Acta Anaesthesiol Scand 2001; 45: 1006–1010 Copyright C Acta Anaesthesiol Scand 2001 Printed in Denmark. All rights reserved ACTA ANAESTHESIOLOGICA SCANDINAVICA ISSN 0001-5172 Dose–response relationship of propofol on mid-latency auditory evoked potentials (MLAEP) in cardiac surgery S. PALM 1 , U. LINSTEDT 1 , A. PETRY 2 and H. WULF 3 1 Clinic for Anaesthesiology and Operative Intensive Care, Christian-Albrechts-University Kiel, 2 Anaesthesia and Intensive Care II, Heart Center Leipzig, 3 Clinic for Anaesthesiology and Intensive Care, Philipps-University Marburg, Germany Background: Propofol-sufentanil anaesthesia has become popu- lar during cardiac surgery for its titrability and short recovery time. Avoidance of awareness is a major goal during cardiac surgery. We therefore investigated the dose–response relation- ship of propofol and cortical responses (mid-latency auditory evoked potentials, MLAEP). Methods: One hundred patients undergoing cardiac surgery were investigated. Basic anaesthesia was performed with sufen- tanil (4.5 mg kg ª1 h ª1 ) / flunitrazepam (9 mg kg ª1 h ª1 ) infusion (control group); the other groups received in addition a loading dose of propofol 2 mg kg ª1 and a maintainance infusion of 1– 3.5 mg kg ª1 h ª1 . MLAEP were evaluated by using Pa/Nb-am- plitudes and Nb-latencies. Haemodynamics were monitored by ECG, arterial blood pressure and cardiac function with pulmon- ary artery catheterization. Results: In the control group, median amplitude of MLAEP de- creased by 50% with a wide range, but were detectable in 90% of patients throughout surgery. Propofol suppressed amplitude Pa/Nb of MLAEP dose dependently. With 3.5 mg kg ª1 h ª1 am- plitudes disappeared in 40% of patients throughout cardiac S INCE THE beginning of the 1990s hypnotics, such as propofol, have been increasingly used in addition to high-dose opioids in cardiac surgery (1, 2). These were introduced in order to shorten anaesthesia re- covery time. High-dose opioids supplemented with benzodiazepines were advocated in the past because of haemodynamic stability during cardiac surgery, even in cases with severely impaired cardiac function (3–6). Side effects of such a regime included hyperten- sive breakthrough, tachycardia and awareness (2). Awareness with recall during general anaesthesia is rare (0.2–0.7%) but troublesome. For cardiac surgery an incidence up to 4% has been reported (7–10). The frequency of such events depends on the anaesthetic technique used. High-dose opioid analgesia supple- mented with benzodiazepines has a higher incidence of awareness than the use of anaesthetics such as iso- flurane and propofol. Unfortunately, these anaes- thetics possess substantial cardiodepressive side 1006 surgery. Median Nb-latencies increased in the control group from 44 to a range from 50 to 60 ms. In groups with propofol 2 mg kg ª1 h ª1 , Nb-latencies, detectable in the patients without complete suppression of MLAEP, increased to median 60 ms. Haemodynamic parameters and cardiac function did not differ among the groups. The use of vasopressors was not increased even with the highest propofol dose used. Conclusion: The dose–response effect of propofol on auditory evoked potentials reveals that combining a loading dose of 2 mg kg ª1 with a consecutive infusion of 3.5 mg kg ª1 h ª1 significant- ly suppresses MLAEP during cardiac surgery. Thus, auditory information may not be processed and awareness with recall becomes unlikely. Received 24 November 2000, accepted for publication 5 May 2001 Key words: Propofol; auditory evoked potentials; awareness; dose–response relation; cardiovascular surgery. c Acta Anaesthesiologica Scandinavica 45 (2001) effects in patients with impaired myocardial func- tion. As intraoperative acoustic perception is of import- ance for postoperative recall, monitoring of mid-la- tency auditory evoked potentials (MLAEP) has been used to investigate the probability of awareness (11, 12). Following auditory stimuli, electrical cerebral ac- tivity in the range from 10 to 100 ms is less influenced by the application of opioids and benzodiazepines, whereas anaesthetics like isoflurane or propofol dose- dependently diminish MLAEP (11, 13, 14). Schwender et al. state that when MLAEP are preserved during general anaesthesia, auditory information may be processed and remembered postoperatively (15). Only in patients without detectable MLAEP were there no cases of explicit or implicit recall at all. Since explicit recall is a rare event and implicit recall testing is dif- ficult to perform, we used the criterion of lost MLAEP assuming lack of awareness. It was thus the aim of