[Monitoring intraoperative awareness. Vegetative signs, isolated forearm technique, electroencephalogram, and acute evoked potentials]

Several methods have been developed to quantify central anaesthetic effects and monitor awareness during general anaesthesia. The most important of these are the PRST score, calculated from changes in blood pressure, heart rate, sweating, and tear production, the isolated forearm technique, where the patient is allowed to move during surgery, the processed electroencephalogram (EEG) and the derived parameters median frequency (MF) and spectral-edge frequency (SEF), and mid-latency auditory evoked potentials (MLAEP). In clinical practice, the application of individual doses of anaesthetics is generally guided by autonomic vegetative clinical signs such as changes in blood pressure, heart rate, sweating, and tear production, quantified as the PRST score. Unfortunately, these parameters are not very reliable with regard to predicting the suppression of consciousness and awareness, especially when high-dose opioids are used in patients with cardiovascular medications and a variety of accompanying diseases. The PRST score probably indicates mainly the autonomic responses to painful stimuli, and seems to be useful in guiding the individual use of analgesics. The isolated forearm technique is a useful test of the patient's responsiveness during general anaesthesia, and thus an instrument for investigating the incidence of awareness during different anaesthetic regimens and when muscle relaxants are employed. A disadvantage is that it can only be used for 20 to 30 min because of pressure-induced nerve blocks or lesions. It can not be employed when long-term relaxation is necessary and consciousness and awareness are to be monitored continuously. The processed EEG and the derived parameters MF and SEF are important scientific tools to quantify central effects of many anaesthetics and opioid analgesics that allow the development of pharmacodynamic-pharmacokinetic models of anaesthetic action. MF has proven to be useful in monitoring closed-loop feedback of intravenous drug administration. Unfortunately, until now there have been no clinical studies that document the usefulness of MF or SEF with regard to predicting intraoperative arousal or awareness. To the contrary, some experimental data failed to predict imminent arousal and response to surgical incision or verbal commands by MF or SEF. Therefore, the EEG seems to be of limited value for monitoring awareness, consciousness, or memory formation during anaesthesia. MLAEP are suppressed in a dose-dependent fashion by many general anaesthetics and correlate with wakefulness, awareness, and explicit and implicit memory during anaesthesia and seem to be a promising method of monitoring awareness during anaesthesia. Nevertheless, future studies will have to determine threshold values for the different MLAEP parameters for intraoperative awareness and explicit and implicit recall of intraoperatively presented information for the different commonly used anaesthetics. Only then will it be possible to determine the usefulness of the method in clinical practice.

[Fluoride-induced nephrotoxicity: factor fiction?]

In the 1960s, the widespread use of the inhalational anaesthetic methoxyflurane was associated with a significant occurrence of postoperative renal dysfunction. This was attributed to hepatic biotransformation of methoxyflurane and subsequent release of inorganic fluoride ions into the circulation. Based upon the clinical experience with methoxyflurane, serum fluoride concentrations exceeding 50 mumol/l were considered to be nephrotoxic. Without further reevaluation, this 50 mumol/l threshold was subsequently applied to other fluorinated anaesthetics as well. Enflurane and even isoflurane may, when used during prolonged operations, also yield anorganic fluoride levels in excess of 50 mumol/l. Nevertheless, no cases of renal dysfunction attributable to prolonged use of these anesthetics have been reported. About 4% of the new inhalational anaesthetic sevoflurane is metabolized, and fluoride concentrations exceeding those after enflurane are frequently measured. Numerous studies have examined the nephrotoxic potential of sevoflurane degradation products. However, fluoride-related toxicity has been observed neither in animal nor in clinical studies, including prolonged administration and patients with pre-existing renal disease. New insights into the intrarenal metabolisation of volatile anaesthetics may well explain the absence of nephrotoxicity after sevoflurane. The threshold for fluoride nephrotoxicity of 50 mumol/l, still given in many medical text-books, can no longer be applied as an indicator of nephrotoxicity after isoflurane, enflurane or sevoflurane. Therefore, the elevated serum fluoride concentrations consistently recorded following anaesthesia with sevoflurane are devoid of clinical significance.

Midlatency auditory evoked potentials during anaesthesia with increasing endexpiratory concentrations of desflurane

BACKGROUND

Under general anaesthesia with the volatile anaesthetics halothane, enflurane and isoflurane, midlatency auditory evoked potentials (MLAEP) are suppressed dose-dependently. Therefore, MLAEP have been used to measure depth of anaesthesia and to indicate intraoperative awareness. Desflurane is a new volatile anaesthetic and its effect on MLAEP have not been studied previously.

METHODS

We have studied MLAEP during general anaesthesia with increasing endexpiratory concentrations of desflurane in 12 patients scheduled for elective gynaecological surgery. Auditory evoked potentials were recorded in the awake state and during anaesthesia with endexpiratory steady state concentrations of 1.5, 3.0, 4.5 and 6.0 vol % of desflurane on vertex (positive) and mastoids on both sides (negative). Latencies of the peaks V, Na, Pa, Nb, Pl (ms) and amplitudes Na/Pa, Pa/Nb and Nb/Pl (micro V) were measured.

