Burst suppression-MAC and burst suppression-CP₅₀ as measures of cerebral effects of anaesthetics

Background: MAC (minimum alveolar concentration of an inhaled anaesthetic) and CP₅₀i (minimum plasma concentration of i.v. anaesthetics) are well-established measures to compare potencies of anaesthetics. The underlying clinical endpoint immobility reflects mainly effects of anaesthetics on the spinal cord, which limits the use of this measure for comparison of effects on the main target organ of general anaesthesia--the brain. The present study determines the median concentration of sevoflurane, isoflurane, and propofol that induce the onset of electroencephalogram (EEG) suppression ('silent second'): MACBS and CP₅₀BS.

Methods: Fifty-five unpremedicated patients (ASA physical status of I or II) undergoing elective surgery were randomly assigned to receive general anaesthesia with sevoflurane, isoflurane, or propofol. A two-channel EEG was continuously recorded to identify 'silent second'. Independent cross-over pairs were analysed using the 'Dixon's up-and-down' method, and MACBS/CP₅₀BS values were calculated by logistic regression.

Results: CP₅₀BS was 4.9 µg ml(-1) for propofol. MACBS was 2.9 vol% for sevoflurane and 1.5 vol% for isoflurane. CP₅₀BS of propofol was less than one-third of CP₅₀i, whereas MACBS of sevoflurane was >1.4-fold of MAC; MACBS of isoflurane was 1.3-fold of MAC.

Conclusions: Immobility and cerebral effects reflect different entities of anaesthetic action. The median concentration of anaesthetic drug (volatile or i.v. agent) required to induce 'silent second' might be a more useful metric than the median concentration required to prevent movement in response to a surgical stimulus in order to compare relative potencies of anaesthetic agents on the brain. Advantage of the 'silent second' is an easy identification of this endpoint, while such a deep level is not required for clinical anaesthesia.