Cerebral oxygen availability by NIR spectroscopy during transient hypoxia in humans

Acute hypoxia elicits prefrontal oxygenation asymmetry in young adults

Significance

Cerebrovascular reactivity can be evaluated by prefrontal cortex (PFC) hemodynamic responses and oxygenation changes secondary to hypoxia using near-infrared spectroscopy (NIRS). However, whether there are hemispheric differences in these NIRS-determined PFC hemodynamic responses and oxygenation changes remains unknown.

Aim

This study was performed to determine whether there are differences in the PFC hemodynamic responses and oxygenation changes secondary to hypoxia between the left and right frontal poles (FPL and FPR, respectively).

Approach

Fifteen young men participated in the study. During conduction of an isocapnic hypoxia protocol with a 10-min hypoxic phase at partial pressure of end-tidal oxygen (PET O 2 ) of 45 Torr, hemodynamic and oxygenation indices comprising oxygenated hemoglobin (oxy-Hb), deoxygenated Hb (deoxy-Hb), total Hb (total-Hb), and tissue oxygen saturation (StO 2 ) over FPL and FPR were measured by NIRS. The heart rate (HR) was evaluated by electrocardiography.

Results

In response to hypoxia, the HR increased, oxy-Hb decreased, deoxy-Hb increased, total-Hb increased above baseline, and StO 2 decreased. There was no difference in the change in total-Hb between FPL and FPR. However, there were greater changes in oxy-Hb, deoxy-Hb, and StO 2 over FPL than over FPR, indicating that PFC oxygenation asymmetry occurs in response to hypoxia. Moreover, the change in total-Hb over FPL was associated with the increase in HR.

Conclusions

NIRS-determined hemodynamic responses and oxygenation changes secondary to hypoxia might not simply reflect the direct effect of hypoxia on cerebral vessels. Although there is no hemispheric difference in the PFC hemodynamic responses to hypoxia as in total-Hb, PFC oxygenation asymmetry occurs in young adults.

Mild poikilocapnic hypoxia increases very low frequency haemoglobin oxygenation oscillations in prefrontal cortex

BACKGROUND

The aim of the study was to investigate the effect of mild cerebral hypoxia on haemoglobin oxygenation (HbO₂), cerebrospinal fluid dynamics and cardiovascular physiology. To achieve this goal, four signals were recorded simultaneously: blood pressure, heart rate / electrocardiogram, HbO₂ from right hemisphere and changes of subarachnoid space (SAS) width from left hemisphere. Signals were registered from 30 healthy, young participants (2 females and 28 males, body mass index = 24.5 ± 2.3 kg/m², age 30.8 ± 13.4 years).

RESULTS

We analysed the recorded signals using wavelet transform and phase coherence. We demonstrated for the first time that in healthy subjects exposed to mild poikilokapnic hypoxia there were increases in very low frequency HbO₂ oscillations (< 0.052 Hz) in prefrontal cortex. Additionally, SAS fluctuation diminished in the whole frequency range which could be explained by brain oedema.

CONCLUSIONS

Consequently the study provides insight into mechanisms governing brain response to a mild hypoxic challenge. Our study supports the notion that HbO₂ and SAS width monitoring might be beneficial for patients with acute lung disease.

The Role of CO2 on Respiration and Metabolism During Hypercapnic and Normocapnic Recovery From Exercise

High intensity exercise can lead to depletion of CO₂ from the body (hypocapnia). This disturbance becomes more noticeable during recovery or between seasons of intermittent exercise, putting the subject in a neural fatigue state. Objectives: A possible hypothesis to address this condition would be to provide high CO₂ mixtures (hypercapnic) during the recovery period from exercise in order to relieve hypocapnia. Methods: Eight men (23.8 ± 1.2 yrs, VO_2max = 45 ± 1.9 ml▪kg-1▪min-1) performed cycling exercise at 80%VO_2max for 6-7 min. During recovery (23 min) they inhaled hypercapnic air (EXP-21%O2, 3%CO₂, and 76%N₂) or normal air (CON-21%O₂, 0.003%CO₂, and 79%N₂). Respiratory parameters were collected with open spirometry and heart rate was measured. Results: Exercise caused mild hypocapnia {9.9 mmHg drop of CO₂ end-expiratory partial pressure (P_ETCO₂)} in CON condition after exercise (p < .005). P_ETCO₂ elevated close to the rest values during the three hypercapnic phases in EXP condition (main effect of condition p < .001 between EXP and CON), but after hypercapnic breathing it returned to hypocapnia similarly with CON. The ventilatory response (V_E▪P_ETCO₂^-1) and the exhaled volume of CO₂ (VCO₂) progressively increased during and also after ventilatory manipulations in EXP compared to CON condition (V_E▪P_ETCO₂^-1: post hoc p < .001, VCO₂: pVCO₂: p < .05-.001), and VO₂ became lower after the end of second hypercapnic manipulation (p < .05 between EXP and CON). Conclusion: It seems that hypercapnic breathing after exercise is not a good strategy to reverse exercise hypocapnia, because of great hyperventilation caused by CO₂ and exercise mechanisms during the recovery period leading to increased CO₂ removal from body. This intervention may also decrease O₂ supply and muscles blood flow.

The Use of Cerebral NIRS Monitoring to Identify Acute Brain Injury in Patients With VA-ECMO

Acute brain injury (ABI) increases morbidity and mortality in patients with veno-arterial extracorporeal membrane oxygenation (VA-ECMO). Optimal neurologic monitoring methods have not been well-explicated. We studied the use of Near-infrared Spectroscopy (NIRS) to monitor cerebral regional oxygenation tissue saturation (rSO2) and its relation to ABI in VA-ECMO. In this prospective, observational cohort study of 39 consecutive patients, we analyzed the ability of rSO2 values from continuous bedside NIRS monitoring to predict ABI during VA-ECMO support. ABI occurred in 24 (61.5%) patients. Those with ABI had a lower pre-ECMO Glasgow Coma Scale, more blood product transfusions of pRBCs and FFP, and higher APACHEII score. Baseline rSO2 values were not significantly different between cohorts (54.25 vs 58.50, p = 0.260), while the minimum rSO2 value was lower for patients who experienced an ABI than those who did not (39.75 vs 44.50, p = 0.039). In patients with ABI, 21 (87.5%) had a drop in rSO2 of 25% from baseline, compared to only 7 (46.7%) patients without ABI (p = 0.017). By ROC analysis, we found that desaturations with >25% drop from the baseline rSO2 on VA-ECMO exhibited 86% sensitivity and 55% specificity to predict ABI, with an area under the curve of 0.68. Patients with ABI were more likely to have withdrawal of life sustaining therapy (17 vs 5, p = 0.049), while neurologic outcome and mortality were not statistically different between patients with or without ABI. Our results support that cerebral NIRS is a useful, real-time bedside neuromonitoring tool to detect ABI in VA-ECMO patients. A >25% drop from the baseline was sensitive in predicting ABI occurrence. Further research is needed to assess how to implement this knowledge to utilize NIRS in developing appropriate intervention strategy in VA-ECMO patients.