RESULTS

In the awake state, MLAEP had high peak-to-peak amplitudes and a periodic waveform. During general anaesthesia with increasing endexpiratory concentration of desflurane, the latency of the brainstem response V increased only slightly. In contrast, MLAEP showed a marked dose-dependent and statistically significant increase in latencies of Na, Pa, Nb and Pl and decrease in amplitudes of Na/Pa, Pa/Nb and Nb/Pl. Under 6.0 vol % of desflurane MLAEP were severely attenuated or even abolished.

CONCLUSION

Based on these observations, endexpiratory concentrations of > or = 4.5 vol % desflurane should suppress awareness phenomena such as auditory perceptions during anaesthesia.

[Intraoperative awareness and auditory evoked potentials]

Midlatency auditory evoked potentials (MLAEP) are suppressed dose-dependently during anaesthesia with a variety of general anaesthetics. Therefore, MLAEP have been proposed to measure depth of anaesthesia and to indicate intraoperative awareness. Several studies give evidence of a close relationship between MLAEP and motor signs of wakefulness, intraoperative awareness, and explicit and implicit memory functions during general anaesthesia. Summarising these data, one may conclude that there is a close hierarchical relation between cognitive function, memory and wakefulness during anaesthesia, and MLAEP latencies. A short Nb latency below 45 ms is consistent with conscious awareness and unimpaired memory function with explicit recall and adequate response to commands. When Nb latency increases to 45-50 ms, it may be associated with conscious awareness. Patients still respond to commands, but memory formation is impaired and explicit recall is lost. A further increase of Nb latencies seems to be consistent with unconscious awareness, characterised by implicit memory of intraoperative events; 60 ms seems to be the threshold value for motor signs of wakefulness during anaesthesia. With a further increase of MLAEP latency during anaesthesia, conscious awareness and memory formation, explicit and implicit recall, response to commands, and spontaneous purposeful movements during anaesthesia are blocked. The new volatile anaesthetic sevoflurane leads to a dose-dependent increase in MLAEP peak latencies and a decrease in MLAEP amplitudes. At about 1.5 vol.% end-expiratory sevoflurane concentration, MLAEP are significantly suppressed and Nb latency is in the range of 68-80 ms. Therefore, from the present data and those from the literature, one may expect that sevoflurane at concentrations greater than 1.5 vol.% for general anaesthesia would be able to suppress awareness phenomena such as purposeful movements, auditory perception, intraoperative wakefulness and awareness, memory formation, and explicit and implicit recall of intraoperative events.

[Awareness during general anesthesia. Definition, incidence, clinical relevance, causes, avoidance and medicolegal aspects]

The possibility that a patient during general anaesthesia is aware of the operation going on and aware of severe pain that might be remembered postoperatively must be very alarming to patients and anaesthetists alike. Furthermore, there is experimental evidence showing that conscious recall of intraoperative events is only the tip of an iceberg; it seems very probable that there is even a higher incidence of unconscious perception during general anaesthesia. Therefore, the following stages of intraoperative awareness must be distinguished: (1) conscious awareness with explicit recall and with severe pain; (2) conscious awareness with explicit recall but no complaints of pains; (3) conscious awareness without explicit recall and possible implicit recall; (4) subconscious awareness without explicit recall and possible implicit recall; (5) no awareness. The incidence of conscious awareness with explicit recall and severe pain has been estimated at less frequent than 1/3000 general anaesthetics. Conscious awareness with explicit recall but no complaints of pain has been reported in the literature with an incidence of 05-2%. With 7-72%, conscious awareness without explicit recall and possible implicit recall shows a very wide range of variation and its occurrence probably depends on the anaesthetic drugs used. Subconscious awareness with possible implicit recall has an incidence of up to 80%, but there are many methodological problems in demonstrating implicit memory of intraoperative events. Reports of intraoperative awareness do not come exclusively from cardiac surgery and obstetrics, but also from all other operative specialties. Postoperatively, patients who experience intraoperative awareness may develop a so-called post-traumatic stress syndrome. Symptoms involve re-experiencing the event awake or in dreams, sleep disturbances, depression, avoidance of stimuli associated with the event. The probability of the development of the post-traumatic stress syndrome seems to coincide with the experience of severe pain. When a patient complains of intraoperative awareness postoperatively the anaesthesiologist should discuss the event frankly with the patient. When the symptoms of the post-traumatic stress syndrome persist a psychotherapy should follow. Causes for intraoperative awareness may be: equipment failure, too-light anaesthesia, e.g. for a caesarean section or for emergency surgery in severely injured or polytraumatized patients, during cardiac surgery, bronchoscopy of difficult intubation. There is interindividual variability in anaesthetic effect; for example, chronic drug or alcohol abuse or overweight may make increased anaesthetic doses necessary. They are at risk for intraoperative awareness. Some general anaesthetics or anaesthetic procedures, e.g. the combination of a relaxant and N2O, opioid mono-anaesthetics, or opioids combined with benzodiazepines, seem to involve a higher risk of intraoperative awareness than do volatile anaesthetics. The bases of litigation are medical malpractice, breach of contract by the anaesthesiologist or lack of informed consent from the patient. Therefore, patients who are at risk of intraoperative awareness should be given detailed information on this special risk before the operation.

