EEG, Cerebral Oximetry, and Arterial to Jugular Venous Lactate to Assess Cerebral Ischemia During Carotid Endarterectomy
EEG, Cerebral Oximetry, and Arterial to Jugular Venous Lactate to Assess Cerebral Ischemia During Carotid Endarterectomy
A highly desired result during carotid endarterectomy (CEA) is the ability to predict and
 warn the surgeon if the brain is at risk of damage during the period of time that the carotid
 artery is cross-clamped for surgical repair of the vessel narrowing. A number of approaches
 for cerebral monitoring have been developed, including EEG, cerebral oximetry, and
 measurement of arterial to jugular venous concentration differences of oxygen, glucose or
 lactate. This study will utilize and compare multiple monitoring approaches for detecting
 when and if the brain is at risk of injury during CEA. As such, this robust approach to
 monitoring may permit a more prompt intervention to prevent or limit damage should cerebral
 ischemia occur. In this study we will compare a processed EEG monitor -- the EEGo, which uses
 nonlinear analysis to a bispectral (BIS) index monitor, and to the FORE-SIGHT cerebral
 oximeter to assess the ability of each to identify cerebral ischemia should it occur with
 carotid artery cross-clamping during CEA. These monitors will be correlated with arterial to
 jugular venous lactate concentration difference, which has recently been shown to be a
 sensitive indicator of hemispheric ischemia during CEA.
Objective: This study is designed to test the hypothesis that the EEGo monitor will be
 superior to the BIS monitor to assess onset of cerebral ischemia if it should arise during
 CEA. The two EEG monitors will be compared with the FORE-SIGHT cerebral oximeter, and all
 three monitors will be correlated with arterial to jugular venous lactate differences before,
 during and after carotid cross-clamping.

 We hypothesize that the EEGo monitor will be able to more accurately indicate if the brain is
 at risk of ischemia and better able to direct therapy to relieve the ischemic episode as
 compared to the BIS monitor. The EEGo will be superior because the raw signal is processed
 using phase delay analysis, with each patient's raw EEG analyzed instead of a proprietary but
 generic signal processing approach on a linear scale as with the BIS monitor. Phase delay
 analysis is a standard approach to display nonlinear signals. A highly reproducible signal
 transition occurs with differing depths of anesthesia. The ability to process the EEG and
 display phase delay plots in 50 msec is what makes the EEGo monitor attractive to acutely
 assess changes in the EEG which can manifest with cerebral ischemia. We also hypothesize that
 the FORE-SIGHT cerebral oximeter will be an effective monitor for detecting cerebral ischemia
 during CEA. Accurate assessment of changes in the EEG and cerebral tissue oxygen saturation
 with carotid cross-clamping will allow better anesthesia management of patients undergoing
 CEA.

 This pilot study will be done to assess the ability of the nonlinear EEG monitor (the EEGo)
 compared to the BIS monitor and the FORE-SIGHT cerebral oximeter to direct therapy if
 cerebral ischemia occurs -- notifying the surgeon of the findings, optimizing the blood
 pressure, arterial oxygenation and carbon dioxide tensions, and permitting consideration of
 placing a carotid shunt. The EEGo processes the standard EEG signal by nonlinear analysis of
 the raw signal by 3 dimensional phase delay plots. A cascade from a point attractor, periodic
 attractor, toroidal attractor to a 3D chaotic attractor occurs from burst suppression to the
 awake state. These resemble phase transitions and occur rapidly from one state to the next.
 An analogy is the phase transition that occurs when water changes to ice and vice versa.
 Monitoring these transitions should permit a rational approach to monitoring cerebral
 well-being during carotid cross-clamping with CEA. The EEGo will be compared directly in real
 time to the bispectral (BIS) monitor and to the FORE-SIGHT cerebral oximeter for
 intraoperative monitoring during CEA.

 Cerebral oximetry (also known as Near Infrared Spectroscopy or NIRS) provides a non-invasive
 measure of regional cerebral tissue oxygen saturation which reflects local oxygen supply and
 demand, and could indicate cerebral ischemia when desaturation occurs. The FORE-SIGHT
 cerebral oximeter is a non-invasive, optically-based near-infrared spectrometer that
 continuously estimates absolute cerebral tissue oxygen saturation, and works based on the
 principal that hemoglobin's two forms -- oxygenated and deoxygenated -- absorb light
 differently. The sensor projects laser into the brain at four different wavelengths (to
 maximize accuracy), with the reflected light being analyzed via algorithm to an absolute
 value of cerebral saturation. The FORE-SIGHT has been validated in several studies that have
 shown good precision of the cerebral tissue oxygen saturation value.

 In addition, as previously stated, the two EEG monitors and FORE-SIGHT cerebral oximeter will
 be correlated with arterial to jugular venous lactate differences. In a recent paper,
 arterial to jugular venous lactate differences were shown to be especially sensitive for
 detecting cerebral ischemia in awake surgery for CEA. In that paper the sensitivity of
 lactate differences to detect ischemic changes was 100% for a difference greater than 0.1
 mmole/L; 100% specificity if the difference exceeded 0.32 mmole/L; and an ROC best fit for a
 difference greater than 0.16 mmole/L. In this study we plan to look at a combined approach
 during carotid artery cross-clamping -- measures of lactate differences with simultaneous
 assessment of cerebral tissue oxygen saturation using the FORE-SIGHT cerebral oximeter, and
 EEG changes using two different processed EEG monitors (the well established BIS monitor and
 the newer EEGo monitor). As such, this is intensified monitoring for these patients at risk
 of cerebral ischemia over the standard of care.
Inclusion Criteria:

 -  All adult patients undergoing carotid endarterectomy will be approached in the
 Pre-Anesthetic Clinic

 Exclusion Criteria:

 -  Patient refusal

 -  A history of asthma requiring routine use of bronchodilators because the study will
 use desflurane as the volatile agent

 -  Pregnancy

 -  Non-elective carotid endarterectomy