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* From the Departments of Anesthesia and
Neurosciences, Toronto Western Hospital, University Health Network, University of Toronto, Toronto, Ontario, Canada.
Address correspondence to: Dr. Pirjo Manninen, Department of Anesthesia, Toronto Western Hospital, 399 Bathurst Street, Toronto, Ontario M5T 2S8, Canada. Phone: 416-603-5118; Fax: 416-603-6494; E mail: Pirjo.Manninen{at}uhn.on.ca
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Abstract |
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Methods: All patients undergoing carotid endarterectomy during three years who had routine bilateral MNSSEP were also monitored with bilateral PTNSSEP. Patients received a shunt if there was a significant change (> 50% decrease in amplitude of cortical peak (N20) in the MNSSEP after cross clamping. The incidence, timing, and duration of all PTNSSEP changes were compared to MNSSEP changes.
Results: One hundred fifty-three patients were studied. Significant changes in MNSSEP after cross clamp lead to insertion of a shunt in six patients. Changes in PTNSSEP occurred at almost the same time in three patients, four minutes before MNSSEP in one, three minutes later in one and no change in one patient. Good quality baseline tracings were obtained in 99% MNSSEP as compared to 88% PTNSSEP (P < 0.05). New postoperative neurological deficits occurred in four patients (2.6%), only one had significant evoked potential changes.
Conclusion: Monitoring of PTNSSEP is feasible and may be considered for an adjunct to MNSSEP or as an alternative modality if there are difficulties with MNSSEP. However, there may be a greater incidence of poor quality baseline tracings for PTNSSEP.
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Introduction |
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Methods |
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All patients received a general anesthetic as determined by the attending anesthesiologist. In addition to standard monitors, an intra-arterial catheter was used for blood pressure measurements. The anesthetic management included the induction of anesthesia with fen-tanyl, propofol, and rocuronium. Maintenance consisted of nitrous oxide, fentanyl, rocuronium, and isoflurane, sevoflurane, or desflurane. Muscle paralysis was maintained. Blood pressure was maintained at the patient’s preoperative normal level and any hypotension was treated with vasopressors, (phenylephrine, ephedrine) and bradycardia was treated with atropine. The temperature and PaCO2 of the patient were both maintained within a normal range.
A trained technologist performed all evoked potential monitoring. The equipment used was an Axon Epoch 2000 neurophysiological recording system (Axon Systems, Hauppauge, NY, USA). This monitor allows for simultaneous asynchronous stimulation and recording of both the MNSSEP and PTNSSEP. Standard parameters were used for both recording and stimulating the upper limb and lower limb SSEP. Subdermal needle electrodes (Rochester Electro-Medical Inc., Tampa, FL, USA) were inserted after the induction of anesthesia and used for both stimulating and recording. The electrode impedance was maintained below 5.0 k ohms and the inter-electrode impedance below 1.0 k ohms. The MNSSEP was stimulated at the wrist with a 250 µsec duration square wave pulse of 20 to 30 mA intensity, 3.1 pulses·sec–1. The PTN was stimulated at the ankle (posterior to the medial malleolus) using the same stimulation parameters. Baseline recordings were acquired soon after the induction of anesthesia and subsequent, comparative recordings were obtained continuously during the procedure. The MNSSEP was recorded from the brachial plexus (Epi-Epc), the upper cervical spine (Cv2-Fpz), and the sensory cortex (CPc-Fpz). The PTNSSEP was recorded from the upper cervical spine (Cv2-Fpz), and the sensory cortex (CPz-Fpz, CPc-Cpi). A common ground electrode was placed on the patient’s sternum.
Patient demographic information was documented including the neurological history, reason for surgery and the immediate preoperative neurological status. SSEP data recorded included the measurements of the amplitude and latency of the cortical peaks (N20, P37) for baseline evoked potentials after induction of anesthesia and prior to cross clamping of the carotid artery. The incidence of any changes occurring during the procedure and especially after cross clamping of the carotid artery were documented. The incidence of shunting was documented. The patients were examined on emergence from anesthesia for any change in their neurological status and new postoperative neurological deficits were noted. The incidence, the timing, and the duration of all PTNSSEP changes were compared to MNSSEP changes. Chi-square testing was performed for statistical analysis of the raw data.
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Results |
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Good quality bilateral MNSSEP baseline tracings after the induction of anesthesia were obtained in 99% of the patients. In two patients there was absence or poor quality of the MNSSEP waveform on one side due to a previous stroke [ipsilateral to side of surgery (one), contralateral (one)]. In contrast good quality bilateral PTNSSEP baseline tracings were present only in 135 patients [88% (P < 0.05)]. Absent or poor baseline tracings occurred bilaterally in 14 patients. The reasons included technical problems (seven), diabetes (four), peripheral vascular disease (two) and leg edema (one). As well in four patients only one side PTNSSEP was useful for monitoring [technical problems ipsilateral (one), contralateral (one); previous amputation, ipsilateral (one), and prepping of the leg for a vein graph, ipsilateral (one)].
