I Keller

Publications of I Keller

• [The effect of thiopental on mid-latency auditory evoked potentials and their frequency analysis]

  Authors: D Schwender, I Keller, S Klasing, C Madler

  Anästhesiologie, Intensivmedizin, Notfallmedizin, Schmerztherapie : AINS. 12/1991; 26(7):375-80.

  The effect of Thiopentone on mid-latency auditory evoked potentials has not yet been examined sufficiently. Therefore, mid-latency auditory evoked potentials during induction of general anaesthesiaThe effect of Thiopentone on mid-latency auditory evoked potentials has not yet been examined sufficiently. Therefore, mid-latency auditory evoked potentials during induction of general anaesthesia with thiopentone were studied. Following informed consent in 10 patients scheduled for elective surgery, anaesthesia was induced with thiopentone (5 mg/kg b.w. i.v.). Assisted ventilation via face mask with 100% O2 was performed until the first purposeful movement of the limbs appeared. Then a second bolus of thiopentone (2 mg/kg b.w. i.v.) was given and general anaesthesia was maintained according to anaesthesiological and operative necessities. Auditory evoked potentials were recorded in the awake and on-line when thiopentone was injected up to 9 min after the onset of general anaesthesia. Latencies of the peaks V, Na, Pa were measured. Using Fast-Fourier-Transform-Analysis, corresponding power spectra were calculated to analyse energy portions of the AEP frequency components. In the awake state peak latencies were in the normal range. Corresponding power spectra indicated energy maxima in the 30-40 Hz frequency range. After induction of general anaesthesia an increase in latencies of the peaks Na, Pa could be observed. The energy in the 30-40 Hz range became suppressed, the AEP energy maxima shifted to the low frequency range. These effects were observable until 3 min after injection. When the first purposeful movement of a limb occurred, Na, Pa latencies returned to normal values and most part of the AEP-energy was in the 30-40 Hz range as in the awake.(ABSTRACT TRUNCATED AT 250 WORDS)

• [Acoustic evoked potentials of medium latency and intraoperative wakefulness during anesthesia maintenance using propofol, isoflurane and flunitrazepam/fentanyl]

  Authors: D Schwender, I Keller, M Schlund, S Klasing, C Madler

  Der Anaesthesist. 05/1991; 40(4):214-21.

  Auditory evoked potentials have been used as an indicator of awareness. During combined local and general anesthesia clinical signs of adequate anesthesia are difficult to evaluate. In the presentAuditory evoked potentials have been used as an indicator of awareness. During combined local and general anesthesia clinical signs of adequate anesthesia are difficult to evaluate. In the present study we combined peridural analgesia with three techniques of general anesthesia. Intraoperative wakefulness was documented and correlated with cardiocirculatory parameters as well as with mid-latency auditory evoked potentials (MLAEP). METHODS. After institutional approval and informed consent 30 patients undergoing elective laparotomy were studied as follows: first, continuous peridural analgesia was instituted in all patients to block painful sensation of surgical stimuli and the anesthetic level was maintained at T5. Then general anesthesia was induced with propofol 2.5 mg/kg i.v. (group I, n = 10), thiopental 5 mg/kg i.v. (group II, n = 10), or etomidate 0.2 mg/kg i.v. (group III, n = 10) and maintained with propofol 3-5 mg/kg per hour i.v. (group I), isoflurane 0.4-0.8 vol.-% (group II), or flunitrazepam 0.005-0.01 mg/kg i.v. and fentanyl 0.0025-0.005 mg/kg i.v. bolus injections every 20-30 min (group III). Heart rate and arterial pressure were registered continuously. Purposeful movements of the limbs, eye-opening, or other movements as well as coughing were documented as signs of intraoperative wakefulness. AEP were recorded in the awake state, after induction, and during maintenance of general anesthesia. Latencies of the peaks V, Na, and Pa were measured. By fast-Fourier transformation corresponding power-spectra were calculated to analyze the energy content of the AEP frequency components. RESULTS. Intraoperative wakefulness occurred statistically significantly more often in the patients of group III than in those of groups I and II. There was no correlation between wakefulness and cardiocirculatory parameters. Latencies of peaks V, Na, and Pa in the awake patients were in the normal range; the corresponding power-spectra had their major energy content in the 30-40-Hz range. After induction of general anesthesia with propofol, thiopentone, and etomidate as well as during maintenance of general anesthesia with propofol and isoflurane peak latencies of Na and Pa increased, frequencies in the 30-40 Hz range became suppressed, and MLAEP energy maxima shifted to the low-frequency range. In contrast, during maintenance of general anesthesia with flunitrazepam/fentanyl peak latencies of Na and Pa returned to awake values and frequencies in the range of 30 Hz regained energy dominance in the corresponding power-spectra. CONCLUSIONS. The maintenance of MLAEP and the primary cortical complex Na/Pa correlates with the incidence of motor signs of wakefulness. During the combination of regional and general anesthesia, isoflurane and propofol seem to provide better suppression of intraoperative wakefulness than bolus injections of flunitrazepam/fentanyl.

