Use of near infrared spectroscopy to estimate cerebral blood flow in conscious and anaesthetized adult subjects

Lawrence K. Direct assessment of extracerebral signal contamination on optical measurements of cerebral blood flow, oxygenation, and metabolism

Significance: Near-infrared spectroscopy (NIRS) combined with diffuse correlation spectroscopy (DCS) provides a noninvasive approach for monitoring cerebral blood flow (CBF), oxygenation, and oxygen metabolism. However, these methods are vulnerable to signal contamination from the scalp. Our work evaluated methods of reducing the impact of this contamination using time-resolved (TR) NIRS and multidistance (MD) DCS. Aim: The magnitude of scalp contamination was evaluated by measuring the flow, oxygenation, and metabolic responses to a global hemodynamic challenge. Contamination was assessed by collecting data with and without impeding scalp blood flow. Approach: Experiments involved healthy participants. A pneumatic tourniquet was used to cause scalp ischemia, as confirmed by contrast-enhanced NIRS, and a computerized gas system to generate a hypercapnic challenge. Results: Comparing responses acquired with and without the tourniquet demonstrated that the TR-NIRS technique could reduce scalp contributions in hemodynamic signals up to 4 times (r SD = 3    cm ) and 6 times (r SD = 4    cm ). Similarly, blood flow responses from the scalp and brain could be separated by analyzing MD DCS data with a multilayer model. Using these techniques, there was no change in metabolism during hypercapnia, as expected, despite large increases in CBF and oxygenation. Conclusion: NIRS/DCS can accurately monitor CBF and metabolism with the appropriate enhancement to depth sensitivity, highlighting the potential of these techniques for neuromonitoring.

Cerebral perfusion and blood-brain barrier assessment in brain trauma using contrast-enhanced near-infrared spectroscopy with indocyanine green: A review

Contrast-enhanced near-infrared spectroscopy (NIRS) with indocyanine green (ICG) can be a valid non-invasive, continuous, bedside neuromonitoring tool. However, its usage in moderate and severe traumatic brain injury (TBI) patients can be unprecise due to their clinical status. This review is targeted at researchers and clinicians involved in the development and application of contrast-enhanced NIRS for the care of TBI patients and can be used to design future studies. This review describes the methods developed to monitor the brain perfusion and the blood-brain barrier integrity using the changes of diffuse reflectance during the ICG passage and the results on studies in animals and humans. The limitations in accuracy of these methods when applied on TBI patients and the proposed solutions to overcome them are discussed. Finally, the analysis of relative parameters is proposed as a valid alternative over absolute values to address some current clinical needs in brain trauma care. In conclusion, care should be taken in the translation of the optical signal into absolute physiological parameters of TBI patients, as their clinical status must be taken into consideration. Discussion on where and how future studies should be directed to effectively incorporate contrast-enhanced NIRS into brain trauma care is given.

Near-Infrared Confocal Laser Reflectance Cytoarchitectural Imaging of the Substantia Nigra and Cerebellum in the Fresh Human Cadaver

BACKGROUND

Cytoarchitectural neuroimaging remains critical for diagnosis of many brain diseases. Fluorescent dye-enhanced, near-infrared confocal in situ cellular imaging of the brain has been reported. However, impermeability of the blood-brain barrier to most fluorescent dyes limits clinical utility of this modality. The differential degree of reflectance from brain tissue with unenhanced near-infrared imaging may represent an alternative technique for in situ cytoarchitectural neuroimaging.

METHODS

We assessed the utility of unenhanced near-infrared confocal laser reflectance imaging of the cytoarchitecture of the cerebellum and substantia nigra in 2 fresh human cadaver brains using a confocal near-infrared laser probe. Cellular images based on near-infrared differential reflectance were captured at depths of 20-180 μm from the brain surface. Parts of the cerebellum and substantia nigra imaged using the probe were subsequently excised and stained with hematoxylin and eosin for histologic correlation.

RESULTS

Near-infrared reflectance imaging revealed the 3-layered cytoarchitecture of the cerebellum, with Purkinje cells appearing hyperreflectant. In the substantia nigra, neurons appeared hyporeflectant with hyperreflectant neuromelanin cytoplasmic inclusions. Cytoarchitecture of the cerebellum and substantia nigra revealed on near-infrared imaging closely correlated with the histology on hematoxylin-eosin staining.

