroimaging, we found larger variability between subjects in changes of cerebral hemodynamics and oxygenation induced by an intricate experimental paradigm involving colored light exposure and a cognitive task. We aimed to investigate the role the activity of the systemic physiology has on individual variations in the fNIRS data. Thirty-two healthy subjects (17 female, 15 male and age: 25.5 ± 4.3 years) were exposed to blue and red light for 9 min (colored light exposure, CLE) while performing a verbal fluency task (VFT). We found that (i), at the group level, the visual cortex showed a stronger deoxyhemoglobin concentration response during blue light exposure than during red light exposure, and (ii) this relationship was influenced by individually different baseline blood pressure values. Furthermore, we found other correlations between changes in fNIRS signals and changes in systemic physiology. Our study demonstrates the usefulness and necessity of the SPA-fNIRS approach to gain insights into the individual variability of hemodynamic responses measured with fNIRS, especially in the case of an intricate experimental paradigm (i.e., CLE-VFT) as used in our study.

Association of Ongoing Cerebral Oxygen Extraction During Deep Hypothermic Circulatory Arrest With Postoperative Brain Injury

Cardiac surgery utilizing circulatory arrest is most commonly performed under deep hypothermia (∼18°C) to suppress tissue oxygen demand and provide neuroprotection during operative circulatory arrest. Studies investigating the effects of deep hypothermic circulatory arrest (DHCA) on neurodevelopmental outcomes of patients with congenital heart disease give conflicting results. Here, we address these issues by quantifying changes in cerebral oxygen saturation, blood flow, and oxygen metabolism in neonates during DHCA and investigating the association of these changes with postoperative brain injury. Neonates with critical congenital heart disease undergoing DHCA were recruited for continuous intraoperative monitoring of cerebral oxygen saturation (ScO₂) and an index of cerebral blood flow (CBF_i) using 2 noninvasive optical techniques, diffuse optical spectroscopy (DOS) and diffuse correlation spectroscopy (DCS). Pre- and postoperative brain magnetic resonance imaging (MRI) was performed to detect white matter injury (WMI). Fifteen neonates were studied, and 11/15 underwent brain MRI. During DHCA, ScO₂ decreased exponentially in time with a median decay rate of -0.04 min^-1. This decay rate was highly variable between subjects. Subjects who had larger decreases in ScO₂ during DHCA were more likely to have postoperative WMI (P = 0.02). Cerebral oxygen extraction persists during DHCA and varies widely from patient-to-patient. Patients with a higher degree of oxygen extraction during DHCA were more likely to show new WMI in postoperative MRI. These findings suggest cerebral oxygen extraction should be monitored during DHCA to identify patients at risk for hypoxic-ischemic injury, and that current commercial cerebral oximeters may underestimate cerebral oxygen extraction.

Predictors of changes in cerebral perfusion and oxygenation during obstructive sleep apnea

Obstructive sleep apnea syndrome (OSAS) is a common sleep disorder. Severe OSAS defined as apnea-hypopnea index (AHI) ≥ 30/h is a risk factor for developing cerebro-cardiovascular diseases. The mechanisms of how repetitive sleep apneas/hypopneas damage cerebral hemodynamics are still not well understood. In this study, changes in blood volume (BV) and oxygen saturation (StO2) in the left forehead of 29 newly diagnosed severe OSAS patients were measured by frequency-domain near-infrared spectroscopy during an incremental continuous positive airway pressure (CPAP) titration protocol together with polysomnography. The coefficients of variation of BV (CV-BV) and the decreases of StO2 (de-StO2) of more than 2000 respiratory events were predicted using linear mixed-effect models, respectively. We found that longer events and apneas rather than hypopneas induce larger changes in CV-BV and stronger cerebral desaturation. Respiratory events occurring during higher baseline StO2 before their onsets, during rapid-eye-movement sleep and those associated with higher heart rate induce smaller changes in CV-BV and de-StO2. The stepwise increased CPAP pressures can attenuate these changes. These results suggest that in severe OSAS the length and the type of respiratory event rather than widely used AHI may be better parameters to indicate the severity of cerebral hemodynamic changes.

Impact of Cutaneous Blood Flow on NIR-DCS Measures of Skeletal Muscle Blood Flow Index

Near-infrared diffuse correlation spectroscopy (NIR-DCS) is an optical technique for estimating relative changes in skeletal muscle perfusion during exercise, but may be affected by changes in cutaneous blood flow, as photons emitted by the laser must first pass through the skin. Accordingly, the purpose of this investigation was to examine how increased cutaneous blood flow affects NIR-DCS blood flow index (BFI) at rest and during exercise using a passive whole-body heating protocol that increases cutaneous, but not skeletal muscle, perfusion in the uncovered limb. BFI and cutaneous perfusion (laser Doppler flowmetry) were assessed in 15 healthy young subjects before (e.g., rest) and during 5-minutes of moderate-intensity hand-grip exercise in normothermic conditions and after cutaneous blood flow was elevated via whole-body heating. Hyperthermia significantly increased both cutaneous perfusion (~7.3-fold; p≤0.001) and NIR-DCS BFI (~4.5-fold; p≤0.001). Although relative BFI (i.e., fold-change above baseline) exhibited a typical exponential increase in muscle perfusion during normothermic exercise (2.81±0.95), there was almost no change in BFI during hyperthermic exercise (1.43±0.44). A subset of 8 subjects were subsequently treated with intradermal injection of botulinum toxin-A (Botox) to block heating-induced elevations in cutaneous blood flow, which 1) nearly abolished the hyperthermia-induced increase in BFI, and 2) restored BFI kinetics during hyperthermic exercise to values that were not different from normothermic exercise (p=0.091). Collectively, our results demonstrate that cutaneous blood flow can have a substantial, detrimental impact on NIR-DCS estimates of skeletal muscle perfusion and highlight the need for technical and/or pharmacological advancements to overcome this issue moving forward.

Cerebral blood volume increment after resuscitation measured by near-infrared time-resolved spectroscopy can estimate degree of hypoxic-ischemic insult in newborn piglets

Neonatal hypoxic–ischemic encephalopathy is a notable cause of neonatal death and developmental disabilities. To achieve better outcomes, it is important in treatment strategy selection to categorize the degree of hypoxia ischemia and evaluate dose response. In an asphyxia piglet model with histopathological brain injuries that we previously developed, animals survived 5 days after insult and showed changes in cerebral blood volume (CBV) that reflected the severity of injuries. However, little is known about the relationship between changes in CBV during and after insult. In this study, an HI event was induced by varying the amount and timing of inspired oxygen in 20 anesthetized piglets. CBV was measured using near-infrared time-resolved spectroscopy before, during, and 6 h after insult. Change in CBV was calculated as the difference between the peak CBV value during insult and the value at the end of insult. The decrease in CBV during insult was found to correlate with the increase in CBV within 6 h after insult. Heart rate exhibited a similar tendency to CBV, but blood pressure did not. Because the decrement in CBV was larger in severe HI, the CBV increment immediately after insult is considered useful for assessing degree of HI insult.

Non-invasive diffuse optical neuromonitoring during cardiopulmonary resuscitation predicts return of spontaneous circulation

Neurologic injury is a leading cause of morbidity and mortality following pediatric cardiac arrest. In this study, we assess the feasibility of quantitative, non-invasive, frequency-domain diffuse optical spectroscopy (FD-DOS) neuromonitoring during cardiopulmonary resuscitation (CPR), and its predictive utility for return of spontaneous circulation (ROSC) in an established pediatric swine model of cardiac arrest. Cerebral tissue optical properties, oxy- and deoxy-hemoglobin concentration ([HbO₂], [Hb]), oxygen saturation (StO₂) and total hemoglobin concentration (THC) were measured by a FD-DOS probe placed on the forehead in 1-month-old swine (8–11 kg; n = 52) during seven minutes of asphyxiation followed by twenty minutes of CPR. ROSC prediction and time-dependent performance of prediction throughout early CPR (< 10 min), were assessed by the weighted Youden index (J_w, w = 0.1) with tenfold cross-validation. FD-DOS CPR data was successfully acquired in 48/52 animals; 37/48 achieved ROSC. Changes in scattering coefficient (785 nm), [HbO₂], StO₂ and THC from baseline were significantly different in ROSC versus No-ROSC subjects (p < 0.01) after 10 min of CPR. Change in [HbO₂] of + 1.3 µmol/L from 1-min of CPR achieved the highest weighted Youden index (0.96) for ROSC prediction. We demonstrate feasibility of quantitative, non-invasive FD-DOS neuromonitoring, and stable, specific, early ROSC prediction from the third minute of CPR.