• [Sensory information processing during general anesthesia-- acoustic-evoked 30-40 Hz oscillations and intraoperative wakefulness during cesarean section]

  Authors: D Schwender, I Keller, B Daschner, C Madler

  Anästhesiologie, Intensivmedizin, Notfallmedizin, Schmerztherapie : AINS. 03/1991; 26(1):17-24.

  Neuropsychological and neurophysiological investigations indicate that the underlying framework of adequate sensory information processing is a 30-40 Hz oscillatory brain mechanism, which also can beNeuropsychological and neurophysiological investigations indicate that the underlying framework of adequate sensory information processing is a 30-40 Hz oscillatory brain mechanism, which also can be observed in mid-latency auditory evoked potentials (MLA-EP). Since high incidence of stimuli perception and wakefulness is a phenomenon during Caesarean section under general anaesthesia it was studied if auditory evoked 30-40 Hz oscillation correlate with intraoperative wakefulness during this surgical procedure. Following informed consent, 21 patients were selected for elective Caesarean section. Anaesthesia was induced with thiopentone (5 mg/kg b.w. i.v.) and maintained with thiopentone bolus injection (1-2 mg/kg b.w.i.v). and O2/N2O 1:1 according to clinical signs of adequate anaesthesia. After delivery, a balanced anaesthetic technique using fentanyl, enflurane and N2O in O2 1:1 was employed. Clinical signs of intraoperative wakefulness were spontaneous movements of the limbs, mimics, eye-opening, wakefulness after auditory stimulation (tape A: crying baby, tape B: classical music), one hour and 24 hours postoperatively reported dreams, hallucinations and detailed reports about intraoperative events. Auditory evoked potentials were recorded on-line before and during general anaesthesia, during the entire surgical procedure. Latencies of the peaks V, Na, Pa were measured. Employing Fast-Fourier transformation analysis, corresponding power spectra were calculated to analyse energy portions of AEP's frequency components. Spontaneous motoric movements occurred in 60% of the patients and did not correlate with heart rate, blood pressure or other clinical signs of inadequate anaesthesia. Provoked motoric reactions were 4 times as often after presentation of tape A as after tape B.(ABSTRACT TRUNCATED AT 250 WORDS)

• Sensory information processing during general anaesthesia: effect of isoflurane on auditory evoked neuronal oscillations.

  Authors: C Madler, I Keller, D Schwender, E Pöppel

  British journal of anaesthesia. 02/1991; 66(1):81-7.

