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Keywords Motor evoked potential Thoracoabdominalaortic aneurysm HypothermiaMonitoring of motor evoked potentials (MEPs) can provideinformation about the functional integrity of a motorpathway and, therefore, has been widely used during sur-gery in which a motor pathway is at risk of injury,including craniotomy, spine/spinal surgery, and thoracic orthoracoabdominal aortic aneurysm (TAA or TAAA) sur-gery [1, 2]. Because myogenic MEPs are very sensitive tosuppression by anesthetic agents and neuromuscularblockade, careful attention should be paid to anestheticmanagement during monitoring of MEPs. In general, totalintravenous anesthesia with propofol and opioids is used,although ketamine-based anesthesia may be applied incases in which recording of myogenic MEPs is not reliablebecause of suppression by anesthetic agents or inuence bypreoperative motor dysfunction.Of the cases in which MEPs monitoring is required,TAA/TAAA surgery is the most challenging for reliableMEP monitoring. In addition to the effects of anesthetics,a variety of factors can affect the recording of MEPs,including hypothermia, aortic clamp, and induction ofcardiopulmonary bypass (CPB). Hypothermia may havedirect effects on the MEP itself and also indirect effects onMEPs by changing the pharmacokinetics and pharmaco-dynamics of anesthetic agents, modied by the effects ofaortic clamp and the induction of CPB. Leslie et al. [3]demonstrated that a temperature reduction of 3 C increasedblood propofol concentration by 30 C during a constantrate infusion. Kakinohana et al. [4] reported that plasmapropofol concentration increased and bispectral indexvalues decreased rapidly after aortic cross-clamping inthoracic aortic aneurysm repair surgery during propofolanesthesia. Yoshitani et al. [5] demonstrated that, in patientsreceiving propofol infusion at a rate of 5 or 6 mg/kg/h,plasma propofol concentration increased and burst sup-pression rate on electroencephalogram increased after theinduction of normothermic CPB, compared with thosevalues before CPB. These ndings suggest that interpre-tation of MEP changes could be difcult because anestheticlevel can easily uctuate during TAAA surgery withhypothermic CPB.Data regarding the effects of hypothermia on myogenicMEPs have been limited to animal experiments. Meylaertset al. [6] investigated the effects of regional spinal cordhypothermia on myogenic MEPs induced by transcranialtrain-pulse stimulation in pigs anesthetized with ketamine,sufentanil, clonidine, and nitrous oxide. They demonstratedthat progressive cooling resulted in an increase of MEPs at28 –30 C and was followed by a progressive decrease inMEP amplitude. MEP amplitude decreased below 25% atapproximately 14 C. Sakamoto et al. [7] also investigatedthe effects of systemic hypothermia on myogenic MEPsinduced by a single pulse and a train of pulses under

journal of anesthesia

Improvement of motor evoked potentials monitoring is required during thoracic or thoracoabdominal aortic aneurysm surgery under hypothermic cardiopulmonary bypass