Individual Differences in Hemodynamic Responses Measured on the Head Due to a Long-Term Stimulation Involving Colored Light Exposure and a Cognitive Task: A SPA-fNIRS Study

When brain activity is measured by neuroimaging, the canonical hemodynamic response (increase in oxygenated hemoglobin ([O2Hb]) and decrease in deoxygenated hemoglobin ([HHb]) is not always seen in every subject. The reason for this intersubject-variability of the responses is still not completely understood. This study is performed with 32 healthy subjects, using the systemic physiology augmented functional near-infrared spectroscopy (SPA-fNIRS) approach. We investigate the intersubject variability of hemodynamic and systemic physiological responses, due to a verbal fluency task (VFT) under colored light exposure (CLE; blue and red). Five and seven different hemodynamic response patterns were detected in the subgroup analysis of the blue and red light exposure, respectively. We also found that arterial oxygen saturation and mean arterial pressure were positively correlated with [O2Hb] at the prefrontal cortex during the CLE-VFT independent of the color of light and classification of the subjects. Our study finds that there is substantial intersubject-variability of cerebral hemodynamic responses, which is partially explained by subject-specific systemic physiological changes induced by the CLE-VFT. This means that both subgroup analyses and the additional assessment of systemic physiology are of crucial importance to achieve a comprehensive understanding of the effects of a CLE-VFT on human subjects.

Kinetic differences between macro- and microvascular measures of reactive hyperemia

Postischemia reperfusion kinetics are markedly dissociated when comparing the macro- versus microvasculature. We used Doppler ultrasound and near-infrared diffuse correlation spectroscopy (NIR-DCS), an emerging technique for continuously and noninvasively quantifying relative changes in skeletal muscle microvascular perfusion (i.e., blood flow index or BFI), to measure macro- and microvascular reactive hyperemia (RH) in the nondominant arm of 16 healthy young adults. First, we manipulated the duration of limb ischemia (3 vs. 6 min) with the limb at heart level (neutral, -N). Then, we reduced/increased forearm perfusion pressure (PP) by positioning the arm above (3 min-A, 60°) or below (3 min-B, 30°) the heart. The major novel findings were twofold: first, changes in the ischemic stimulus similarly affected peak macrovascular (i.e., conduit, mL/min) and microvascular (i.e., peak NIR-DCS-derived BFI) reperfusion during reactive hyperemia (6 min-N > 3 min-N, P < 0.05, both) but did not affect the rate at which microvascular reperfusion occurs (i.e., BFI slope). Second, changing forearm PP predictably affected both peak macro- and microvascular reperfusion during RH (3 min-B > N > A, P < 0.05, all), as well as the rate at which microvascular reperfusion occurred (BFI slope; 3 min-B >N > A, P < 0.05). Together, the data suggest that kinetic differences between macro- and microvascular reperfusion are largely determined by differences in fluid mechanical energy (i.e., pressure, gravitational, and kinetic energies) between the two compartments that work in tandem to restore pressure across the arterial tree following a period of tissue ischemia.NEW & NOTEWORTHY We extend our understanding of macro- versus microvascular hemodynamics in humans, by using near-infrared diffuse correlation spectroscopy (micro-) and Doppler ultrasound (macro-) to characterize reperfusion hemodynamics following experimental manipulation of the ischemic stimulus and tissue perfusion pressure. Our results suggest kinetic differences between macro- and microvascular reperfusion are largely determined by differences in fluid mechanical energy (i.e., pressure, gravitational, and kinetic energies) between the two compartments, rather than inherent differences between the macro- and microvasculature.

A Randomized, Crossover Study of the Acute Cognitive and Cerebral Blood Flow Effects of Phenolic, Nitrate and Botanical Beverages in Young, Healthy Humans

Background: In whole foods, polyphenols exist alongside a wide array of other potentially bioactive phytochemicals. Yet, investigations of the effects of combinations of polyphenols with other phytochemicals are limited. Objective: The current study investigated the effects of combining extracts of beetroot, ginseng and sage with phenolic-rich apple, blueberry and coffee berry extracts. Design: This randomized, double-blind, placebo-controlled crossover design investigated three active beverages in 32 healthy adults aged 18–49 years. Each investigational beverage comprised extracts of beetroot, ginseng and sage. Each also contained a phenolic-rich extract derived from apple (containing 234 mg flavanols), blueberry (300 mg anthocyanins) or coffee berry (440 mg chlorogenic acid). Cognition, mood and CBF parameters were assessed at baseline and then again at 60, 180 and 360 min post-drink. Results: Robust effects on mood and CBF were seen for the apple and coffee berry beverages, with increased subjective energetic arousal and hemodynamic responses being observed. Fewer effects were seen with the blueberry extract beverage. Conclusions: Either the combination of beetroot, ginseng and sage was enhanced by the synergistic addition of the apple and coffee berry extract (and to a lesser extent the blueberry extract) or the former two phenolic-rich extracts were capable of evincing the robust mood and CBF effects alone.

Non-invasive optical neuromonitoring of the temperature-dependence of cerebral oxygen metabolism during deep hypothermic cardiopulmonary bypass in neonatal swine

Management of deep hypothermic (DH) cardiopulmonary bypass (CPB), a critical neuroprotective strategy, currently relies on non-invasive temperature to guide cerebral metabolic suppression during complex cardiac surgery in neonates. Considerable inter-subject variability in temperature response and residual metabolism may contribute to the persisting risk for postoperative neurological injury. To characterize and mitigate this variability, we assess the sufficiency of conventional nasopharyngeal temperature (NPT) guidance, and in the process, validate combined non-invasive frequency-domain diffuse optical spectroscopy (FD-DOS) and diffuse correlation spectroscopy (DCS) for direct measurement of cerebral metabolic rate of oxygen (CMRO₂). During CPB, n = 8 neonatal swine underwent cooling from normothermia to 18℃, sustained DH perfusion for 40 min, and then rewarming to simulate cardiac surgery. Continuous non-invasive and invasive measurements of intracranial temperature (ICT) and CMRO₂ were acquired. Significant hysteresis (p < 0.001) between cooling and rewarming periods in the NPT versus ICT and NPT versus CMRO₂ relationships were found. Resolution of this hysteresis in the ICT versus CMRO₂ relationship identified a crucial insufficiency of conventional NPT guidance. Non-invasive CMRO₂ temperature coefficients with respect to NPT (Q₁₀ = 2.0) and ICT (Q₁₀ = 2.5) are consistent with previous reports and provide further validation of FD-DOS/DCS CMRO₂ monitoring during DH CPB to optimize management.

Evaluation of hyperspectral NIRS for quantitative measurements of tissue oxygen saturation by comparison to time-resolved NIRS

Near-infrared spectroscopy (NIRS) is considered ideal for brain monitoring during preterm infancy because it is non-invasive and provides a continuous measure of tissue oxygen saturation (StO2). Hyperspectral NIRS (HS NIRS) is an inexpensive, quantitative modality that can measure tissue optical properties and oxygen saturation (StO2) by differential spectroscopy. In this study, experiments were conducted using newborn piglets to measure StO2 across a range of oxygenation levels from hyperoxia to hypoxia by HS and time-resolved (TR) NIRS for validation. A strong correlation between StO2 measurements from the two techniques was observed (R2 = 0.98, average slope of 1.02 ± 0.28); however, the HS-NIRS estimates were significantly higher than the corresponding TR-NIRS values. These regression results indicate that HS NIRS could become a clinically feasible method for monitoring StO2 in preterm infants.