  There is evidence from neuropsychological and psychophysical measurements that sensory information is processed in discrete time segments. The segmentation process may be described as neuronalThere is evidence from neuropsychological and psychophysical measurements that sensory information is processed in discrete time segments. The segmentation process may be described as neuronal oscillation at a frequency of 30-40 Hz. Stimulus-induced neuronal oscillations of this frequency are found in the middle latency range of the auditory evoked potential (AEP). We have studied the effect of different end-tidal concentrations of isoflurane on auditory evoked 30-40 Hz neuronal oscillations. We studied 13 patients undergoing intra-abdominal urological and gynaecological procedures. AEP were recorded in the awake state and during end-expiratory steady state isoflurane concentrations of 0.3, 0.6 and 1.2 vol%. These incremental doses of isoflurane caused a stepwise decrease in frequency of oscillations. The decrease in oscillation frequency and sometimes the disappearance of oscillatory components may be interpreted as suppression of sensory information processing. The measurement of auditory evoked neuronal oscillations in the AEP appears to be a promising tool to monitor both sensory information processing capacity and depth of anaesthesia.

• [Mid-latency auditory evoked potentials during induction of intravenous anesthesia using midazolam, diazepam and flunitrazepam]

  Authors: D Schwender, I Keller, S Klasing, C Madler

  Anästhesie, Intensivtherapie, Notfallmedizin. 01/1991; 25(6):383-90.

  Since intraoperative awareness is not infrequently observed under balanced anaesthetic regimens employing benzodiazepines for suppression of consciousness, we studied the effect of intravenousSince intraoperative awareness is not infrequently observed under balanced anaesthetic regimens employing benzodiazepines for suppression of consciousness, we studied the effect of intravenous induction of general anaesthesia using the benzodiazepines midazolam, diazepam and flunitrazepam on mid-latency auditory evoked potentials and auditory evoked neuronal 30-40 Hz oscillation. Following informed consent in 30 patients scheduled for minor gynaecological procedures, anaesthesia was induced with midazolam (0.2-0.3 mg/kg b.w. i.v., group I n = 10), diazepam (0.3-0,4 mg/kg b.w., i.v., group II n = 10) or flunitrazepam (0.03-0.04 mg/kg b.w., i.v., group III n = 10). Auditory evoked potentials were recorded before, during and after induction of general anaesthesia on vertex (positive) and mastoides on both sides (negative). Auditory clicks were presented binaurally at 70 dBnHL with a frequency of 9.3 Hz. Using the electrodiagnostic system Pathfinder I (Nicolet), 1000 successive stimuli were averaged over a 100 ms poststimulus period and analysed off-line. Latencies of the peak V, Na, Pa were measured. By means of Fast-Fourier transformation-analysis corresponding power spectra were calculated to analyse energy portions of the AEP frequency components. In the awake state AEP showed an oscillatory component between 20 and 100 ms poststimulus latency. Corresponding power spectra indicated a predominant 30-40 Hz frequency. After induction of general anaesthesia using midazolam, diazepam and flunitrazepam, there was no increase in latencies of the peaks V, Na, Pa, but only a small decrease in amplitudes Na/Pa without statistical significance. The auditory evoked mid-latency neuronal oscillation persisted under induction of general anaesthesia with midazolam, diazepam, flunitrazepam.(ABSTRACT TRUNCATED AT 250 WORDS)

• Readiness potentials preceding spontaneous motor acts: voluntary vs. involuntary control.

  Authors: I Keller, H Heckhausen

  Electroencephalography and clinical neurophysiology. 11/1990; 76(4):351-61.

  Libet et al. (1983) developed a method to compare the onset time of a readiness potential (RP) with the onset time of the corresponding intention to perform a spontaneous voluntary motor act. InLibet et al. (1983) developed a method to compare the onset time of a readiness potential (RP) with the onset time of the corresponding intention to perform a spontaneous voluntary motor act. In relation to the onset of the RP, the time of conscious intention to move followed 350 msec later. From these results Libet (1985) concluded that the cerebral initiation of a spontaneous voluntary act begins unconsciously. We investigated the alternative interpretation that with the instruction to pay attention to feelings of 'wanting to move,' automatic and normally unconscious motor acts might have been brought to a level of conscious awareness. Therefore we conducted 3 kinds of experiment. In the first, RPs were measured from subjects performing unconscious movements. The second experiment was a replication of Libet's study while the third was a resting condition in which subjects looked for intentions to move introspectively. The results showed that RPs beginning approximately 500 msec before movement onset can be obtained with unconsciously as well as consciously performed spontaneous motor acts. The different scalp distributions of the two types of RP indicate that unconscious movements can be attributed to the activation of a contralateral process (lateral premotor system (LPS), primary motor cortex), whereas voluntary spontaneous motor acts seemed to be predominated by the medial premotor system (MPS). It is proposed that in the Libet situation focused attention on internal events led to the conscious detection of a normally unconscious process. This resulted in the activation of the MPS, especially the supplementary motor area (SMA), which released the starting signal for the execution of the motor act. We believe that the activation of the SMA and the urge to move occurred at the same time.