The effects of anesthesia with increasing end-expiratory concentrations of sevoflurane on midlatency auditory evoked potentials

We studied midlatency auditory evoked potentials (MLAEP) during general anesthesia with increasing end-expiratory concentrations of sevoflurane in 12 patients scheduled for elective gynecologic surgery. After oral premedication with 20 mg clorazepate dipotassium, anesthesia was induced with etomidate (0.2 mg/kg intravenously [IV]). Vecuronium (0.1 mg/kg) was given for neuromuscular block, and controlled ventilation with sevoflurane in 100% O2 was instituted. Auditory evoked potentials were recorded in the awake state and during anesthesia with end-expiratory steady-state concentrations of 0.5, 1.0, 1.5, and 2.0 vol% of sevoflurane on vertex (positive) and mastoids on both sides (negative). Latencies of peaks V, Na, Pa, Nb, and P1 (ms) and amplitudes of Na/Pa, Pa/Nb, and Nb/P1 (microV) were measured. In the awake state, MLAEP had high peak-to-peak amplitudes and a periodic waveform. During general anesthesia with increasing end-expiratory concentrations of sevoflurane, the latency of the brainstem response V increased slightly. In contrast, MLAEP showed marked dose-dependent, statistically significant increases in the latencies of Na, Pa, Nb, and P1 and decreases in the amplitudes of Na/Pa, Pa/Nb, and Nb/P1. Under 2 vol% of sevoflurane, MLAEPs were severely attenuated or abolished. Based on these observations, > or = 1.5 vol% sevoflurane should suppress phenomena such as auditory perceptions, intraoperative wakefulness, and awareness.

[Acoustic evoked potentials mid-latency following anesthesia with sufentanil]

PATIENTS AND METHODS

We have studied mid-latency auditory evoked potentials (MLAEP) during general anaesthesia with sufentanil in ten patients scheduled for elective major urological surgery. Anaesthesia was induced with sufentanil 2-3 micrograms/kg; for maintenance of anaesthesia a further bolus of sufentanil (1-2 micrograms/kg) 10 min before the start of surgery (skin incision) was given. MLAEP were recorded before and 10 min after the last sufentanil bolus on the vertex (positive) and mastoids on both sides (negative). Latencies of the peaks V, Na, Pa, Nb, and Pl (ms) and amplitudes Na/Pa, Pa/Nb, and Nb/Pl (microV) were measured.

RESULTS

In the awake state, MLAEP had high peak-to-peak amplitudes and a periodic wave form. During general anaesthesia with sufentanil the brainstem response V was stable. There was a marked increase in latency and a decrease in the amplitude of Nb and Pl. In contrast, for the early cortical potentials Na and Pa only small increases in latencies and decreases in amplitudes were observed. Na and Pa showed a similar pattern to that in awake patients.

CONCLUSIONS

There is no substantial difference of sufentanil's effect on MLAEP compared with the opioids alfentanil, fentanyl, and morphine. Because Na, Pa, and Nb are generated in the primary auditory cortex of the temporal lobe, it must be concluded that during general anaesthesia with sufentanil primary cortical processing of auditory stimuli may be preserved.

Anesthesia with increasing doses of sufentanil and midlatency auditory evoked potentials in humans

Our interest focused on the question whether sufentanil differs from alfentanil, fentanyl, and morphine with regard on its effects on midlatency auditory evoked potentials (MLAEP). Therefore, we studied MLAEP during general anesthesia with increasing doses of sufentanil in 16 patients scheduled for elective major urologic surgery. Anesthesia was induced with sufentanil (1 microgram/kg every 7 min to a total dose of 3 micrograms/kg). In 8 of 16 patients, further incremental doses of sufentanil were given to a total dose of 5 micrograms/kg. Auditory evoked potentials were recorded before and 5 min after every sufentanil dose on vertex (positive) and mastoids on both sides (negative). Latencies of the peaks V, Na, Pa, Nb, and P1 (ms), and amplitudes Na/Pa, Pa/Nb, and Nb/P1 (microV) were measured. In the awake state, MLAEP had high peak-to-peak amplitudes and a periodic waveform. During general anesthesia the brainstem response V was stable to increasing doses of sufentanil. There was a marked statistically significant increase in latency and decrease in amplitude of Nb and P1 after 1-2 micrograms/kg sufentanil, which remained stable under further sufentanil application. In contrast, the early cortical potentials Na and Pa increased only slightly in latencies. This increase was statistically significant at 4 micrograms/kg for Na and at 3 and 4 micrograms/kg for Pa. For the amplitudes Na/Pa and Pa/Nb there was only a slight and statistically insignificant reduction. After the largest dose of sufentanil (3-5 micrograms/kg) Na and Pa showed a similar pattern as in awake patients. We conclude that sufentanil does not differ essentially from alfentanil, fentanyl, and morphine with regard on its effects on MLAEP.

Mid-latency auditory evoked potentials and wakefulness during caesarean section

We investigated intra-operative wakefulness and mid-latency auditory evoked potentials (MLAEP) in 20 patients undergoing elective Caesarean section under general anaesthesia. Anaesthesia was induced with thiopentone 5 mg kg-1 i.v. and succinylcholine 1-1.5 mg kg-1 i.v. After delivery, a balanced anaesthetic technique was maintained using fentanyl 0.2-0.3 mg i.v., enflurane 0.4-1.0 vol% end-expired concentration and 50% N2O in oxygen. Purposeful movements were interpreted as signs of inadequate anaesthesia and intra-operative wakefulness. They were recorded as either spontaneous movements or in response to one of two audio tapes (tape A: sound of a crying baby; tape B: classical music). Post-operatively, intra-operative dreams, hallucinations and detailed reports about intra-operative events were evaluated. Auditory evoked potentials were recorded online before and during general anaesthesia. Twenty spontaneous purposeful movements were observed in 12 patients, seven before or during delivery and 13 after delivery. Four purposeful movements were observed after presentation of the sound of a crying baby but only one after classical music. Dreams and hallucinations were reported by nine patients. Two patients reported experiencing surgical manipulations. In the awake state MLAEPs had great peak-to-peak amplitudes and a periodic waveform. Under adequate levels of general anaesthesia MLAEPs showed a marked increase in latency and decrease in amplitude or were even suppressed completely. This increase in latencies and decrease in amplitude of MLAEP was absent in patients who reported intraoperative events and during spontaneous or provoked motor reactions.