CONCLUSIONS

We showed that unenhanced near-infrared reflectance imaging of fresh human cadaver brain can reliably identify and distinguish neurons and detailed cytoarchitecture of the cerebellum and substantia nigra.

Depth selective acousto-optic flow measurement

Optical based methods for non-invasive measurement of regional blood flow tend to incorrectly assess cerebral blood flow, due to contribution of extra-cerebral tissues to the obtained signal. We demonstrate that spectral analysis of phase-coded light signals, tagged by specific ultrasound patterns, enables differentiation of flow patterns at different depths. Validation of the model is conducted by Monte Carlo simulation. In-vitro experiments demonstrate good agreement with the simulations' results and provide a solid validation to depth discrimination ability. These results suggest that signal contamination originating from extra-cerebral tissue may be eliminated using spectral analysis of ultrasonically tagged light.

Near-infrared spectroscopy for the evaluation of anesthetic depth

The standard-of-care guidelines published by the American Society of Anesthesiologists (ASA) recommend monitoring of pulse oximetry, blood pressure, heart rate, and end tidal CO2 during the use of anesthesia and sedation. This information can help to identify adverse events that may occur during procedures. However, these parameters are not specific to the effects of anesthetics or sedatives, and therefore they offer little, to no, real time information regarding the effects of those agents and do not give the clinician the lead-time necessary to prevent patient "awareness." Since no "gold-standard" method is available to continuously, reliably, and effectively monitor the effects of sedatives and anesthetics, such a method is greatly needed. Investigation of the use of functional near-infrared spectroscopy (fNIRS) as a method for anesthesia or sedation monitoring and for the assessment of the effects of various anesthetic drugs on cerebral oxygenation has started to be conducted. The objective of this paper is to provide a thorough review of the currently available published scientific studies regarding the use of fNIRS in the fields of anesthesia and sedation monitoring, comment on their findings, and discuss the future work required for the translation of this technology to the clinical setting.

Quantifying cerebral blood flow in an adult pig ischemia model by a depth-resolved dynamic contrast-enhanced optical method

Dynamic contrast-enhanced (DCE) near-infrared (NIR) methods have been proposed for bedside monitoring of cerebral blood flow (CBF). These methods have primarily focused on qualitative approaches since scalp contamination hinders quantification. In this study, we demonstrate that accurate CBF measurements can be obtained by analyzing multi-distance time-resolved DCE data with a combined kinetic deconvolution optical reconstruction (KDOR) method. Multi-distance time-resolved DCE-NIR measurements were made in adult pigs (n=8) during normocapnia, hypocapnia and ischemia. The KDOR method was used to calculate CBF from the DCE-NIR measurements. For validation, CBF was measured independently by CT under each condition. The mean CBF difference between the techniques was -1.7 mL/100 g/min with 95% confidence intervals of -16.3 and 12.9 mL/100 g/min; group regression analysis revealed a strong agreement between the two techniques (slope=1.06±0.08, y-intercept=-4.37±4.33 mL/100 g/min, p<0.001). The results of an error analysis suggest that little a priori information is needed to recover CBF, due to the robustness of the analytical method and the ability of time-resolved NIR to directly characterize the optical properties of the extracerebral tissue (where model mismatch is deleterious). The findings of this study suggest that the DCE-NIR approach presented is a minimally invasive and portable means of determining absolute hemodynamics in neurocritical care patients.