Measurement of CMRO2 and its relationship with CBF in hypoxia with an extended calibrated BOLD method

Cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO₂) are physiological parameters that not only reflect brain health and disease but also jointly contribute to blood oxygen level-dependent (BOLD) signals. Nevertheless, unsolved issues remain concerning the CBF-CMRO₂ relationship in the working brain under various oxygen conditions. In particular, the CMRO₂ responses to functional tasks in hypoxia are less studied. We extended the calibrated BOLD model to incorporate CMRO₂ measurements in hypoxia. The extended model, which was cross-validated with a multicompartment BOLD model, considers the influences of the reduced arterial saturation level and increased baseline cerebral blood volume (CBV) and deoxyhemoglobin concentration on the changes of BOLD signals in hypoxia. By implementing a pulse sequence to simultaneously acquire the CBV-, CBF- and BOLD-weighted signals, we investigated the effects of mild hypoxia on the CBF and CMRO₂ responses to graded visual stimuli. Compared with normoxia, mild hypoxia caused significant alterations in both the amplitude and the trend of the CMRO₂ responses but did not impact the corresponding CBF responses. Our observations suggested that the flow-metabolism coupling strategies in the brain during mild hypoxia were different from those during normoxia.

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.

Multi-channel multi-distance broadband near-infrared spectroscopy system to measure the spatial response of cellular oxygen metabolism and tissue oxygenation

We present a multi-channel, multi-distance broadband near-infrared spectroscopy (NIRS) system with the capability of measuring changes in haemoglobin concentrations (Δ[HbO₂], Δ[HHb]), oxidation state of cytochrome-c-oxidase (Δ[oxCCO]) and tissue oxygen saturation (TOI) in the adult human brain. The main components of the instrument are two customized spectrographs and two light sources. Each spectrograph is lens-based to improve light throughput, has a grating enhanced to optimise reflection in the near-infrared (NIR) spectral region and uses a front illuminated cooled CCD camera (-70° C) with a square chip dimension of 12.3 x 12.3 mm (512 x 512 pixels). Each light source uses a 50W halogen bulb with a gold plated mirror to increase the intensity of the NIR light. Each light source was connected to a custom-built bifurcated fibre bundle to create two source fibre bundles (3.2 mm diameter each). Each spectrograph received light input from another custom-built fibre bundle comprised of six individual bundles (one with 0.6 mm diameter and the other five with 1.5 mm diameter). All fibre bundles were fixed on a 3D printed optode holder (two light sources x two fibre bundles each = four probes; and two spectrographs x six fibre bundles each = 12 probes) that allowed 24 multi-distance channels across the forehead (six channels at 20 mm, three channels at 30 mm and 15 channels at 35 mm) and six TOI measurements. We demonstrated the use of the system in a cohort of nine healthy adult volunteers during prefrontal cortex functional activation using the Stroop task. We have observed functional responses identified as significant increase in Δ[HbO₂], decrease in Δ[HHb] and increase in Δ[oxCCO] in five channels (out of 12), that overlay the left and right dorsolateral prefrontal cortices. There was no observable TOI functional response and we have shown small variations in TOI across different sites within the same subject and within the same site across subjects.

Residual chemosensitivity to ventilatory challenges in genotyped congenital central hypoventilation syndrome

Congenital central hypoventilation syndrome (CCHS) is a neurodevelopmental disorder characterized by life-threatening hypoventilation, possibly resulting from disruption of central chemosensory integration. However, animal models suggest the possibility of residual chemosensory function in the human disease. Cardioventilatory function in a large cohort with CCHS and verified paired-like homeobox 2B (PHOX2B) mutations was assessed to determine the extent and genotype dependence of any residual chemosensory function in these patients. As part of inpatient clinical care and evaluation, 64 distinct studies from 32 infants, children, and young adults with the disorder were evaluated for physiological response to three different inspired steady-state gas exposures of 3 min each: hyperoxia [100% oxygen (O2)]; hyperoxic hypercapnia [95% O2 and 5% carbon dioxide (CO2)]; and hypoxic hypercapnia [14% O2 and 7% CO2 balanced with nitrogen (N2)]. These were followed by a hypoxia challenge consisting of five or seven breaths of N2 (100% N2). In addition, a control group of 15 young adults was exposed to all but the hypoxic challenge. Comprehensive monitoring was used to derive breath-to-breath and beat-to-beat measures of ventilatory, cardiovascular, and cerebrovascular function. On average, patients showed a residual awake ventilatory response to chemosensory challenge, independent of the specific patient PHOX2B genotype. Graded dysfunction in cardiovascular regulation was found to associate with genotype, suggesting differential effects on different autonomic subsystems. In addition, differences between cases and controls in the cerebrovascular response to chemosensory challenge may indicate alterations in cerebral autoregulation. Thus residual cardiorespiratory responses suggest partial preservation of central nervous system networks that could provide a fulcrum for potential pharmacological interventions.

Modelling of mitochondrial oxygen consumption and NIRS detection of cytochrome oxidase redox state

In recent years there has been widespread use of near infrared spectroscopy (NIRS) to monitor the brain. The signals of interest include changes in the levels of oxygenated and deoxygenated haemoglobin and tissue oxygen saturation. In addition to oxy- and deoxy-haemoglobin, the Cu(A) centre in cytochrome-c-oxidase (CCO) is a significant NIR absorber, giving rise to another signal termed the DeltaoxCCO signal. This signal has great potential as a marker of cellular oxygen metabolism, but is also the hardest to interpret. Here we use a recently constructed model to predict NIRS signal changes, and compare the model output to data from an in vivo hypoxia study in healthy adults. Our findings indicate strongly that the DeltaoxCCO signal contains useful information despite the noise, and has responses consistent with the known physiology.

A model of brain circulation and metabolism: NIRS signal changes during physiological challenges

We construct a model of brain circulation and energy metabolism. The model is designed to explain experimental data and predict the response of the circulation and metabolism to a variety of stimuli, in particular, changes in arterial blood pressure, CO(2) levels, O(2) levels, and functional activation. Significant model outputs are predictions about blood flow, metabolic rate, and quantities measurable noninvasively using near-infrared spectroscopy (NIRS), including cerebral blood volume and oxygenation and the redox state of the Cu(A) centre in cytochrome c oxidase. These quantities are now frequently measured in clinical settings; however the relationship between the measurements and the underlying physiological events is in general complex. We anticipate that the model will play an important role in helping to understand the NIRS signals, in particular, the cytochrome signal, which has been hard to interpret. A range of model simulations are presented, and model outputs are compared to published data obtained from both in vivo and in vitro settings. The comparisons are encouraging, showing that the model is able to reproduce observed behaviour in response to various stimuli.

Effect of age on brain oxygenation regulation during changes in position

INTRODUCTION

Reports indicate that brain regulation of oxygenation is inhibited in patients with low baseline oxyhemoglobin concentrations and that brain oxyhemoglobin concentrations are decreased with aging. The purpose of this study was to determine if regulation of brain oxygenation to changes in blood pressure is inhibited by normal aging.

METHODS

Brain oxyhemoglobin (OHb) and deoxyhemoglobin (HHb) concentrations were determined from the forehead using a frequency domain near infrared spectroscopy in 27 healthy volunteers. Subjects were separated into two groups by age (20-39, n=16; 40-60, n=11). Brain hemoglobin and non-invasive blood pressure were measured in (1) supine, (2) sitting, (3) supine and (4) sitting positions with 10-min equilibration intervals between each determination. Statistical differences were determined by two way repeated measures analysis of variance.

RESULTS

Young subjects were 28+/-5 years (mean+/-S.D.) and older subjects were 48+/-6 years. In supine position, OHb and HHb were 28.4+/-8.3 and 15.4+/-2.4micromol/L, respectively, in young; 22.4+/-5.7 and 13.4+/-2.9micromol/L, respectively, in older subjects, both P<0.05 between groups. Changing position from supine to sitting decreased OHb 5% and increased HHb 5% with no difference between groups.