Effects of surgical stimulation on midlatency auditory evoked potentials during general anaesthesia with propofol/fentanyl, isoflurane/fentanyl and flunitrazepam/fentanyl

During general anaesthesia, midlatency auditory evoked potentials are suppressed in a dose dependent manner by a number of general anaesthetics. The activating effects of surgical stimuli on midlatency auditory evoked potentials have been demonstrated during light inhalational anaesthesia, and indicate that midlatency auditory evoked potentials reflect the activity of the central nervous system and not only anaesthetic concentrations. We investigated the effect of surgical stimulation (skin incision, sternotomy) on midlatency auditory evoked potentials under high dose opioid analgesia in 30 patients undergoing elective cardiac surgery. High dose opioid analgesia was maintained using fentanyl (1.2 mg.h-1) and combined with either propofol (4-8 mg.kg-1.h-1) (group I, n = 10), isoflurane (0.6-1.2 vol%) (group II, n = 10) or flunitrazepam (1.2 mg.h-1) (group III, n = 10). Midlatency auditory evoked potentials were recorded in the awake state, during general anaesthesia before skin incision, after skin incision and after sternotomy. During general anaesthesia there were marked statistically significant increases in latencies and decreases in amplitudes of midlatency auditory evoked potentials in the propofol/fentanyl and isoflurane/fentanyl groups. In contrast, in the flunitrazepam/fentanyl group there were only small changes of midlatency auditory evoked potentials. The latencies of the early cortical potentials were similar to those in the awake state. After skin incision as well as after sternotomy no significant changes of midlatency auditory evoked potentials could be observed in any of the experimental groups. These results indicate that activation of the auditory pathway by surgical stimuli may be blocked when analgesia is provided by high dose fentanyl.(ABSTRACT TRUNCATED AT 250 WORDS)

Motor signs of wakefulness during general anaesthesia with propofol, isoflurane and flunitrazepam/fentanyl and midlatency auditory evoked potentials

Auditory evoked potentials have been used as an indicator of awareness. In the present study we combined epidural analgesia with three techniques of general anaesthesia. Motor signs of intra-operative wakefulness were documented and assessed along with cardiovascular changes and with midlatency auditory evoked potentials. Thirty patients undergoing elective laparotomy were studied as follows: first continuous epidural analgesia was used in all patients to block painful sensation to the level of T5. Intravenous general anaesthesia was induced with propofol (2.5 mg.kg-1 b.w., group 1, n = 10), thiopentone (5 mg.kg-1 b.w., group 2, n = 10) or etomidate (0.2 mg.kg-1 b.w., group 3, n = 10) and maintained with a propofol (3-5 mg.kg-1, group 1), isoflurane (0.4-0.8 Vol%, group 2), flunitrazepam and fentanyl (0.005 mg.kg-1 b.w.) bolus injection every 20 to 30 s (group 3). Heart rate and arterial pressure were recorded continuously. Purposeful movements of the limbs, eye-opening or other movements as well as coughing were documented as motor signs of intra-operative wakefulness. Auditory evoked potentials were recorded in the awake state, after induction and during maintenance of general anaesthesia. Motor signs of intra-operative wakefulness occurred statistically significantly more often in the patients of the flunitrazepam/fentanyl group than in those of the propofol and isoflurane group. There was no correlation between wakefulness and cardiocirculatory parameters. In the awake patients midlatency auditory evoked potentials had high peak to peak amplitudes and a periodic waveform.(ABSTRACT TRUNCATED AT 250 WORDS)

[Anesthesia with flunitrazepam/fentanyl and isoflurane/fentanyl. Unconscious perception and mid-latency auditory evoked potentials]

