Processed electroencephalogram during combined extradural and general anaesthesia

Neurophysiological techniques to assess pain in animals

Neurophysiological techniques are widely applied to animals, both in the search as a monitor for adequacy of anaesthesia, and studies to assess the efficacy of analgesic agents. Laboratory animals have been extensively used in models to investigate pain in man. However a substantial number of studies have also used neurophysiological techniques to increase knowledge of pain in specific animal species, with the aim of improving animal welfare. This review provides an overview of neurophysiological techniques involving the brain that have been used in the assessment of pain in animals. An explanation of the methodology of EEG recording, with particular emphasis on veterinary studies, is given. Neurophysiological models developed to assess pain in different species are described, and their relevance to advancements in animal welfare or best clinical practice indicated.

Changes in the EEG during castration in horses and ponies anaesthetized with halothane

OBJECTIVE

To identify changes in the amplitude spectrum of the electroencephalogram (EEG) during a standardized surgical model of nociception in horses.

ANIMALS

Thirteen entire male horses and ponies referred to Division of Clinical Veterinary Science, Bristol (n = 9) and Department of Clinical Veterinary Medicine (n = 4) for castration.

MATERIALS AND METHODS

Following pre-anaesthetic medication with acepromazine, anaesthesia was induced with guaiphenesin and thiopental and maintained with halothane in oxygen. The EEG was recorded continuously using subcutaneous needle electrodes. Additional monitoring comprised ECG, arterial blood pressure, blood gas analysis, airway gases, and body temperature. All animals were castrated using a closed technique. The raw EEG was analysed after completion of each investigation and the EEG variables median frequency (F50), spectral edge frequency (SEF) 95% and total amplitude were derived from the spectra using standard techniques. The mean values of EEG variables recorded during a baseline time period (recorded before the start of surgery) and castration of each testicle were compared using analysis of variance for repeated measures.

RESULTS

Total amplitude (Atot) decreased and F50 increased during castration of each testicle compared to the baseline time period [(89.0 +/- 7.8% testicle 1, 87.0 +/- 7.8% testicle 2) and (110.0 +/- 15.0% testicle 1, 109.0 +/- 15.0% testicle 2), respectively]. Changes in SEF 95% were not significant.

CONCLUSIONS

De-synchronization was identified in the EEG during the nociceptive stimulus of castration. The results suggest that an increase in F50 may be a specific marker for nociception in the horse.

CLINICAL RELEVANCE

Studies investigating the efficacy of analgesic agents in horses are limited by difficulties in peri-operative pain assessment. This model, using EEG changes associated with nociceptive stimulation, can be used to investigate the anti-nociceptive efficacy of different anaesthetic agents in the horse.

Isoflurane dosage for equivalent intraoperative electroencephalographic suppression in patients with and without epidural blockade

We conducted a prospective, randomized, controlled trial to establish the effect of epidural blockade on isoflurane requirements for equivalent intraoperative electroencephalographic (EEG) suppression. Fifty patients undergoing abdominal hysterectomy received combined epidural and general anesthesia or general anesthesia alone with isoflurane and alfentanil. Isoflurane was administered by computer-controlled closed-loop feedback to maintain an EEG 95% spectral edge frequency of 17.5 Hz, a target chosen on the basis of a pilot study. In epidural patients, end-tidal isoflurane concentration (FE'(ISO)) was 0.19% smaller (95% confidence interval [CI], -0.32% to -0.06%; P < 0.01), mean arterial blood pressure was 17 mm Hg lower (95% CI, -24 to -9 mm Hg; P < 0.0001), and body temperature was 0.4 degrees C lower (95% CI, -0.7 to 0 degrees C; P < 0.05) than in controls. EEG bispectral index (BIS) was 4 points higher (95% CI, 1 to 7; P < 0.05). EEG median frequency and heart rate were similar in both groups. Epidural patients were 76% more likely (95% CI, 58% to 94%; P < 0.001) to require metaraminol for hypotension and were 28% more likely (95% CI, 3% to 53%; P < 0.05) to require glycopyrrolate for bradycardia. After surgery, the time to eye opening in epidural patients was 2.3 min shorter (95% CI, -4.2 to -0.5 min; P < 0.05). Time to eye opening correlated better with FE'(ISO) in the last 30 s of anesthesia (FE'(ISO) = 0.07 x time to eye opening + 0.31; r(2) = 0.59; P < 0.0001) than with BIS from the same period (BIS = 64 - 1.25 x time to eye opening; r(2) = 0.22; P < 0.001) (P < 0.0001). To maintain similar intraoperative spectral edge frequency, patients receiving combined epidural and general anesthesia require 21% less isoflurane than those receiving general anesthesia alone. This smaller isoflurane dose is associated with faster emergence from anesthesia.