Differential pathlength factor in continuous wave functional near-infrared spectroscopy: reducing hemoglobin's cross talk in high-density recordings

Functional near-infrared spectroscopy (fNIRS) estimates the functional oscillations of oxyhemoglobin and deoxyhemoglobin in the cortex through scalp-located multiwavelength recordings. Hemoglobin oscillations are inferred through temporal changes in continuous-wave (CW) light attenuation. However, because of the diffusive multilayered head tissue structures, the photon path is longer than the source-detector separation, complicating hemoglobin evaluation. This aspect is incorporated in the modified Beer-Lambert law where the source-detector distance is multiplied by the differential pathlength factor (DPF). Since DPF estimation requires photons' time-of-flight information, DPF is assumed a priori in CW-fNIRS. Importantly, errors in the DPF spectrum induce hemoglobin cross talk, which is detrimental for fNIRS. We propose to estimate subject-specific DPF spectral dependence relying on multidistance high-density measurements. The procedure estimates the effective attenuation coefficient (EAC), which is proportional to the geometric mean of absorption and reduced scattering. Since DPF depends on the scattering-to-absorption ratio, EAC limits the spectral dependence assumption to scattering. This approach was compared to a standard frequency-domain multidistance procedure. A good association between the two methods ( r 2 = 0.69 ) was obtained. This approach could estimate low-resolution maps of the DPF spectral dependence through large field of view, high-density systems, reducing hemoglobin cross talk, and increasing fNIRS sensitivity and specificity to brain activity without instrumentation modification.

Effect of assuming constant tissue scattering on measured tissue oxygenation values during tissue ischemia and vascular reperfusion

The purpose of this study was to determine the effects of assuming constant tissue scattering properties on tissue oxygenation measurements during a vascular occlusion test (VOT). Twenty-one subjects (21.8 ± 1.9 yr) completed a VOT [1 min baseline (BL), 5 min of tissue ischemia (TI), and 3 min of vascular reperfusion (VR)]. Absolute concentrations of oxygenated heme (oxy-[heme]), deoxygenated heme (deoxy-[heme]), total heme (total [heme), tissue oxygen saturation (StO2), and heme difference [heme]diff) were measured using frequency domain near-infrared spectroscopy (FD-NIRS) while 1) continuously measuring and incorporating tissue scattering ([Formula: see text]) and 2) assuming scattering remained constant. FD-NIRS measured [Formula: see text] increased during TI at 692 nm ( P < 0.001) and decreased at 834 nm ( P < 0.001). During VR, [Formula: see text] decreased at 692 nm ( P < 0.001) and increased at 834 nm ( P < 0.001). When assuming constant scattering, oxy-[heme] was significantly less at TIpeak ( P < 0.05) while deoxy-[heme] and StO2 were significantly altered at BL, TIpeak, and VRpeak (all P < 0.001). Total [heme] did not change during the VOT. Absolute changes in deoxy-[heme], oxy-[heme], and StO2 in response to TI and VR were significantly exaggerated (all P < 0.001) and the rates of change during TI ( slope 1) and VR ( slope 2) in deoxy-[heme], oxy-[heme], StO2, and [heme]diff were significantly increased (all P < 0.05) when constant tissue scattering was assumed. These findings demonstrate the need for caution when interpreting NIRS data without continuously measuring tissue optical properties. Further, assuming tissue optical properties remain constant may have important consequences to experimental data and clinical conclusions made using NIRS.

NEW & NOTEWORTHY NIRS measurements provide significant experimental and clinical insight. We demonstrate that absolute changes in tissue oxygenation measurements made with NIRS are overestimated and the kinetic responses of NIRS measurements are exaggerated by varying degrees among individuals if tissue scattering characteristics are assumed to remain constant during vascular occlusion tests.

Understanding near infrared spectroscopy and its application to skeletal muscle research

Near infrared spectroscopy (NIRS) is a powerful noninvasive tool with which to study the matching of oxygen delivery to oxygen utilization and the number of new publications utilizing this technique has increased exponentially in the last 20 yr. By measuring the state of oxygenation of the primary heme compounds in skeletal muscle (hemoglobin and myoglobin), greater understanding of the underlying control mechanisms that couple perfusive and diffusive oxygen delivery to oxidative metabolism can be gained from the laboratory to the athletic field to the intensive care unit or emergency room. However, the field of NIRS has been complicated by the diversity of instrumentation, the inherent limitations of some of these technologies, the associated diversity of terminology, and a general lack of standardization of protocols. This Cores of Reproducibility in Physiology (CORP) will describe in basic but important detail the most common methodologies of NIRS, their strengths and limitations, and discuss some of the potential confounding factors that can affect the quality and reproducibility of NIRS data. Recommendations are provided to reduce the variability and errors in data collection, analysis, and interpretation. The goal of this CORP is to provide readers with a greater understanding of the methodology, limitations, and best practices so as to improve the reproducibility of NIRS research in skeletal muscle.

Multiwavelength time-resolved near-infrared spectroscopy of the adult head: assessment of intracerebral and extracerebral absorption changes

An optical technique based on diffuse reflectance measurement combined with indocyanine green (ICG) bolus tracking is extensively tested as a method for the clinical assessment of brain perfusion at the bedside. We report on multiwavelength time-resolved diffuse reflectance spectroscopy measurements carried out on the head of a healthy adult during the intravenous administration of a bolus of ICG. Intracerebral and extracerebral changes in absorption were estimated from an analysis of changes in statistical moments (total number of photons, mean time of flight and variance) of the distributions of times of flight (DTOF) of photons recorded simultaneously at 16 wavelengths from the range of 650-850 nm using sensitivity factors estimated by diffusion approximation based on a layered model of the studied medium. We validated the proposed method in a series of phantom experiments and in-vivo measurements. The results obtained show that changes in the concentration of the ICG can be assessed as a function of time of the experiment and depth in the tissue. Thus, the separation of changes in ICG concentration appearing in intra- and extracerebral tissues can be estimated from optical data acquired at a single source-detector pair of fibers/fiber bundles positioned on the surface of the head.

Metaboreceptor activation in heart failure with reduced ejection fraction: Linking cardiac and peripheral vascular haemodynamics

NEW FINDINGS

What is the central question of this research? Do patients with heart failure with reduced ejection fraction (HFrEF) exhibit a greater dependence on cardiac or peripheral vascular haemodynamics across multiple levels of muscle metaboreflex activation provoked by postexercise circulatory occlusion? What is the main finding and its importance? The metaboreflex-induced pressor response in HFrEF patients is governed almost entirely by the peripheral circulation, which places a substantial haemodynamic load on the failing heart. This maladaptive response exacerbates the disease-related impairment of systolic function that is a hallmark feature of HFrEF and may therefore contribute to exercise intolerance in this patient group.