Far red/near infrared light treatment promotes femoral artery collateralization in the ischemic hindlimb

Nitric oxide (NO) is a crucial mediator of hindlimb collateralization and angiogenesis. Within tissues there are nitrosyl-heme proteins which have the potential to generate NO under conditions of hypoxia or low pH. Low level irradiation of blood and muscle with light in the far red/near infrared spectrum (670 nm, R/NIR) facilitates NO release. Therefore, we assessed the impact of red light exposure on the stimulation of femoral artery collateralization. Rabbits and mice underwent unilateral resection of the femoral artery and chronic R/NIR treatment. The direct NO scavenger carboxy-PTIO and the nitric oxide synthase (NOS) inhibitor L-NAME were also administered in the presence of R/NIR. DAF fluorescence assessed R/NIR changes in NO levels within endothelial cells. In vitro measures of R/NIR induced angiogenesis were assessed by endothelial cell proliferation and migration. R/NIR significantly increased collateral vessel number which could not be attenuated with L-NAME. R/NIR induced collateralization was abolished with c-PTIO. In vitro, NO production increased in endothelial cells with R/NIR exposure, and this finding was independent of NOS inhibition. Similarly R/NIR induced proliferation and tube formation in a NO dependent manner. Finally, nitrite supplementation accelerated R/NIR collateralization in wild type C57Bl/6 mice. In an eNOS deficient transgenic mouse model, R/NIR restores collateral development. In conclusion, R/NIR increases NO levels independent of NOS activity, and leads to the observed enhancement of hindlimb collateralization.

Frontal cerebral cortex blood flow, oxygen delivery and oxygenation during normoxic and hypoxic exercise in athletes

During maximal hypoxic exercise, a reduction in cerebral oxygen delivery may constitute a signal to the central nervous system to terminate exercise. We investigated whether the rate of increase in frontal cerebral cortex oxygen delivery is limited in hypoxic compared to normoxic exercise. We assessed frontal cerebral cortex blood flow using near-infrared spectroscopy and the light-absorbing tracer indocyanine green dye, as well as frontal cortex oxygen saturation (S(tO2)%) in 11 trained cyclists during graded incremental exercise to the limit of tolerance (maximal work rate, WRmax) in normoxia and acute hypoxia (inspired O2 fraction (F(IO2)), 0.12). In normoxia, frontal cortex blood flow and oxygen delivery increased (P < 0.05) from baseline to sub-maximal exercise, reaching peak values at near-maximal exercise (80% WRmax: 287 ± 9 W; 81 ± 23% and 75 ± 22% increase relative to baseline, respectively), both leveling off thereafter up to WRmax (382 ± 10 W). Frontal cortex S(tO2)% did not change from baseline (66 ± 3%) throughout graded exercise. During hypoxic exercise, frontal cortex blood flow increased (P = 0.016) from baseline to sub-maximal exercise, peaking at 80% WRmax (213 ± 6 W; 60 ± 15% relative increase) before declining towards baseline at WRmax (289 ± 5 W). Despite this, frontal cortex oxygen delivery remained unchanged from baseline throughout graded exercise, being at WRmax lower than at comparable loads (287 ± 9 W) in normoxia (by 58 ± 12%; P = 0.01). Frontal cortex S(tO2)% fell from baseline (58 ± 2%) on light and moderate exercise in parallel with arterial oxygen saturation, but then remained unchanged to exhaustion (47 ± 1%). Thus, during maximal, but not light to moderate, exercise frontal cortex oxygen delivery is limited in hypoxia compared to normoxia. This limitation could potentially constitute the signal to limit maximal exercise capacity in hypoxia.

Quantitative measurement of cerebral blood flow in a juvenile porcine model by depth-resolved near-infrared spectroscopy

Nearly half a million children and young adults are affected by traumatic brain injury each year in the United States. Although adequate cerebral blood flow (CBF) is essential to recovery, complications that disrupt blood flow to the brain and exacerbate neurological injury often go undetected because no adequate bedside measure of CBF exists. In this study we validate a depth-resolved, near-infrared spectroscopy (NIRS) technique that provides quantitative CBF measurement despite significant signal contamination from skull and scalp tissue. The respiration rates of eight anesthetized pigs (weight: 16.2+/-0.5 kg, age: 1 to 2 months old) are modulated to achieve a range of CBF levels. Concomitant CBF measurements are performed with NIRS and CT perfusion. A significant correlation between CBF measurements from the two techniques is demonstrated (r(2)=0.714, slope=0.92, p<0.001), and the bias between the two techniques is -2.83 mL min(-1)100 g(-1) (CI(0.95): -19.63 mL min(-1)100 g(-1)-13.9 mL min(-1)100 g(-1)). This study demonstrates that accurate measurements of CBF can be achieved with depth-resolved NIRS despite significant signal contamination from scalp and skull. The ability to measure CBF at the bedside provides a means of detecting, and thereby preventing, secondary ischemia during neurointensive care.

