AUTHOR(S): Taniguchi, Makoto, M.D.; Nadstawek, Joachim, M.D.; Pechstein, Ulrich, M.D.; Schramm, Johannes, M.D. Departments of Neurological Surgery (MT, UP, JS) and Anesthesiology (JN), University of Bonn, Bonn, Germany Neurosurgery 31; 891-897, 1992 ABSTRACT: TWO ANESTHETIC REGIMENS for monitoring somatosensory evoked potentials (SEPs) during intracranial aneurysm surgery were compared. Eighty-four sequential cases of intracranial aneurysms were operated on employing SEP monitoring. The first group of 22 cases was anesthetized with "balanced anesthesia" and the second group of 62 cases received total intravenous anesthesia (TIVA) consisting of propofol and alfentanil. In the TIVA group, the amplitude of early cortical SEP responses (N20-P25, or P40-N50) was significantly higher than that of responses in the balanced anesthesia group. In median nerve SEPs, the averaged amplitude of N20-P25 was 3.22 µV with TIVA and 1.69 µV with balanced anesthesia (P = 0.006). Similarly, posterior tibial nerve SEPs showed a P40-N50 response of 1.85 µV and 1.00 µV, respectively (P = 0.017). The superior signal-to-noise ratio obtained with TIVA allowed more frequent and reliable intraoperative SEP recordings than was possible with balanced anesthesia, resulting in rapid and reliable feedback for the surgeon. In 19% of median nerve SEPs recorded with TIVA, the cortical responses were over 5 µV in amplitude, so that reproducible N20-P25 responses were obtainable by averaging only 10 to 50 serial responses, that is, two to three recordings per minute. The higher amplitude of posterior tibial nerve SEPs recorded with TIVA made monitoring during surgery for anterior communicating artery aneurysms possible in all cases. This was not always the case with balanced anesthesia. The late deflection of median nerve SEPs (N30) was more frequently observed with TIVA. It is a more sensitive indicator of cortical hypoperfusion than the N20-P25 complex, which adds additional information. This new anesthetic protocol consisting of propofol and alfentanil was useful for intraoperative SEP monitoring and was safe to use during aneurysm surgery. KEY WORDS: Alfentanil; Aneurysm surgery; Intraoperative monitoring; Propofol; Somatosensory evoked potentials; Total intravenous anesthesia Intraoperative recording of somatosensory evoked potentials (SEPs) is a useful indicator of cortical
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
tolerance during occlusion of large vessels in intracranial aneurysm surgery (5,6,12,21). Deterioration in cortical SEP responses (N20-P25, P40-N50) during vessel occlusion can alarm the surgeon and prompt corrective action, for example, removal of temporary clips. Conversely, preserved SEP responses can indicate good collateral flow to the corresponding cortical area, allowing the surgeon to consider all optional tactics, perhaps even permanent vessel occlusion. To support prompt and clear decision making, SEP recordings should be consistent and frequent, that is, an adequate signal-to-noise ratio is necessary. In addition to the usual effort to eliminate electrical interference, the chosen anesthetic regime should only minimally suppress SEPs. "Balanced anesthesia" is widely used in neurosurgery and allows a high incidence of acceptable SEP recordings, at least of median nerve SEPs (6,11,16,22). However, monitoring of posterior tibial nerve SEPs is often difficult with balanced anesthesia (22,23). The cortical responses of posterior tibial nerve SEPs are smaller in amplitude than those of median nerve SEPs, even in awake patients (4)--that is, the same level of anesthetic suppression shows a more intense effect on tibial nerve SEPs. In our experience with balanced anesthesia, the early cortical responses could be readily monitored in 95% of median nerve SEPs, but only in 71% of posterior tibial nerve SEPs (20). This implies that SEP monitoring is less reliable in surgery for anterior communicating aneurysms, where the blood supply to the medial hemispheric region is of concern. In some cases in which balanced anesthesia was employed, even successfully monitored cases, cortical SEPs were so small that extensive averaging was necessary to obtain a reliable response. This process could require several minutes, diminishing its practicability as a real-time monitoring tool. This point can be of great importance during aneurysm surgery, where prompt information regarding the collateral blood flow and deterioration are absolutely necessary. Therefore, improvement of the anesthetic technique appears necessary to upgrade the quality of the monitoring. It should suppress SEPs only minimally, supply a better signal-to-noise ratio, and should be applicable as a routine method without adding any clinical complications. Recently, the method of anesthesia for SEP monitoring in our institution was changed from balanced anesthesia to total intravenous anesthesia (TIVA) (13,24). This change had a favorable effect on intraoperative SEP recording. The purpose of this paper is to outline the advantage of this new intravenous anesthetic method for SEP monitoring. PATIENTS AND METHODS Eighty-four consecutive patients with intracranial aneurysms were monitored with SEP intraoperatively since the previous summary of 134 patients (20). All of the patients reported here were examined by the same person (M.T.) using the same recording technique that was standardized during the above-mentioned study. Thirty-one patients had aneurysms of the
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
Neurosurgery 1992-98 November 1992, Volume 31, Number 5 891 Total Intravenous Anesthesia for Improvement of Intraoperative Monitoring of Somatosensory Evoked Potentials during Aneurysm Surgery Clinical Study
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
the early cortical responses was calculated by averaging all the examined responses in the series, and the recordings obtained during intervening surgical events (e.g., vessel occlusion) were excluded to analyze the effects of the anesthetics alone. RESULTS Amplitude of somatosensory evoked potentials To record median nerve SEPs, balanced anesthesia was used in 11 patients and TIVA was used in 42. The cortical responses (N20-P25) of all patients were always obtainable during the entire operation, regardless of the method of anesthesia used. Although the amplitude of the SEPs differed from individual to individual, it remained fairly constant in the range of ±25% of the control value during the whole operation, as long as no further surgical events (e.g., temporary vessel occlusion) had occurred. The amplitude of the N20-P25 response differed between the two groups. The TIVA group had larger cortical responses as compared with the balanced anesthesia group. The mean amplitudes for N20-P25 were 1.69 µV and 3.22 µV for the balanced anesthesia group and the TIVA group, respectively. The difference between these two means was statistically significant [Z0 = 2.7 = Z(0.006), Mann-Whitney U-test] (Fig. 1, top). Special attention should be paid to eight patients (19%) who underwent recording of median nerve SEPs under TIVA. N20-P25 complex measuring greater than 5 µV could be observed directly on the oscilloscope without averaging, due to the large signal-to-noise ratio. As few as 10 to 50 responses