Perioperative clinicians use monitors to enhance the availability of diagnostic data that in turn could help positively alter patient management. However, although pulse oximetry may come close to aiding clinicians in improving outcome,1 no present monitor has been shown to definitively improve morbidity and mortality.2 Factors that may improve outcome include adequacy, appropriateness, and timeliness of care as well as instituting care in the right population and based on the individual patient response.3 Currently, there are a variety of modalities in which to monitor or indirectly examine cerebral blood flow: brain tissue oxygen tension, transcranial doppler, electroencephalogram, jugular venous oxygen saturation, and transcranial cerebral oximetry.4,5 All these monitors have inherent limitations. For instance, both brain tissue oxygen tension and jugular venous oxygen saturation require invasive techniques to achieve measurements. Transcranial doppler does not easily provide continuous measurements, whereas electroencephalogram typically requires a specialist at the bedside to interpret results. Cerebral oximetry is an attractive alternative because it is noninvasive and provides an approximation of regional tissue cerebral oxygen saturation. However, it is important to recognize this technology’s limitations.
Near-infrared spectroscopy (NIRS) devices have a variety of limitations that should be considered, which also may explain why there is no direct evidence as to whether interventions directed toward correcting cerebral oxygen desaturations clearly improve patient outcomes. The typical NIRS device has a light emitter and 2 light detectors (1 proximal and 1 distal to the emitter).5,6 Through a subtraction algorithm (and the use of spatial resolution), skin, subcutaneous tissue, and scalp are intended to be excluded to accurately estimate cerebral tissue oxygen saturation.5 Whether the subtraction algorithms are accurate across a range of different patients’ scalp thickness, cranial bone thickness and sinus size still have yet to be clearly delineated.
A variety of recognized issues must be understood when using NIRS clinically. First, NIRS devices measure light absorption of chromophores from a small segment of tissue within the path of emitted light and its sensors. Thus, NIRS provides a measure of localized regional oxygen saturation and not global brain oxygenation. High melanin skin concentrations also may affect readings displayed on some of the commercially available devices.7 Alterations in intra- and extracranial contents also may affect readings. Some researchers even suggest that high cerebral oximetry readings are observed in cadavers, begging the question of accuracy (versus no oxygen extraction in the setting of ceased blood flow).8 Last, all NIRS devices are subject to extracranial contamination. The following brief review focuses on literature addressing the concept of extracranial contamination among NIRS devices.
In 1995, Germon et al9 first investigated the potential contribution of extracranial tissue to cerebral oxygen saturation measurements. The authors applied a forehead pneumatic cuff just below the INVOS 3100™ Sensor (Medtronic, Minneapolis, MN) device in 8 patients undergoing either cervical or lumbar discectomy. They observed a significant decline in cerebral oxygen saturations with cuff inflation when compared with without cuff inflation and attributed this observation to device measurement of extracranial tissue. They concluded that if the subtraction algorithm designed to account for extracranial tissue contamination was effective, there would be no decrease in cerebral oxygen saturations. In 2012, Davie and Grocott10 performed an observational study in 12 healthy volunteers examining the presence of extracranial contamination with 3 commercially available NIRS devices (INVOS 5100 [Medtronic], FORE-SIGHT [CAS Medical Systems Inc, Bradford, CT], and EQUANOX [Nonin Medical Inc, Plymouth, MN]). Their results suggested that all the tested NIRS devices were subject to significant cerebral oxygen saturation decline when an inflated cuff was applied just below the sensors. The INVOS 5100 cerebral oximeter appeared to be most affected by cuff inflation at 5 minutes. A more recent study by Sørensen et al11 exposed healthy subjects to a variety of different physiological conditions (including administration of phenylephrine and norepinephrine). The authors concluded that approximately 35% of the INVOS 4100 (Medtronic) saturation signal could be attributed to extracerebral oxygenation and that the alternative NIRO-200NX (Hamamatsu, Japan; SniroO2) device did not correlate with estimates of cerebral capillary oxygenation, as directly measured by using arterial and jugular venous blood samples.