CONCLUSIONS

There was a small but significant decrease in OHb and an increase in HHb from supine to sitting position, and this effect was similar between young and older subjects. Regulation of brain oxygenation during modest decreases in blood pressure did not change in normal aging to 60 years compared to young adults.

Effects of skin on bias and reproducibility of near-infrared spectroscopy measurement of cerebral oxygenation changes in porcine brain

The influence of skin on the bias and reproducibility of regional cerebral oxygenation measurements is investigated using cw near-infrared spectroscopy (NIRS). Receiving optodes are placed over the left and right hemispheres of a piglet (C3, C4 EEG placement code) and one transmitting optode centrally (Cz position). Optical densities (OD) are measured during stable normo, mild, and deep hypoxemia. This is done for skin condition 1: all optodes on the skin; skin condition 2: transmitting optode on the skin and one receiving optode on the skull; and skin condition 3: all optodes on the skull. Absolute changes of oxy- (cO2Hb), deoxyhemoglobin (cHHb), and total hemoglobin (ctHb) concentrations [micromolL] are calculated from the ODs. These absolute changes are calculated for each skin condition with respect to normoxic condition. Additionally, for skin condition 2, the difference of concentration changes between receiver 1 (skull) and receiver 2 (skin) is calculated. The effect of skin removal is an average increase of attenuation changes by a factor of 1.66 (=0.51 OD) and of the concentration changes due to the arterial oxygen saturation steps by 23%. We conclude that skin significantly influences regional oxygenation measurements. Nevertheless, it is hypothesized that the estimated concentration changes are dominated by changes of the oxygenation in the brain.

Trends in monitoring patients with aneurysmal subarachnoid haemorrhage

After aneurysmal subarachnoid haemorrhage (SAH), the clinical outcome depends upon the primary haemorrhage and a number of secondary insults in the acute post-haemorrhagic period. Some secondary insults are potentially preventable but prevention requires prompt recognition of cerebral or systemic complications. Currently, several neuro-monitoring techniques are available; this review describes the most frequently used techniques and discusses indications for their use, and their value in diagnosis and prognosis. None of the techniques, when considered in isolation, has proved sufficient after SAH. Furthermore, the use of multi-modality monitoring is hampered by a lack of clinical studies that identify combinations of specific techniques in terms of clinical information and reliability. However, ischaemia at the tissue level can be detected by intracerebral microdialysis technique. Used together with the conventional monitoring systems, for example intracranial pressure measurements, transcranial Doppler ultrasound and modern neuro-imaging, direct assessment of biochemical markers by intracerebral microdialysis is promising in the advancement of neurointensive care of patients with SAH. A successfully implemented monitoring system provides answers but it also raises valuable new questions challenging our current understanding of the brain injury after SAH.

Effect of acute exposure to 3660 m altitude on orthostatic responses and tolerance

Orthostatic reflexes were examined at 375 m and after 60 min of exposure in a hypobaric chamber at 3660 m using a 20-min 70 degrees head-up tilt (HUT) test. Mean arterial blood pressure, R wave-R wave interval (RRI), and mean cerebral blood flow velocity (MFV) were examined with coarse-graining spectral analysis. Of 14 subjects, 7 at 375 m and 12 at 3660 m were presyncopal. Immediately on arrival to high altitude, breathing frequency and MFV increased, and endtidal PCO2, RRI, RRI complexity, and the parasympathetic nervous system indicator decreased. MFV was similar in HUT at both altitudes. The sympathetic nervous system indicator increased with tilt at 3660 m, whereas parasympathetic nervous system indicator decreased with tilt at both altitudes. Multiple regression analysis of supine variables from either 375 or 3660 m and the time to presyncope at 3660 m indicated that, after 1 h of exposure, increased presyncope at altitude was the result of 1). ineffective peripheral vasoconstriction, despite increased cardiac sympathetic nervous system activity with HUT, and 2). insufficient cerebral perfusion owing to cerebral vasoconstriction as the result of hypoxic hyperventilation-induced hypocapnia.

[Cognitive functions and cerebral oxygenation of older patients after general and regional anaesthesia]

OBJECTIVE

The aim of the present study was to show the influence of cerebral oxygenation (regional cerebral oxygen saturation rSO(2) by near infrared spectroscopy) and of the nocturnal arterial oxygen saturation (SatO(2) by pulse oximetry) on the restitution of cognitive functions in patients aged between 40 and 85 years scheduled for elective hip arthroplasty.

METHODS

A total of 40 patients (ASA II) were randomized to be operated either in general anaesthesia or regional anaesthesia. The patients were additionally classified by age (40-64 years and 65-85 years). Cognitive functions were tested 14-16 h pre-operatively (t0), 1.5 h post-operatively (t1) and at the first and third postoperative days (t2 and t3). During testing, as well as during surgery and postoperatively until t1, rSO(2) was continuously measured. SatO(2) was measured in the night before surgery (N0) and for 3 nights after surgery (N1, N2, N3) between 22.00 p.m. and 5.00 a.m. These measurements were divided into interval groups (80-83%, 84-87%, 88-91%, 92-95%, 96-100%).

RESULTS

Almost all cognitive functions were significantly reduced at t1 in all groups compared to t0, but recovered up to the third postoperative day (t3). RSO(2) in contrast was significantly reduced in all groups compared to t0 at the third postoperative day (t3). The relative proportion of the intervals compared to the total measurement time for SatO(2) shifted in both anaesthesia procedures: before surgery (N0) the most frequented interval was 96-100%, after surgery (N1, N2) it was 88-91%. There was no correlation between rSO(2), the restitution of the tested cognitive functions and SatO(2).

CONCLUSIONS

Cognitive functions recovered completely during the first 3 postoperative days in patients scheduled for elective hip surgery under general or regional anaesthesia regardless of age and type of anaesthesia. This restitution of cognition occurred despite a significant decrease of cerebral oxygenation (rSO(2)) and despite an increase of nocturnal hypoxaemic intervals. Changes of the rsO(2) up to 3% below the baseline values (measured by NIRS) do not predict cognitive restitution. A minimal limiting value of the rSO(2) could not be defined.

The Oxygen Dependency of Cerebral Oxidative Metabolism in the Newborn Piglet Studied with 31P NMRS and NIRS

Mean cerebral saturation and changes in the oxidation state of the CuA centre of cytochrome oxidase were measured by near infra-red spectroscopy simultaneously with phosphorous metabolites and intracellular pH measured using 31P NMR spectroscopy during transient anoxia (inspired oxygen fraction = 0.0 for 105 seconds) in the newborn piglet brain. By collecting high quality 31P spectra every 10 seconds, it was possible to resolve the delay between the onset of anoxia and the fall in PCr and to show that the CuA centre of cytochrome oxidase reduced simultaneously with the fall in PCr. From these observations it is concluded that, at normoxia, oxygen tension at the mitochondrial level is substantially above a critical value at which oxidative metabolism becomes oxygen dependent.

Regional response of cerebral blood volume to graded hypoxic hypoxia in rat brain

BACKGROUND

The response of cerebral blood flow to hypoxic hypoxia is usually effected by dilation of cerebral arterioles. However, the resulting changes in cerebral blood volume (CBV) have received little attention. We have determined, using susceptibility contrast magnetic resonance imaging (MRI), changes in regional CBV induced by graded hypoxic hypoxia.