needed to be averaged for suitable recording. Such cortical responses are ideal for real-time monitoring, as two to three recordings can be obtained per minute (Fig. 2). Posterior tibial nerve SEPs were examined in 11 patients who had balanced anesthesia and 20 in whom TIVA was used. The smaller amplitude of the P40-N50 response was more difficult to interpret when compared with that of the N20-P25 response. Responses smaller than 0.5 µV could not be accurately detected. This problem occurred only in the balanced anesthesia group, where P40-N50 was preserved only in six patients (55%), recorded only on occasion in three patients (27%), and not at all in two patients (18%). The mean amplitude of the P40-N50 response in this group was 1.0 µV (for statistical purposes, an arbitrary value of 0.5 µV was assigned to the five patients in whom the peak could not be detected). In contrast, early cortical responses were obtainable throughout the operation in all 20 TIVA patients. The mean amplitude of the P40-N50 response obtained in the TIVA group was 1.85 µV. The difference between the two groups was also statistically significant [Z0 = 2.38 = Z(0.017), MannWhitney U-test] (Fig. 1, bottom). Changes during surgical "events" Twenty-six surgical "events" were encountered during surgery (7 in patients under balanced anesthesia, 19 in patients under TIVA), including the following: 16 temporary clippings of vessels (3 in patients under balanced anesthesia, 11 in patients
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
anterior communicating artery or distal portion of the anterior cerebral artery, 27 had aneurysms of the internal carotid artery, 15 had aneurysms of the middle cerebral artery, and 11 had aneurysms of the posterior circulation. Posterior tibial nerve SEPs were monitored in patients with aneurysms of the anterior cerebral artery, and median nerve SEPs were monitored in the other patients. The indications for surgery, perioperative management, etc. were the same as described before (20). In cases requiring more than one operation for multiple aneurysms, we included only one series of examinations per patient in this study, in order to emphasize interindividual difference. The first 22 patients--11 of whom had median nerve SEP monitoring and 11 of whom had posterior tibial nerve SEP monitoring--were anesthetized with "balanced anesthesia." This comprised 66% nitrous oxide; halogenated anesthetics (enflurane or isoflurane; 0.5 minimal alveolar concentration); fentanyl (intravenously, 0.1-0.2 mg for initial analgesia, and 0.1 mg every hour, up to 1.0 mg in total); and pancuronium bromide or vecuronium (2 mg intravenously) every hour for relaxation. The next 62 patients--42 of whom had median nerve SEP monitoring and 20 of whom had posterior tibial nerve SEP monitoring--were anesthetized using only intravenous agents. A continuous infusion of propofol was used as a narcotic agent. A 60-mg bolus was given for induction, and maintenance was achieved with 15 mg/min for the first 60 minutes and 6 mg/min thereafter. In cases where induced hypotension was requested by the surgeon (usually during dissection around the aneurysm) a high dose of 15 mg/min of propofol was administered in addition to the usual antihypertensive drugs (e.g., a 2.5-mg bolus injection of urapidil). For analgesia, a continuous infusion of alfentanil was employed. A 1mg intra-venous bolus was given for induction, followed by 0.3 mg/min for the first hour and 0.1 mg/min thereafter for maintenance. Muscle relaxants were administered according to the same regimen followed in the first group. Recently, this scheme was slightly modified to reduce the total amount of alfentanil administered. SEPs were recorded using the Nicolet Pathfinder I (Nicolet Biomedical Instruments Co., Ltd., Madison, WI) with the following stimulation and recording parameters: rectangular constant current pulses delivered to the median nerve or to the posterior tibial nerve (duration, 300 µs; intensity, 20 mA; frequency, 5.3 Hz). Two-channel recordings were obtained from cortical location--2 cm posterior to C3 and C4-Fpz for median nerve SEPs; Cz-Fpz for tibial nerve SEPs--and cervical location. The sensitivity was 5 µV/DIV, with filter settings of 30 Hz to 3 kHz, and an analysis time of 50 ms for median nerve SEPs and 100 ms for tibial nerve SEPs (repetition, 50-300). All recordings were stored on a hard disk for later analysis. The amplitude of the early cortical responses (N20P25 for median nerve SEPs, P40-N50 for tibial nerve SEPs) were used for comparison between the two groups. A set of several hundred cortical SEPs was acquired during each operation. The mean value of
Anesthesiological management Neither of the two methods of anesthesia used in this study resulted in major anesthesiological complications. The anesthetic effect was sufficient in both groups. No intraoperative awareness was reported. Recovery from anesthesia was equally rapid in both groups, with the exception of 3 of the 62 patients in the TIVA group (4.8%). These patients had such a long recovery period that computed tomographic scanning was necessary postoperatively; this showed no abnormality in any of them. This prolongation of the recovery period was supposed to be due to an alfentanil hangover. The patients remained unconscious for several hours before being extubated, but meanwhile responded adequately to
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
pain stimuli with both extremities. Five cases (8.1%) of prominent bradycardia (50%) of SEPs (in 4 TIVA patients). No significant difference in the incidence of SEP changes during surgical events was seen between groups. Late deflections of SEPs are not routinely seen under general anesthesia. Normally, sufficient doses of inhalation anesthetics seem to suppress the N30 component of median nerve SEPs almost completely. Intravenous agents, however, allow monitoring of the N30 component more frequently, while still achieving satisfactory hypnotic effects. N30 was preserved throughout surgery and served as a useful monitoring parameter in 3 of 11 patients receiving balanced anesthesia (27%) and in 15 of 42 patients receiving TIVA (36%). The N30 component is likely to provide information on insufficient cerebral blood flow in addition to that provided by the the N20-P25 complex. Figure 3 illustrates a case in which hypotension was induced to control a small hemorrhage from an aneurysm. This caused a loss of the N30 component first, followed by a loss of the N20-P25 complex. As blood pressure increased, the N20-P25 complex recovered first, followed by the N30 response. Direct pharmacological suppression by hypotensive agents or other drugs seemed unlikely to be the cause, since contralateral SEPs at that moment were unchanged. This suggests that the N30 component may be a more sensitive parameter for monitoring cerebral ischemia. Similar changes were also observed during temporary clipping of the middle cerebral artery in a patient with an aneurysm of that artery. In this case, the middle cerebral artery was clipped for 7 minutes with no change to the N20-P25 complex, but with loss of the N30 component during vessel occlusion only. This patient recovered from anesthesia with transient hemiparesis, which persisted for half a day.
