Investigation of changes in the middle latency auditory evoked potential during anesthesia with sevoflurane in dogs

Composite-, plain-auditory evoked potentials index and bispectral index to measure the effects of sevoflurane

The composite auditory evoked potentials index (cAAI) uses both cortical electroencephalogram (EEG) and response to auditory stimuli, while the bispectral index (BIS) uses only the cortical EEG and auditory evoked potentials index (AAI) uses only response to auditory stimuli. We expected that the cAAI was more useful to monitor anesthetic effect of sevoflurane than the BIS and AAI. The present study compared the changes of cAAI, AAI, and BIS in different sevoflurane concentration. Forty-five adult patients were anesthetized with sevoflurane in 50 % nitrous oxide. AAI (AEP version 1.4), cAAI (AEP version 1.6), and BIS (A-2000) were compared (each 15 patients in AAI, cAAI, and BIS groups) before induction, just before and after intubation, at 10 min since sevoflurane was set to 1.0, 1.5 and 2.0 %, and after extubation. All three indices decreased significantly before intubation. The cAAI was significantly higher than the AAI at sevoflurane 1.0 and 1.5 %. The AAI and BIS were significantly lower at sevoflurane 2.0 % than those at sevoflurane 1.0 %, but the cAAI did not. The cAAI had the largest and AAI had the smallest inter-individual variation. In sevoflurane-nitrous oxide anesthesia, cAAI was inferior to AAI and BIS to discriminate different anesthetic effect. The cAAI had larger inter-individual variation than the AAI and BIS.

General anesthesia changes gap-evoked auditory responses in guinea pigs

CONCLUSION

General anesthesia induced by sodium pentobarbital reduces temporal resolution as represented by an increase in the threshold of gap-evoked auditory responses in guinea pigs.

OBJECTIVES

To explore the potential impact of general anesthesia by sodium pentobarbital (PB), a common anesthetic used in animal research, on gap-evoked responses.

MATERIALS AND METHODS

The evoked potentials in response to gaps formed by bursts of broadband noise were recorded from electrodes implanted in the inferior colliculus (IC) and the auditory cortex (AC) of guinea pigs. The gap responses were compared in three conditions: unanesthetized and anesthetized with two doses of sodium pentobarbital (40 mg/kg and 20 mg/kg).

RESULTS

PB increased the gap response thresholds, especially when applied at the higher dose. The threshold shift induced by PB was greater in the AC than in the IC. In addition, the higher dose of PB significantly increased the gap response latency in both IC and AC, and decreased response amplitude in IC only.

Why we still use intravenous drugs as the basic regimen for neurosurgical anaesthesia

PURPOSE OF REVIEW

Evolution of neurosurgery mainly trends towards minimally invasive and functional procedures including endoscopies, small-size craniotomies, intraoperative imaging and stereotactic interventions. Consequently, new adjustments of anaesthesia should aim at providing brain relaxation, minimal interference with electrophysiological monitoring, rapid recovery, patients' cooperation during surgery and neuroprotection.

RECENT FINDINGS

In brain tumour patients undergoing craniotomy, propofol anaesthesia is associated with lower intracranial pressure and cerebral swelling than volatile anaesthesia. Hyperventilation used to improve brain relaxation may decrease jugular venous oxygen saturation below the critical threshold. It decreases the cerebral perfusion pressure in patients receiving sevoflurane, but not in those receiving propofol. The advantage of propofol over volatile agents has also been confirmed regarding interference with somatosensory, auditory and motor evoked potentials. Excellent and predictable recovery conditions as well as minimal postoperative side-effects make propofol particularly suitable in awake craniotomies. Finally, the potential neuroprotective effect of this drug could be mediated by its antioxidant properties which can play a role in apoptosis, ischaemia-reperfusion injury and inflammatory-induced neuronal damage.

SUMMARY

Although all the objectives of neurosurgical anaesthesia cannot be met by one single anaesthetic agent or technique, propofol-based intravenous anaesthesia appears as the first choice to challenge the evolution of neurosurgery in the third millennium.

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.