ABSTRACT

We sought to evaluate the muscle metaboreflex in heart failure with reduced ejection fraction (HFrEF) patients, with an emphasis on the interaction between cardiac and peripheral vascular haemodynamics across multiple levels of metaboreceptor activation. In 23 HFrEF patients (63 ± 2 years of age) and 15 healthy control subjects (64 ± 3 years of age), we examined changes in mean arterial pressure, cardiac output, systemic vascular conductance, effective arterial elastance, stroke work and forearm deoxyhaemoglobin concentration during metaboreceptor activation elicited by postexercise circulatory occlusion (PECO) after three levels of static-intermittent handgrip exercise (15, 30 and 45% maximal voluntary contraction). Across workloads, the metaboreflex-induced increase in deoxyhaemoglobin and mean arterial pressure were similar between groups. However, in control subjects, the pressor response was driven by changes (Δ) in cardiac output  (Δ495 ± 155, Δ564 ± 156 and Δ666 ± 217 ml min-1 ), whereas this change was accomplished by intensity-dependent reductions in systemic vascular conductance in patients with HFrEF (Δ-4.9 ± 1.5, Δ-9.1 ± 1.9 and Δ-12.7 ± 1.8 ml min mmHg-1 ). This differential response contributed to the exaggerated increases in effective arterial elastance in HFrEF patients compared with control subjects, coupled with a blunted response in stroke work in the HFrEF patients. Together, these findings indicate a preserved role of the metaboreflex-induced pressor response in HFrEF but suggest that this response is governed by changes in the peripheral circulation. The net effect of this response appears to be maladaptive, as it places a substantial haemodynamic load on the left ventricle that may exacerbate left ventricular systolic dysfunction and contribute to exercise intolerance in this patient population.

Effects of isoflurane anesthesia on physiological parameters in murine subcutaneous tumor allografts measured via diffuse reflectance spectroscopy

Diffuse reflectance spectroscopy (DRS) has been used in murine studies to quantify tumor perfusion and therapeutic response. These studies frequently use inhaled isoflurane anesthesia, which depresses the respiration rate and results in the desaturation of arterial oxygen saturation, potentially affecting tissue physiological parameters. However, there have been no controlled studies quantifying the effect of isoflurane anesthesia on DRS-derived physiological parameters of murine tissue. The goal of this study was to perform DRS on Balb/c mouse (n = 10) tissue under various anesthesia conditions to quantify effects on tissue physiological parameters, including total hemoglobin concentration, tissue oxygen saturation, oxyhemoglobin and reduced scattering coefficient. Two independent variables were manipulated including metabolic gas type (pure oxygen vs. medical air) and isoflurane concentration (1.5 to 4.0%). The 1.5% isoflurane and 1 L/min oxygen condition most closely mimicked a no-anesthesia condition with oxyhemoglobin concentration within 89% ± 19% of control. The time-dependent effects of isoflurane anesthesia were tested, revealing that anesthetic induction with 4.0% isoflurane can affect DRS-derived physiological parameters up to 20 minutes post-induction. Finally, spectroscopy with and without isoflurane anesthesia was compared for colon tumor Balb/c-CT26 allografts (n = 5) as a representative model of subcutaneous murine tumor allografts. Overall, isoflurane anesthesia yielded experimentally-induced depressed oxyhemoglobin, and this depression was both concentration and time dependent. Investigators should understand the dynamic effects of isoflurane on tissue physiological parameters measured by DRS. These results may guide investigators in eliminating, limiting, or managing anesthesia-induced physiological changes in DRS studies in mouse models.

Preoperative cerebral hemodynamics from birth to surgery in neonates with critical congenital heart disease

BACKGROUND

Hypoxic-ischemic white matter brain injury commonly occurs in neonates with critical congenital heart disease. Recent work has shown that longer time to surgery is associated with increased risk for this injury. In this study we investigated changes in perinatal cerebral hemodynamics during the transition from fetal to neonatal circulation to ascertain mechanisms that might underlie this risk.

METHODS

Neonates with either transposition of the great arteries (TGA) or hypoplastic left heart syndrome (HLHS) were recruited for preoperative noninvasive optical monitoring of cerebral oxygen saturation, cerebral oxygen extraction fraction, and cerebral blood flow using diffuse optical spectroscopy and diffuse correlation spectroscopy, 2 noninvasive optical techniques. Measurements were acquired daily from day of consent until the morning of surgery. Temporal trends in these measured parameters during the preoperative period were assessed with a mixed effects model.

RESULTS

Forty-eight neonates with TGA or HLHS were studied. Cerebral oxygen saturation was significantly and negatively correlated with time, and oxygen extraction fraction was significantly and positively correlated with time. Cerebral blood flow did not significantly change with time during the preoperative period.

CONCLUSIONS

In neonates with TGA or HLHS, increasing cerebral oxygen extraction combined with an abnormal cerebral blood flow response during the time between birth and heart surgery leads to a progressive decrease in cerebral tissue oxygenation The results support and help explain the physiological basis for recent studies that show longer time to surgery increases the risk of acquiring white matter injury.

Hyperspectral imaging solutions for brain tissue metabolic and hemodynamic monitoring: past, current and future developments

Hyperspectral imaging (HSI) technologies have been used extensively in medical research, targeting various biological phenomena and multiple tissue types. Their high spectral resolution over a wide range of wavelengths enables acquisition of spatial information corresponding to different light-interacting biological compounds. This review focuses on the application of HSI to monitor brain tissue metabolism and hemodynamics in life sciences. Different approaches involving HSI have been investigated to assess and quantify cerebral activity, mainly focusing on: (1) mapping tissue oxygen delivery through measurement of changes in oxygenated (HbO2) and deoxygenated (HHb) hemoglobin; and (2) the assessment of the cerebral metabolic rate of oxygen (CMRO2) to estimate oxygen consumption by brain tissue. Finally, we introduce future perspectives of HSI of brain metabolism, including its potential use for imaging optical signals from molecules directly involved in cellular energy production. HSI solutions can provide remarkable insight in understanding cerebral tissue metabolism and oxygenation, aiding investigation on brain tissue physiological processes.

Oxygen-sensing neurons reciprocally regulate peripheral lipid metabolism via neuropeptide signaling in Caenorhabditis elegans

The mechanisms by which the sensory environment influences metabolic homeostasis remains poorly understood. In this report, we show that oxygen, a potent environmental signal, is an important regulator of whole body lipid metabolism. C. elegans oxygen-sensing neurons reciprocally regulate peripheral lipid metabolism under normoxia in the following way: under high oxygen and food absence, URX sensory neurons are activated, and stimulate fat loss in the intestine, the major metabolic organ for C. elegans. Under lower oxygen conditions or when food is present, the BAG sensory neurons respond by repressing the resting properties of the URX neurons. A genetic screen to identify modulators of this effect led to the identification of a BAG-neuron-specific neuropeptide called FLP-17, whose cognate receptor EGL-6 functions in URX neurons. Thus, BAG sensory neurons counterbalance the metabolic effect of tonically active URX neurons via neuropeptide communication. The combined regulatory actions of these neurons serve to precisely tune the rate and extent of fat loss to the availability of food and oxygen, and provides an interesting example of the myriad mechanisms underlying homeostatic control.

Long-Term Changes in Optical Properties (μa, μ's, μeff and DPF) of Human Head Tissue During Functional Neuroimaging Experiments

Frequency-domain near-infrared spectroscopy (FD-NIRS) enables to measure absolute optical properties (i.e. the absorption coefficient, μ_a, and the reduced scattering coefficient, [Formula: see text]) of the brain tissue. The aim of this study was to investigate how the optical properties changed during the course of a functional NIRS experiment. The analyzed dataset comprised of FD-NIRS measurements of 14 healthy subjects (9 males, 5 females, aged: 33.4 ± 10.5 years, range: 24-57 years old). Each measurement lasted 33 min, i.e. 8 min baseline in darkness, 10 min intermittent light stimulation, and 15 min recovery in darkness. Optical tissue properties were obtained bilaterally over the prefrontal cortex (PFC) and visual cortex (VC) with FD-NIRS (Imagent, ISS Inc., USA). Changes in μ_a and [Formula: see text] were directly measured and two parameters were calculated, i.e. the differential pathlength factor (DPF) and the effective attenuation coefficient (μ_eff). Differences in the behavior of the optical changes were observed when comparing group-averaged data versus single datasets: no clear overall trend was presented in the group data, whereas a clear long-term trend was visible in almost all of the single measurements. Interestingly, the changes in [Formula: see text] statistically significantly correlated with μ_a, positively in the PFC and negatively in the VC. Our analysis demonstrates that all optical brain tissue properties (μ_a, [Formula: see text], μ_eff and DPF) change during these functional neuroimaging experiments. The change in [Formula: see text] is not random but follows a trend, which depends on the single experiment and measurement location. The change in the scattering properties of the brain tissue during a functional experiment is not negligible. The assumption [Formula: see text] ≈ const during an experiment is valid for group-averaged data but not for data from single experiments.