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
supported by Kalkman et al. (10) who compared the effect of nitrous oxide and propofol in the same patients during alfentanil anesthesia. They reported that posterior tibial nerve SEPs were almost twice as large in case of propofol. Scheepstra et al. (18) reported 10 cases of ASA Grade I patients who were anesthetized with a combination of propofol and intermittent injections of fentanyl for minor surgery of short duration. They found no significant suppressive effects of anesthesia on SEPs compared with preinduction values (18). Their protocol with TIVA using fentanyl as an analgesic may be better suited for comparison of SEPs with those of our balanced anesthesia group, although incompatible with lengthy neurosurgical procedures. They reported slightly larger amplitudes of N20-P25 (4.09-5.03 µV) than we observed. The effect of different analgesic agents, fentanyl and alfentanil, seems not to be excluded in our study. However, other reports have concluded that surgical doses of fentanyl and alfentanil have little suppressive effect on SEPs (8,9, 19) . The similar amplitudes of SEP responses under propofol anesthesia reported by different authors, regardless of the analgesics used, leads us to conclude that the suppressive effect of various analgesic agents is so minimal that it would be negligible compared with the larger difference between propofol and volatile anesthetics. Better intraoperative SEP monitoring was achieved with TIVA. Further technical refinement of this regime will help to increase its value as a new practical method of anesthesia. ACKNOWLEDGMENTS This study and M. Taniguchi were supported by a grant from the Deutsche Forschungsgemeinschaft (DFG Schr 285/1-3). The authors thank E. Heunemann, K. Lehrmann, M. Melchers, and M. Wisniewski for their technical assistance. These results were presented in part at the Fourth International Evoked Potentials Symposium in Toronto, Canada, October 1990, and at the annual meeting of the American Society of Evoked Potentials (ASEP), Minneapolis, Minnesota, May 1991. Received, December 3, 1991. Accepted, June 8, 1992. Reprint requests: Johannes Schramm, M.D., Neurochirurgische Klinik, Universität Bonn, Sigmund-Freud-Str. 25, W-5300 Bonn 1, Germany. REFERENCES: (1-25) 1.
2.
Abrahamian HA, Allison T, Goff WR, Rosner BS: Effects of thiopental on human cerebral somatic response. Anesthesiology 24:650657, 1963. Allison T, Goff WR, Abrahamian HA, Rosner BS: The effects of barbiturate anesthesia upon human somatosensory evoked responses. Electroencephalogr Clin Neurophysiol
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
examinations. Both this limitation from the averaging time and the intraoperative noise level enabled us to detect only responses greater than 0.5 µV. According to this criteria of 0.5 µV for clinically useful SEP responses, the rate of successful SEP recording in the series of Pathak et al. does not differ from ours. Lower frequency (0.5-1 Hz) stimulation of the peripheral nerves has been proposed by other authors as a solution to the problem of SEP suppression (8,14, 17) . Unfortunately, this sacrifices the capability of obtaining frequent responses in short intervals, which is required for adequate monitoring during aneurysm surgery. In contrast, good cortical responses were obtained in all patients under TIVA without altering the stimulation and recording parameters, so that reliable information could be gathered without additional delay. An equally high rate of successful monitoring using tibialis nerve SEPs has been reported for other types of anesthesia (1-3). However, we consider TIVA to be the most practicable anesthesia technique among them, because it meets an important criteria for neurosurgical cases, namely, prompt recovery from anesthesia. Postoperative neurological evaluation of the patient is easily possible with TIVA. Thiopental is well known to produce minimal suppression of SEPs (1,2); however, it is not practical to use this regime during lengthy aneurysm operations, because of its relatively long half-life and, therefore, hangover effects. A use of moderate hypothermic anesthesia consisting of a meperidine relaxant technique and 65% nitrous oxide was reported by Buchthal and colleagues (3,12). They reported 100% successful monitoring of posterior tibial nerve SEPs. However, this method also seems to require a long recovery phase in the intensive care unit before extubation. Compared with the two abovementioned methods of anesthesia, recovery from TIVA was almost always as prompt as from balanced anesthesia, and thus better fits the requirements of neurosurgery. After our early experiences with alfentanil hangover in a few patients (4.8%), we modified the infusion scheme for alfentanil. No further problem with anesthetic hangover was encountered thereafter. Late deflections of median nerve SEPs (N30 component) were more susceptible to anesthetic agents than early components (N20-P25 complex), making them unreliable as an intraoperative monitoring parameter. However, under TIVA, the N30 component was more consistently obtained and was a more sensitive indicator of cortical hypoperfusion. It is difficult to determine which drug used for TIVA resulted in this favorable effect on SEP monitoring, since both the anesthetics and the analgesics were changed at the same time. Propofol is known to have little analgesic action, and therefore, continuous administration of a strong analgesic was necessary to introduce this anesthetic protocol clinically. Alfentanil was chosen over fentanyl because its rapid turnover seemed to be favorable for reducing the risk of hangover. For several reasons, we might conclude that the larger amplitude of SEPs with TIVA derives mainly from the different anesthetics, such as propofol. This is strongly
4.
5.
6.
7. 8.
9.
10.
11.
12.
13.
14.