Significant MNSSEP changes (a greater than 50% decrease in the cortical peak (N20) after cross clamping of the carotid artery occurred in seven patients (Table
). Six of these patients had a shunt inserted by the surgeon. Significant PTNSSEP changes also occurred in five of these patients. No patient had only a PTNSSEP change. The time of onset of each SSEP change varied greatly. In two patients the PTNSSEP changes occurred before there was a change recorded in the MNSSEP. After the insertion of the shunt the SSEP changes recovered rapidly in five patients, but in one patient they persisted for 15 min and this patient had a transient deficit on awakening. In one patient there was a significant SSEP change in both modalities after 18 min of cross clamping, however the procedure was then quickly finished and the evoked potentials recovered immediately with release of the cross clamp at 21 min. The patient had no deficit. In another patient with no SSEP changes the surgeon decided to shunt prophylactically based on the patient’s clinical history and the fact that technical problems with the evoked potential monitoring had developed 30 min into the procedure. Examples of the time of onset of the SSEP changes are shown in Figures 1 and 2.
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New postoperative neurological deficits occurred in four patients. One patient who had received a shunt and had prolonged changes in both MNSSEP and PTNSSEP woke up with a deficit that persisted for 48 hr. Two patients who had transient deficits (lasting less than 24 hr) had had no intraoperative SSEP changes. One other patient who had no significant evoked potential changes woke up with a major persistent deficit (hemiplegia and aphasia). Patients were discharged from hospital at two days (median), (range 1–32). Overall the outcome of the patients at discharge from hospital was good for all except three who had cardiorespiratory complications and one patient who died from intracranial hemorrhage that occurred on day three postoperatively.
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Discussion |
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Though there is an associated risk of cerebral ischemia during carotid endarterectomy, there is controversy regarding the need for monitoring and there is no consensus on the most appropriate monitor.1–14 The decision to insert a shunt during cross clamping of the carotid artery is also controversial and each surgeon or institution has its own criteria.1,4–13,1516 Even when using SSEP monitoring, there are no universally recognized criteria for a significant change.2–5,7–16 However, the most commonly accepted and used is a greater than 50% decrease in amplitude of the cortical peak (N20) of the MN. In our study, the patients received a shunt on the basis of this change in MNSSEP. The number of patients with a significant change requiring a shunt was small (3.9%). All of these patients also had a change in their PTNSSEP except one. The time of onset of a MNSSEP change following cross clamping as well as the recovery of the changes after shunting when compared to the PTNSSEP changes varied. Most changes were close in time, though in one patient there was a very definite earlier change in the PTNSSEP. Some of the explanations of the variation in onset time include the state of collateral circulation and intracranial disease of each patient, as well as the technical aspects of evoked potential monitoring. Even with the use of sophisticated equipment to do simultaneous monitoring of different modalities, interferences may occur causing artifacts that impede monitoring.
SSEP changes were classified as non-significant when the decrease in amplitude of the cortical peak (N20 or P37) was less than 50% but greater than 30%. These changes were ipsilateral to the side of surgery and mostly related to decreases in blood pressure. The change occurred only in the MNSSEP in half of the patients. Because the blood pressure was treated immediately it could be that the change in the PTNSSEP did not have time to develop. As well it is not known whether these changes would have progressed to a significant change if the blood pressure had not been aggressively treated. The influence of anesthetic agents cannot be completely eliminated. However anesthetic agents should affect both ipsilateral and contralateral sides. In our practice whenever there are SSEP changes that occur bilaterally, the anesthetic management of the patient is examined and adjusted if needed.
We did not attempt to assess the relationship of an intraoperative evoked potential change to the occurrence of a postoperative deficit. All patients received a shunt whenever there was a significant evoked potential change after cross clamping, and any other changes of lesser severity were treated by increasing the patient’s blood pressure. This could influence the outcome. Overall, new postoperative deficits occurred in four patients (2.6%), only one of whom had significant SSEP changes.
In conclusion the monitoring of PTNSSEP is feasible and may be considered for an adjunct to MNSSEP or as an alternative modality if there are difficulties with MNSSEP during carotid endarterectomy. However there may be a greater incidence of absent or poor quality baseline tracings for PTNSSEP due to the common medical problems that are present in patients undergoing a carotid endarterectomy.
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Footnotes |
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Accepted for publication April 7, 2004. Revision accepted August 2, 2004.
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References |
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2 Schweiger H, Kamp HD, Dinkel M. Somatosensory-evoked potentials during carotid artery surgery: experience in 400 operations. Surgery 1991; 109: 602–9.[Medline]
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