METHODS

Six anaesthetized rats were subjected to incremental reduction in the fraction of inspired oxygen: 0.35, 0.25, 0.15, and 0.12. At each episode, CBV was determined in five regions of each hemisphere after injection of a contrast agent: superficial and deep neocortex, striatum, corpus callosum and cerebellum. A control group (n = 6 rats) was studied with the same protocol without contrast agent, to determine blood oxygenation level dependent (BOLD) contribution to the MRI changes.

RESULTS

Each brain region exhibited a significant graded increase in CBV during the two hypoxic episodes: 10-27% of control values at 70% SaO2, and 26-38% at 55% SaO2. There was no difference between regions in their response to hypoxia. The mean CBV of all regions increased from 3.6 (SD 0.6) to 4.1 (0.6) ml (100 g)-1 and to 4.7 (0.7) ml (100 g)-1 during the two hypoxic episodes, respectively (Scheffé F-test; P < 0.01). Over this range, CBV was inversely proportional to SaO2 (r2 = 0.80). In the absence of the contrast agent, changes due to the BOLD effect were negligible.

CONCLUSIONS

These findings imply that hypoxic hypoxia significantly raises CBV in different brain areas, in proportion to the severity of the insult. These results support the notion that the vasodilatory effect of hypoxia is deleterious in patients with reduced intracranial compliance.

Optical spectroscopy and prevention of deleterious cerebral vascular effects of ethanol by magnesium ions

Previously, it has been suggested that acute ethanol (alcohol) administration can result in concentration-dependent vasoconstriction and decreased cerebral blood flow. Here, we present in vivo results using rapid (240 nm/min) optical backscatter measurements, with an intact cranial preparation in the rat, indicating that acute infusion of ethanol directly into the rat brain rapidly produces dose-dependent vasoconstriction of the cerebral microcirculation associated with a pronounced reduction in tissue blood content, pronounced rises in deoxyhemoglobin, significantly increased levels of reduced cytochrome oxidase and microvascular damage as the dose increases. Furthermore, we present in vivo experiments demonstrating the capability of magnesium ions (Mg(2+)) to attenuate and prevent these deleterious responses. Optical backscatter spectra (500-800 nm) were obtained by directing a single sending and receiving fiber to a portion of the left parietal cranium (in anesthetized rats), shaved to a translucent appearance to facilitate optical penetration. In the absence of added Mg(2+), infusion of a 10% solution of ethanol at 0.34 ml/min ( approximately 26.8 mg/min) produced prompt vasoconstriction as evidenced by a greater than 90% loss of oxyhemoglobin from the field-of-view and increases in levels of reduced cytochrome oxidase to between 50% and >90%. These effects were partially, to nearly completely, attenuated by the addition of MgCl(2) to the infusate containing added ethanol. Of special interest was the observation that attenuation of the vasoconstrictive effect of ethanol by Mg(2+) persisted despite a subsequent ethanol challenge without added Mg(2+). The results obtained demonstrate that, depending on dose, ethanol can produce prompt and severe vasoconstriction of the intact cerebral microcirculation and that infusion of moderate doses of Mg(2+) can largely attenuate and prevent this response. We conclude that appreciable, graded changes in cerebral cytochrome oxidase aa(3), blood volume and the state of hemoglobin occur at minimal tissue levels of ethanol which can be modulated by Mg(2+).

Monitors of cerebral oxygenation

None of the monitors of cerebral oxygenation discussed above has proven to be effective enough to have become a standard of care in any given area of medical treatment. As described above, each has specific and well-defined shortcomings that prevent its widespread use. These shortcomings may not be so much a failure of technology as an acknowledgement of the complexity of our goal: a monitor that can divide the entire brain into small, focal, and discrete areas and accurately measure the oxygen tension in each one. Because we are asking for the functional equivalent of 30 or 40 simultaneous PbtO2 probes, it is small wonder that we are not yet satisfied. Of the three monitors discussed here, the greatest potential may lie with the transcranial cerebral oximetry. The cerebral oximeter has the biggest potential for improvement because it holds the most potential for technical advancement. Although, for instance, jugular venous bulb oximetric catheters may become somewhat more accurate, the biggest drawbacks in that monitor's usefulness lie in human anatomy and intracerebral blood mixing, not catheter accuracy. PbtO2 probes, also, have little room for improvement. Although every technology can be refined, the PbtO2 probes are already accurate. The fact that they are an invasive monitor, and a regional one at that, will relegate them to a limited number of cases. Cerebral oximeters hold more potential. Their greatest limitations lie in technical aspects that can be, and hopefully will be, improved upon in terms of computer technology as well as algorithm accuracy. The fact that cerebral oximeters can be used on any patient, at any time, on almost any case, makes it, potentially, truly an ideal monitor for anesthesiologists and intensivists alike. There is no certainty that any of these limitations will be surmounted, at least to the degree necessary to achieve desired accuracy. But there is much to anticipate.

Orthostatic hypotension in elderly persons during passive standing: a comparison with young persons

BACKGROUND

The present study was aimed at clarifying the mechanism of orthostatic hypotension (OH) that occurs in elderly persons and at investigating assisting methods to prevent OH by evaluating changes in autonomic nervous system (ANS) activity and cerebral circulation of elderly persons when engaged in passive standing.

METHODS

Eight elderly volunteers and 9 young volunteers gave informed consent to participate in the study. Two experimental conditions were established: (i) "active standing," in which the subjects stood on their own with guidance from an assistant, and (ii) "passive standing," in which the subjects were placed in a standing position completely by an assistant. ANS was determined before and after standing by measuring the heart rate variability. The reaction of the ANS was evaluated on the basis of low-frequency power (LF: 0.05--0.15 Hz) and high-frequency power (HF: 0.15--0.4 Hz), which were separated from the R-R interval data by power spectral analysis using the fast Fourier transformation. Cerebral perfusion was measured over the right frontal region using a near-infrared spectroscopy cerebral oxygen monitor.

RESULTS

The main findings were: (i) Transient decreases in blood pressure occurred immediately after standing in both the young and elderly subjects. (ii) The LF:HF ratio increased significantly ( p <.05) immediately after active standing in the young subjects, whereas this ratio increased in the elderly subjects after some delay. (iii) The LF:HF ratio increased significantly ( p <.01) immediately after passive standing in the young subjects, whereas this ratio decreased significantly ( p <.05) in the elderly subjects. (iv) In the elderly subjects, the total hemoglobin (HbT) and oxyhemoglobin showed the greatest decrease during the 15-second period after standing. The maximum changes in the HbT with passive standing differed significantly ( p <.01) from those observed during active standing.

CONCLUSIONS

Our findings emphasize the need to devise bioengineered means that allow elderly persons to exert themselves, to maintain or improve muscle contractility and ANS function, while providing minimum assistance for standing.

Monitoring neurologic patients in intensive care

The brain is sensitive to changes in substrate delivery. In neurologically critically ill patients (e.g., those with head injury, subarachnoid hemorrhage, or stroke), interruption of this supply causes ischemic brain damage and thus impairs the outcome. To prevent, detect, and treat these ischemic events as soon as possible, the cerebral blood flow is continuously monitored, its coupling or not with the consumption of oxygen and so forth, and the detected derangements of normal physiology. Intracranial pressure and cerebral perfusion pressure are two parameters that often reflect ischemic events, and thus it is mandatory to continuously measure them. To better assess cerebral hemodynamics, jugular bulb oxymetry and brain pressure tissue oxygen monitoring are two neuromonitoring techniques that allow for a better understanding of the balance between oxygen supply and consumption, and therefore are useful in directing therapy. Transcranial Doppler ultrasonography is a noninvasive technique with the same purpose but with less clinical relevance. The new neuromonitoring technique, microdialysis, is useful for understanding the mechanisms involved in brain ischemia. However, it is clear that the physician who interprets the measurements given by devices and the clinical data (e.g., temperature, glycemia) is still the cornerstone in the management of neurologically critically ill patients.