[Suppl] 24:68-75, 1963. Buchthal A, Belopavlovic M, Mooij JJA: Somatosensory evoked in temporary vascular occlusion for aneurysm surgery under moderate hypothermia, in Schramm J, Møller AR (eds): Intraoperative Neurophysiologic Monitoring in Neurosurgery. Berlin and Heidelberg, Springer-Verlag, 1991, pp 172183. Desmedt JE: Somatosensory evoked potentials in neuromonitoring, in Desmedt JE (ed): Neuromonitoring in Surgery. Amsterdam, Elsevier Science Publishers B.V. (Biomedical Division), 1989, pp 1-21. Ducati A, Landi A, Cenzato M, Fava E, Rampini P, Giovanelli M, Villani R: Monitoring of brain function by means of evoked potentials in cerebral aneurysm surgery. Acta Neurochir (Wien) [suppl] 42:813, 1988. Friedman WA, Chadwick GM, Verhoeven FJS, Mahla M, Day AL: Monitoring of somatosensory evoked potentials during surgery for middle cerebral artery aneurysms. Neurosurgery 29:83-88, 1991. Grundy BL: Intraoperative monitoring of sensory-evoked potentials, Anesthesiology 58:72-87, 1983. Harper CM, Daube JR: Surgical monitoring with evoked potentials: The Mayo Clinic experience, in Desmedt JE (ed): Neuromonitoring in Surgery. Amsterdam, Elsevier Science Publishers B.V. (Biomedical Division), 1989, pp 275-301. Kalkman CJ, Rheineck Leyssius AT, Bovill JG: Influence of high- dose opioid anesthesia on posterior tibial nerve somatosensory cortical evoked potentials: Effects of fentanyl, sufentanil, alfentil. J Cardiothorac Anesth 2:764-785, 1988. Kalkman CJ, Traast H, Zuurmond WW, Bovill JG: Differential effects of propofol and nitrous oxide on posterior tibial nerve somatosensory cortical evoked potentials during alfentanil anesthesia. Br J Anaesth 66:483-489, 1991. McPherson RW, Mahla M, Johnson R, Traystman RJ: Effects of enflurane, isoflurane, and nitrous oxide on somatosensory evoked potentials during fentanyl anesthesia. Anesthesiology 62:62633, 1985. Mooij JJA, Buchthal A, Belpavlovic M: Somatosensory evoked potential monitoring of temporary middle cerebral artery occlusion during aneurysm operation. Neurosurgery 21:492-496, 1987. Nadstawek J, Taniguchi M, Ruta U, Limberg N, Schramm J: Totale intravenöse Anästhesie (TIVA) mit Alfentanil und Propofol versus balancierte Anästhesie mit Isoflurane und Fentanyl in der Neurochirurgie. Anaesthesist 40(Suppl 2):43, 1991. Nuwer MR: Technical problems, artifacts, and
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
quality assessment of intraoperative evoked potentials, in Schramm J, Møller AR (eds): Intraoperative Neurophysiologic Monitoring in Neurosurgery. Berlin and Heidelberg, Springer-Verlag, 1991, pp 18-27. Pathak KS, Ammadio M, Kalamchi A, Scoles N, Schaffer JW, Mackay W: Effects of somatosensory evoked potentials during nitrous oxide anesthesia. Anesthesiology 66:753-57, 1987. Peterson DO, Drummond JC, Todd MM: Effects of halothane, enflurane, isoflurane, and nitrous oxide on somatosensory evoked potentials in humans. Anesthesiology 65:3540, 1986. Salzman SK, Beckman AL, Marks HG, Naidu R, Bunnell WP, MacEwen GD: Effects of halothane on intraoperative scalp-recorded somatosensory evoked potentials to posterior tibial nerve stimulation in man. Electroencehalogr Clin Neurophysiol 65:3645, 1986. Scheepstra GL, De Lange JJ, Booij LHDJ, Ros HH: Median nerve evoked potentials during propofol anesthesia. Br J Anaesth 62:92-94, 1989. Schramm J: Intraoperative monitoring with evoked potentials in cerebral vascular surgery and posterior fossa surgery, in Desmedt JE (ed): Neuromonitoring in Surgery. Amsterdam, Elsevier Science Publishers B.V. (Biomedical Division), 1989, pp 243-262. Schramm J, Koht A, Schmidt G, Pechstein U, Taniguchi M, Fahlbusch R: Surgical and electrophysiological observations during clipping of 134 aneurysms with evoked potentials monitoring. Neurosurgery 26:6170, 1990. Symon L, Wang AD, Costa e Silva IE, Gentili F: Perioperative use of somatosensory evoked responses in aneurysm surgery. J Neurosurg 60:269-275, 1984. Symon L, Murota T: Intraoperative monitoring of somatosensory evoked potentials during intracranial vascular surgery, in Desmedt JE (ed): Neuromonitoring in Surgery. Amsterdam, Elsevier Science Publishers B.V. (Biomedical Division), 1989, pp 263-274. Symon L, Jellinek DA: Experiences with intraoperative temporary vessel occlusions and monitoring in aneurysm surgery, in Schramm J, Møller AR (eds): Intraoperative Neurophysiologic Monitoring in Neurosurgery. Berlin and Heidelberg, Springer-Verlag, 1991, pp 162-171. Van Aken H, Van Hemelrijck J, Merckx L, Möllhoff T, Muler J, Lubbesmeyer HJ: Totale intravenöse Anästhesie mit Propofol und Alfentanil im Vergleich zur balancierten Anästhesie in der Neurochirurgie. Anaesth Intensivther Notfallmed 25:54-58, 1990. York DH: A critical evaluation of the 50% criterion in SEP monitoring. Presented at the
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
3.
COMMENTS The authors studied the effect of two anesthetic regimens on the monitoring of somatosensory evoked potentials (SEPs) during surgery for intracranial aneurysms. Twenty-two patients anesthetized with nitrous oxide and enflurane or isoflurane were compared with 62 patients anesthetized with total intravenous anesthesia (TIVA) using propofol and alfentanil. Median nerve SEPs were obtainable with either regimen; however, TIVA resulted in larger cortical responses, thus requiring fewer averages per recording and allowing a greater number of recordings per minute. For patients with anterior cerebral artery aneurysms, TIVA provided a significantly better ability to record posterior tibial SEPs than the technique of balanced anesthesia. There were no anesthesia-related complications, although three patients in the TIVA group (4.8%) had prolonged anesthetic recovery necessitating a postoperative computed tomographic scan. The authors suspect alfentanil and have recently modified their regimen to decrease the total administration of this drug. The increased strength of signals possible with TIVA allows more frequent feedback and should provide the surgeon valuable information to guide surgery, particularly regarding the length of time of temporary clipping. If confirmed by other laboratories, this regimen should provide a significant therapeutic advantage. John Oro Columbia, Missouri The report by Taniguchi et al. clearly demonstrates the advantages of using intravenous anesthesia without inhalation agents for monitoring neural function with somatosensory evoked potentials in those patients for whom this anesthetic regimen is appropriate. This approach is particularly useful in those patients whose baseline somatosensory evoked potentials are abnormal because of the underlying neurological disease. Jasper R. Daube Rochester, Minnesota
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
second annual convention of the American Society of Evoked Potentials Monitoring, Minneapolis, Minnesota, May 9-11, 1991.
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
Figure 1. Top: Amplitude of N20-P25 responses recorded under different methods of anesthesia: 11 patients who underwent inhalation anesthesia and 42 cases who underwent intravenous anesthesia were compared. The mean values of the amplitude of N20-P25 under each regime were 1.69 µV and 3.22 µV, respectively. This difference was statistically significant. Bottom: The same comparison for posterior tibial nerve SEPs in 11 patients who received balanced anesthesia and 20 who received TIVA. The mean values of the amplitude of P40-N50 were 1.00 µV and 1.85 µV, respectively. In cases of unsuccessful monitoring of patients due to strong suppression of SEPs, a tentative value of 0.5 µV was arbitrarily assigned for statistical analysis. Therefore, the true mean value of P40-N50 with balanced anesthesia might be lower. This difference was also statistically significant.
