From individual to population: the minimum alveolar concentration curve

End-tidal Anesthetic Concentration: Monitoring, Interpretation, and Clinical Application

Age-adjusted fraction of minimum alveolar concentration derived from end-tidal anesthetic partial pressure measurement remains a useful drug advisory display to help prevent awareness if interpreted with proper understanding of the quantal and probabilistic nature of minimum alveolar concentration, semantics, drug interactions, and hysteresis.

Resistance to state transitions in responsiveness is differentially modulated by different volatile anaesthetics in male mice

BACKGROUND

Recent studies point to a fundamental distinction between population-based and individual-based anaesthetic pharmacology. At the population level, anaesthetic potency is defined as the relationship between drug concentration and the likelihood of response to a stimulus. At the individual level, even when the anaesthetic concentration is held constant, fluctuations between the responsive and unresponsive states are observed. Notably, these spontaneous fluctuations exhibit resistance to state transitions R_st. Therefore, the response probability in each individual depends not just upon the drug concentration, but also upon responses to previous stimuli. Here, we hypothesise that R_st is distinct from drug potency and is differentially modulated by different anaesthetics.

METHODS

Adult (14-24 weeks old) C57BL/6J male mice (n=60) were subjected to repeated righting reflex (RR) assays at equipotent steady-state concentrations of isoflurane (0.6 vol%), sevoflurane (1.0 vol%), and halothane (0.4 vol%).

RESULTS

Fluctuations in RR were observed for all tested anaesthetics. Analysis of these fluctuations revealed that R_st was differentially modulated by different anaesthetics (F[2, 56.01]=49.59; P<0.0001). Fluctuations in RR were modelled using a stochastic dynamical system. This analysis confirmed that the amount of noise that drives behavioural state transitions depends on the anaesthetic agent (F[2, 42.86]=16.72; P<0.0001).

CONCLUSIONS

Whilst equipotent doses of distinct anaesthetics produce comparable population response probabilities, they engage dramatically different dynamics in each individual animal. This manifests as a differential aggregate propensity to exhibit state transitions. Thus, resistance to state transitions is a fundamentally distinct, novel measure of individualised anaesthetic pharmacology.

Sevoflurane-induced isoelectric EEG and burst suppression: differential and antagonistic effect of added nitrous oxide

The objective of this study was to investigate whether nitrous oxide influenced the ED50 of sevoflurane for induction of isoelectric electroencephalogram (ED50_isoelectric ) differently from its influence on the ED50 of sevoflurane for electroencephalogram burst suppression (ED50_burst ). In a prospective, randomised, double-blind, parallel group, up-down sequential allocation study, 77 ASA physical status 1 and 2 patients received sevoflurane induction and, after tracheal intubation, were randomly allocated to receive sevoflurane with either 40% oxygen in air (control group) or 60% nitrous oxide in oxygen mixture (nitrous group). The ED50_isoelectric in the two groups was determined using Dixon's up and down method, starting at 2.5% with 0.2% step size of end-tidal sevoflurane. The electroencephalogram was considered as isoelectric when a burst suppression ratio of 100% lasted > 1 min. The subsequent concentrations of sevoflurane administered were determined by the presence or absence of isoelectric electroencephalogram in the previous patient in the same group. The ED50_isoelectric in the nitrous group 4.08 (95%CI, 3.95-4.38)% was significantly higher than that in the control group 3.68 (95%CI, 3.50-3.78)% (p < 0.0001). The values for ED50_burst were 3.05 (95%CI, 2.66-3.90)% and 3.02 (95%CI, 3.00-3.05)% in nitrous group and control group, respectively (p = 0.52). The addition of 60% nitrous oxide increases ED50_isoelectric , but not the ED50_burst of sevoflurane. Neither result indicates an additive effect of anaesthetic agents, as might be expected, and possible reasons for this are discussed.