Propofol EC50, BIS and AEPI at induction of anaesthesia

A.M. Troy, S.E. Milne , M.G. Irwin, G.N.C. Kenny

University Department of Anaesthesia, Royal Infirmary, Glasgow, G31 2ER, UK

Background and Goal of study: The EC₅₀ is the blood concentration of an anaesthetic at which no reflex movement occurs following incision in 50% of patients. Blood EC₅₀ for propofol has been determined¹. We have measured effect site EC₅₀ for propofol along with the Bispectral Index (BIS) and Auditory Evoked Potential Index (AEPI)².

Materials and Methods: Following ethics committee approval and written informed consent, 40 ASA I or II non-premedicated patients were enrolled in the study. Anaesthesia was induced with a target controlled infusion of propofol starting at 2.5 mg/ml and increasing by 0.5 mg/ml every 30 seconds until: 1) loss of response to verbal command, 2) loss of eyelash reflex and 3) no purposeful movement elicited to a 50 Hz tetanic stimulus. Haemodynamic data, calculated blood and effect site propofol concentration, BIS and AEPI were recorded before induction, and at each endpoint.

Results and Discussion: Loss of verbal response and eyelash movement occurred simultaneously in 37.5% of patients.

	Awake	Loss of verbal response	Loss of eyelash reflex	No movement
Mean BP (mmHg)	95.6 (15.6)	85.2 (12.5)	84.5 (11.8)	82.5 (11.0)
Blood prop (ug/ml)	_	5.3 (1.2)	5.4 (1.4)	8.2 (2.2)
Effect prop (ug/ml)	_	2.7 (0.8)	2.8 (0.8)	5.2 (1.2)
BIS	96.6(2.6)	66.8(14.0)	63.7(15.4)	37.0(9.2)
AEPI	70.3(11.8)	53.2(8.6)	51.3(9.5)	39.9(10.7)

Values are mean (SD)

The mean blood pressure decreased after induction of anaesthesia, but there was no change in heart rate or oxygen saturation. BIS and AEPI decreased after induction of anaesthesia and between loss of consciousness and no movement (p<0.001). Blood and effect site propofol concentrations were higher at no movement compared with loss of consciousness (p<0.001). The effect site EC₅₀ concentrations for loss of eyelash reflex and no movement were 2.8 and 5.0 mg/ml respectively.

Conclusion: BIS and AEPI monitors may be useful in the monitoring of adequate depth of anaesthesia. The site of action of anaesthetic drugs is the brain and estimation of the effect site EC₅₀ of propofol and its relationship with the BIS and the AEPI should allow anaesthetists to administer propofol more effectively.

References:

1. Davidson JAH, Macleod AD, Howie JC, et al. Acta. Anaes. Scand. 1993;37:458-464.

2. Mantzaridis H, Kenny GNC. Anaes. 1997;52:1030-36.