• [Middle-latency auditory evoked potentials during high-dose opioid analgesia]

  Authors: D Schwender, I Keller, S Klasing, C Madler

  Der Anaesthesist. 07/1990; 39(6):299-305.

  Neuropsychological and neurophysiological investigations indicate that a 30-40 Hz oscillatory brain mechanism is necessary for the adequate uptake and processing of elementary successive sensoryNeuropsychological and neurophysiological investigations indicate that a 30-40 Hz oscillatory brain mechanism is necessary for the adequate uptake and processing of elementary successive sensory events. An oscillatory component of that particular frequency range can be observed in the mid-latency auditory evoked potential (MLAEP). It is suppressed under non-specific anesthetic agents (agents not acting on specific structures of the brain or receptors). Opioids produce analgesia, sedation, and are also used for induction and maintenance of general anesthesia. The effect of opioids on MLAEP has not yet been examined sufficiently. Because intraoperative wakefulness and awareness is not rarely observed under high-dose opioid analgesia, the effect of general anesthesia with fentanyl on MLAEP and auditory evoked neuronal 30-40 Hz oscillation was studied. Following informed consent in 20 patients scheduled for elective surgery, anesthesia was induced with fentanyl 0.01-0.02 mg/kg i.v. Auditory evoked potentials (AEP) were recorded before and 5 min after the start of general anesthesia over the vertex (positive) and mastoids on both sides (negative). Auditory clicks were presented binaurally at 70 dBnHL with a frequency of 9.3 Hz. Using the electrodiagnostic system Pathfinder I (Nicolet Co.), 1000 successive stimuli were averaged over a 100-ms post-stimulus period and analyzed off-line. Latencies of the peaks V, Na, and Pa were measured. By Fast-Fourier Transform-analysis corresponding power spectra were calculated to analyze energy portions of the AEP frequency components. In the awake state AEP showed an oscillatory component between 20- and 100-ms post-stimulus latency. Corresponding power spectra indicated a dominant 30-40 Hz frequency.(ABSTRACT TRUNCATED AT 250 WORDS)

• Analysis of oscillatory components in perioperative AEP-recordings: a nonparametric procedure for frequency measurement.

  Authors: I Keller, C Madler, D Schwender, E Pöppel

  Clinical EEG (electroencephalography). 05/1990; 21(2):88-92.

  Auditory evoked potentials (AEPs) were recorded in 15 patients scheduled for minor gynecological procedures. The suppression of a middle latency oscillation about 40 Hz was observed during 2 minutesAuditory evoked potentials (AEPs) were recorded in 15 patients scheduled for minor gynecological procedures. The suppression of a middle latency oscillation about 40 Hz was observed during 2 minutes after injection of the anesthetic agent propofol. The decrease in oscillation frequency during anesthetic induction was evaluated with a fast Fourier transformation (FFT) and a rank-correlation procedure (RCP). With AEPs having a short epoch of 100 ms the frequency resolution of the RCP was better than the frequency resolution of the FFT. The rank-correlation function of the RCP indicates an initial drop of oscillation frequency from 44 Hz to 37 Hz during 1 minute after anesthetic induction. During the second minute after the infusion of propofol, the oscillation frequency decreases from 37 Hz to 19 Hz. It is suggested that the suppression of neuronal oscillations in the AEP represents a loss of sensory information processing. The measurement of middle latency oscillations in the AEP is proposed to be a promising indicator for the intraoperative stage of anesthesia.