Toward real-time diffuse optical tomography: accelerating light propagation modeling employing parallel computing on GPU and CPU

Parameter recovery in diffuse optical tomography is a computationally expensive algorithm, especially when used for large and complex volumes, as in the case of human brain functional imaging. The modeling of light propagation, also known as the forward problem, is the computational bottleneck of the recovery algorithm, whereby the lack of a real-time solution is impeding practical and clinical applications. The objective of this work is the acceleration of the forward model, within a diffusion approximation-based finite-element modeling framework, employing parallelization to expedite the calculation of light propagation in realistic adult head models. The proposed methodology is applicable for modeling both continuous wave and frequency-domain systems with the results demonstrating a 10-fold speed increase when GPU architectures are available, while maintaining high accuracy. It is shown that, for a very high-resolution finite-element model of the adult human head with ∼600,000 nodes, consisting of heterogeneous layers, light propagation can be calculated at ∼0.25  s/excitation source.

Effect of short-term colored-light exposure on cerebral hemodynamics and oxygenation, and systemic physiological activity

There is not yet a comprehensive view of how the color of light affects the cerebral and systemic physiology in humans. The aim was to address this deficit through basic research. Since cerebral and systemic physiological parameters are likely to interact, it was necessary to establish an approach, which we have termed "systemic-physiology-augmented functional near-infrared spectroscopy (SPA-fNIRS) neuroimaging." This multimodal approach measures the systemic and cerebral physiological response to exposure to light of different colors. In 14 healthy subjects (9 men, 5 women, age: [Formula: see text] years, range: 24 to 57 years) exposed to red, green, and blue light (10-min intermittent wide-field visual color stimulation; [Formula: see text] blocks of visual stimulation), brain hemodynamics and oxygenation were measured by fNIRS on the prefrontal cortex (PFC) and visual cortex (VC) simultaneously, in addition with systemic parameters. This study demonstrated that (i) all colors elicited responses in the VC, whereas only blue evoked a response in the PFC; (ii) there was a color-dependent effect on cardiorespiratory activity; (iii) there was significant change in neurosystemic functional connectivity; (iv) cerebral hemodynamic responses in the PFC and changes in the cardiovascular system were gender and age dependent; and (v) electrodermal activity and psychological state showed no stimulus-evoked changes, and there was no dependence on color of light, age, and gender. We showed that short-term light exposure caused color-dependent responses in cerebral hemodynamics/oxygenation as well as cardiorespiratory dynamics. Additionally, we showed that neurosystemic functional connectivity changes even during apparently stress-free tasks-an important consideration when using any of the hemodynamic neuroimaging methods (e.g. functional magnetic resonance imaging, positron emission tomography, and fNIRS). Our findings are important for future basic research and clinical applications as well as being relevant for everyday life.

Cerebral blood volume and oxygen supply uniformly increase following various intrathoracic pressure strains

Intrathoracic pressure (ITP) swings challenge many physiological systems. The responses of cerebral hemodynamics to different ITP swings are still less well-known due to the complexity of cerebral circulation and methodological limitation. Using frequency-domain near-infrared spectroscopy and echocardiography, we measured changes in cerebral, muscular and cardiac hemodynamics in five graded respiratory maneuvers (RM), breath holding, moderate and strong Valsalva maneuvers (mVM/sVM) with 20 and 40 cmH2O increments in ITP, moderate and strong Mueller maneuvers (mMM/sMM) with 20 and 40 cmH2O decrements in ITP controlled by esophageal manometry. We found cerebral blood volume (CBV) maintains relative constant during the strains while it increases during the recoveries together with increased oxygen supply. By contrast changes in muscular blood volume (MBV) are mainly controlled by systemic changes. The graded changes of ITP during the maneuvers predict the changes of MBV but not CBV. Changes in left ventricular stroke volume and heart rate correlate to MBV but not to CBV. These results suggest the increased CBV after the ITP strains is brain specific, suggesting cerebral vasodilatation. Within the strains, cerebral oxygen saturation only decreases in sVM, indicating strong increment rather than decrement in ITP may be more challenging for the brain.

Determining the light scattering and absorption parameters from forward-directed flux measurements in cardiac tissue

We describe a method to accurately measure the light scattering model parameters from forward-directed flux (FDF) measurements carried out with a fiber-optic probe (optrode). Improved determination of light scattering parameters will, in turn, permit better modeling and interpretation of optical mapping in the heart using voltage-sensitive dyes. Using our optrode-based system, we carried out high spatial resolution measurements of FDF in intact and homogenized cardiac tissue, as well as in intralipid-based tissue phantoms. The samples were illuminated with a broad collimated beam at 660 and 532 nm. Measurements were performed with a plunge fiber-optic probe (NA=0.22) at a spatial resolution of up to 10  μm. In the vicinity of the illuminated surface, the FDF consistently manifested a fast decaying exponent with a space constant comparable with the decay rate of ballistic photons. Using a Monte Carlo model, we obtained a simple empirical formula linking the rate of the fast exponent to the scattering coefficient, the anisotropy parameter g, and the numerical aperture of the probe. The estimates of scattering coefficient based on this formula were validated in tissue phantoms. Potential applications of optical fiber-based FDF measurements for the evaluation of optical parameters in turbid media are discussed.

Modelling confounding effects from extracerebral contamination and systemic factors on functional near-infrared spectroscopy

Haemodynamics-based neuroimaging is widely used to study brain function. Regional blood flow changes characteristic of neurovascular coupling provide an important marker of neuronal activation. However, changes in systemic physiological parameters such as blood pressure and concentration of CO2 can also affect regional blood flow and may confound haemodynamics-based neuroimaging. Measurements with functional near-infrared spectroscopy (fNIRS) may additionally be confounded by blood flow and oxygenation changes in extracerebral tissue layers. Here we investigate these confounds using an extended version of an existing computational model of cerebral physiology, ‘BrainSignals’. Our results show that confounding from systemic physiological factors is able to produce misleading haemodynamic responses in both positive and negative directions. By applying the model to data from previous fNIRS studies, we demonstrate that such potentially deceptive responses can indeed occur in at least some experimental scenarios. It is therefore important to record the major potential confounders in the course of fNIRS experiments. Our model may then allow the observed behaviour to be attributed among the potential causes and hence reduce identification errors.

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Confounding of fNIRS haemoglobin signals is simulated using a computational model.

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Model is extended to simulate scalp haemodynamics.

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Changes in blood pressure and CO₂ can mimic and mask functional activation.

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Experimental recording of systemic factors is recommended to aid interpretation.

Evidence for cerebral edema, cerebral perfusion, and intracranial pressure elevations in acute mountain sickness

INTRODUCTION

We hypothesized that cerebral alterations in edema, perfusion, and/or intracranial pressure (ICP) are related to the development of acute mountain sickness (AMS).

METHODS

To vary AMS, we manipulated ambient oxygen, barometric pressure, and exercise duration. Thirty-six subjects were tested before, during and after 8 h exposures in (1) normobaric normoxia (NN; 300 m elevation equivalent); (2) normobaric hypoxia (NH; 4400 m equivalent); and (3) hypobaric hypoxia (HH; 4400 m equivalent). After a passive 15 min ascent, each subject participated in either 10 or 60 min of cycling exercise at 50% of heart rate reserve. We measured tissue absorption and scattering via radio-frequency near-infrared spectroscopy (NIRS), optic nerve sheath diameter (ONSD) via ultrasound, and AMS symptoms before, during, and after environmental exposures.

