Direct Current Auditory Evoked Potentials--论文代写范文精选

统计上显著的差异被发现，对于类似的效果,从动物实验报告,可以推测到人类的实验。这些结果说明DC-AEPs潜在有用的评估皮质功能，在麻醉和可能有资格的方法。下面的report代写范文进行详述。

Introduction 
Direct current auditory evoked potentials (DC-AEPs) are a sensitive indicator of depth of anesthesia in animals. However, they have never been investigated in humans. To assess the potential usefulness of DC-AEPs as an indicator of anesthesia in humans, we performed an explorative study in which DC-AEPs were recorded during propofol and methohexital anesthesia in humans. DC-AEPs were recorded via 22 scalp electrodes in 19 volunteers randomly assigned to receive either propofol or methohexital. DC-AEPs were evoked by binaurally presented 2-s, 60-dB, 800-Hz tones; measurements were taken during awake baseline, anesthesia, and emergence. Statistical analysis included analysis of variance and discriminant analysis of data acquired during these three conditions. 

About 500 ms after stimulus presentation, DC-AEPs could be observed. These potentials were present only during baseline and emergence—not during anesthesia. Statistically significant differences were found between baseline and anesthesia and between anesthesia and emergence. In conclusion, similar effects, as reported in animal studies of anesthetics on the DC-AEPs, could be observed in anesthetized humans. These results demonstrate that DC-AEPs are potentially useful in the assessment of cortical function during anesthesia and might qualify the method for monitoring anesthesia in humans.

Monitoring depth of anesthesia remains a problem in the management of anesthesia. Several approaches have been taken, mostly based on measuring cortical electrical activity, to develop monitoring devices that combine high sensitivity, reliability, and easy handling in an operation room setting. Auditory evoked potentials (AEPs) have been investigated as possible monitoring variables, and most of the short-, middle-, and long-latency components of AEPs have already been examined for their sensitivity to anesthesia (1). 

In addition to these transient, or AC, components, event-related potentials also contain sustained, or direct current (DC), components. DC potential responses to sustained acoustic stimuli are an excellent indicator of the integrity of cortical sensory processing in the cat. The cortical DC response of the primary acoustic cortex showed a dose-dependent reduction to anesthetics, and it disappeared during deep anesthesia (2,3). Thus, DC-AEPs could be used for monitoring depth of anesthesia. However, their sensitivity to anesthesia in humans has not been investigated. In an explorative study, we aimed to investigate whether similar effects on DC-AEPs could be observed in anesthetized humans. We administered anesthesia for 20 min in 19 healthy volunteers and simultaneously recorded DC-AEPs via 22 scalp channels. Because of the growing importance of total IV anesthesia and to exclude effects specific for a single anesthetic substance, we used two different shortlasting IV anesthetics. We hypothesized that DC-AEPs would show significant differences between the awake, anesthetized, and emergent states.

Methods 
After approval from our medical ethics committee and the obtaining of signed written informed consent, 19 male right-handed volunteers were included in the study (median age, 23 yr; range, 20–28 yr). Handedness was determined by using the Marian Annett Handedness Inventory (4). All subjects were classified as ASA physical status I, showing no sign or history of acute or chronic disease, organ impairment, or use or abuse of drugs known to affect cerebral activity. Single-blinded randomization to the Propofol or Methohexital Group was accomplished by means of a computer generated randomization list (Microsoft Excel™ 7.0; Microsoft, Redmond, WA). Experiments took place in a separate room of an intensive care unit and were isolated to the surroundings, thus assuring undisturbed conduct of the protocol. In the first condition, 60-dB 800-Hz tones of 2-s duration were presented binaurally to the awake and fully aware subject via standard stereo earphones. 

To allow comparison with anesthesia, subjects had to close their eyes during this baseline condition. The interstimulus interval of the tones varied from 5 to 14 s, the mean interval being 9.5 s. In total, 75 tones were presented to each subject (duration approximately 15 min). After this condition, anesthesia was induced by using propofol in 10 and methohexital in 9 subjects. Auditory stimuli were presented in the same manner as during the first condition (except that the total number of presented tones was higher because of the longer duration of this part of the experiment). Tone presentation and electroencephalogram (EEG) recordings began shortly before the induction of anesthesia and continued until the reoccurrence of the eyelash reflex.

Venous blood samples were drawn after the anesthesia induction, during stable anesthesia, and after return of the eyelash reflex. Samples were centrifuged and stored for further processing at 220°C. Because of technical reasons (defect of the refrigerator), samples from only 10 volunteers (7 with propofol and 3 with methohexital) could be used for measurements. Therefore, no statistical evaluation was performed, and these data will be presented only descriptively. Propofol and methohexital concentrations were measured with gas chromatography and mass spectrometry. Calibration curves were linear over a range measured from 0.01 to 10 mg/mL, and a limit of detection of 0.01 mg propofol and 0.01 mg methohexital per milliliter of plasma could be achieved. 

Because only trials judged to be free of artifacts should be used for averaging, data were visually inspected off-line, excluding trials assessed to contain artifacts from the analysis. Artifacts caused by eye movements and blinks were removed with a linear regression approach (7). After removal of artifact-containing trials, stimuluslinked averages of trials of baseline, anesthesia, and emergence from anesthesia were calculated for each subject. Because background EEG amplitude increased during anesthesia compared with baseline, trials of all three conditions were digitally low-pass filtered (frequency range, DC to 5 Hz) before averaging. 

