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• W2031666075 abstract "In this issue, Saito et al. [1] report on the use of a two-wavelength near-infrared (NIR) spectroscope (INVOS 3100; Somanetics Corp., Troy, MI) to demonstrate that electroconvulsive therapy (ECT) decreases regional brain oxygen saturation (rSO (2)) immediately after application of the current, then increases rSO2 to values exceeding those obtained before ECT. Because this monitor of cerebral oxygenation has now been approved by the U.S. Food and Drug Administration for clinical use, the report by Saito et al. provides an opportunity to briefly review what we might expect of this technology and what we should not expect. This device can very likely meet some modest expectations but certainly will fail if expected to meet unrealistic goals. It is essential to note that near-infrared spectroscopy (NIRS) cannot at present provide precise, quantitative data regarding cerebral venous saturation. Therefore, measurements of regional brain saturation cannot be considered equivalent to measurements of jugular venous hemoglobin oxygen saturation for routine clinical purposes, nor can data acquired with this device be considered appropriate for quantitative measurement of cerebral hemoglobin saturation for physiologic research. However, NIRS shows promise as a monitor of trends in cerebral oxygenation in patients in whom the adequacy of cerebral oxygenation cannot be predicted by knowing the key physiologic variables of systemic blood pressure, PaCO2 and PaO2. NIRS, first described by Jobsis in 1977 [2], is based on a modification of the Beer-Lambert law, which states that changes in absorbance as light passes through tissue are proportional to the concentration of light-absorbing pigments or chromophores (oxy- and deoxyhemoglobin), the chromophore absorption coefficient constant (x), and the distance that light has traveled in tissue (mean optical path length). As biological tissues are relatively transparent to NIR light at wavelengths between 700 and 1000 nm [2,3], the relationship between absorbed light and chromophore concentration may be processed in an algorithm designed to reduce path length dependence and emphasize brain parenchyma, which may be used to estimate brain hemoglobin oxygen saturation [4]. However, the validity of the data acquired by the INVOS 3100 and by NIRS technology in general is limited by several necessary assumptions required in constructing algorithms that convert changes in optical density to hemoglobin saturation. These include assignment (or measurement) of path length values, assumptions regarding the relative path lengths through extracranial tissue versus deeper brain tissue, and estimation of the proportions of arterial and venous blood in the path of the light. In neonates, transmission spectroscopy uses skull transillumination to reduce problems with path length estimation [5,6]. In adults, in whom reflectance spectroscopy is used to estimate brain hemoglobin oxygen saturation, studies have yielded conflicting results [7,8]. Optical path length, a key variable in the calculation, is neither similar between individuals nor stable within individuals. Benaron et al. [9], using phase-modulated NIRS, recently demonstrated in infants that optical path length varies with age, head size, emitter-to-detector distance, and emitted wavelengths. Because NIR light travels in an elliptical pathway through tissue [10], it theoretically should be possible to separate brain tissue from superficial tissue (skin, subcutaneous tissue, and skull). The INVOS 3100 uses an NIR light-emitting diode and two light detectors, one nearer the light-emitting diode (presumably more influenced by extracranial hemoglobin saturation) and one more distant (presumably reflecting more intracranial hemoglobin saturation). The distance between the detectors and the source may influence the extent of extracerebral contamination [4]. Earlier studies, using probes with emitter-to-detector distances of 1 and 2.7 cm, suggested that extracerebral contribution could substantially contaminate the saturation recorded by the oximeter [7]. Similar studies using probes with emitter-to-detector separation of 3 and 4 cm have shown improved results [8]. Using emitter-to-detector distances of 3 and 4 cm, Germon et al. [11] applied a scalp tourniquet to reduce extracranial blood flow and improve detection of brain hypoxia; however, extracranial oxygenation still appeared to confound the results. Some extracranial contamination appears to occur even with a 5-cm separation [12]. Although Saito et al. [1] mention that extracranial muscle desaturation may have contributed to the initial decrease in rSO2 observed after the application of electrical current, they do not discuss the possibility that reactive hyperemia in the facial muscles could have increased extracranial contamination and contributed to the later, sustained increase in rSO2. However, perhaps the greatest difficulty in interpreting the value of NIRS in the present study is that ECT is precisely the type of situation in which the monitor is unlikely to provide quantitatively valid data. Because the algorithm depends on the assumption that the proportions of arterial and venous blood remain constant, the ideal challenge for the monitor is a situation in which only cerebral venous saturation changes but the volumes and proportions of arterial and venous blood remain constant. Although such circumstances are difficult to imagine, perhaps the closest in clinical practice is carotid clamping and unclamping during carotid endarterectomy; under these conditions, a detector positioned over the ipsilateral parietal region (the middle cerebral artery territory) correlated highly with changes in jugular venous saturation [8]. The correlation, however, deteriorated if the detector was placed over the frontal region. In contrast, ECT increases frontal cerebral blood flow [8] and middle cerebral artery flow velocity [13], presumably increases oxygen uptake, acutely increases blood pressure (from a mean of 86 mm Hg to a mean of 105 mm Hg in the study of Saito et al.), and may change the extent of extracranial contamination. This challenge exceeds the design constraints of the INVOS 3100 algorithm. The recent approval by the U.S. Food and Drug Administration of the INVOS 3100 marks the release of the first commercial cerebral oximeter. The monitor must not be expected to provide quantitative data regarding cerebral venous saturation under an infinite variety of physiologic and pharmacologic circumstances. It is a trend monitor of greatest value in situations in which intracranial hemoglobin saturation could dangerously change and in which changes in systemic hemodynamics and oxygenation would not predict that change. At present, cerebral oximetry complements rather than replaces established techniques for measuring cerebral perfusion and oxygenation. Much additional work must be done in patients with established neurological disease. Despite rapid technological advances over the last decade, NIRS technology remains in its infancy. Further research is needed to determine which methodology and instrumentation will best suit patient needs, and careful research design is essential to accurately distinguish cerebral from extracerebral oxygenation." @default.
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• W2031666075 title "Cerebral Near-Infrared Spectroscopy" @default.
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