RESULTS

We observed significant increases in NIRS tissue scattering of 0.35 ± 0.11 cm(-1) (P = 0.001) in subjects with AMS (i.e., AMS+), consistent with mildly increased cerebral edema. We also noted a small, but significant increase in total hemoglobin concentrations with AMS+, 3.2 ± 0.8 μmolL(-1) (P < 0.0005), consistent with increased cerebral perfusion. No effect of exercise duration was found, nor did we detect differences between NH and HH. ONSD assays documented a small but significant increase in ONSD (0.11 ± 0.02 mm; P < 0.0005) with AMS+, suggesting mildly elevated ICP, as well as further increased ONSD with longer exercise duration (P = 0.005).

CONCLUSION

In AMS+, we found evidence of cerebral edema, elevated cerebral perfusion, and elevated ICP. The observed changes were small but consistent with the reversible nature of AMS.

Pressure modulation algorithm to separate cerebral hemodynamic signals from extracerebral artifacts

We introduce and validate a pressure measurement paradigm that reduces extracerebral contamination from superficial tissues in optical monitoring of cerebral blood flow with diffuse correlation spectroscopy (DCS). The scheme determines subject-specific contributions of extracerebral and cerebral tissues to the DCS signal by utilizing probe pressure modulation to induce variations in extracerebral blood flow. For analysis, the head is modeled as a two-layer medium and is probed with long and short source-detector separations. Then a combination of pressure modulation and a modified Beer-Lambert law for flow enables experimenters to linearly relate differential DCS signals to cerebral and extracerebral blood flow variation without a priori anatomical information. We demonstrate the algorithm's ability to isolate cerebral blood flow during a finger-tapping task and during graded scalp ischemia in healthy adults. Finally, we adapt the pressure modulation algorithm to ameliorate extracerebral contamination in monitoring of cerebral blood oxygenation and blood volume by near-infrared spectroscopy.

Frequency domain analysis of cerebral near infrared spectroscopy signals during application of an impedance threshold device in spontaneously ventilating volunteers

Currently available near infrared spectroscopy (NIRS) devices are unable to discriminate between arterial and venous blood, a potential source of artifact. The purpose of this study was to test the hypothesis that oscillations in NIR signals at the respiratory and cardiac frequency could be attributed to venous and arterial blood, respectively, and thereby isolated. After written informed consent was obtained, a two-wavelength NIRS device was placed over the left frontal cortex in 20 volunteers. After 5 min of unimpeded spontaneous ventilation, an impedance threshold device (ITD, average resistance-7 cm H2O) was applied and an additional two minutes of data recorded. Tissue saturation (StO2) calculated at the ventilatory and cardiac frequencies was compared to non-pulsatile StO2, before and after application of the ITD using spectral peak and power algorithms. The ITD increased non-pulsatile cerebral saturation by 3.6 %. The ITD had no discernable effect on pulsatile estimates of StO2 at either the ventilatory or cardiac frequencies. StO2 estimated at the NIRS spectral peak from 0.75 to 1.75 Hz was 24 % higher than non-pulsatile StO2 (p = 0.0013). There were no other significant differences between pulsatile and non-pulsatile algorithms in the estimation of StO2. In 64 % of cases, both the low (ventilator) and high (cardiac) frequency estimates of StO2 were either both larger or both smaller than non-pulsatile StO2, suggesting that they were interrogating the same vascular bed. Frequency domain analysis cannot reliably separate NIRS waveforms into arterial and venous components.

Calibration of diffuse correlation spectroscopy blood flow index with venous-occlusion diffuse optical spectroscopy in skeletal muscle

We investigate and assess the utility of a simple scheme for continuous absolute blood flow monitoring based on diffuse correlation spectroscopy (DCS). The scheme calibrates DCS using venous-occlusion diffuse optical spectroscopy (VO-DOS) measurements of arm muscle tissue at a single time-point. A calibration coefficient (γ) for the arm is determined, permitting conversion of DCS blood flow indices to absolute blood flow units, and a study of healthy adults (N=10) is carried out to ascertain the variability of γ. The average DCS calibration coefficient for the right (i.e., dominant) arm was γ=(1.24±0.15)×10(8) (mL·100  mL(−1)·min(−1))/(cm(2)/s). However, variability can be significant and is apparent in our site-to-site and day-to-day repeated measurements. The peak hyperemic blood flow overshoot relative to baseline resting flow was also studied following arm-cuff ischemia; excellent agreement between VO-DOS and DCS was found (R(2)=0.95, slope=0.94±0.07, mean difference=−0.10±0.45). Finally, we show that incorporation of subject-specific absolute optical properties significantly improves blood flow calibration accuracy.

Oxygen delivery-utilization mismatch in contracting locomotor muscle in COPD: peripheral factors

Central cardiorespiratory and gas exchange limitations imposed by chronic obstructive pulmonary disease (COPD) impair ambulatory skeletal muscle oxygenation during whole body exercise. This investigation tested the hypothesis that peripheral factors per se contribute to impaired contracting lower limb muscle oxygenation in COPD patients. Submaximal neuromuscular electrical stimulation (NMES; 30, 40, and 50 mA at 50 Hz) of the quadriceps femoris was employed to evaluate contracting skeletal muscle oxygenation while minimizing the influence of COPD-related central cardiorespiratory constraints. Fractional O₂ extraction was estimated by near-infrared spectroscopy (deoxyhemoglobin/myoglobin concentration; deoxy-[Hb/Mb]), and torque output was measured by isokinetic dynamometry in 15 nonhypoxemic patients with moderate-to-severe COPD (SpO2 = 94 ± 2%; FEV₁ = 46.4 ± 10.1%; GOLD II and III) and in 10 age- and gender-matched sedentary controls. COPD patients had lower leg muscle mass than controls (LMM = 8.0 ± 0.7 kg vs. 8.9 ± 1.0 kg, respectively; P < 0.05) and produced relatively lower absolute and LMM-normalized torque across the range of NMES intensities (P < 0.05 for all). Despite producing less torque, COPD patients had similar deoxy-[Hb/Mb] amplitudes at 30 and 40 mA (P > 0.05 for both) and higher deoxy-[Hb/Mb] amplitude at 50 mA (P < 0.05). Further analysis indicated that COPD patients required greater fractional O₂ extraction to produce torque (i.e., ↑Δdeoxy-[Hb/Mb]/torque) relative to controls (P < 0.05 for 40 and 50 mA) and as a function of NMES intensity (P < 0.05 for all). The present data obtained during submaximal NMES of small muscle mass indicate that peripheral abnormalities contribute mechanistically to impaired contracting skeletal muscle oxygenation in nonhypoxemic, moderate-to-severe COPD patients.

Exploring diazepam's effect on hemodynamic responses of mouse brain tissue by optical spectroscopic imaging

In this study, a simple duel-optical spectroscopic imaging apparatus capable of simultaneously determining relative changes in brain oxy-and deoxy-hemoglobin concentrations was used following administration of the anxiolytic compound diazepam in mice with strong dominant (Dom) and submissive (Sub) behavioral traits. Three month old mice (n = 30) were anesthetized and after 10 min of baseline imaging, diazepam (1.5 mg/kg) was administered and measurements were taken for 80 min. The mouse head was illuminated by white light based LED's and diffused reflected light passing through different channels, consisting of a bandpass filter and a CCD camera, respectively, was collected and analyzed to measure the hemodynamic response. This work's major findings are threefold: first, Dom and Sub animals showed statistically significant differences in hemodynamic response to diazepam administration. Secondly, diazepam was found to more strongly affect the Sub group. Thirdly, different time-series profiles were observed post-injection, which can serve as a possible marker for the groups' differentiation. To the best of our knowledge, this is the first report on the effects of an anxiolytic drug on brain hemodynamic responses in mice using diffused light optical imaging.

