Wakeful response to command indicates memory potential during emergence from general anesthesia

Objective. An important aspect of assessing anesthetic depth is determining whether a patient will remember events during surgery. We looked for a clinical sign that would indicate a patient's potential for memory formation during emergence from anesthesia. A clinical sign indicating memory potential could be a useful endpoint for measuring the performance of anesthetic depth monitors and for titrating administration of anesthetic agents.Methods. We evaluated patients' responses to commands to open the eyes, squeeze the hand four times, and count 20 numbers. These responses were correlated with results on recall, cued recall, and multiple-choice memory tests.Main Results. Patients did not have evidence of memory formation until they sustained wakefulness sufficiently long to complete at least four hand squeezes or count four numbers. Of 28 patients, 13 (46%) with this sustained wakeful response had memory. Of 22 patients, 0 (0%) had evidence of memory formation when they demonstrated a brief wakeful response, defined as being responsive to command but unable to complete more than one hand squeeze or count, or an intermediate response, defined as two or three hand squeezes or counts.Conclusions. We conclude that a brief wakeful response to command indicates that a patient is unlikely to form memories, while a sustained wakeful response indicates that a patient may form memories. Thus, a patient's wakeful response to command could be a useful indicator of potential for memory.This work was supported by the Kaiser Foundation Research Institute. Elements of this work were presented at the Society for Technology in Anesthesia, January 1992, San Diego, CA.     

EEG predicts movement response to surgical stimuli during general anesthesia with combinations of isoflurane, 70% N2O,and fentanyl

Objective. Our objective was to evaluate the performance of the EEG as an indicator of anesthetic depth by measuring EEG prediction of movement response to surgical stimuli.Methods. While using 5 different combinations of isoflurane, 70% N₂O, and fentanyl, we measured the EEG of 246 patients during pelvic laparoscopy and observed their movement responses to opening stimuli (defined as skin incision, CO₂ needle insertion, or trocar insertion) and also to closing stimuli (defined as sutures during incision closure). The EEG was expressed asF95, the frequency in hertz below which resides 95% of the power in the EEG frequency spectrum. The relations betweenF95 and movement response were expressed as logistic regression curves.F95-response logistic regression curves, which are analogous to dose-response curves, were calculated for each of the 2 stimuli administered during each of the 5 anesthetic techniques. The prediction of patient responsiveness byF95 was tested using (beta), a measure of the slope of anF95-response logistic curve. The presence of shifts among theF95-response logistic curves was tested using the differences inF95 values between curves. Hypothesis tests used a level of significance ofP = 0.05.Main Results. The slopes of theF95-response logistic regression curves showed a statistically significant ability to predict movement response to stimuli for 9 of the 10 combinations of stimuli and anesthetic techniques. We did not calculate anF95-response logistic curve for the tenth combination because it contained burst suppression, which our EEG analysis method was not designed to process. TheF95-response logistic curves were shifted relative to each other, and the shifts were affected by the type of stimulus and the combination of anesthetic agents. Referenced to opening curves, the mean shift of the closing curves was ± 4.2 ± 0.3 Hz (mean ± SD). With increasing doses of fentanyl, the use of 70% N₂O, or both, the curves shifted to higher values ofF95; the range in shifts was 0.2 to 8.1 Hz. The slope values of theF95-response logistic curves and the shifts among the curves were similar to the values and shifts that might be expected from changes in anesthetic agent doses.Conclusions. The EEG, expressed asF95, predicted movement response to surgical stimuli during combinations of isoflurane, 70% N₂O, and fentanyl. TheF95-response curves shifted upward on the frequency scale for the less intense stimuli and for anesthetic techniques using 70% N₂O, fentanyl, or both.F95 prediction of movement response appeared to be related to anesthetic agent doses. OurF95-response curves may provide helpful guidelines for usingF95 to titrate the administration of anesthetic agents and for assessing the depth of general anesthesia.     

