Influence of acute normovolaemic haemodilution on bispectral index monitoring and propofol dose requirements

Background: Numerous medical and physiological conditions that might alter electroencephalography (EEG), such as hypoglycaemia, hypothermia or hypovolaemia, were shown to result in the bispectral Index (BIS) indicating an incorrect hypnotic state. Recently, acute normovolaemic haemodilution (ANH) was shown to be associated with significant impairment of cognitive functions that could alter EEG and consequently BIS monitoring, an EEG derived parameter.
Methods: In a randomised clinical study, we assessed the effect of ANH on BIS monitoring before induction and after propofol target controlled infusion (TCI) anaesthesia in 45 unmedicated patients randomly allocated to ANH with oxygen insufflation (oxygen group), ANH with air insufflation (air group), or control group.
Results: With ANH, mean BIS values briefly declined in the oxygen group (82±4) and air group (84±3) before returning to baseline values. The loss of consciousness time was significantly shorter, with fewer propofol TCI dose requirements, and BIS was significantly higher in the oxygen group (1.3±0.5 min, 2.41±0.15 μg/ml, 73±7) and air group (1.2±0.6 min, 2.44±0.17 μg/ml, 75±5), compared with the control group (1.7±0.4 min, 2.75±0.17 μg/ml, 61±5), respectively. Whereas, there was no significant difference in BIS values between the oxygen group (38±7), air group (36±5) and control group (40±6) at propofol TCI 4 μg/ml anaesthesia maintenance.
Conclusions: BIS values briefly declined with ANH before returning to baseline values before anaesthesia induction. Despite transient ANH enhancement of propofol effect during induction, there was no significant difference in BIS values with or without ANH during propofol maintenance of anaesthesia.