Comparison of a layered slab and an atlas head model for Monte Carlo fitting of time-domain near-infrared spectroscopy data of the adult head

Near-infrared spectroscopy (NIRS) estimations of the adult brain baseline optical properties based on a homogeneous model of the head are known to introduce significant contamination from extracerebral layers. More complex models have been proposed and occasionally applied to in vivo data, but their performances have never been characterized on realistic head structures. Here we implement a flexible fitting routine of time-domain NIRS data using graphics processing unit based Monte Carlo simulations. We compare the results for two different geometries: a two-layer slab with variable thickness of the first layer and a template atlas head registered to the subject's head surface. We characterize the performance of the Monte Carlo approaches for fitting the optical properties from simulated time-resolved data of the adult head. We show that both geometries provide better results than the commonly used homogeneous model, and we quantify the improvement in terms of accuracy, linearity, and cross-talk from extracerebral layers.

Depth sensitivity and source-detector separations for near infrared spectroscopy based on the Colin27 brain template

Understanding the spatial and depth sensitivity of non-invasive near-infrared spectroscopy (NIRS) measurements to brain tissue–i.e., near-infrared neuromonitoring (NIN) – is essential for designing experiments as well as interpreting research findings. However, a thorough characterization of such sensitivity in realistic head models has remained unavailable. In this study, we conducted 3,555 Monte Carlo (MC) simulations to densely cover the scalp of a well-characterized, adult male template brain (Colin27). We sought to evaluate: (i) the spatial sensitivity profile of NIRS to brain tissue as a function of source-detector separation, (ii) the NIRS sensitivity to brain tissue as a function of depth in this realistic and complex head model, and (iii) the effect of NIRS instrument sensitivity on detecting brain activation. We found that increasing the source-detector (SD) separation from 20 to 65 mm provides monotonic increases in sensitivity to brain tissue. For every 10 mm increase in SD separation (up to ∼45 mm), sensitivity to gray matter increased an additional 4%. Our analyses also demonstrate that sensitivity in depth (S) decreases exponentially, with a “rule-of-thumb” formula S = 0.75*0.85^depth. Thus, while the depth sensitivity of NIRS is not strictly limited, NIN signals in adult humans are strongly biased towards the outermost 10–15 mm of intracranial space. These general results, along with the detailed quantitation of sensitivity estimates around the head, can provide detailed guidance for interpreting the likely sources of NIRS signals, as well as help NIRS investigators design and plan better NIRS experiments, head probes and instruments.

Monitoring angiogenesis using a human compatible calibration for broadband near-infrared spectroscopy

Angiogenesis is a hallmark of many conditions, including cancer, stroke, vascular disease, diabetes, and high-altitude exposure. We have previously shown that one can study angiogenesis in animal models by using total hemoglobin (tHb) as a marker of cerebral blood volume (CBV), measured using broadband near-infrared spectroscopy (bNIRS). However, the method was not suitable for patients as global anoxia was used for the calibration. Here we determine if angiogenesis could be detected using a calibration method that could be applied to patients. CBV, as a marker of angiogenesis, is quantified in a rat cortex before and after hypoxia acclimation. Rats are acclimated at 370-mmHg pressure for three weeks, while rats in the control group are housed under the same conditions, but under normal pressure. CBV increased in each animal in the acclimation group. The mean CBV (%volume/volume) is 3.49%± 0.43% (mean ± SD) before acclimation for the experimental group, and 4.76%± 0.29% after acclimation. The CBV for the control group is 3.28%± 0.75%, and 3.09%± 0.48% for the two measurements. This demonstrates that angiogenesis can be monitored noninvasively over time using a bNIRS system with a calibration method that is compatible with human use and less stressful for studies using animals.

Assessment of brain oxygenation in term and preterm neonates using near infrared spectroscopy

PURPOSE

The aim of this study was to determine brain oxygenation in full-term and preterm neonates using near infrared spectroscopy.

MATERIAL AND METHODS

A total of 88 full-term and preterm newborn infants without hypoxic-ischaemic disorders admitted to the NICU were examined using NIRS on the first day of life and on day 28 of life. Additional measurements were taken at the end of the first week of life in the premature neonates group. Measurements of oxyhaemoglobin (HbO2), deoxyhaemoglobin (Hb), total haemoglobin (HbT) concentration and tissue oxygen saturation (Ox) were performed in 5 brain regions. Right and left frontal areas, the occipital area and right and left temporal areas were measured.

RESULTS

In full-term healthy neonates a marked decrease in HbO, Hb and HbT values was observed on day 28 of life in all brain regions except the occipital area. In the neonatal period the greatest changes in brain oxygenation occurred in the right and left frontal regions of the brain. In preterm neonates constant values of HbO2 and Ox were observed in the first 28 days of life. In preterm newborn infants, as well as in full term newborn infants, similar Ox and HbO2 values were obtained on day 28 of life.

CONCLUSIONS

NIRS is a safe method and can be used to evaluate brain oxygenation in newborn infants. The results of these measurements are in accordance with changes in brain oxygenation in the first month of life, which are predicated on the basis of the neonate's physiology.

Early postoperative changes in cerebral oxygen metabolism following neonatal cardiac surgery: effects of surgical duration

OBJECTIVE

The early postoperative period following neonatal cardiac surgery is a time of increased risk for brain injury, yet the mechanisms underlying this risk are unknown. To understand these risks more completely, we quantified changes in postoperative cerebral metabolic rate of oxygen (CMRO(2)), oxygen extraction fraction (OEF), and cerebral blood flow (CBF) compared with preoperative levels by using noninvasive optical modalities.

METHODS

Diffuse optical spectroscopy and diffuse correlation spectroscopy were used concurrently to derive cerebral blood flow and oxygen utilization postoperatively for 12 hours. Relative changes in CMRO(2), OEF, and CBF were quantified with reference to preoperative data. A mixed-effect model was used to investigate the influence of total support time and deep hypothermic circulatory arrest duration on relative changes in CMRO(2), OEF, and CBF.

RESULTS

Relative changes in CMRO(2), OEF, and CBF were assessed in 36 patients, 21 with single-ventricle defects and 15 with 2-ventricle defects. Among patients with single-ventricle lesions, deep hypothermic circulatory arrest duration did not affect relative changes in CMRO(2), CBF, or OEF (P > .05). Among 2-ventricle patients, total support time was not a significant predictor of relative changes in CMRO(2) or CBF (P > .05), although longer total support time was associated significantly with greater increases in relative change of postoperative OEF (P = .008).

CONCLUSIONS

Noninvasive diffuse optical techniques were used to quantify postoperative relative changes in CMRO(2), CBF, and OEF for the first time in this observational pilot study. Pilot data suggest that surgical duration does not account for observed variability in the relative change in CMRO(2), and that more comprehensive clinical studies using the new technology are feasible and warranted to elucidate these issues further.

Taming the "sleeping giant": the role of endothelin-1 in the regulation of skeletal muscle blood flow and arterial blood pressure during exercise

The cardiovascular response to exercise is governed by a combination of vasodilating and vasoconstricting influences that optimize exercising muscle perfusion while protecting mean arterial pressure (MAP). The degree to which endogenous endothelin (ET)-1, the body's most potent vasoconstrictor, participates in this response is unknown. Thus, in eight young (24 ± 2 yr), healthy volunteers, we examined leg blood flow, MAP, tissue oxygenation, heart rate, leg arterial-venous O(2) difference, leg O(2) consumption, pH, and net ET-1 and lactate release at rest and during knee extensor exercise (0, 5, 10, 15, 20, and 30 W) before and after an intra-arterial infusion of BQ-123 [ET subtype A (ET(A)) receptor antagonist]. At rest, BQ-123 did not evoke a change in leg blood flow or MAP. During exercise, net ET-1 release across the exercising leg increased approximately threefold. BQ-123 increased leg blood flow by ~20% across all work rates (changes of 113 ± 76, 176 ± 83, 304 ± 108, 364 ± 130, 502 ± 117, and 570 ± 178 ml/min at 0, 5, 10, 15, 20, and 30 W, respectively) and attenuated the exercise-induced increase in MAP by ~6%. The increase in leg blood flow was accompanied by a ~9% increase in leg O(2) consumption with an unchanged arterial-venous O(2) difference and deoxyhemoglobin, suggesting a decline in intramuscular efficiency after ET(A) receptor blockade. Together, these findings identify a significant role of the ET-1 pathway in the cardiovascular response to exercise, implicating vasoconstriction via the ET(A) receptor as an important mechanism for both the restraint of blood flow in the exercising limb and maintenance of MAP in healthy, young adults.

