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The influence of brain tumours in BIS and in propofol predicted effect-site concentrations at loss of consciousness

 Ferreira DA, Nunes CS, Antunes LM, Lobo F, Santos IA, Casal M, Amorim P

Serviço de Anestesiologia, Hospital Geral de Santo António, Porto, Portugal; CECAV UTAD- Vila Real, Portugal 

Introduction

TCI pharmacokinetic models are widely used to predict drug concentrations in anesthetic procedures. The influence of cerebral tumours in loss of consciousness (LOC) and its correlation with the propofol predicted effect concentrations (PropCe) and BIS in the same moment is addressed in this study.

Methods

GROUP 1: Patients scheduled for spinal surgeries, Glasgow 15, ASA 1/3. GROUP 2: Patients with cerebral tumours scheduled for surgery, Glasgow 15, ASA 1/3. Induction of anaesthesia in both groups was with a propofol constant infusion rate of 200 ml hr^-1 until LOC. Rugloop II^â TCI software using Schnider¹ model was used to predict propofol effect-site concentrations and to collect data from a Datex AS3 (heart rate (HR) and non invasive mean arterial pressure (MAP)) and from an Aspect A2000XP (BIS) monitors. All patients were premedicated with 10 mg of diazepam per os 2 hours before the induction of anaesthesia. LOC was defined as loss of eye opening in response to a tap in the forehead and name calling. Data between both groups were analyzed at LOC using Student t-Test. Correlation analysis was performed between BIS and PropCe at LOC, in both groups. Data is expressed in mean±SD.

Results

 GROUP 1: 20 patients, 47±11 years (20-72), 66.3±13.1 Kg, 164±9 cm, 12 females. Before the beginning of propofol infusion (T0), BIS was 94.6±5.1, HR was 70±17 bpm and MAP was 103±13 mmHg. Time since the beginning of propofol infusion and LOC was 3.62±0.9 minutes. GROUP 2: 20 patients, 54±12 years (29-74), 70.9±10.3 Kg, 165±9 cm, 11 females. At T0, BIS was 93±5.4, HR was 71±14 bpm and MAP was 107±11 mmHg. Time since the beginning of propofol infusion and LOC was 3.61±0.7 minutes.

Table 1- G1- Patients without brain tumours; G2- Patients with brain tumours. Propofol predicted effect-site concentrations (PropCe; µg ml^-1), BIS, heart rate (HR; bpm) and mean arterial pressure (MAP; mmHg) at loss of consciousness (LOC). Data are expressed as mean±sd. *Statistically significant difference (P<0.05) between groups.

BIS was significantly higher (p=0.03) at LOC in patients with brain tumours. There was no correlation between BIS at LOC and PropCe at LOC in any of the two groups.

Discussion

Due to the cerebral depression usually associated to patients with brain tumours, one would expect that these patients had LOC at higher BIS values and lower propofol predicted effect-site concentrations than patients without brain tumours. In fact, LOC in patients with brain tumours occurred at BIS values significantly higher than those observed with patients without brain tumours, but with similar propofol predicted effect-site concentrations and similar time to LOC. It is possible that the increased intracranial pressure usually associated to brain tumours combined with the decrease in arterial pressure during induction of anaesthesia, may lead to a reduction in the cerebral blood flow and propofol delivery to the brain.

Acknowledgements

The authors wish to acknowledge the Portuguese Foundation for Science and Technology (Lisbon, Portugal) for their financial support under project POSC/EEA-SRI/57607/2004.

References

1.       Anesthesiology 1999, 90:1502-16.