• [Sensory information processing during general anesthesia. The effect of propofol and ketamine on mid-latency auditory evoked potentials]

  Authors: D Schwender, S Klasing, I Keller, C Madler

  Der Anaesthesist. 01/1990; 38(12):664-72.

  Experimental evidence from various neuropsychological and neurophysiological fields indicates that an oscillatory brain mechanism in a frequency range of 30-40 Hz is necessary for adequate sensoryExperimental evidence from various neuropsychological and neurophysiological fields indicates that an oscillatory brain mechanism in a frequency range of 30-40 Hz is necessary for adequate sensory information processing. An oscillatory component of that particular frequency range can also be observed in the mid-latency auditory evoked potentials. Thus general anesthesia can be defined as a suppression of sensory information processing, and the effect of the i.v. anesthetics, Propofol and ketamine, on auditory perception and auditory-evoked potentials was therefore studied. In 22 patients anesthesia was induced with Propofol (2.5 mg/kg body weight, group I: n = 10) or ketamine (2 mg/kg body weight, group II: n = 12). Auditory-evoked potentials were recorded before, during, and after induction of general anesthesia. In the awake state, an oscillatory component with an energetically dominant 30-40 Hz frequency was present between 20 and 100 ms poststimulus latency. Propofol suppressed the mid-latency oscillatory component, whereas the component persisted under ketamine. Under Propofol, the 30-40 Hz leading frequency shifted to the lower frequency range, whereas under ketamine, the AEP were still dominated by a 30-40 Hz oscillation. Propofol suppresses the oscillatory brain mechanism necessary for adequate sensory information processing like various other general anesthetics. In contrast to Propofol, ketamine does not alter auditory perception. Suppression of sensory information processing must take place at a higher cortical level in a dissociative manner. The persistence of a 30-40 Hz oscillation must be seen in connection with dreams and hallucinations as reported for the drug and may be interpreted as insufficient suppression of sensory information processing under ketamine anesthesia.

• Induction of anaesthesia in patients undergoing aortocoronary bypass surgery: propofol versus etomidate.

  Authors: R Haessler, I Keller, C Madler

  Journal of cardiothoracic anesthesia. 11/1989; 3(5 Suppl 1):28.

• Readiness potentials preceding spontaneous motor acts: voluntary vs. involuntary control

  Authors: I. Keller, H. Heckhausen

  Electroencephalography and Clinical Neurophysiology.

  Libet et al. (1983) developed a method to compare the onset time of a readiness potential (RP) with the onset time of the corresponding intention to perform a spontaneous voluntary motor act. InLibet et al. (1983) developed a method to compare the onset time of a readiness potential (RP) with the onset time of the corresponding intention to perform a spontaneous voluntary motor act. In relation to the onset of the RP, the time of conscious intention to move followed 350 msec later. From these results Libet (1985) concluded that the cerebral initiation of a spontaneous voluntary act begins unconsciously. We investigated the alternative interpretation that with the instruction to pay attention to feelings of ‘wanting to move’, automatic and normally unconscious motor acts might have been brought to a level of conscious awareness. Therefore we conducted 3 kinds of experiments. In the first, RPs were measured from subjects performing unconscious movements. The second experiment was a replication of Libet's study while the third was a resting condition in which subjects looked for intentions to move introspectively. The results showed that RPs beginning approximately 500 msec before movement onset can be obtained with unconsciously as well as consciously performed spontaneous motor acts. The different scalp distributions of the two types of RP indicate that unconscious movements can be attributed to the activation of a contralateral process (lateral premotor system (LPS), primary motor cortex), whereas voluntary spontaneous motor acts seemed to be predominated by the medial premotor system (MPS). It is proposed that in the Libet situation focused attention on internal events led to the conscious detection of a normally unconscious process. This resulted in the activation of the MPS, especially the supplementary motor area (SMA), which released the starting signal for the execution of the motor act. We believe that the activation of the SMA and the urge to move occurred at the same time.