End-tidal CO2: an important parameter for a correct interpretation in functional brain studies using speech tasks

The aim was to investigate the effect of different speech tasks, i.e. recitation of prose (PR), alliteration (AR) and hexameter (HR) verses and a control task (mental arithmetic (MA) with voicing of the result on end-tidal CO2 (PETCO2), cerebral hemodynamics and oxygenation. CO2 levels in the blood are known to strongly affect cerebral blood flow. Speech changes breathing pattern and may affect CO2 levels. Measurements were performed on 24 healthy adult volunteers during the performance of the 4 tasks. Tissue oxygen saturation (StO2) and absolute concentrations of oxyhemoglobin ([O2Hb]), deoxyhemoglobin ([HHb]) and total hemoglobin ([tHb]) were measured by functional near-infrared spectroscopy (fNIRS) and PETCO2 by a gas analyzer. Statistical analysis was applied to the difference between baseline before the task, 2 recitation and 5 baseline periods after the task. The 2 brain hemispheres and 4 tasks were tested separately. A significant decrease in PETCO2 was found during all 4 tasks with the smallest decrease during the MA task. During the recitation tasks (PR, AR and HR) a statistically significant (p<0.05) decrease occurred for StO2 during PR and AR in the right prefrontal cortex (PFC) and during AR and HR in the left PFC. [O2Hb] decreased significantly during PR, AR and HR in both hemispheres. [HHb] increased significantly during the AR task in the right PFC. [tHb] decreased significantly during HR in the right PFC and during PR, AR and HR in the left PFC. During the MA task, StO2 increased and [HHb] decreased significantly during the MA task. We conclude that changes in breathing (hyperventilation) during the tasks led to lower CO2 pressure in the blood (hypocapnia), predominantly responsible for the measured changes in cerebral hemodynamics and oxygenation. In conclusion, our findings demonstrate that PETCO2 should be monitored during functional brain studies investigating speech using neuroimaging modalities, such as fNIRS, fMRI to ensure a correct interpretation of changes in hemodynamics and oxygenation.

Regional and hemispheric asymmetries of cerebral hemodynamic and oxygen metabolism in newborns

Understanding the evolution of regional and hemispheric asymmetries in the early stages of life is essential to the advancement of developmental neuroscience. By using 2 noninvasive optical methods, frequency-domain near-infrared spectroscopy and diffuse correlation spectroscopy, we measured cerebral hemoglobin oxygenation (SO(2)), blood volume (CBV), an index of cerebral blood flow (CBF(i)), and the metabolic rate of oxygen (CMRO(2i)) in the frontal, temporal, and parietal regions of 70 premature and term newborns. In concordance with results obtained using more invasive imaging modalities, we verified both hemodynamic (CBV, CBF(i), and SO(2)) and metabolic (CMRO(2i)) parameters were greater in the temporal and parietal regions than in the frontal region and that these differences increased with age. In addition, we found that most parameters were significantly greater in the right hemisphere than in the left. Finally, in comparing age-matched males and females, we found that males had higher CBF(i) in most cortical regions, higher CMRO(2i) in the frontal region, and more prominent right-left CBF(i) asymmetry. These results reveal, for the first time, that we can detect regional and hemispheric asymmetries in newborns using noninvasive optical techniques. Such a bedside screening tool may facilitate early detection of abnormalities and delays in maturation of specific cortical areas.

Application of a time-resolved optical brain imager for monitoring cerebral oxygenation during carotid surgery

Recent studies have shown that time-resolved optical measurements of the head can estimate changes in the absorption coefficient with depth discrimination. Thus, changes in tissue oxygenation, which are specific to intracranial tissues, can be assessed using this advanced technique, and this method allows us to avoid the influence of changes to extracerebral tissue oxygenation on the measured signals. We report the results of time-resolved optical imaging that was carried out during carotid endarterectomy. This surgery remains the "gold standard" treatment for carotid stenosis, and intraoperative brain oxygenation monitoring may improve the safety of this procedure. A time-resolved optical imager was utilized within the operating theater. This instrument allows for the simultaneous acquisition of 32 distributions of the time-of-flight of photons at two wavelengths on both hemispheres. Analysis of the statistical moments of the measured distributions of the time-of-flight of photons was applied for estimating changes in the absorption coefficient as a function of depth. Time courses of changes in oxy- and deoxyhemoglobin of the extra- and intracerebral compartments during cross-clamping of the carotid arteries were obtained. A decrease in the oxyhemoglobin concentration and an increase in the deoxyhemoglobin concentrations were observed in a large area of the head. Large changes were observed in the hemisphere ipsilateral to the site of clamped carotid arteries. Smaller amplitude changes were noted at the contralateral site. We also found that changes in the hemoglobin signals, as estimated from intracerebral tissue, are very sensitive to clamping of the internal carotid artery, whereas its sensitivity to clamping of the external carotid artery is limited. We concluded that intraoperative multichannel measurements allow for imaging of brain tissue hemodynamics. However, when monitoring the brain during carotid surgery, a single-channel measurement may be sufficient.

Measuring brain oxygenation in humans using a multiparametric quantitative blood oxygenation level dependent MRI approach

Quantitative blood oxygenation level dependent approaches have been designed to obtain quantitative oxygenation information using MRI. A mathematical model is usually fitted to the time signal decay of a gradient-echo and spin-echo measurements to derive hemodynamic parameters such as the blood oxygen saturation or the cerebral blood volume. Although the results in rats and human brain have been encouraging, recent studies have pointed out the need for independent estimation of one or more variables to increase the accuracy of the method. In this study, a multiparametric quantitative blood oxygenation level dependent approach is proposed. A combination of arterial spin labeling and dynamic susceptibility contrast methods were used to obtain quantitative estimates of cerebral blood volume and cerebral blood flow. These results were combined with T 2 and T(2) measurements to derive maps of blood oxygen saturation or cerebral metabolic rate of oxygen. In 12 normal subjects, a mean cerebral blood volume of 4.33 ± 0.7%, cerebral blood flow of 43.8 ± 5.7 mL/min/100 g, blood oxygen saturation of 60 ± 6% and cerebral metabolic rate of oxygen 157 ± 23 μmol/100 g/min were found, which are in agreement with literature values. The results obtained in this study suggest that this methodology could be applied to study brain hypoxia in the setting of pathology.

Evaluation of a quantitative blood oxygenation level-dependent (qBOLD) approach to map local blood oxygen saturation

Blood oxygen saturation (SO(2)) is a promising parameter for the assessment of brain tissue viability in numerous pathologies. Quantitative blood oxygenation level-dependent (qBOLD)-like approaches allow the estimation of SO(2) by modelling the contribution of deoxyhaemoglobin to the MR signal decay. These methods require a high signal-to-noise ratio to obtain accurate maps through fitting procedures. In this article, we present a version of the qBOLD method at long TE taking into account separate estimates of T(2), total blood volume fraction (BV(f)) and magnetic field inhomogeneities. Our approach was applied to the brains of 13 healthy rats under normoxia, hyperoxia and hypoxia. MR estimates of local SO(2) (MR_LSO(2)) were compared with measurements obtained from blood gas analysis. A very good correlation (R(2) = 0.89) was found between brain MR_LSO(2) and sagittal sinus SO(2).