Oxygen dependency and precision of cytochrome oxidase signal from full spectral NIRS of the piglet brain

Oxidation changes of the copper A (Cu(A)) center of cytochrome oxidase in the brain were measured during brief anoxic swings at both normocapnia and hypercapnia (arterial PCO(2) approximately 55 mmHg). Hypercapnia increased total hemoglobin from 37.5 +/- 9.1 to 50.8 +/- 12.9 micromol/l (means +/- SD; n = 7), increased mean cerebral saturation (Smc(O(2))) from 65 +/- 4 to 77 +/- 3%, and oxidized Cu(A) by 0.43 +/- 0.23 micromol/l. During the onset of anoxia, there were no significant changes in the Cu(A) oxidation state until Smc(O(2)) had fallen to 43 +/- 5 and 21 +/- 6% at normocapnia and hypercapnia, respectively, and the maximum reduction during anoxia was not significantly different at hypercapnia (1.49 +/- 0.40 micromol/l) compared with normocapnia (1.53 +/- 0.44 micromol/l). Residuals of the least squares fitting algorithm used to convert near-infrared spectra to concentrations are presented and shown to be small compared with the component of attenuation attributed to the Cu(A) signal. From these observations, we conclude that there is minimal interference between the hemoglobin and Cu(A) signals in this model, the Cu(A) oxidation state is independent of cerebral oxygenation at normoxia, and the oxidation after hypercapnia is not the result of increased cerebral oxygenation.

Near-infrared spectroscopy of the adult head: effect of scattering and absorbing obstructions in the cerebrospinal fluid layer on light distribution in the tissue

Previous modeling of near-infrared (NIR) light distribution in models of the adult head incorporating a clear nonscattering cerebrospinal fluid (CSF) layer have shown the latter to have a profound effect on the resulting photon measurement density function (PMDF). In particular, the presence of the CSF limits the PMDF largely to the outer cortical gray matter with little signal contribution from the deeper white matter. In practice, the CSF is not a simple unobstructed clear layer but contains light-scattering membranes and is crossed by various blood vessels. Using a radiosity-diffusion finite-element model, we investigated the effect on the PMDF of introducing intrusions within the clear layer. The results show that the presence of such obstructions does not significantly increase the light penetration into the brain tissue, except immediately adjacent to the obstruction and that its presence also increases the light sampling of the adjacent skull tissues, which would lead to additional contamination of the NIR spectroscopy signal by the surface tissue layers.

Effects of nitric oxide and peroxynitrite on the cytochrome oxidase K(m) for oxygen: implications for mitochondrial pathology

This review summarises current knowledge about the effect of oxygen on cytochrome oxidase activity in vitro and in vivo. Cytochrome oxidase normally operates above its K(m) for oxygen in vivo. However, decreases in the intracellular oxygen concentration (hypoxia) under physiological extremes, or during pathophysiology, can cause mitochondrial respiration to become oxygen limited. Inhibitors that raise the enzyme's K(m) will induce oxygen limitation under apparently normoxic conditions. It is known that the concentrations of nitric oxide and peroxynitrite are raised in a number of pathophysiological conditions. These compounds are capable of reversibly and irreversibly raising the cytochrome oxidase K(m) for oxygen. Therefore, measurements of cell and mitochondrial respiration in vitro that fail to systematically vary oxygen through the range of physiological concentrations are likely to underestimate the effects of nitric oxide and peroxynitrite in vivo.

[Monitoring of severely head-injured patients during the first 24 hours]

Monitoring of severely head-injured patients is essential to optimize cerebral haemodynamics and thus to limit intracranial hypertension and to prevent the occurrence of secondary systemic cerebral injuries. It includes continuous measurement of intracranial pressure, mean arterial pressure and venous jugular oxygen saturation. Assessment of circulatory velocity in the arteries of the polygon of Willis allows identification of intracranial hypertension, and later, a vasospasm in case of subarachnoid haemorrhage. Near infrared spectroscopy is an indirect indicator of cerebral oxygenation. This technique has not yet been validated. Direct tissue measurement of cerebral oxygen content is the most recently developed monitoring tool. Its clinical range of application has still to be specified. These basic monitoring techniques are a standard. They are essential for an undelayed and efficient treatment of complications occurring in head trauma patients.

Oxygen dependency of cerebral oxidative phosphorylation in newborn piglets

Changes in hemoglobin oxygenation and oxidation state of the CuA centre of cytochrome oxidase were measured with full spectral near infrared spectroscopy simultaneously with phosphorus metabolites using nuclear magnetic resonance 31P spectroscopy at high time resolution (10 seconds) during transient anoxia (FiO2 = 0.0 for 105 seconds) in the newborn piglet brain. During the onset of anoxia, there was no change in either phosphocreatine (PCr) concentration or the oxidation state of the CuA centre of cytochrome oxidase until there was a substantial fall in cerebral hemoglobin oxygenation, at which point the CuA centre reduced simultaneously with the decline in PCr. At a later time during the anoxia, intracellular pH decreased rapidly, consistent with a fall in cerebral metabolic rate for O2 and reduced flux through the tricarboxylic acid cycle. The simultaneous reduction of CuA and decline in PCr can be explained in terms of the effects of the falling mitochondrial electrochemical potential. From these observations, it is concluded that, at normoxia, oxidative phosphorylation and the oxidation state of the components of the electron transport chain are independent of cerebral oxygenation and that the reduction in the CuA signal occurs when oxygen tension limits the capacity of oxidative phosphorylation to maintain the phosphorylation potential.

Near-infrared oximetry of the brain

Near-infrared (IR) light easily penetrates biological tissue, and the information offered by in vivo spectroscopy of cerebral oxygenation is detailed and comes with a high temporal resolution. Near-IR light spectroscopy (NIRS) reflects cerebral oxygenation during arterial hypotension, hypoxic hypoxaemia and hypo- and hypercapnia. As determined by dual-wavelength NIRS, the cerebral O2 saturation integrates the arterial O2 content and the cerebral perfusion, and as established for skeletal muscle, NIRS obtains information on tissue oxygenation and metabolism beyond that obtained by venous blood sampling. Caveats of cerebral NIRS include insufficient light shielding, optode displacement and a sample volume including muscle or the frontal sinus mucous membrane. The relative influence from the extracranial tissue is minimized by optode separation and correction for an extracranial sample volume, or both. The natural pigment melatonin and also water are of little influence to spectroscopic analysis of cerebral oxygenation, whereas bilirubin systematically lowers ScO2 and attenuates the detection of changes in cerebral oxygenation. By NIRS, reduction of cytochrome oxidase is demonstrated during hypoxic hypoxaemia and head-up tilt-induced arterial hypotension, but the changes are small. In the clinical setting, NIRS offers useful information in patients with both systemic and local cerebral circulatory impairment, for example, during cranial trauma, surgery on the cerebral arteries, orthostasis and acute heart failure. Whereas mapping of the brain circulation is needed for jugular venous sampling to reflect either global or local oxygenation, the determination of cerebral oxygenation by NIRS has the advantage of localized monitoring of the cerebral cortex.