There is a high incidence of intraoperative awareness during cardiac surgery. Mid-latency auditory evoked potentials (MLAEP) reflect the primary cortical processing of auditory stimuli. In the present study, we investigated MLAEP and explicit and implicit memory for information presented during cardiac anaesthesia. PATIENTS AND METHODS. Institutional approval and informed consent was obtained in 30 patients scheduled for elective cardiac surgery. Anaesthesia was induced in group I (n = 10) with flunitrazepam/fentanyl (0.01 mg/kg) and maintained with flunitrazepam/fentanyl (1.2 mg/h). The patients in group II (n = 10) received etomidate (0.25 mg/kg) and fentanyl (0.005 mg/kg) for induction and isoflurane (0.6-1.2 vol%)/fentanyl (1.2 mg/h) for maintenance of general anaesthesia. Group III (n = 10) served as a control and patients were anaesthetized as in I or II. After sternotomy an audiotape that included an implicit memory task was presented to the patients in groups I and II. The story of Robinson Crusoe was told, and it was suggested to the patients that they remember Robinson Crusoe when asked what they associated with the word Friday 3-5 days postoperatively. Auditory evoked potentials were recorded awake and during general anaesthesia before and after the audiotape presentation on vertex (positive) and mastoids on both sides (negative). Auditory clicks were presented binaurally at 70 dBnHL at a rate of 9.3 Hz. Using the electrodiagnostic system Pathfinder I (Nicolet), 1000 successive stimulus responses were averaged over a 100 ms poststimulus interval and analyzed off-line. Latencies of the peak V, Na, Pa were measured. V belongs to the brainstem-generated potentials, which demonstrates that auditory stimuli were correctly transduced. Na, Pa are generated in the primary auditory cortex of the temporal lobe and are the electrophysiological correlate of the primary cortical processing of the auditory stimuli. RESULTS. None of the patients had an explicit memory of intraoperative events. Five patients in group I, one patient in group II, and no patients in group III showed implicit memory of the intraoperative tape message. They remembered Robinson Crusoe spontaneously when they were asked their associations with Friday. In the awake state AEP peak latencies were in the normal range. During general anaesthesia in group I, the peaks Na, Pa did not increase in latency or decrease in amplitude before and after the audiotape presentation. The primary cortical complex Na/Pa could be identified as in the awake state. In contrast, in group II Na, Pa showed a marked increase in latency and a decrease in amplitude or were completely suppressed. CONCLUSIONS. During general anaesthesia auditory information can be processed and remembered postoperatively by an implicit memory function, when the electrophysiological conditions of primary cortical stimuli processing is preserved. Implicit memory can be observed more often when high-dose opioid analgesia is combined with receptor-binding agents like the benzodiazepines than under non-specific anaesthetics like isoflurane. Non-specific anaesthetics seem to provide a more effective suppression of auditory stimuli processing than receptor-specific agents.

Midlatency auditory evoked potentials and explicit and implicit memory in patients undergoing cardiac surgery

BACKGROUND

A high incidence of intraoperative awareness during cardiac surgery has been reported. Midlatency auditory evoked potentials (MLAEP) have been used recently as an indicator of awareness. In the current study, memory for information presented during anesthesia was investigated using MLAEP as one experimental indicator in 45 patients scheduled for elective cardiac surgery.

METHODS

In all patients general anesthesia was maintained using high-dosage fentanyl (1.2 mg.h-1). In addition, the patients of group 1 (n = 10) received flunitrazepam (1.2 mg.h-1), the patients of group 2 (n = 10) isoflurane (0.6-1.2 vol%), and the patients of group 3 (n = 10) propofol (4-8 mg.kg-1.h-1). Group 4 (n = 15) served as a control, and those patients were assigned randomly to one of the anesthetic regimes. After sternotomy and before cardiopulmonary bypass, an audiotape, which included an implicit memory task, was presented to the patients of groups 1-3. Auditory evoked potentials were recorded while the patients were awake and during general anesthesia immediately before and after the audiotape presentation. Latencies of the brain stem peak V and the early cortical potentials Na and Pa were measured.

RESULTS

Three to 5 days postoperatively no patient had a clear explicit memory of intraoperative events. However, there were statistically significant differences in the incidence of implicit recall among the groups. Five patients in the flunitrazepam-fentanyl group, 1 patient in the isoflurane-fentanyl group, 1 patient in the propofol-fentanyl group, and no patient in the control group showed an implicit memory of the intraoperative tape message. In the awake state, MLAEP showed high peak-to-peak amplitudes and a periodic waveform. In the patients with implicit memory postoperatively, MLAEP continued to show this pattern during general anesthesia. The early cortical potentials Na and Pa did not increase in latency or decrease in amplitude before or after the audiotape presentation. In contrast, in the patients without implicit memory, MLAEP waveform was severely attenuated or abolished. Na and Pa showed marked increases in latencies and decreases in amplitudes or were completely suppressed. In 9 patients, including all patients (7 of 9) with implicit memory, Pa latency increased less than 12 ms, and 21 of 23 patients without implicit memory showed a Pa latency increase of greater than 12 ms during anesthesia and the audiotape presentation. Therefore, the Pa latency increase of greater or less than 12 ms may provide sensitivity of 100% and specificity of 77% in distinguishing patients with implicit memory from patients without implicit memory postoperatively.

CONCLUSIONS

When the early cortical potentials of MLAEP are preserved during general anesthesia, auditory information may be processed and remembered postoperatively by an implicit memory task.