The Effect of Auditory Stimulation upon Cerebral Blood Oxygenation in Infants: Measurements by Light Emitting Diode (LED) Near Infrared Spectroscopy

This study assessed the feasibility of neonatal cerebral oxygen monitoring by near-infrared light spectroscopy (NIRS) using a light emitting diode (LED) based system. We aimed to measure the changes in cerebral oxygen saturation, as regional oxygen saturation and tissue oxygenation index, in response to auditory stimuli. Documenting changes in oxygenation in response to such stimuli will help validate the usefulness of LED-NIRS as a tool in the study of cerebral oxygen saturation in neonates.

The effects of the extent of spinal block on the BIS score and regional cerebral oxygen saturation in elderly patients: A prospective, randomized, and double-blinded study

BACKGROUND

Patients may become sedated with spinal anesthesia; however, the effect of the extent of spinal block on the Bispectral index (BIS), a processed electroencephalographic variable, has not been fully investigated. We evaluated the influence of the extent of spinal block on BIS values and on regional cerebral oxygen saturation (rSO(2)) in elderly patients.

METHODS

A prospective, randomized, double-blinded study was performed in 55 ASA II patients undergoing urological surgery. The patients were randomly allocated into one of two groups to receive 2.7 ml of 0.5% hyperbaric bupivacaine or 1.5 ml, and then divided into two groups according to level of spinal blockade: high spinal group (Th6 and above) or low spinal group (Th12 and below). Systolic blood pressure (SBP), heart rate (HR), cardiac output (CO), stroke volume (SV), BIS values, and rSO(2) were measured for 30 min. CO and SV were evaluated using impedance cardiograph methods.

RESULTS

The level of spinal blockade was Th4.7 +/- 1.0 in high spinal group (n = 20) and L2.5 +/- 2.2 in low spinal group (n = 20). High spinal anesthesia produced a significant decrease in SBP (p < 0.01) and SV (p < 0.01), but had no effect on CO. High spinal anesthesia significantly decreased BIS values (p < 0.01) without affecting rSO(2). There was relationship between level of spinal blockade and BIS values (r = 0.566). In contrast, no changes in above parameters were found in low spinal group.

CONCLUSIONS

This study provides evidence that the extent of spinal block may have significant influence on BIS values without affecting rSO(2) in elderly patients.

The influence of sevoflurane on the bispectral index, regional cerebral oxygen saturation, and propofol concentration during propofol/N2O anesthesia

OBJECTIVE

This study was undertaken to investigate the influence of sevoflurane on the bispectral index (BIS), regional cerebral oxygen saturation (rSO(2)), and serum propofol concentration during propofol/N(2)O anesthesia. This study tested the hypothesis that sevoflurane affect BIS values, rSO(2), and the pharmacokinetics of propofol during propofol/ N(2)O anesthesia.

METHODS

General anesthesia was administered to 15 ASA I-II patients with a continuous infusion of propofol to maintain a BIS value of 45 +/- 5. After recording baseline values, patients were assigned to receive sevoflurane (2.0%, 20 min). BIS values, rSO(2) using near-infrared spectroscopy, and hemodynamic parameters were recorded for 60 min. Cardiac output (CO) and stroke volume (SV) were evaluated using impedance cardiograph methods. Propofol concentration was determined using high-performance liquid chromatography.

RESULTS

Sevoflurane (2.0%, 20 min) decreased the BIS score from 47.4 +/- 8.2 to 27.3 +/- 5.9 (P < 0.01, n = 15) without affecting rSO(2). Sevoflurane decreased systolic blood pressure from 112.1 +/- 14.0 mmHg to 96.5 +/- 13.2 mmHg (P < 0.001, n = 15) without affecting heart rate. Both CO and SV were significantly decreased during sevoflurane application. Propofol concentration was increased from 2.71 +/- 0.51 microg/ml to 3.30 +/- 0.57 microg/ml (P < 0.05) after sevoflurane application, and was returned to baseline after sevoflurane washout.