Redox status of cytochrome a,a3: a noninvasive indicator of dysoxia in regional hypoxic or ischemic hypoxia

OBJECTIVE

Multiwavelength near infrared (NIR) spectrophotometry can monitor the redox state of cytochrome a,a3 (cyt a,a3) in vivo. Because cyt a,a3 is the most immediate reductant of oxygen, this technique has been proposed to evaluate tissue oxygenation. The purpose of this study was to examine the relationship between cyt a,a3 oxidation level as an indicator of dysoxia and oxygen uptake (VO2) when oxygen delivery (DO2) was progressively lowered in an in situ vascularly isolated hindlimb.

DESIGN

Prospective, randomized, laboratory study.

SETTING

University research laboratory.

SUBJECTS

Fourteen pigs.

INTERVENTIONS

Measurement of critical values for both VO2 and cyt a,a3 oxidation during ischemic and hypoxic hypoxia.

MEASUREMENTS AND MAIN RESULTS

The right hindlimb of anesthetized, paralyzed, and ventilated pigs was subjected to progressive ischemic or hypoxic hypoxia for 100 mins by ten stepwise decreases in DO2. In ischemic hypoxia (n = 7), arterial inflow (Q) from a pump-membrane oxygenator system was lowered from 50 to 0 mL/min, with PaO2 maintained at 100 mm Hg. In hypoxic hypoxia (n = 6), PaO2 was lowered from 100 mm Hg to 0 mm Hg. Hindlimb DO2 was calculated as the product of Q and arterial oxygen content, and VO2 as the product of Q and arteriovenous difference. The cyt a,a3 oxidation level was measured every 10 secs with a four-wavelength spectrophotometer. These parameters were measured 9 mins after each change of DO2. Critical values for both VO2 and cyt a,a3 oxidation level as a function of DO2 were determined in each animal by dual linear regression analysis. In ischemic and hypoxic hypoxia, a strong correlation was found between cyt a,a3 oxidation level and VO2 in both ischemic and hypoxic hypoxia (r2 =.90 and .87, respectively). Hindlimb vascular resistance increased in ischemic hypoxia and decreased in hypoxic hypoxia when DO2 reached critical DO2.

CONCLUSIONS

From these results, we concluded that monitoring the cyt a,a3 redox state by NIR spectrophotometry is, in this experimental setting, a sensitive indicator of dysoxia during regional hypoxic or ischemic hypoxia.

Experimental and theoretical comparison of NIR spectroscopy measurements of cerebral hemoglobin changes

Two near-infrared spectroscopy (NIRS) methods are available for measuring changes (Delta) in total cerebral hemoglobin concentration (CHC): 1) a continuous measurement of the changes in total hemoglobin concentration (Delta[Hb]tot) and 2) the difference between two absolute measurements of CHC, each derived from a small, controlled change in inspired O2 fraction. This paper investigates the internal consistency of these two methods by using an experimental and theoretical comparison. NIRS was used to measure [Hb]tot in five newborn piglets before and after a change in arterial PCO2. Delta[Hb]tot demonstrated a low coefficient of variation of 2.8 +/- 2.8 (SD) % which allowed changes in CO2-cerebral blood volume reactivity to be clearly discriminated. However, a high coefficient of variation of 22.8 +/- 3.5% on the DeltaCHC measurements obscured any CO2 reactivity changes. A theoretical analysis demonstrates the effects of optical pathlength, background absorption, scatter, and blood vessel diameter on both methods. For more accurate monitoring of CHC, individual measurements of optical pathlength and more accurate pulse oximetry are required.

The relationship of oxygen delivery to absolute haemoglobin oxygenation and mitochondrial cytochrome oxidase redox state in the adult brain: a near-infrared spectroscopy study

Near-infrared spectroscopy was used to determine the effect of changes in the rate of oxygen delivery to the adult rat brain on the absolute concentrations of oxyhaemoglobin, deoxyhaemoglobin and the redox state of the CuA centre in mitochondrial cytochrome oxidase. The cytochrome oxidase detection algorithm was determined to be robust to large changes in haemoglobin oxygenation and concentration. By assuming complete haemoglobin deoxygenation and CuA reduction following mechanical ventilation on 100% N2O, the absolute concentration of oxyhaemoglobin (35 microM), deoxyhaemoglobin (27 microM) and the redox state of CuA (82% oxidized) were calculated in the normal adult brain. The mean arterial blood pressure was decreased by exsanguination. When the pressure reached 100 mmHg, haemoglobin oxygenation started to fall, but the total haemoglobin concentration and oxidized CuA levels only fell when cerebral blood volume autoregulation mechanisms failed at 50 mmHg. Haemoglobin oxygenation fell linearly with decreases in the rate of oxygen delivery to the brain, but the oxidized CuA concentration did not start to fall until this rate was 50% of normal. The results suggest that the brain maintains more than adequate oxygen delivery to mitochondria and that near-infrared spectroscopy may be a good measure of oxygen insufficiency in vivo.

Preoperative carbon dioxide reactivity studies do not predict the hemodynamic changes seen during carotid endarterectomy after internal carotid artery clamping

BACKGROUND AND PURPOSE

A preoperative warning of progressive cerebral ischemia during surgery may be of use in planning carotid endarterectomy (CE). We have set out to determine whether preoperative cerebrovascular reactivity studies predict the late hemodynamic changes detected during the procedure.

PATIENTS AND METHODS

Sixty-seven patients awaiting CE for high-grade carotid stenosis underwent preoperative cerebral CO(2) reactivity assessment using standard transcranial Doppler (TCD) methods. During subsequent surgery, hemodynamic changes were monitored with TCD, near-infrared spectroscopy, and a cerebral function monitor. Signals of mean MCA flow velocity (FV) and of cerebral oxyhemoglobin and deoxyhemoglobin concentration were recorded continuously, and the mean values were measured at 5, 10, and 15 minutes after internal carotid clamping. For each variable, the maximum signal change was measured 5 minutes after clamp application (Max(5)), and in addition the spontaneous hemodynamic recovery over the next 10 minutes (I(rec)) was calculated. Scatterplots of Max(5) and I(rec) against preoperative CO(2) reactivity indices (ipsilateral, bihemispheric mean, and interhemispheric assymmetry) were generated for correlative analysis.

RESULTS

The Max(5) for oxyhemoglobin and deoxyhemoglobin were inversely related (r=-.302, P=.015). There was no relationship between ipsilateral preoperative CO(2) reactivity index and Max(5) for any intraoperative variable measured. In addition, the reactivity did not predict the hemodynamic recovery (I(rec)) observed during clamp application. However, the interhemispheric asymmetry did show a modest inverse correlation to Max(5) FV (r=-.256, P=.046).

CONCLUSION

Preoperative TCD cerebral CO(2) reactivity cannot be used clinically to predict the intraoperative hemodynamic changes seen during CE.

