To the Editor:—

Kreuer et al.  1reported advantages of the new electroencephalographic system the Narcotrend monitor (MonitorTechnik, Bad Bramstedt, Germany) and the Bispectral Index from the BIS® monitor (Aspect Medical Systems, Inc., Newton, MA) to consider faster emergence from anesthesia and a reduction of propofol–remifentanil consumption.

We want to congratulate Kreuer et al.  for the interesting and detailed results about monitoring of anesthesia. They indicate the possibility of using the Narcotrend monitor as an alternative to the BIS® monitor in this issue. A recent study underlines the limitations of the Bispectral Index. 2 

Nevertheless, we think the study design should be debated. The propofol infusion was adjusted during the surgical procedure and maintenance of anesthesia to stages assumed as deeper Narcotrend stages (D0), whereas anesthesia was reduced to lighter stages (C1) 15 min before the end of surgery. In general, it should be questioned why the depth of anesthesia should be lightened at the end of surgery. Kreuer et al.  reported that no patient had explicit intraoperative recall. The authors could not have known of the very recent results from Münte et al.  3with propofol–remifentanil anesthesia that were published at the same time. They reported about increasing implicit memory function in the absence of explicit memory function during Narcotrend stages C1to D2compared with deeper stages.

In our opinion, electroencephalographic monitoring during anesthesia is a useful tool to assess the drug effects, but electroencephalographic monitoring should not cause the risk of memory function resulting in potential awareness. 4Münte et al.  demonstrated that even in the absence of explicit memories, implicit memory may occur during light anesthesia as used by Kreuer et al.  Electroencephalographic monitoring should not become a risk for the patient because of aspired minimal depth of anesthesia to reduce the drug consumption.

Kreuer S, Biedler A, Larsen R, Altman S, Wilhelm W: Narcotrend monitoring allows faster emergence and a reduction of drug consumption in propofol–remifentanil anesthesia. A nesthesiology 2003; 99: 34–41
Messner M, Beese U, Romstöck J, Dinkel M, Tschaikowsky K: The Bispectral Index declines during neuromuscular block in fully awake persons. Anesth Analg 2003; 97: 488–91
Münte S, Münte TF, Grotkamp J, Haeseler G, Raymondos K, Piepenbrock S, Kraus G: Implicit memory varies as a function of hypnotic electroencephalogram stage in surgical patients. Anesth Analg 2003; 97: 132–8
Kalkman CJ, Drummond JC: Monitors of depth of anesthesia, quo vadis? A nesthesiology 2002; 96: 784–7