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
Figure 2. Recording of median nerve SEPs during surgery for a middle cerebral artery aneurysm in a patient receiving TIVA. The averaged amplitude of the N20-P25 complex in this particular patient was 8.25 µV. Shown is some plotting of SEP recordings of the case, routinely made after every operation for the purpose of documentation and analysis. Reproducible SEPs were obtainable two to three times per minute by averaging 50 serial responses.
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
Figure 3. Loss of the N30 component, and later of the N20-P25 component. The N30 component appeared to be a sensitive indicator of insufficient cortical blood supply.
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
Figure 4. Changes in amplitude of the N20-P25 response during the operation. The patient was first anesthetized with TIVA. The addition of inhalation agents, both isoflurane and nitrous oxide, resulted in a constant and strong suppression of the amplitude of the N20-P25 response.
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
tolerance during occlusion of large vessels in intracranial aneurysm surgery (5,6,12,21). Deterioration in cortical SEP responses (N20-P25, P40-N50) during vessel occlusion can alarm the surgeon and prompt corrective action, for example, removal of temporary clips. Conversely, preserved SEP responses can indicate good collateral flow to the corresponding cortical area, allowing the surgeon to consider all optional tactics, perhaps even permanent vessel occlusion. To support prompt and clear decision making, SEP recordings should be consistent and frequent, that is, an adequate signal-to-noise ratio is necessary. In addition to the usual effort to eliminate electrical interference, the chosen anesthetic regime should only minimally suppress SEPs. "Balanced anesthesia" is widely used in neurosurgery and allows a high incidence of acceptable SEP recordings, at least of median nerve SEPs (6,11,16,22). However, monitoring of posterior tibial nerve SEPs is often difficult with balanced anesthesia (22,23). The cortical responses of posterior tibial nerve SEPs are smaller in amplitude than those of median nerve SEPs, even in awake patients (4)--that is, the same level of anesthetic suppression shows a more intense effect on tibial nerve SEPs. In our experience with balanced anesthesia, the early cortical responses could be readily monitored in 95% of median nerve SEPs, but only in 71% of posterior tibial nerve SEPs (20). This implies that SEP monitoring is less reliable in surgery for anterior communicating aneurysms, where the blood supply to the medial hemispheric region is of concern. In some cases in which balanced anesthesia was employed, even successfully monitored cases, cortical SEPs were so small that extensive averaging was necessary to obtain a reliable response. This process could require several minutes, diminishing its practicability as a real-time monitoring tool. This point can be of great importance during aneurysm surgery, where prompt information regarding the collateral blood flow and deterioration are absolutely necessary. Therefore, improvement of the anesthetic technique appears necessary to upgrade the quality of the monitoring. It should suppress SEPs only minimally, supply a better signal-to-noise ratio, and should be applicable as a routine method without adding any clinical complications. Recently, the method of anesthesia for SEP monitoring in our institution was changed from balanced anesthesia to total intravenous anesthesia (TIVA) (13,24). This change had a favorable effect on intraoperative SEP recording. The purpose of this paper is to outline the advantage of this new intravenous anesthetic method for SEP monitoring. PATIENTS AND METHODS Eighty-four consecutive patients with intracranial aneurysms were monitored with SEP intraoperatively since the previous summary of 134 patients (20). All of the patients reported here were examined by the same person (M.T.) using the same recording technique that was standardized during the above-mentioned study. Thirty-one patients had aneurysms of the
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
Neurosurgery 1992-98 November 1992, Volume 31, Number 5 891 Total Intravenous Anesthesia for Improvement of Intraoperative Monitoring of Somatosensory Evoked Potentials during Aneurysm Surgery Clinical Study
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
the early cortical responses was calculated by averaging all the examined responses in the series, and the recordings obtained during intervening surgical events (e.g., vessel occlusion) were excluded to analyze the effects of the anesthetics alone. RESULTS Amplitude of somatosensory evoked potentials To record median nerve SEPs, balanced anesthesia was used in 11 patients and TIVA was used in 42. The cortical responses (N20-P25) of all patients were always obtainable during the entire operation, regardless of the method of anesthesia used. Although the amplitude of the SEPs differed from individual to individual, it remained fairly constant in the range of ±25% of the control value during the whole operation, as long as no further surgical events (e.g., temporary vessel occlusion) had occurred. The amplitude of the N20-P25 response differed between the two groups. The TIVA group had larger cortical responses as compared with the balanced anesthesia group. The mean amplitudes for N20-P25 were 1.69 µV and 3.22 µV for the balanced anesthesia group and the TIVA group, respectively. The difference between these two means was statistically significant [Z0 = 2.7 = Z(0.006), Mann-Whitney U-test] (Fig. 1, top). Special attention should be paid to eight patients (19%) who underwent recording of median nerve SEPs under TIVA. N20-P25 complex measuring greater than 5 µV could be observed directly on the oscilloscope without averaging, due to the large signal-to-noise ratio. As few as 10 to 50 responses needed to be averaged for suitable recording. Such cortical responses are ideal for real-time monitoring, as two to three recordings can be obtained per minute (Fig. 2). Posterior tibial nerve SEPs were examined in 11 patients who had balanced anesthesia and 20 in whom TIVA was used. The smaller amplitude of the P40-N50 response was more difficult to interpret when compared with that of the N20-P25 response. Responses smaller than 0.5 µV could not be accurately detected. This problem occurred only in the balanced anesthesia group, where P40-N50 was preserved only in six patients (55%), recorded only on occasion in three patients (27%), and not at all in two patients (18%). The mean amplitude of the P40-N50 response in this group was 1.0 µV (for statistical purposes, an arbitrary value of 0.5 µV was assigned to the five patients in whom the peak could not be detected). In contrast, early cortical responses were obtainable throughout the operation in all 20 TIVA patients. The mean amplitude of the P40-N50 response obtained in the TIVA group was 1.85 µV. The difference between the two groups was also statistically significant [Z0 = 2.38 = Z(0.017), MannWhitney U-test] (Fig. 1, bottom). Changes during surgical "events" Twenty-six surgical "events" were encountered during surgery (7 in patients under balanced anesthesia, 19 in patients under TIVA), including the following: 16 temporary clippings of vessels (3 in patients under balanced anesthesia, 11 in patients
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