Mid-latency auditory evoked potentials and circulatory response to loud sounds

We investigated in 60 patients scheduled for elective aorto-coronary bypass grafting if loud sounds by themselves can induce cardiovascular responses and if these could be related to mid-latency auditory evoked potentials (MLAEP). Anaesthesia was induced in group I (n = 20) with flunitrazepam-fentanyl 0.01 mg kg-1 and maintained with flunitrazepam-fentanyl 1.2 mg h-1. Patients in groups II (n = 20) and III (n = 20) received etomidate 0.25 mg kg-1 and fentanyl 0.005 mg kg-1 for induction and 0.6-1.2 vol% isoflurane and fentanyl 1.2 mg h-1, or propofol 4-8 mg kg-1 h-1 and fentanyl 1.2 mg h-1 for maintenance of general anaesthesia. After preparation of the sternum the operation was stopped for several minutes. Then, as a loud auditory stimulus, the sound of the running sternotomy saw was presented to the patients by putting the saw inverted on the sternum for several seconds. Heart rate (HR), arterial pressure (SAP), pulmonary capillary wedge pressure (PCWP), cardiac index, systemic vascular resistance and MLAEP were measured in the awake state, before and after presentation of the sound. Latencies of the peak V, Na, Pa, Nb and P1 were measured. In group I there were statistically significant increases in HR (63.5-70.2 beat min-1), SAP (123.9-146-5 mm Hg) and PCWP (9.2-11.7 mm Hg) after presentation of the sound. These haemodynamic changes were not observed in patients in groups II and III. In the awake state, AEP had high peak -to-peak amplitudes and a periodic waveform.(ABSTRACT TRUNCATED AT 250 WORDS)

Midlatency auditory evoked potentials and purposeful movements after thiopentone bolus injection

The effect of thiopentone on the middle latency auditory evoked potentials was investigated in 12 patients during induction of anaesthesia with thiopentone 5 mg.kg-1. 100% oxygen was administered throughout the induction, and when the patient moved purposefully a further bolus (2 mg.kg-1) was given, and anaesthesia continued in the normal way. The middle latency auditory evoked potentials were elicited before and during the induction. Binaural clicks (70 dB above normal hearing threshold) were presented at a rate of 9.3 per s. Averages of 1000 responses were analysed off line, and a fast Fourier transformation of the middle latency auditory evoked potentials were used to calculate the power spectrum of the averages. Awake, the patients had large peak to peak amplitudes and a normal waveform. The power spectra showed a high energy between 30-40 Hz. After induction the latencies of waves Na, Pa, Nb and P1 and the amplitudes of the waves Na/Pa, Pa/Nb, and Nb/P1 were decreased or completely attenuated. Both effects lasted for 4 min. When movement occurred (after 4-6 min), the values returned to awake. The second bolus repeated the changes.

Mid-latency auditory evoked potentials in humans during anesthesia with S (+) ketamine--a double-blind, randomized comparison with racemic ketamine

Mid-latency auditory evoked potentials (MLAEP) reflect the primary cortical processing of auditory stimuli. They are suppressed widely during general anesthesia. Under ketamine, in contrast, MLAEP seem to be preserved. Ketamine exists in two optical isomers, S (+) ketamine and R (-) ketamine, which differ in their pharmacodynamic properties. S (+) ketamine has a higher anesthetic-hypnotic and analgesic potency than R (-) ketamine or the racemic mixture of S (+) ketamine and R (-) ketamine. In a blinded, randomized evaluation we compared the effect of induction of general anesthesia with the more potent ketamine compound--S (+) ketamine--to induction with the racemic ketamine on MLAEP in 60 patients scheduled for minor gynecologic procedures. Anesthesia was induced with S (+) ketamine (1 mg/kg Group I, n = 30) or an equi-anesthetic dose of racemic ketamine (2 mg/kg, Group II, n = 30). Auditory evoked potentials (AEP) were recorded before, during, and after induction of general anesthesia. Latencies of the peaks V, Na, Pa, Nb, and P1 and amplitudes Na/Pa, Pa/Nb, and Nb/P1 were measured. A fast-Fourier transform was used to calculate the power spectra of the AEP. The baseline MLAEP peaks of the awake patients were of normal amplitude and demonstrated a characteristic periodic wave form morphology. Power spectra indicated high energy in the 30-40 Hz frequency range. After induction of general anesthesia with S (+) ketamine or racemic ketamine, there was no increase in latencies of peaks V, Na, Pa, Nb, or P1. No decrease in amplitudes Na/Pa, Pa/Nb, or Nb/P1 could be observed. There was no significant change in the power spectra.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of benzodiazepines on mid-latency auditory evoked potentials

Midlatency auditory evoked potentials (MLAEP) reflect primary cortical processing of auditory stimuli. The effects of benzodiazepines on MLAEP have not yet been studied. We examined the effects of intravenous induction of general anaesthesia using the benzodiazepines midazolam, diazepam and flunitrazepam on MLAEP in 30 patients scheduled for minor gynaecological procedures. Anaesthesia was induced with midazolam (0.2-0.3 mg.kg-1, Group I, n = 10), diazepam (0.3-0.4 mg.kg-1, Group II, n = 10) or flunitrazepam (0.03-0.04 mg.kg-1, Group III, n = 10). Auditory-evoked potentials were recorded before and five to ten minutes after induction of general anaesthesia. Latencies of the peak V, Na, Pa, Nb and Pl (ms) and amplitudes Na/Pa, Pa/Nb and Nb/P1 (microV) were measured. In the awake state, MLAEP had high peak to peak amplitudes and a periodic waveform. After induction of anaesthesia there was no or only a small increase in latencies of the peaks Na, Pa, Nb and P1, which was significant only for P1 in the midazolam group. Amplitudes Na/Pa, Pa/Nb and Nb/P1 decreased only slightly and which reached statistical significance only for Na/Pa in the flunitrazepam group. The MLAEPs do not change markedly in amplitude or latency during induction of general anaesthesia with benzodiazepines. Primary cortical processing of auditory stimuli seems to be preserved under benzodiazepines. This may be seen in connection with cases of intraoperative awareness and especially the perception of auditory stimuli during anaesthetic regimens where benzodiazepines are used to suppress consciousness.