CONCLUSIONS

We have shown that sevoflurane decreases BIS values during propofol/N(2)O anesthesia without affecting rSO(2) and that this change is accompanied by an increase in serum propofol concentration. Changes in propofol concentration may be due to, at least in part, hemodynamic changes such as decreased CO produced by sevoflurane.

Extraction of depth-dependent signals from time-resolved reflectance in layered turbid media

We try a new approach with near-IR time-resolved spectroscopy, to separate optical signals originated in the upper layer from those in the lower layer and to selectively determine the absorption coefficient (mu(a)) of each layer in a two-layered turbid medium. The difference curve in the temporal profiles of light attenuation between a target and a reference medium is divided into segments along the time axis, and a slope of each segment is calculated to determine the depth-dependent mu(a). The depth-dependent mu(a) values are estimated under various conditions in which mu(a) and the reduced scattering coefficient (mu(s)') of each layer are changed with a Monte Carlo simulation and in phantom experiments. Temporal variation of them represents the difference in mu(a) between two layers when mu(s)' of a reference is the same as that of the upper layer of the target. The discrepancies between calculated mu(a) and the real mu(a) depend on the ratio of the real mu(a) of the upper layer to that of the lower layer, and our approach enables us to estimate the ratio of mu(a) between the two layers. These results suggest the potential that mu(a) of the lower layer can be determined by our procedure.

Noninvasive measurement of cerebral blood flow in adults using near-infrared spectroscopy and indocyanine green: a pilot study

This pilot study was designed to determine the feasibility of measuring cerebral blood flow noninvasively after an intravenous bolus of indocyanine green using near-infrared spectroscopy and pulse dye-densitometry. Feasibility aside, this study did not attempt to validate the measured values of cerebral blood flow against an established method of measurement. Twelve healthy volunteers were investigated after peripheral intravenous injection of indocyanine green. Arterial and cerebral changes in indocyanine green concentration were measured using pulse dye-densitometry and near-infrared spectroscopy, respectively. Two methods of calculating cerebral blood flow were used, and a blood flow index was also estimated. Absolute cerebral blood flow was calculated using a modification of the Fick principle and a deconvolution algorithm to derive the impulse residue function. Mean (range) estimated cerebral blood flow for the Fick method was 8.2 mL/100 g/min (4.2-16.2 mL/100 g/min) and 8.3 mL/100 g/min (4.7-15.3 mL/100 g/min) for the impulse residue function method. The impulse residue function method provided a more precise intrasubject estimation of cerebral blood flow compared with the modified Fick principle, with a coefficient of variation of 10.1% versus 25.5%. The blood flow index was 8.6 mg/sec (range: 5.6-17.3 mg/sec) with an intrasubject coefficient of variation of 12.0%. Estimation of cerebral blood flow using near-infrared spectroscopy and pulse dye-densitometry can be made at the bedside after intravenous injection of indocyanine green, and the precision can be improved using a deconvolution algorithm. Notwithstanding the low values obtained for absolute cerebral blood flow, further investigation and validation of this bedside technique is warranted.

A comparison of the INVOS 4100 and the NIRO 300 near-infrared spectrophotometers

We determined whether two different devices for measuring near-infrared spectroscopy (NIRS)---the INVOS 4100 and the NIRO 300---produce similar cerebral oxygenation data during the CO(2) challenge test. Nineteen patients anesthetized with sevoflurane, 67% nitrous oxide in oxygen, and fentanyl were studied. A series of measurements of regional cerebral oxygen saturation (rSO(2)), measured by the INVOS 4100, and tissue oxygen index (TOI), measured by the NIRO 300, were performed in the following conditions: 1) normocapnia (PaCO(2), 35--45 mm Hg); 2) hypocapnia (PaCO(2), 25--35 mm Hg); 3) normocapnia; and 4) hypercapnia (PaCO(2), 45--55 mm Hg). Hemodynamic variables, including arterial blood gases and cerebral blood flow velocity, were measured at the same time with transcranial Doppler. The values and percentage changes of rSO(2) and TOI were compared by using regression analysis and Bland and Altman analysis. The rSO(2) showed a significant positive correlation with TOI (r = 0.58, P < 0.01). The percentage change of rSO(2) also showed a significant positive correlation with the percentage change of TOI during the CO(2) challenge (r = 0.85, P < 0.01). Bland and Altman analysis revealed a bias of -0.5% with 2 SD of 15.6% when comparing the rSO(2) value with the TOI value, and it showed a bias of -3.4% with 2 SD of 15.2% when comparing the percentage change of rSO(2) with the percentage change of TOI, indicating unacceptable disagreement of these data. These results indicate that cerebral oxygen saturation and its relative change during the CO(2) challenge may vary depending on the type of NIRS used. Because the measurement technique and algorithm were different in each device, we should carefully consider the clinical application of the values produced by NIRS.