anterior communicating artery or distal portion of the anterior cerebral artery, 27 had aneurysms of the internal carotid artery, 15 had aneurysms of the middle cerebral artery, and 11 had aneurysms of the posterior circulation. Posterior tibial nerve SEPs were monitored in patients with aneurysms of the anterior cerebral artery, and median nerve SEPs were monitored in the other patients. The indications for surgery, perioperative management, etc. were the same as described before (20). In cases requiring more than one operation for multiple aneurysms, we included only one series of examinations per patient in this study, in order to emphasize interindividual difference. The first 22 patients--11 of whom had median nerve SEP monitoring and 11 of whom had posterior tibial nerve SEP monitoring--were anesthetized with "balanced anesthesia." This comprised 66% nitrous oxide; halogenated anesthetics (enflurane or isoflurane; 0.5 minimal alveolar concentration); fentanyl (intravenously, 0.1-0.2 mg for initial analgesia, and 0.1 mg every hour, up to 1.0 mg in total); and pancuronium bromide or vecuronium (2 mg intravenously) every hour for relaxation. The next 62 patients--42 of whom had median nerve SEP monitoring and 20 of whom had posterior tibial nerve SEP monitoring--were anesthetized using only intravenous agents. A continuous infusion of propofol was used as a narcotic agent. A 60-mg bolus was given for induction, and maintenance was achieved with 15 mg/min for the first 60 minutes and 6 mg/min thereafter. In cases where induced hypotension was requested by the surgeon (usually during dissection around the aneurysm) a high dose of 15 mg/min of propofol was administered in addition to the usual antihypertensive drugs (e.g., a 2.5-mg bolus injection of urapidil). For analgesia, a continuous infusion of alfentanil was employed. A 1mg intra-venous bolus was given for induction, followed by 0.3 mg/min for the first hour and 0.1 mg/min thereafter for maintenance. Muscle relaxants were administered according to the same regimen followed in the first group. Recently, this scheme was slightly modified to reduce the total amount of alfentanil administered. SEPs were recorded using the Nicolet Pathfinder I (Nicolet Biomedical Instruments Co., Ltd., Madison, WI) with the following stimulation and recording parameters: rectangular constant current pulses delivered to the median nerve or to the posterior tibial nerve (duration, 300 µs; intensity, 20 mA; frequency, 5.3 Hz). Two-channel recordings were obtained from cortical location--2 cm posterior to C3 and C4-Fpz for median nerve SEPs; Cz-Fpz for tibial nerve SEPs--and cervical location. The sensitivity was 5 µV/DIV, with filter settings of 30 Hz to 3 kHz, and an analysis time of 50 ms for median nerve SEPs and 100 ms for tibial nerve SEPs (repetition, 50-300). All recordings were stored on a hard disk for later analysis. The amplitude of the early cortical responses (N20P25 for median nerve SEPs, P40-N50 for tibial nerve SEPs) were used for comparison between the two groups. A set of several hundred cortical SEPs was acquired during each operation. The mean value of
Anesthesiological management Neither of the two methods of anesthesia used in this study resulted in major anesthesiological complications. The anesthetic effect was sufficient in both groups. No intraoperative awareness was reported. Recovery from anesthesia was equally rapid in both groups, with the exception of 3 of the 62 patients in the TIVA group (4.8%). These patients had such a long recovery period that computed tomographic scanning was necessary postoperatively; this showed no abnormality in any of them. This prolongation of the recovery period was supposed to be due to an alfentanil hangover. The patients remained unconscious for several hours before being extubated, but meanwhile responded adequately to
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
pain stimuli with both extremities. Five cases (8.1%) of prominent bradycardia (50%) of SEPs (in 4 TIVA patients). No significant difference in the incidence of SEP changes during surgical events was seen between groups. Late deflections of SEPs are not routinely seen under general anesthesia. Normally, sufficient doses of inhalation anesthetics seem to suppress the N30 component of median nerve SEPs almost completely. Intravenous agents, however, allow monitoring of the N30 component more frequently, while still achieving satisfactory hypnotic effects. N30 was preserved throughout surgery and served as a useful monitoring parameter in 3 of 11 patients receiving balanced anesthesia (27%) and in 15 of 42 patients receiving TIVA (36%). The N30 component is likely to provide information on insufficient cerebral blood flow in addition to that provided by the the N20-P25 complex. Figure 3 illustrates a case in which hypotension was induced to control a small hemorrhage from an aneurysm. This caused a loss of the N30 component first, followed by a loss of the N20-P25 complex. As blood pressure increased, the N20-P25 complex recovered first, followed by the N30 response. Direct pharmacological suppression by hypotensive agents or other drugs seemed unlikely to be the cause, since contralateral SEPs at that moment were unchanged. This suggests that the N30 component may be a more sensitive parameter for monitoring cerebral ischemia. Similar changes were also observed during temporary clipping of the middle cerebral artery in a patient with an aneurysm of that artery. In this case, the middle cerebral artery was clipped for 7 minutes with no change to the N20-P25 complex, but with loss of the N30 component during vessel occlusion only. This patient recovered from anesthesia with transient hemiparesis, which persisted for half a day.
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
supported by Kalkman et al. (10) who compared the effect of nitrous oxide and propofol in the same patients during alfentanil anesthesia. They reported that posterior tibial nerve SEPs were almost twice as large in case of propofol. Scheepstra et al. (18) reported 10 cases of ASA Grade I patients who were anesthetized with a combination of propofol and intermittent injections of fentanyl for minor surgery of short duration. They found no significant suppressive effects of anesthesia on SEPs compared with preinduction values (18). Their protocol with TIVA using fentanyl as an analgesic may be better suited for comparison of SEPs with those of our balanced anesthesia group, although incompatible with lengthy neurosurgical procedures. They reported slightly larger amplitudes of N20-P25 (4.09-5.03 µV) than we observed. The effect of different analgesic agents, fentanyl and alfentanil, seems not to be excluded in our study. However, other reports have concluded that surgical doses of fentanyl and alfentanil have little suppressive effect on SEPs (8,9, 19) . The similar amplitudes of SEP responses under propofol anesthesia reported by different authors, regardless of the analgesics used, leads us to conclude that the suppressive effect of various analgesic agents is so minimal that it would be negligible compared with the larger difference between propofol and volatile anesthetics. Better intraoperative SEP monitoring was achieved with TIVA. Further technical refinement of this regime will help to increase its value as a new practical method of anesthesia. ACKNOWLEDGMENTS This study and M. Taniguchi were supported by a grant from the Deutsche Forschungsgemeinschaft (DFG Schr 285/1-3). The authors thank E. Heunemann, K. Lehrmann, M. Melchers, and M. Wisniewski for their technical assistance. These results were presented in part at the Fourth International Evoked Potentials Symposium in Toronto, Canada, October 1990, and at the annual meeting of the American Society of Evoked Potentials (ASEP), Minneapolis, Minnesota, May 1991. Received, December 3, 1991. Accepted, June 8, 1992. Reprint requests: Johannes Schramm, M.D., Neurochirurgische Klinik, Universität Bonn, Sigmund-Freud-Str. 25, W-5300 Bonn 1, Germany. REFERENCES: (1-25) 1.
