Correlation between clinical signs of depth of anaesthesia and cerebral state index responses in dogs during induction of anaesthesia with propofol.

The cerebral state index (CSI) is used for monitoring EEG and depth of anaesthesia. The objective of this study was to analyse the correlation between ocular reflexes, CSI and estimated propofol plasma concentrations (PropCP) in dogs during induction of anaesthesia with propofol. Fourteen dogs were premedicated with acepromazine 0.05 mg kg(-1) IM. Anaesthesia was induced with a 200 ml h(-1) propofol 1% constant infusion rate until loss of corneal reflex using RugLoop II software with Beths' pharmacokinetic model to estimate PropCp. Palpebral reflex (PR) and the corneal reflex (CR) were tested every 30s and classified as present (+) or absent (-), and eyeball position was registered as rotated ventromedialy (ERV) or centred (EC). Heart rate (HR), mean arterial pressure (MAP) and CSI values were analyzed from baseline before the beginning of propofol infusion (T0) until loss of CR; CSI and PropCp, CSI and anaesthetic planes, and PropCp and anaesthetic planes were compared using correlation analysis. PropCp reached 7.65+/-2.1 microg ml(-1) at the end of the study. CSI values at T0 were 89.2+/-3.8. Based on the observation of ocular reflexes and eyeball position, it was possible to define five anaesthetic planes: A (superficial) to E (deep), being A (PR+/CR+/EC), B (PR+/ERV/CR+), C (PR-/ERV/CR+), D (PR-/EC/CR+) and E (PR-/EC/CR-). There was a significant correlation between PropCp and the anaesthetic planes (R=0,861; P<0.01). No significant correlation was observed between CSI and the anaesthetic planes or between CSI and PropCp. MAP decreased significantly from T0 until loss of corneal reflex (from 98+/-14 mm Hg to 82+/-12 mm Hg); HR did not change significantly (from 101+/-30 bpm to 113+/-16 bpm). The CSI monitoring was not consistent with the clinical observations observed in the different stages of depth anaesthesia. This could limit the use of CSI for monitoring depth of anaesthesia with propofol.