Effects of increasing doses of alfentanil, fentanyl and morphine on mid-latency auditory evoked potentials

We have studied dose-dependent effects of alfentanil, fentanyl and morphine on mid-latency auditory evoked potentials (MLAEP). Anaesthesia was induced with alfentanil 100 micrograms kg-1 every 5 min to a total dose of 500 micrograms kg-1 (group I, n = 10), fentanyl 10 micrograms kg-1 every 7 min to a total dose of 50 micrograms kg-1 (group II, n = 10) or morphine 1 mg kg-1 for induction and 0.5 mg kg-1 every 15 min to a total dose of 3 mg kg-1 (group III, n = 10). MLAEP were recorded before and 3-15 min after every opioid dose on vertex (positive) and mastoids on both sides (negative). Latencies of the peaks V, Na, Pa, Nb, P1 (ms) and amplitudes Na/Pa, Pa/Nb and Nb/P1 (microV) were measured. Fast-Fourier transformation was used to calculate power spectra of the AEP. In the awake state, MLAEP had high peak-to-peak amplitudes and a periodic waveform. Power spectra indicated high energy in the 30-40 Hz frequency range. During general anaesthesia with increasing doses of alfentanil, fentanyl and morphine, the brainstem response V was stable. There was a marked increase only in latency and decrease in amplitude of P1. In contrast, for the early cortical potentials Na and Pa, only small increases in latencies and decreases in amplitudes were observed. After the largest doses of alfentanil (500 micrograms kg-1), fentanyl (50 micrograms kg-1) and morphine (3 mg kg-1), Na, Pa and Nb showed a similar pattern as in awake patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Mid-latency auditory evoked potentials during ketamine anaesthesia in humans

We studied mid-latency auditory evoked potentials (MLAEP) during induction of general anaesthesia with ketamine 2 mg kg-1. MLAEP were recorded before, during and after induction of general anaesthesia on the vertex (positive) and mastoid (negative) positions. Latencies of the peak V, Na, Pa, Nb, P1 and amplitudes Na/Pa, Pa/Nb and Nb/P1 were measured. Fast-Fourier transformation was used to calculate power spectra of the MLAEP. In the awake state, MLAEP had large peak-to-peak amplitudes and a periodic waveform. Peak latencies remained within the normal range. Power spectra indicated high energy in the 30-40 Hz frequency range. After induction of general anaesthesia with ketamine, there was no change in latency of peaks V, Na, Pa, Nb, P1 and no apparent reduction in amplitudes Na/Pa, Pa/Nb and Nb/P1. In the power spectra, frequencies in the range of 30-40 Hz retained high energy. Amplitudes and latencies of MLAEP did not change during induction of general anaesthesia with ketamine. Primary processing of auditory stimuli in the primary auditory cortex seemed to be preserved under ketamine. Suppression of sensory (auditory) information processing must take place at a higher cortical level in a dissociative manner.

[Monitoring intraoperative processing of acoustic stimuli with auditory evoked potentials]

130 Patients undergoing elective intra-abdominal, gynaecological, urological or cardiac surgery were studied after institutional approval and informed consent. In all patients auditory-evoked potentials (AEP) were recorded in the awake state and during general anaesthesia. Latencies of the peaks V, Na, Pa, Nb and P1 were measured. V belongs to the brainstem-generated potentials, which demonstrates that auditory stimuli were correctly transduced. Na, Pa, Nb, P1 are generated in the primary auditory cortex of the temporal lobe. During anaesthesia with isoflurane, enflurane, thiopentone, etomidate and propofol the peak V remains unchanged, whereas the mid-latency auditory-evoked potentials (MLAEP) show marked increases in latencies and decreases in amplitudes or are even completely suppressed. This indicates a successful stimulus transmission up to the level of the brainstem and midbrain. However, stimulus processing in the primary auditory cortex is blocked. Under increasing end-expiratory concentrations of isoflurane MLAEP show a dose-dependent increase of latencies and decrease of amplitudes. Under surgical anaesthesia with 1.2 vol%, MLAEP are nearly completely suppressed. A different picture can be seen when MLAEP were recorded during anaesthesia with the receptor-specific anaesthetics midazolam, flunitrazepam, diazepam, fentanyl and ketamine. During anaesthesia with receptor-specific anaesthetics, the brainstem peak V as well as the mid-latency components remain nearly unchanged compared with AEP from awake patients. This indicates that auditory stimuli reach the primary auditory cortex and are processed at a primary cortical level. With increasing doses of fentanyl one can observe only a significant decrease of amplitudes for the late component P1.(ABSTRACT TRUNCATED AT 250 WORDS)

[Recovery of psychomotor and cognitive functions following anesthesia. Propofol/alfentanil and thiopental/isoflurane/ alfentanil]