2.
Abrahamian HA, Allison T, Goff WR, Rosner BS: Effects of thiopental on human cerebral somatic response. Anesthesiology 24:650657, 1963. Allison T, Goff WR, Abrahamian HA, Rosner BS: The effects of barbiturate anesthesia upon human somatosensory evoked responses. Electroencephalogr Clin Neurophysiol
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
examinations. Both this limitation from the averaging time and the intraoperative noise level enabled us to detect only responses greater than 0.5 µV. According to this criteria of 0.5 µV for clinically useful SEP responses, the rate of successful SEP recording in the series of Pathak et al. does not differ from ours. Lower frequency (0.5-1 Hz) stimulation of the peripheral nerves has been proposed by other authors as a solution to the problem of SEP suppression (8,14, 17) . Unfortunately, this sacrifices the capability of obtaining frequent responses in short intervals, which is required for adequate monitoring during aneurysm surgery. In contrast, good cortical responses were obtained in all patients under TIVA without altering the stimulation and recording parameters, so that reliable information could be gathered without additional delay. An equally high rate of successful monitoring using tibialis nerve SEPs has been reported for other types of anesthesia (1-3). However, we consider TIVA to be the most practicable anesthesia technique among them, because it meets an important criteria for neurosurgical cases, namely, prompt recovery from anesthesia. Postoperative neurological evaluation of the patient is easily possible with TIVA. Thiopental is well known to produce minimal suppression of SEPs (1,2); however, it is not practical to use this regime during lengthy aneurysm operations, because of its relatively long half-life and, therefore, hangover effects. A use of moderate hypothermic anesthesia consisting of a meperidine relaxant technique and 65% nitrous oxide was reported by Buchthal and colleagues (3,12). They reported 100% successful monitoring of posterior tibial nerve SEPs. However, this method also seems to require a long recovery phase in the intensive care unit before extubation. Compared with the two abovementioned methods of anesthesia, recovery from TIVA was almost always as prompt as from balanced anesthesia, and thus better fits the requirements of neurosurgery. After our early experiences with alfentanil hangover in a few patients (4.8%), we modified the infusion scheme for alfentanil. No further problem with anesthetic hangover was encountered thereafter. Late deflections of median nerve SEPs (N30 component) were more susceptible to anesthetic agents than early components (N20-P25 complex), making them unreliable as an intraoperative monitoring parameter. However, under TIVA, the N30 component was more consistently obtained and was a more sensitive indicator of cortical hypoperfusion. It is difficult to determine which drug used for TIVA resulted in this favorable effect on SEP monitoring, since both the anesthetics and the analgesics were changed at the same time. Propofol is known to have little analgesic action, and therefore, continuous administration of a strong analgesic was necessary to introduce this anesthetic protocol clinically. Alfentanil was chosen over fentanyl because its rapid turnover seemed to be favorable for reducing the risk of hangover. For several reasons, we might conclude that the larger amplitude of SEPs with TIVA derives mainly from the different anesthetics, such as propofol. This is strongly
4.
5.
6.
7. 8.
9.
10.
11.
12.
13.
14.
[Suppl] 24:68-75, 1963. Buchthal A, Belopavlovic M, Mooij JJA: Somatosensory evoked in temporary vascular occlusion for aneurysm surgery under moderate hypothermia, in Schramm J, Møller AR (eds): Intraoperative Neurophysiologic Monitoring in Neurosurgery. Berlin and Heidelberg, Springer-Verlag, 1991, pp 172183. Desmedt JE: Somatosensory evoked potentials in neuromonitoring, in Desmedt JE (ed): Neuromonitoring in Surgery. Amsterdam, Elsevier Science Publishers B.V. (Biomedical Division), 1989, pp 1-21. Ducati A, Landi A, Cenzato M, Fava E, Rampini P, Giovanelli M, Villani R: Monitoring of brain function by means of evoked potentials in cerebral aneurysm surgery. Acta Neurochir (Wien) [suppl] 42:813, 1988. Friedman WA, Chadwick GM, Verhoeven FJS, Mahla M, Day AL: Monitoring of somatosensory evoked potentials during surgery for middle cerebral artery aneurysms. Neurosurgery 29:83-88, 1991. Grundy BL: Intraoperative monitoring of sensory-evoked potentials, Anesthesiology 58:72-87, 1983. Harper CM, Daube JR: Surgical monitoring with evoked potentials: The Mayo Clinic experience, in Desmedt JE (ed): Neuromonitoring in Surgery. Amsterdam, Elsevier Science Publishers B.V. (Biomedical Division), 1989, pp 275-301. Kalkman CJ, Rheineck Leyssius AT, Bovill JG: Influence of high- dose opioid anesthesia on posterior tibial nerve somatosensory cortical evoked potentials: Effects of fentanyl, sufentanil, alfentil. J Cardiothorac Anesth 2:764-785, 1988. Kalkman CJ, Traast H, Zuurmond WW, Bovill JG: Differential effects of propofol and nitrous oxide on posterior tibial nerve somatosensory cortical evoked potentials during alfentanil anesthesia. Br J Anaesth 66:483-489, 1991. McPherson RW, Mahla M, Johnson R, Traystman RJ: Effects of enflurane, isoflurane, and nitrous oxide on somatosensory evoked potentials during fentanyl anesthesia. Anesthesiology 62:62633, 1985. Mooij JJA, Buchthal A, Belpavlovic M: Somatosensory evoked potential monitoring of temporary middle cerebral artery occlusion during aneurysm operation. Neurosurgery 21:492-496, 1987. Nadstawek J, Taniguchi M, Ruta U, Limberg N, Schramm J: Totale intravenöse Anästhesie (TIVA) mit Alfentanil und Propofol versus balancierte Anästhesie mit Isoflurane und Fentanyl in der Neurochirurgie. Anaesthesist 40(Suppl 2):43, 1991. Nuwer MR: Technical problems, artifacts, and
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
