Suppressive actions of volatile anaesthetics on the response capability in cats

Suppression of response to a given stimulus by anaesthetics might be considered as a summation of the suppression of basal (pre-stimulus) activity and response capability (increased by stimulus). Anaesthetic suppression of each component in brain and cardiovascular variables by halothane, isoflurane or sevoflurane was compared in cats.

Thirty cats were allocated to one of three groups (n = 10 in each) according to the anaesthetic given. The sciatic nerve was stimulated after maintaining the end-tidal concentration of the anaesthetic at 1.3 or 2.0 MAC for at least 30 min. Cortical electroencephalogram (EEG), multi-unit activity in the mid-brain reticular formation (R-MUA), mean arterial pressure (MAP) and heart rate (HR) were measured before and after electrical sciatic nerve stimulation.

The EEG patterns and R-MUA indicated greater suppression of activity in the brain by isoflurane (31 +/- 4% of awake state at 1.3 MAC, mean +/- SEM) and sevoflurane (38 +/- 5%) than by halothane (61 +/- 5%, P < 0.05), before stimulation. The R-MUA following the stimulation was not different among agents. The MAP and HR were not different among groups before stimulation, but following stimulation were greater in the sevoflurane group (137 +/- 9 and 103 +/- 9 mmHg at 1.3 and 2.0 MAC) than in the halothane group (103 +/- 5 and 76 +/- 3 mmHg, P < 0.05).

Isoflurane and sevoflurane have greater suppressive action on the basal CNS activity than halothane at the same MAC, and that these two anaesthetics have a weak suppressive action on the response capability to peripheral stimulation.