Most recently, our research group performed an observational trial among 20 healthy volunteers, similar to the previous Davie and Grocott trial. The objective was to investigate whether the newer FORE-SIGHT ELITE device (CAS Medical Systems Inc), which uses 5 wavelengths of light (680, 730, 770, 805, and 870 nm), reduces extracranial contamination when compared with the previous INVOS 5100 device, which uses 2 wavelengths (730 and 810 nm).12 Our results suggest that both devices are subject to extracranial contamination on pneumatic cuff inflation. However, it appeared that there was a significant decrease in the amount of contamination as defined by a drop from baseline saturations on pneumatic cuff inflation in the FORE-SIGHT ELITE versus INVOS (median decrease ScO2 from baseline at 5 minutes 8.6% [4.0–12.3] vs 15.1% [12.6–17.6], respectively, median difference, 7.9%; 99% confidence interval, 1.9–16.5; P = 0.002). When comparing our results with those of Davie and Grocott, it appeared that the FORE-SIGHT ELITE had a smaller mean reduction from baseline than the previous FORE-SIGHT device, although this was not directly studied in our trial. Therefore, updated algorithms combined with additional wavelengths of light (eg, FORE-SIGHT ELITE) may reduce this phenomenon, but it has yet to be eliminated.12
Currently, there is no absolute “gold standard” to validate a specific range of NIRS device measurements related to different cellular conditions. The closest correlate, used in research and to validate these devices for clinical use, is the ratio of venous blood (jugular bulb) to arterial blood oxygen saturation. Typically, device manufacturers set a fixed ratio at approximately 70%/30% venous to arterial blood as verified in volunteer studies. However, the assumption that NIRS devices reflect an exact mixture of 70%/30% venous to arterial blood does not hold true in all clinical circumstances.13 Bickler et al13 studied 5 commercially available NIRS devices among 23 healthy volunteers and during isocapnic hypoxic conditions. Simultaneous jugular venous and radial arterial blood samples were obtained and compared with cerebral oximeter readings. The authors suggested that there was a large variation in reading errors among all devices studied.13 They further speculated that the variations may have been because of individual variation in the ratios of venous to arterial blood in the brain.13 If variations in venous/arterial ratios (V/A) are significant, then the weighted V/A blood reference values may no longer be reliable, which clouds whether deviations are because of the reference methodology or the device. Ito et al14 also suggested that changes in cerebral blood volume during hypo- or hypercapnia were because of changes in arterial blood volume and not venous or capillary volumes. Future improved technologies may adjust for interpatient variability in ratios of V/A blood and thereby improve the accuracy of these devices.
Another unanswered question is whether extracranial contamination simulates any real-time clinical situations. The clinical significance of extracranial contamination has yet to be clearly examined in clinical scenarios where NIRS is practically used nor have any researchers definitively investigated the effect of forehead pneumatic cuff compression on cerebral oxygen saturation measurements in alternative positions (ie, sitting or Trendelenburg positions). Recently, Shintani et al15 published abstract 2156 at the 2015 Annual American Society of Anesthesiologists meeting, suggesting that the Trendelenburg position in 20 patients undergoing robotic prostate surgery has no effect on cerebral oxygen saturations. Still, clearly there need to be more studies on position changes and their effect on cerebral oxygen saturations.
Lam et al16 used skin laser doppler flowmetry to investigate the contribution of cutaneous blood flow to NIRS measurements in 50 patients undergoing carotid endarterectomy. The authors suggested that laser doppler flowmetry may be used to estimate the extracerebral contribution of the NIRS signal. Therefore, this technology may be further incorporated into clinical studies with complex patients to determine whether there is a true clinical effect of extracranial contamination. It is also important to mention that pneumatic cuff inflation may produce a situation in which significant venous congestion occurs creating a predominance of venous blood (as indicated by a blue forehead, seen in several volunteers in our study). Venous congestion produced by forehead cuff inflation also may produce altered measurements, similar to when a noninvasive blood pressure cuff is inflated causing venous congestion and pulsations. Our study among others did not compare the NIRS device measurements with the weighted ratios of jugular V/A blood oxygen saturations when exposed to pneumatic cuff inflation conditions. One could surmise that if the spatial resolution of these devices was optimized to account for extracranial contamination, then the ScO2 values would solely reflect cerebral tissue oxygen saturation levels alone. Subsequently, there would be no decrease in ScO2 values during pneumatic cuff compression of the forehead. However, if these blood value measurements do not change on pneumatic cuff inflation, this may further substantiate the argument that a significant contribution of cutaneous vasoconstriction contributes to the NIRS measurements. Further studies may investigate this matter while incorporating hemodynamic and positional changes.
It is unclear as to what effect, if any, head pneumatic cuff inflation has on global cerebral tissue oxygen saturation. However, it is clear that this pneumatic cuff compression does alter cerebral oxygen saturation measurements. Still, at the present time, it is unknown as to what clinical impact extracranial contamination has on the use of these NIRS devices. Further research is required to delineate a clinical correlation.
Name: Steven Greenberg, MD, FCCP, FCCM.
Contribution: This author helped design and prepare the manuscript.
Conflicts of Interest: Steven Greenberg has served as a consultant for CASMED, the developer of the FORE-SIGHT ELITE cerebral oximeter.
Name: Torin Shear, MD.
Contribution: This author helped design and prepare the manuscript.
Conflicts of Interest: None.
Name: Glenn Murphy, MD.
Contribution: This author helped design and prepare the manuscript.
Conflicts of Interest: Glenn Murphy has served as a consultant for CASMED, the developer of the FORE-SIGHT ELITE cerebral oximeter.
This manuscript was handled by: Maxime Cannesson, MD, PhD.
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