Craniofacial Electromyogram Activation Response: Another Indicator of Anesthetic Depth

Objective. After finding that craniofacial EMG preceding a stimulus was a poor predictor of movement response to that stimulus, we evaluated an alternative relation between EMG and movement: the difference in anesthetic depth between the endpoint of EMG responsiveness to a stimulus and endpoint of movement responsiveness to that stimulus. We expressed this relation as the increment of isoflurane between the two endpoints. Methods. We measured EMG over the frontalis muscle, over the corrugator muscle, and between the F_p2 and the mastoid process as patients emerged from general anesthesia during suture closing of the surgical incision. Anesthesia was decreased by controlled washout of isoflurane while maintaining 70% N₂O, and brain isoflurane concentrations (^Ciso_Brain) were calculated. We studied a control group of 10 patients who received only surgical stimulation, and 30 experimental patients who intermittently received test stimuli in addition to the surgical stimulation. Patients were observed for movement responses and EMG records were evaluated for EMG activation responses. We defined an EMG activation response to be a rapid voltage increase of at least 1.0 µV RMS above baseline, with a duration of at least 30 s, in at least one of the three EMG channels. Patient responses to stimuli were classified as either an EMG activation response without a move response (EMG⁺, a move response without an EMG activation response (MV⁺), both an EMG activation response and a move response (EMG⁺MV⁺), or no response. We defined the EMG⁺ endpoint to be the threshold between EMG⁺ response and nonresponse to a stimulus, and estimated^C iso_Brain at this endpoint. We similarly defined the move endpoint and estimated the move endpoint^C iso_Brain. We then calculated the increment of^C iso_Brain at the EMG⁺ endpoint relative to the move endpoint. Main results. For the 30 experimental patients, the initial response to a test stimulus was an EMG⁺ in 14 patients (47%), an EMG⁺MV⁺ in 12 patients (40%), and a MV⁺ in 1 patient (3%); no response occurred by the time surgery was completed in 3 patients (10%). No response occurred in 7 of the control patients (70%). Of the 14 patients with an initial EMG⁺ response to a test stimulus, 9 patients later had a move response. For these 9 patients, the increment of ^Ciso_Brain between the EMG⁺ endpoint and move endpoint was 0.11 ± 0.04 vol% (mean ± SD). Conclusions. Our results suggest that, given the circumstances of our study, an EMG activation response by a nonmoving patient indicates that the patient is at an anesthetic level close to that at which movement could occur. However, because the first EMG activation response may occur simultaneously with movement, the EMG activation response cannot be relied upon to always herald a move response before it occurs. Our results also suggest that EMG responsiveness to a test stimulus may be used to estimate the anesthetic depth of an individual patient.     

Response of lower esophageal contractility to changing concentrations of halothane or isoflurane: A multicenter study

A multiple-center study was performed to determine the relationship between lower esophageal contractility, clinical signs, and anesthetic concentration as expressed by minimum alveolar concentration (MAC). One hundred four American Society of Anesthesiologists Class I through III patients were exposed to isoflurane (with and without nitrous oxide) or halothane in concentrations of 0.5, 1.0, and 1.5 MAC. Heart rate and systolic blood pressure were continuously monitored. Both the amplitude and frequency of spontaneous and provoked lower esophageal contractions were measured in situ by using a 24-F probe equipped with provoking and measuring balloons. Combined results demonstrated statistically significant correlations (P<0.001) between lower esophageal contractility and MAC. Spontaneous lower esophageal contractions decreased from 1.10±0.12 (SEM) contractions per minute (0.5 MAC) to 0.42±0.05 (1 MAC) to 0.18±0.05 (1.5 MAC). Provoked lower esophageal contractility values decreased from 45±4 mm Hg (0.5 MAC) to 29±3 (1 MAC) to 19±2 (1.5 MAC). Heart rate changes did not correlate with MAC, and systolic blood pressure correlated in only one of three centers. Intracenter and intercenter analyses failed to demonstrate a significant relationship between lower esophageal contractility and heart rate or systolic blood pressure. No intracenter differences in either amplitude or frequency of lower esophageal contractions were observed, despite differences in volatile agents, induction techniques and agents, patient populations, and duration of anesthesia. Our studies indicate that lower esophageal contractility may be an indicator of anesthetic depth as reflected by MAC, but further studies are needed to quantify the effects of surgical stimulus, intravenous anesthetics, vasodilators, anticholinergics, calcium channel blockers, beta-adrenergic agonists, and the presence of a nasogastric tube.