IMPLICATIONS

The dose of general anesthetic required to maintain similar intraoperative suppression of brain electrical activity is 21% less in patients with nerve blockade than in those without. This dose reduction results in faster waking times in patients with nerve blockade, which may reflect lighter intraoperative anesthesia. The dose of general anesthetic required to maintain similar intraoperative suppression of brain electrical activity is 21% less in patients with nerve blockade than in those without. This dose reduction results in faster waking times in patients with nerve blockade, which may reflect lighter intraoperative anesthesia.

How reliable is the Bispectral Index in critically ill patients? A prospective, comparative, single-blinded observer study

OBJECTIVE

To establish a correlation between a reliable subjective measure, the Sedation-Agitation Scale (SAS), and an objective tool, the Bispectral Index (BIS), for monitoring critically ill patients with a decreased level of consciousness.

DESIGN

Prospective, comparative, single-blinded observer study.

SETTING

Surgical and medical intensive care units of the Tufts-New England Medical Center, a 349-bed tertiary care, academic medical center.

PATIENTS

A convenience sample of 20 adult, critically ill patients with a decreased level of consciousness. The data from one patient were excluded because the patient did not meet inclusion criteria.

MEASUREMENTS AND MAIN RESULTS

Patients were prospectively evaluated by a blinded observer using the SAS to subjectively determine their level of consciousness. Sedation levels varied from unarousable (SAS score of 1), to very sedated (SAS score of 2), to mildly sedated (SAS score of 3). Simultaneously, the patients were continuously monitored for 4-6 hrs with the BIS device. There was wide variability in BIS scores for any given level of consciousness as compared with the SAS. Unarousable patients had BIS scores ranging from 23 to 97, with a median score of 50 and an interquartile range of 24. Very sedated patients had BIS scores ranging from 35 to 98, with a median score of 68 and an interquartile range of 36. Mildly sedated patients had BIS scores ranging from 67 to 91, with a median score of 76 and an interquartile range of 8. Overall, there was a less than satisfactory correlation between BIS values and SAS scores (r =.36, p <.001). However, the correlation improved with subgroup analysis when BIS values associated with excessive muscle movement were excluded (r =.50, p <.001).

CONCLUSIONS

The correlation between SAS and BIS scores was suboptimal and inconsistent in a heterogeneous group of critically ill patients. The generation of BIS hardware and software, studied herein, is neither reliable nor valid for routinely monitoring the level of consciousness in the critically ill patient. Excessive muscle movement by the patient is an important and spurious influence on BIS values and seriously undermines BIS reliability.

Assessing sedation during intensive care unit mechanical ventilation with the Bispectral Index and the Sedation-Agitation Scale

OBJECTIVE

To describe the level of sedation for a cohort of mechanically ventilated adult intensive care unit (ICU) patients using validated subjective and objective tools.

DESIGN

Prospective convenience sample.

SETTING

Multidisciplinary 34-bed ICU at Maine Medical Center, a 599-bed nonuniversity, academic medical center.

PATIENTS

Sixty-three adult ICU patients were monitored during 64 episodes of ventilatory support.

MEASUREMENTS AND MAIN RESULTS

Patients were prospectively evaluated by one trained investigator using the revised Sedation-Agitation Scale (SAS) and were simultaneously monitored for 1 to 5 hrs using the Bispectral Index (BIS), a numeric scale from 0 to 100 derived from the electroencephalogram. BIS values were assigned to baseline, stimulated, and average conditions for each patient by a separate investigator blinded to SAS scores. Ventilator settings, medications, and the lung injury severity (LIS) score were also recorded. Sedation levels varied from very deep sedation (SAS score = 1, BIS score = 43) to mild agitation (SAS score = 5, BIS score = 100). Heavily sedated patients (SAS score = 1-2, n = 20) had higher FIO2 (0.52 vs. 0.42, p = .008), oxygenation index (9.4 vs. 5.4, p = .03), and LIS scores (1.3 vs. 0.7, p = .004) and lower baseline (66 vs. 78, p = .01), average (66 vs. 81, p < .001), and stimulated (89 vs. 96, p = .016) BIS scores compared with more awake patients. Patients with intermittent neuromuscular blockade use (n = 4) had higher FIO2 (0.65 vs. 0.44, p = .006), minute ventilation (14.6 vs. 9.9 L/min, p = .005), positive end-expiratory pressure (7.5 vs. 4.8 cm H2O, p = .05), oxygenation index (15.7 vs. 6.0, p < .001), and LIS scores (3.3 vs. 1.0, p = .036) and were more sedated, with higher suppression ratios (3.5 vs. 0.6, p = .05) and lower SAS scores (1.5 vs. 4, p = .035). The average BIS values correlated well with SAS (r2 = .21, p < .001).

CONCLUSIONS

SAS and BIS work well to describe the depth of sedation for ventilated ICU patients. Deeper sedation and intermittent neuromuscular blockade were used for patients with greater ventilatory requirements and more severe lung disease. The correlation between subjective and objective scales varied in medical, surgical, and trauma patients. Further research with SAS and BIS may facilitate the development of quantitative sedation guidelines for the ICU.