Measurement of cytochrome oxidase and mitochondrial energetics by near-infrared spectroscopy

Cytochrome oxidase is the terminal electron acceptor of the mitochondrial respiratory chain. It is responsible for the vast majority of oxygen consumption in the body and essential for the efficient generation of cellular ATP. The enzyme contains four redox active metal centres; one of these, the binuclear CuA centre, has a strong absorbance in the near-infrared that enables it to be detectable in vivo by near-infrared spectroscopy. However, the fact that the concentration of this centre is less than 10% of that of haemoglobin means that its detection is not a trivial matter. Unlike the case with deoxyhaemoglobin and oxyhaemoglobin, concentration changes of the total cytochrome oxidase protein occur very slowly (over days) and are therefore not easily detectable by near-infrared spectroscopy. However, the copper centre rapidly accepts and donates an electron, and can thus change its redox state quickly; this redox change is detectable by near-infrared spectroscopy. Many factors can affect the CuA redox state in vivo (Cooper et al. 1994), but most significant is likely to be the molecular oxygen concentration (at low oxygen tensions, electrons build up on CuA as reduction of oxygen by the enzyme starts to limit the steady-state rate of electron transfer). The factors underlying haemoglobin oxygenation, deoxygenation and blood volume changes are, in general, well understood by the clinicians and physiologists who perform near-infrared spectroscopy measurements. In contrast, the factors that control the steady-state redox level of CuA in cytochrome oxidase are still a matter of active debate, even amongst biochemists studying the isolated enzyme and mitochondria. Coupled with the difficulties of accurate in vivo measurements it is perhaps not surprising that the field of cytochrome oxidase near-infrared spectroscopy has a somewhat chequered past. Too often papers have been written with insufficient information to enable the measurements to be repeated and few attempts have been made to test the algorithms in vivo. In recent years a number of research groups and commercial spectrometer manufacturers have made a concerted attempt to not only say how they are attempting to measure cytochrome oxidase by near-infrared spectroscopy but also to demonstrate that they are really doing so. We applaud these attempts, which in general fall into three areas: first, modelling of data can be performed to determine what problems are likely to derail cytochrome oxidase detection algorithms (Matcher et al. 1995); secondly haemoglobin concentration changes can be made by haemodilution (using saline or artificial blood substitutes) in animals (Tamura 1993) or patients (Skov & Greisen 1994); and thirdly, the cytochrome oxidase redox state can be fixed by the use of mitochondrial inhibitors and then attempts make to cause spurious cytochrome changes by dramatically varying haemoglobin oxygenation, haemoglobin concentration and light scattering (Cooper et al. 1997). We have previously written reviews covering the difficulties of measuring the cytochrome near-infrared spectroscopy signal in vivo (Cooper et al. 1997) and the factors affecting the oxidation state of cytochrome oxidase CuA (Cooper et al. 1994). In this article we would like to strike a somewhat more optimistic note--we will stress the usefulness this measurement may have in the clinical environment, as well as describing conditions under which we can have confidence that we are measuring real changes in the CuA redox state.

Internal and external carotid contributions to near-infrared spectroscopy during carotid endarterectomy

BACKGROUND AND PURPOSE

The internal carotid (ICA) and external carotid (ECA) contributions to changing concentrations of oxyhemoglobin (Hbo2) and deoxyhemoglobin (Hb) during carotid endarterectomy were assessed with the use of near-infrared spectroscopy (NIRS).

METHODS

NIRS optodes were placed on the forehead with an interoptode distance of 6 cm, and laser-Doppler flowmetry (LDF) was used to monitor the change in skin blood flow between the optodes. Hb, Hbo2, LDF, arterial blood pressure, and middle cerebral artery flow velocity were recorded continuously. The ECA was clamped 2 minutes before the ICA was clamped. Suitable multimodal recordings were achieved in 44 patients.

RESULTS

When the ECA was clamped, 76% of patients showed a fall in Hbo2 and 65% an increase in Hb. When corrected for changes in arterial blood pressure, an accompanying fall in cutaneous LDF predicted the fall in Hbo2 with high sensitivity (100%) and specificity (100%). Among those with no NIRS changes during ECA clamping, 56% had severe ECA stenosis or occlusion; none of these showed an accompanying fall in LDF. In contrast, when the ICA was clamped, substantial additional changes in NIRS occurred in 55% of cases, all of which were associated with a fall in flow velocity, but none with a change in LDF. Patients with a constant flow velocity after ICA clamping also showed no change in NIRS.

CONCLUSIONS

Both the ECA and ICA vascular territories contribute to NIRS changes during carotid endarterectomy. The external carotid contribution to NIRS can be monitored with cutaneous LDF.

Near-infrared spectroscopy: continuous measurement of cytochrome oxidation during hemorrhagic shock

OBJECTIVE

Mitochondrial cytochrome a,a3 redox shifts can be determined by near-infrared wavelength reflection. Since near-infrared wavelengths penetrate skin and bone, a potential exists to noninvasively measure mitochondrial oxidation using this phenomenon. The purpose of this study was to compare conventional parameters of resuscitation with regional measurements of spectroscopically derived cytochrome redox state in a hemorrhagic shock model.

DESIGN

Prospective, controlled laboratory investigation.

SETTING

Animal research laboratory of a university medical center.

SUBJECTS

New Zealand white rabbits (n = 23), weighing 2 to 3 kg.

INTERVENTIONS

After anesthesia and instrumentation, the subjects underwent laparotomy with placement of near-infrared spectroscopy probes on the stomach, liver, kidney, and hamstring muscle. Baseline measurements were obtained, and phlebotomy was used to reduce cardiac output by 60% for 30 mins. Animals were resuscitated with shed autologous blood and crystalloid to reach baseline cardiac output (0.9%), and were monitored for an additional 60 mins.

MEASUREMENTS AND MAIN RESULTS

Significant correlations between mitochondrial cytochrome a,a3 redox state, cardiac output, and oxygen delivery were observed throughout shock and resuscitation. However, gastric cytochrome oxidation did not recover after shock, despite systemic evidence of adequate resuscitation (p < .05).

CONCLUSIONS

Resuscitation from severe hemorrhagic shock may not uniformly restore cellular oxygenation, despite normalization of traditional parameters of resuscitation. Direct monitoring of cytochrome oxidation may be useful in identifying regional areas of dysoxia.

Theoretical and experimental investigation of near-infrared light propagation in a model of the adult head

Near-infrared light propagation in various models of the adult head is analyzed by both time-of-flight measurements and mathematical prediction. The models consist of three- or four-layered slabs, the latter incorporating a clear cerebrospinal fluid (CSF) layer. The most sophisticated model also incorporates slots that imitate sulci on the brain surface. For each model, the experimentally measured mean optical path length as a function of source-detector spacing agrees well with predictions from either a Monte Carlo model or a finite-element method based on diffusion theory or a hybrid radiosity-diffusion theory. Light propagation in the adult head is shown to be highly affected by the presence of the clear CSF layer, and both the optical path length and the spatial sensitivity profile of the models with a CSF layer are quite different from those without the CSF layer. However, the geometry of the sulci and the boundary between the gray and the white matter have little effect on the detected light distribution.

The effect of scalp ischaemia on measurement of cerebral blood volume by near-infrared spectroscopy

Near-infrared spectroscopy (NIRS) is a noninvasive method of quantifying changes in cerebral haemodynamics from changes in the absorption of near-infrared light by oxyhaemoglobin and deoxyhaemoglobin. Measurement of neonatal cerebral blood volume (CBV) by NIRS was described in 1990 but it has been suggested that, in adults, scalp and skull blood content contribute a significant amount to the cerebral haemodynamic variables quantifiable by NIRS. To investigate this, CBV was measured in nine adult subjects, in the frontal region of the head, before and after inflating a pneumatic tourniquet proximal to the measurement site. Because a change in scalp blood content could potentially alter the pathlength of light passing through the head and hence affect the measured CBV, the optical pathlength factor was therefore also measured before and after tourniquet inflation. Blood flow occlusion was confirmed by laser Doppler velocimetry. The results showed that tourniquet inflation had no effect on the estimated value of CBV or the differential pathlength factor. We conclude that, provided the distance between light entry and exit on the surface of the scalp is sufficiently large, changes in scalp blood flow have no effect on NIRS measurement of cerebral haemodynamics.

