Carotid surgery: CNS monitoring
Clinical - Cardiovascular, Physics, Monitoring, & Devices
Carotid surgery typically requires temporary occlusion of the internal carotid artery. This would almost certainly result in a large stroke if the parts of the brain supplied by the occluded internal carotid artery were not adequately perfused by collateral circulation. Collateral circulation may exist via the circle of Willis and multiple smaller pathways. Many carotid surgery patients receive preoperative angiography or radiography to demonstrate the existence of collateral circulation.
Intraoperative evidence of adequate cerebral perfusion can be collected through multiple techniques. The detection of insufficient perfusion may encourage the operating room team to increase blood pressure or institute a shunt to bypass the occluded vessel.
• Electroencephalography (EEG) records spontaneous electrical activity at the level of the scalp and reflects underlying neurologic electrical activity. Significant slowing lateralized to the occluded hemisphere may represent inadequate perfusion. EEG requires skilled application and interpretation, and the avoidance of high-doses of anesthetic agents that can suppress EEG activity.
• Stump pressures are blood pressure measurements taken from the occluded carotid artery distal to the point of occlusion. These reflect back pressure from collateral circulation. Cerebral ischemia is unlikely with stump pressures > 60mmHg, but using this high threshold may result in unnecessary shunting.
• Cerebral oximetry involves the placement of sensors on the forehead that aim to measure oxygen saturation in the superficial frontal cortex. An ipsilateral decrease suggests inadequate perfusion.
• Somatosensory evoked potentials (SSEPs) monitor the integrity of the nervous system from peripheral sensory nerve to cerebral cortex. Like EEG, this monitoring modality requires skilled application and interpretation, and the avoidance of anesthetic agents that can suppress evoked potentials.
• Transcranial doppler (TCD) measures blood flow velocity in the middle cerebral artery. Additionally, it can be used to detect microemboli, which may help surgeons modify their surgical technique. It too requires skilled application and interpretation, and is difficult to implement and maintain intraoperatively due to the setup and positioning required.
• Performing surgery under regional block with the patient awake is a sensitive method for detecting intraoperative ischemia, but requires a cooperative patient and surgeon.
No single monitoring method provides 100% sensitivity and 100% specificity, and choice of modality often depends on availability and surgeon or anesthesiologist preference. Few head-to-head studies have been performed, though one showed best results with TCDs.
Moritz S, Kasprzak P, Arlt M, Taeger K, Metz C (2007) Accuracy of cerebral monitoring in detecting cerebral ischemia during carotid endarterectomy: a comparison of transcranial Doppler sonography, near-infrared spectroscopy, stump pressure, and somatosensory evoked potentials. Anesthesiology 107 (4):563-569. doi:10.1097/01.anes.0000281894.69422.ff