Recent changes in the medical system have resulted in a significant increase of ambulatory surgical procedures. Therefore, a safe and short postoperative recovery period and, especially, the full recovery of complex psychological function after general anaesthesia have become increasingly important. In the present study we investigated the recovery of psychomotor and cognitive function after general anaesthesia with propofol/alfentanil and thiopentone/isoflurane/alfentanil. PATIENTS AND METHODS. Institutional approval and informed consent was obtained in 40 female ASA I or II patients undergoing diagnostic laparoscopy. As oral premedication the patients received chloracepat (10-20 mg) 45 min before the start of anaesthesia. Anaesthesia was induced in group I with propofol (2.5 mg/kg) and maintained with propofol (6-12 mg/kg/h)/alfentanil (0.05 mg/kg) and 50% N2O in O2. The patients of group II received thiopentone (5 mg/kg) for induction and isoflurane (0.5-1.5 vol%)/alfentanil (0.05 mg/kg) and 50% N2O in O2 for maintenance of general anaesthesia. In particular we measured the following parameters: (1) The recovery time, defined as the interval between the termination of the anesthetic and the patient's correct recall of her birth date. (2) The choice reaction times to optical stimuli (red or green light), which was used as a parameter for attention and psychomotor function. (3) The score in the "Zahlen-Verbindungs-Test" in which the patients had to connect numbers from 1 to 90 in correct order. This is also a parameter to quantify attention and psychomotor function. (4) The digit span which is a value derived from the number of correctly reproduced digits from a list presented to the patients. It is a measure of numerical memory. (5) The Munich Verbal Learning Test, which is the German version of the California Verbal Learning Test. It represents the number of correctly reproduced words from a previously presented list and is a measure of the verbal memory. (6) The Wisconsin Card Sorting Test, which serves to test the ability to plan and act and to form terms and concepts. (7) The State-Trait Anxiety Inventory, to quantify state anxiety. (8) Pain score, using a visual analogue scale. The tests were performed at four measurement points: the day before the operation and 30, 60, and 240 min after recovery. The "Zahlen-Verbindungs-Test", the digit span and the Munich Verbal Learning Test were presented in four parallel forms to minimize learning effects. For statistical analysis of the data the Wilcoxon test was employed within groups and the Mann-Whitney test between groups. RESULTS. The groups were comparable in age, weight, height and level of education. No significant difference was found between them in operation or anaesthesia time or in the total dosage of alfentanil. Recovery time in the propofol group was, at 10 min, significantly shorter than in the isoflurane group, with 16 min. Choice reaction times were significantly increased 30 min after recovery from anaesthesia in both groups. In the propofol group they returned to normal after 60 min, whereas in the isoflurane group significant increases could be observed even 240 min after recovery from the anaesthetic. Choice reaction times were significantly longer in the isoflurane group than in the propofol group 60 min and 240 min after anaesthesia. In the "Zahlen-Verbindungs-Test" the patients showed significantly worse results 30 min and 60 min after anaesthesia. The propofol group tended to be better than the isoflurane group, but the difference did not reach statistical significance. Also in the digit span, the scores were significantly lower 30 min after recovery from the anaesthetic. Here again the propofol group tended to be a little better than the isoflurane group 30 min, 60 min and 240 min after anaesthesia. In the Munich Verbal Learning Test both groups had lower scores 30 min and 60 min, the isoflurane group also 240 min, after recovery...

[Mid-latency auditory evoked potentials during increasing doses of fentanyl]

OBJECTIVE

Intraoperative awareness, and especially the perception of auditory stimuli occur occasionally under general anaesthesia with high-dose opioids. Mid-latency auditory evoked potentials (MLAEP) reflect the primary cortical processing of auditory stimuli. Hence, we studied the effects of fentanyl on MLAEP.

METHODS

Institutional approval and informed consent was obtained in 20 patients scheduled for cardiac surgery. Anaesthesia was induced with fentanyl (10 micrograms/kg every 7[ up to a total dosage of 50 micrograms/kg). Auditory evoked potentials were recorded before and 5[ after every fentanyl dose on vertex (positive) and mastoids on both sides (negative). Auditory clicks were presented binaurally at 70 dBnHL at a rate of 9.3 Hz. Using the electrodiagnostic system Pathfinder I (Nicolet), 1000 successive stimulus responses were averaged over a 100 ms post-stimulus interval and analysed off-line. Latencies of the peak V, Na, Pa, Nb P1 and amplitudes Na/Pa, Pa/Nb, Nb/P1 were measured. V belongs to the brainstem generated potentials, which demonstrates that auditory stimuli were correctly transduced. Na, Pa, Nb, P1 are generated in the primary auditory cortex of the temporal lobe and are the electrophysiological correlate of the primary cortical processing of the auditory stimuli. By means of a Fast-Fourier transformation power spectra of the AEP were calculated.

RESULTS

In the awake state AEP peak latencies were in the normal range. Power spectra indicated high energy in the 30-40 Hz frequency range. During increasing dosages of fentanyl the brainstem response V was stable. P1 increased in latency and Nb/P1 decreased in amplitude after 10 micrograms/kg of fentanyl significantly. The primary cortical potentials Na, Pa, Nb changed only very slightly in latencies or amplitudes even under highest doses of fentanyl (50 micrograms/kg) and could be identified like in the awake patients. In the power spectra high energy persisted in the 30 Hz frequency range.

CONCLUSION

MLAEP and especially the primary cortical potentials Na, Pa, Nb did not change markedly in amplitude or latency during high-dose fentanyl analgesia. There is no dose-dependent effect of fentanyl on MLAEP as it can be observed under volatile anaesthetics (isoflurane, enflurane). The primary cortical processing of auditory stimuli can be completely blocked by volatile anaesthetics, but is still preserved under highest doses of fentanyl. This may be seen in connection with cases of awareness and perception of auditory stimuli during high-dose fentanyl analgesia.