The effects of cognitive impairment on anaesthetic requirement in the elderly

Abstract

Context: Patients with dementia have a lower bispectral index score (BIS) when awake than age-matched healthy controls.

Objectives: The primary aim was to compare the BIS and the dose of propofol required for induction in patients suffering from cognitive impairment with that in those who had normal cognitive function. This study also evaluated the effects of cognitive impairment in the elderly on anaesthetic agent consumption during surgery and on emergence from anaesthesia.

Design and setting: This randomised controlled study was carried out in a university hospital. Patients over 65 years of age, ASA I-II and scheduled for elective orthopaedic procedures were allocated to one of two groups.

Interventions: Patients (n = 92) were allocated according to their Mini Mental State Examination score: 25 or higher (group 1) or 21 or less (group 2). All patients received propofol 0.5 mg kg(-1) following the commencement of a remifentanil infusion at 0.5 μg kg(-1) min(-1). After incremental doses of propofol up to loss of consciousness, a propofol infusion was started at 75 μg kg(-1) min(-1). Propofol and remifentanil infusion doses were adjusted to keep the BIS value between 45 and 60 during surgery.

Main outcome measure: MMSE score was evaluated 24 h before and after surgery. The anaesthetic consumption, mean arterial pressure, HR and BIS values of the patients were recorded.

Results: Before surgery, mean Mini Mental State Examination scores were 26.8 ± 1.6 and 16.6 ± 4.2 in group 1 and 2, respectively. These returned to baseline value 24 h after surgery in group 1 (26.6 ± 1.5) and group 2 (15.6 ± 4.3). Before induction, four of 45 patients (8.9%) in group 1 had a BIS value less than 93 compared with 13 of 47 (27.7%) in group 2 (P = 0.02). The mean BIS value was significantly lower in group 2 than in group 1 before induction, during loss of consciousness, 3 and 5 min after discontinuation of the anaesthetic agents and before extubation (P < 0.05). The induction dose of propofol was lower in group 2 than in group 1 (P = 0.02). The eye opening time was significantly longer in group 2 than in group 1 (P = 0.03).

Conclusion: The baseline BIS value was lower in patients with cognitive impairment than in those with normal cognitive function. The former received less propofol during induction and eye opening time was longer. On the basis of our findings from the recovery period, we suggest that the recommended target BIS value for adequate anaesthesia in the general population is inappropriate for patients with cognitive impairment.