Near-infrared spectroscopy in experimental pneumococcal meningitis in the rabbit: cerebral hemodynamics and metabolism

Near-infrared spectroscopy is a noninvasive technique which measures oxidized cytochrome aa3, oxygenated Hb, and deoxygenated Hb and calculates total Hb in tissue. This technique, in conjunction with measurement of cerebral blood flow, was used in rabbits with experimental bacterial meningitis to determine whether there was evidence for cerebral energy depletion and alterations in the cerebral vascular bed with infection. Rabbits with meningitis had a significant reduction in cerebral blood flow, cerebral oxidized cytochrome aa3 and a relative increase in the deoxygenated Hb fraction and a decrease in the oxygenated Hb fraction compared with uninfected controls. Total Hb was not significantly different between the two groups. These findings may help clarify the mechanism for some of the intracranial pathophysiologic abnormalities in meningitis.

Near-infrared spectroscopy. Clinical applications

NIRS is an attractive monitoring technology because it is a noninvasive, real-time, repeatable method that allows for regional assessment of the adequacy of tissue oxygenation. It is able to evaluate the oxygenation state of hemoglobin in tissue and redox state of cyt a1,a3, which reflects the overall activity of oxidative metabolism in the cells. The primary technical limitation of current technology is the inability to measure accurately the optical pathlength online, limiting the ability to quantify precisely concentrations of oxygen-dependent chromophores. Even with this limitation, NIRS can provide unique and valuable in vivo metabolic information without invasive intervention.

Cerebral Monitoring by Near-Infrared Spectroscopy

Near-infrared spectroscopy (NIRS) is a noninvasive optical monitoring technology that can provide information on relative cerebral oxyhemoglobin, deoxyhemoglobin, and oxidized cytochrome aa3 concentrations. Unlike other bedside clinical monitoring technologies, NIRS provides direct information on cerebral hemoglobin oxygenation and blood volume changes. NIRS recordings have documented changes in cerebral oxygenation in fetuses during labor, critically ill premature infants, cardiopulmonary bypass patients, and adult surgical patients. This information may improve understanding of the causes of brain injury and allow detection of inadequate oxygen delivery before brain injury occurs. Future developments in NIRS technology are likely to produce practical, quantitative bedside monitors of global and regional cerebral oxygenation.

Near-infrared monitoring of cerebral oxygenation state during carotid endarterectomy

BACKGROUND

Recent studies have indicated that near-infrared spectroscopy (NIRS) could continuously and noninvasively observe the changes in cerebral oxygenation state during hypoxia and ischemia, using their optical properties. Its validity and usefulness during cerebrovascular surgery, however, still remain to be clarified.

METHODS

Using NIRS, we continuously monitored the changes in the concentration of oxyhemoglobin, deoxyhemoglobin, and total hemoglobin ([oxy-HB], [deoxy-Hb], and [total Hb], respectively) and redox state of cytochrome oxidase (cyt ox) during carotid endarterectomy for 22 patients, and we compared the NIRS responses with those of intraoperative somatosensory evoked potentials (SEP) and regional cerebral blood flow (rCBF).

RESULTS

In 9 of 22 patients, cross-clamping of the carotid artery caused a continuous decrease [oxy-Hb] and [total Hb], and an increase in [deoxy-Hb]. Cyt ox was partially reduced during the clamping. These NIRS responses demonstrated the occurrence of severe hypoxia in the ipsilateral cerebral tissue. These patients showed a marked decrease in the N20 amplitude of SEP and rCBF. In contrast, the other 13 patients did not show a significant decrease in the cerebral oxygenation state, which showed no remarkable changes in either SEP or in rCBF.

CONCLUSIONS

NIRS could successfully jude the cerebral oxygenation state noninvasively during carotid surgery and was more sensitive to ischemic crisis than other indirect methods.

Measurement of cranial optical path length as a function of age using phase resolved near infrared spectroscopy

Near infrared spectroscopy (NIRS) has been used to measure concentration changes of cerebral hemoglobin and cytochrome in neonates, children, and adults, to study cerebral oxygenation and hemodynamics. To derive quantitative concentration changes from measurements of light attenuation, the optical path length must be known. This is obtained by multiplying the source/ detector separation by a laboratory measured differential path length factor (DPF) which accounts for the increased distance traveled by light due to scattering. DPF has been measured by time of flight techniques on small populations of adults and postmortem infants. The values for adults are greater than those for newborns, and it is not clear how to interpolate the present data for studies on children. Recent developments in instrumentation using phase resolved spectroscopy techniques have produced a bedside unit which can measure optical path length on any subject. We have developed an intensity modulated optical spectrometer which measures path length at four wavelengths. Two hundred and eighty three subjects from 1 d of age to 50 y were studied. Measurements were made at a fixed frequency of 200 MHz and a source detector separation of 4.5 cm. Results suggest a slowly varying age dependence of DPF, following the relation DPF690 = 5.38 + 0.049A0.877, DPF744 = 5.11 + 0.106A0.723, DPF807 = 4.99 + 0.067A0.814, and DPF832 = 4.67 + 0.062A0.819, where DPF690 is the DPF measured at 690 nm and A is age is expressed in years from full term. There was a wide scatter of values, however, implying that ideally DPF should be measured at the time of each study.

Near-infrared spectroscopy: theory and applications

In conclusion, NIRS appears to offer both a new monitoring modality and new information about cerebral oxygenation. Technical problems in the application of this technology persist, most notably determination of pathlength and the volume of tissue interrogated. Those familiar with the history of pulse oximetry will recall that although Millikan developed an ear oximeter in 1947, it was not until Aoyagi combined recognition of the pulse signal with spectroscopy in the 1970s that oximetry was transformed into a clinically applicable monitor. In much the same way, NIRS may find the same tremendous usefulness as a noninvasive monitor of cerebral oxygen utilization, pending resolution of the remaining technical problems.

Multimodal monitoring in neurointensive care

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The influence of carbon dioxide and body position on near-infrared spectroscopic assessment of cerebral hemoglobin oxygen saturation

Near-infrared spectroscopy may allow continuous and noninvasive monitoring of regional brain hemoglobin oxygen saturation by measuring the differential absorption of infrared light by oxyhemoglobin and deoxyhemoglobin. We have previously examined the correlation between the spectroscopic signal generated by a prototype cerebral oximeter (Invos 3100; Somanetics, Troy, MI), and global brain hemoglobin oxygen saturation calculated from arterial and jugular venous bulb oxygen saturations. Because the technology does not distinguish between arterial and venous hemoglobin saturation, changes in the proportion of cerebral arterial and venous blood volume, which may result from changes in blood flow or venous distending pressure, may confound measurements. In eight conscious volunteers breathing hypoxic oxygen mixtures, we examined the influence of supine, 20 degrees Trendelenburg, and 20 degrees reverse Trendelenburg positions on the correlation of the spectroscopic measurement of cerebral oxygen saturation in the field assessed by the probe (CSfO2) and the calculated brain hemoglobin oxygen saturation (CScombO2), estimated as 0.25 x arterial saturation plus 0.75 x jugular venous bulb oxygen saturation. We found that changes in position did not influence the association between CSfO2 and CScombO2 (r2 = 0.69-0.885) during hypoxic challenge. In a second set of eight volunteers, we studied the influence of hypercapnia and hypocapnia and body position on the association between CSfO2 and CScombO2, and found that they were less well correlated (r2 = 0.366-0.976) in individual patients. Because changes in body position and Paco2 confound the relationship between CSfO2 and CScombO2, changes in CSfO2 can best be assessed if position and Paco2 are constant.