The number of trials included in each average was kept constant to make signal-to-noise ratios comparable among averages. For the baseline condition, the average was computed by randomly selecting 40 of the 75 available trials. Forty consecutive trials during steady-state anesthesia were selected for the anesthesia average, whereas the 40 trials before the reoccurrence of the eyelash reflex were used for the emergence average. In addition, to make sure that only trials representative of the respective conditions were used for averaging, every trial was visually characterized for frequency, amplitude, and topography and classified as belonging to one of four phases (baseline, initial changes after bolus, steady-state anesthesia, and return to baseline patterns). In addition to the subject who had to be excluded because of a laryngeal spasm, 2 of the remaining 18 subjects had to be excluded from the analysis because of technical problems. The final sample consisted of nine subjects in whom propofol and seven in whom methohexital had been used as the anesthetic.

Results 
Duration of anesthesia was 24 6 4 min in the Propofol and 26 6 4 min in the Methohexital Group. Hemodynamic data showed significant differences between the groups, whereas oxygenation was not different (Table 1). Anesthetic blood concentrations were as follows: propofol (median [min; max]) was 10.8 (9; 30) mg/mL after induction and decreased to 5.5 (1.5; 16.8) mg/mL during anesthesia. At emergence, 1.3 (1.1; 4.6) mg/mL was measured. Methohexital decreased from 14.8 (12.2; 18.3) mg/mL to 5.1 (3.4; 8.1) mg/mL and further to 2.8 (2.4; 6.5) mg/mL. The DC-AEPs in Figures 1 and 2 present the main results. In the baseline condition, starting at approximately 500 ms poststimulus onset, a sustained response clearly distinguishable from the transient potentials after onset and offset of stimulus presentation was evoked over frontal, central, and parietal areas with maximal amplitude at fronto-medial electrode sites. During anesthesia, no such response was observed. It reappeared, however, during emergence from anesthesia. Although having a slightly lower amplitude and showing a less sharp onset and offset compared with DC-AEPs during baseline, this component showed a topography almost identical to the one during baseline (Fig. 1 and 2), and its maximum amplitude was also located at electrode Fz.

Discussion 
AEPs are classically divided, on the basis of their latency, into first, fast, middle, slow, and late components (13). Sustained potentials of single polarity may be discriminated from these transient, or AC, components when longer acoustic stimulation is used (14). Most studies on the relationship of AEP components to anesthesia in humans have focused on transient components. Small increases in the latencies of waves III and V of the fast (brainstem) auditory evoked response have repeatedly been observed (15,16). However, these early components seem to be largely unaffected by most anesthetics (17). Both increases in latencies and decreases of peak-to-peak amplitudes of the middle-latency components Na, Pa, Nb, and Pb have been observed with different anesthetics (18–20). During sufentanil anesthesia, reduced latencies and amplitudes of P2 and disappearance of P3 were observed compared with the resting state (21), and it was reported that N1 and P3 could be recorded neither during anesthesia nor during emergence (22). 

Another approach is the recording of the 40-Hz auditory steady-state response (23). This response is attenuated in a concentration-dependent manner by isoflurane (24). One advantage of analyzing potentials evoked by sustained stimulation only is that they are less susceptible to short events (external as well as internal) that occur quite often during surgical operations (e.g., electrocoagulation, noise produced by alarm of the monitor or ventilator, etc.). This is the first study that examines the DC component of evoked potential during anesthesia in humans. We investigated the effects of propofol- and methohexital-induced anesthesia on the DC auditory evoked response. During a preanesthetic baseline condition, we recorded highly plausible and reproducible DC potentials starting about 500 ms after stimulus onset and dissolving shortly after stimulus offset. 

With regard to the time course and to the scalp distribution, this observation is in excellent agreement with other reports (11). During anesthesia, a DC potential could not be identified. This might be seen as an equivalent to the disappearance of the DC response in the anesthetized cat (2,3). Nevertheless, at the end of anesthesia when subjects were regaining consciousness, the DC potential reappeared. Although the mean amplitudes of this condition were slightly lower than those of the baseline condition, the topographical distribution of this condition is almost identical to the scalp potential pattern during baseline (Fig. 2) but clearly distinguishable from the pattern during anesthesia. The latter is important, because it demonstrates that the differences between anesthesia, baseline, and emergence are not solely caused by an unspecific difference in DC activity during anesthesia. 

The significant interactions of the location factor of the ANOVAs, together with the linear contrasts for the fronto-medial versus the occipital region, corroborate the interpretation that the effects we have observed were not caused by a global difference in cortical DC activity but by a location-specific difference, reflecting the presentation of auditory, but not visual, stimuli. Although differences in hemodynamic data were observed, the DCAEP effects were not specific for the anesthetic used. However, our sample size was not large enough to allow definite conclusions on the interaction of anesthetic and cortical electrical activity. Nevertheless, if one follows the small-sample explanation, the effect size of the anesthetics must be small. If it were moderate or large, we would have obtained significance despite the small sample size. Hence, we can tentatively exclude that the effects seen were specific to the anesthetic.(report代写)

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