EDITOR:

We thank Dr McCahon and colleagues for their interest in our study [Reference Takizawa, Takizawa and Miyoshi1]. Dr McCahon enquires whether the effect of ephedrine on bispectral index scores (BIS) values were clinically significant or not. In this study, we examined whether ephedrine increased BIS values during propofol anaesthesia as it does during sevoflurane anaesthesia [Reference Ishiyama, Oguchi, Iijima, Matsukawa, Kashimoto and Kumazawa2] and whether ephedrine decreased propofol concentrations.

Johnson and colleagues [Reference Johnson, Andrzejowski and Sikiotis3] reported arousal following isoprenaline administration during propofol anaesthesia. They also examined the effect of intravenous epinephrine on BIS and sedation, reporting that mean BIS values increased from 63 to 76 and exogenous catecholamines seemed to display an arousal effect [Reference Andrzejowski, Sleigh, Johnson and Sikiotis4]. This could be due to changes in neurotransmitter levels in the brain. The adrenergic system has a role to play in the process of arousal from anaesthesia, and this has been previously demonstrated [Reference Berridge and Foote5]. β-receptors in the reticular-activating system interact with the information processing in the thalamus. However, another possible explanation that propofol concentration decreased due to increased cardiac output on administration of a catecholamine had not been examined. In our previous study, we showed decreases in propofol concentration with increased cardiac output following dopamine administration [Reference Takizawa, Nishikawa and Sato6].

Ephedrine exerts a potent stimulating effect on the central nervous system [Reference Malec and Langwinski7]. Ishiyama and colleagues [Reference Ishiyama, Oguchi, Iijima, Matsukawa, Kashimoto and Kumazawa2] reported that ephedrine increased BIS during sevoflurane anaesthesia. We examined the effect of ephedrine on propofol concentrations and on BIS. Our study showed that ephedrine increased BIS to >60 in eight of 20 patients without changing the propofol concentrations. In our study, post-operative interviews revealed that there were no patients who had any recall during the surgical procedure. No other study reports arousal following ephedrine administration. As suggested by McCahon and colleagues, the clinical significance in the effect of ephedrine on BIS is not clear from our study. Further work on anaesthetic depth is required concerning the clinical significance of the effect of drugs that act on the central nervous system.