IMPLICATIONS

Near-infrared spectroscopy (NIRS) has been proposed as a noninvasive clinical method for assessing cerebral oxygenation. The acceptable reliability and validity of NIRS values have not been established despite their widespread use. The INVOS 4100 and the NIRO 300 can display cerebral oxygen saturation as regional cerebral oxygen saturation and tissue oxygenation index, but they produce differing results.

Spatially resolved spectroscopy (NIRO-300) does not agree with jugular bulb oxygen saturation in patients undergoing warm bypass surgery

PURPOSE

Near infrared spectroscopy (NIRS) is a promising non-invasive method for continuous monitoring of cerebral oxygenation during cardiac surgery with cardiopulmonary bypass (CPB). This study was designed to study the agreement between tissue oxygen index (TOI) measured by spatially resolved spectroscopy (NIRO-300) and jugular bulb oxygen saturation (SjO2) in patients undergoing warm coronary bypass surgery.

METHODS

Seventeen patients undergoing warm coronary artery bypass surgery were studied. NIRS was continuously monitored and was averaged before CPB, five, 20, 40, 60 min on CPB, five minutes before end of CPB and ten minutes after CPB to coincide with SjO2 measurements. Bypass temperature was maintained at 34-37 degrees C.

RESULTS

Bland and Altman analysis showed a bias (TOI-SjO2) of -6.7%, and wide limits of agreement (from 16% to -28%) between the two methods. In addition, mean TOI was lower than mean SjO2 during and after CPB. We observed a statistically significant correlation between arterial carbon dioxide and SjO2 measurements (r2=0.33; P=0.0003), but the former did not correlate with TOI values (r2=0.001; P=0.7).

CONCLUSION

Our results demonstrate a lack of agreement between SjO2 and TOI for monitoring cerebral oxygenation during cardiac surgery. We conclude that the two methods are not interchangeable.

Comparison of cerebral blood volume measured by near infrared spectroscopy and contrast enhanced magnetic resonance imaging

Cerebral blood volume (CBV) can be quantified by both near infrared spectroscopy (NIRS) and magnetic resonance imaging (MRI). The aim is to compare CBV results obtained by NIRS and MRI in adult patients. 10 adult patients, 6 females and 4 males, age median 24 (range 21 to 76) years, were included in this study. All needed a MRI investigation with contrast medium for clinical reasons. The NIRS instrument, the Cerebral RedOx Monitor 2020 from Critikon, quantifies cerebral haemoglobin concentration using a sensor with two receiving channels at different distances. Geometrical detector arrangements of this type enable a ratio measurement to be achieved, which reduces the contribution of the skull and skin, thus allowing quantification. Cerebral haemoglobin concentration can be converted in CBV, as the haemoglobin concentration in the blood is known. CBV can be quantified by MRI using an indicator dilution method. The method requires an injection of a paramagnetic contrast agent. The input function can be measured at the throat and thus perfusion images can be quantified. CBV was measured by NIRS just before the patient entered the magnet and after he had left it. The sensor for the NIRS measurement was applied to the patients front three times for 1 minute to each side, avoiding the sinuses. CBV was determined by contrast enhanced MRI between the NIRS measurements. The mean CBV (NIRS) was 8.6 (SD 1.3) ml/100 g and CBV (MRI) was 7.1 (SD 2.5). The correlation between CBV (NIRS) and CBV (MRI) was Pearson's correlation coefficient -0.297 (p = 0.204) respectively Spearman's rho (nonparametric) -0.266 (p = 0.257). The CBV values obtained by NIRS and MRI, even though they are in the same range, do not correlate.