quality assessment of intraoperative evoked potentials, in Schramm J, Møller AR (eds): Intraoperative Neurophysiologic Monitoring in Neurosurgery. Berlin and Heidelberg, Springer-Verlag, 1991, pp 18-27. Pathak KS, Ammadio M, Kalamchi A, Scoles N, Schaffer JW, Mackay W: Effects of somatosensory evoked potentials during nitrous oxide anesthesia. Anesthesiology 66:753-57, 1987. Peterson DO, Drummond JC, Todd MM: Effects of halothane, enflurane, isoflurane, and nitrous oxide on somatosensory evoked potentials in humans. Anesthesiology 65:3540, 1986. Salzman SK, Beckman AL, Marks HG, Naidu R, Bunnell WP, MacEwen GD: Effects of halothane on intraoperative scalp-recorded somatosensory evoked potentials to posterior tibial nerve stimulation in man. Electroencehalogr Clin Neurophysiol 65:3645, 1986. Scheepstra GL, De Lange JJ, Booij LHDJ, Ros HH: Median nerve evoked potentials during propofol anesthesia. Br J Anaesth 62:92-94, 1989. Schramm J: Intraoperative monitoring with evoked potentials in cerebral vascular surgery and posterior fossa surgery, in Desmedt JE (ed): Neuromonitoring in Surgery. Amsterdam, Elsevier Science Publishers B.V. (Biomedical Division), 1989, pp 243-262. Schramm J, Koht A, Schmidt G, Pechstein U, Taniguchi M, Fahlbusch R: Surgical and electrophysiological observations during clipping of 134 aneurysms with evoked potentials monitoring. Neurosurgery 26:6170, 1990. Symon L, Wang AD, Costa e Silva IE, Gentili F: Perioperative use of somatosensory evoked responses in aneurysm surgery. J Neurosurg 60:269-275, 1984. Symon L, Murota T: Intraoperative monitoring of somatosensory evoked potentials during intracranial vascular surgery, in Desmedt JE (ed): Neuromonitoring in Surgery. Amsterdam, Elsevier Science Publishers B.V. (Biomedical Division), 1989, pp 263-274. Symon L, Jellinek DA: Experiences with intraoperative temporary vessel occlusions and monitoring in aneurysm surgery, in Schramm J, Møller AR (eds): Intraoperative Neurophysiologic Monitoring in Neurosurgery. Berlin and Heidelberg, Springer-Verlag, 1991, pp 162-171. Van Aken H, Van Hemelrijck J, Merckx L, Möllhoff T, Muler J, Lubbesmeyer HJ: Totale intravenöse Anästhesie mit Propofol und Alfentanil im Vergleich zur balancierten Anästhesie in der Neurochirurgie. Anaesth Intensivther Notfallmed 25:54-58, 1990. York DH: A critical evaluation of the 50% criterion in SEP monitoring. Presented at the
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
3.
COMMENTS The authors studied the effect of two anesthetic regimens on the monitoring of somatosensory evoked potentials (SEPs) during surgery for intracranial aneurysms. Twenty-two patients anesthetized with nitrous oxide and enflurane or isoflurane were compared with 62 patients anesthetized with total intravenous anesthesia (TIVA) using propofol and alfentanil. Median nerve SEPs were obtainable with either regimen; however, TIVA resulted in larger cortical responses, thus requiring fewer averages per recording and allowing a greater number of recordings per minute. For patients with anterior cerebral artery aneurysms, TIVA provided a significantly better ability to record posterior tibial SEPs than the technique of balanced anesthesia. There were no anesthesia-related complications, although three patients in the TIVA group (4.8%) had prolonged anesthetic recovery necessitating a postoperative computed tomographic scan. The authors suspect alfentanil and have recently modified their regimen to decrease the total administration of this drug. The increased strength of signals possible with TIVA allows more frequent feedback and should provide the surgeon valuable information to guide surgery, particularly regarding the length of time of temporary clipping. If confirmed by other laboratories, this regimen should provide a significant therapeutic advantage. John Oro Columbia, Missouri The report by Taniguchi et al. clearly demonstrates the advantages of using intravenous anesthesia without inhalation agents for monitoring neural function with somatosensory evoked potentials in those patients for whom this anesthetic regimen is appropriate. This approach is particularly useful in those patients whose baseline somatosensory evoked potentials are abnormal because of the underlying neurological disease. Jasper R. Daube Rochester, Minnesota
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
second annual convention of the American Society of Evoked Potentials Monitoring, Minneapolis, Minnesota, May 9-11, 1991.
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
Figure 1. Top: Amplitude of N20-P25 responses recorded under different methods of anesthesia: 11 patients who underwent inhalation anesthesia and 42 cases who underwent intravenous anesthesia were compared. The mean values of the amplitude of N20-P25 under each regime were 1.69 µV and 3.22 µV, respectively. This difference was statistically significant. Bottom: The same comparison for posterior tibial nerve SEPs in 11 patients who received balanced anesthesia and 20 who received TIVA. The mean values of the amplitude of P40-N50 were 1.00 µV and 1.85 µV, respectively. In cases of unsuccessful monitoring of patients due to strong suppression of SEPs, a tentative value of 0.5 µV was arbitrarily assigned for statistical analysis. Therefore, the true mean value of P40-N50 with balanced anesthesia might be lower. This difference was also statistically significant.
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
Figure 2. Recording of median nerve SEPs during surgery for a middle cerebral artery aneurysm in a patient receiving TIVA. The averaged amplitude of the N20-P25 complex in this particular patient was 8.25 µV. Shown is some plotting of SEP recordings of the case, routinely made after every operation for the purpose of documentation and analysis. Reproducible SEPs were obtainable two to three times per minute by averaging 50 serial responses.
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
Figure 3. Loss of the N30 component, and later of the N20-P25 component. The N30 component appeared to be a sensitive indicator of insufficient cortical blood supply.
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/31/5/891/2549008 by Frankfurt Univesity Library user on 04 March 2018
Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.
Figure 4. Changes in amplitude of the N20-P25 response during the operation. The patient was first anesthetized with TIVA. The addition of inhalation agents, both isoflurane and nitrous oxide, resulted in a constant and strong suppression of the amplitude of the N20-P25 response.