Measurement of cerebral blood volume using near-infrared spectroscopy and indocyanine green elimination

Methods for measuring cerebral blood volume (CBV) have traditionally used radioisotopes. More recently, near-infrared spectroscopy (NIRS) has been used to measure CBV by using a technique involving O(2) desaturation of cerebral tissue, where the observed change in the concentration of oxygenated hemoglobin is a marker of the volume of blood contained within the brain. A new integration method employing NIRS is described by using indocyanine green (ICG) as the intravascular marker. After bolus injection, concentration-time integrals of cerebral tissue ICG concentration ([ICG](tissue)) measured by NIRS are compared with corresponding integrals of the cerebral blood ICG concentrations ([ICG](blood)) estimated by high-performance liquid chromatography of peripheral blood samples with allowance for cerebral-to-large-vessel hematocrit ratio. It is shown that CBV = integral [ICG]tissue/[ICG]blood. Measurements in 10 adult volunteers gave a mean value of 1.1 +/- 0.39 (SD) ml/100 g illuminated tissue. This result, although lower than previous NIRS estimations, is consistent with the long extracerebral path of light in the adult head. Scaling of results is required to take into account this component of the optical pathlength.

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.

A comparison of the Hamamatsu NIRO 500 and the INVOS 3100 near-infrared spectrophotometers

Near-infrared spectroscopy is a technique used for non-invasive measurement of cerebral oxygenation and a number of commercial devices are currently available for use. We compared measurements of cerebral oxygenation made with two near-infrared spectrophotometers--the Somanetics Invos 3100 cerebral oximeter and the Hamamatsu NIRO-500 near-infrared spectrophotometer. Hypoxia was induced in six healthy male volunteers with and without occlusion of scalp blood flow. Oxygen saturation, end-tidal carbon dioxide tension, regional cerebral oxygen saturation, change in regional cerebral oxyhaemoglobin concentration and change in regional cerebral total haemoglobin concentration were measured. The INVOS 3100 displays cerebral oxygen saturation directly. The NIRO-500 displays change in total haemoglobin concentration and oxyhaemoglobin concentration, and the cerebral oxygen saturation was calculated offline. Statistical analysis disproved the assumption that the INVOS 3100 and the NIRO-500 were measuring the same changes in cerebral oxygenation. Neither machine can be confirmed for reliability against a gold standard and operational difficulties mean that neither can be recommended for routine clinical use.

Cerebral oxygenation during prostaglandin E1 induced hypotension

PURPOSE

To evaluate the cerebral oxygenation effects of hypotension induced by prostaglandin E1 (PGE1) during fentanyl-oxygen anaesthesia.

METHODS

Ten patients who underwent elective cardiac surgery received infusion of PGE1. After measuring the baseline arterial, mixed venous and internal jugular vein blood gases, systemic haemodynamics, and regional cerebral oxygen saturation (rSO2) estimated by INVOS 3100R, PGE1 was continuously infused at 0.25-0.65 microgram.kg-1.min-1 (mean dosage: 410 +/- 41.4 mg.kg-1.min-1) intravenously. Ten, 20 and 30 minutes after the start of drug infusions, blood gases described above were obtained simultaneously with the measurement of systemic haemodynamics and rSO2. Thirty minutes from the start of drug infusions, administration of PGE1 was stopped. The same parameters were measured again 10, 30 minutes after the stop of drug infusion.

RESULTS

PGE1 decreased mean arterial pressure (MAP) to approximately 70% of the baseline value (P < 0.05). PGE1 increased mixed venous saturation, but in contrast did not affect internal jugular pressure, internal jugular oxygen saturation and rSO2.

CONCLUSIONS

These results suggest that PGE1 is a suitable drug for induced hypotension because flow/metabolism coupling of brain and regional cerebral oxygenation were well maintained during hypotension.

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.