Narcotrend-assisted propofol/remifentanil anaesthesia for prevention of awareness
Bury St Edmonds, UK
E-mail: oswald20577{at}yahoo.co.uk
Editor—I read with interest about the comparison of the use of Narcotrend and clinical assessment in judging the depth of anaesthesia while using total i.v. anaesthesia (TIVA).1 I fully agree with the authors that the use of clinical assessment alone would lead to a greater deviation from a defined target while running TIVA. The Narcotrend, a computer-based EEG programme, is easy to use and has low running costs, besides in terms of prediction probability, the performance of the Narcotrend index and the bispectral index to predict propofol effect-site concentrations was comparable. I recently conducted a small study to evaluate the number of cases who developed awareness under anaesthesia, out of the 25 anaesthetists who replied, five had cases of awareness under anaesthesia, and out of these five, four were under TIVA. The most significant of these cases was a 52-yr-old lady having an abdominal hysterectomy, anaesthetized by myself using target-controlled infusion (TCI) with propofol and remifentanil. Despite using a 1 µg kg–1 bolus of remifentanil followed by an infusion of 0.2 µg kg–1 h–1and running propofol at a target-controlled level of 6 µg ml–1, clinically evaluating that the patient is asleep such as loss of eyelash reflex, at induction, fall in arterial pressure and heart rate, during maintenance of anaesthesia, the patient complained about awareness under anaesthesia for a short while after induction. Since awareness under anaesthesia is indeed a serious issue, I would indeed welcome the use of Narcotrend if indeed it could decrease the incidence of awareness, and decrease incidence of nausea and vomiting as stated by the authors.
Berlin, Germany
E-mail: ingrid.rundshagen{at}charite.de
Editor—I thank Dr D'Mello for his interesting comment. With respect to the case of awareness he mentioned, one may speculate that the patient experienced a period of wakefulness shortly after induction because propofol infusion was stopped due to the automatic lockout interval of the TCI device. This lockout mechanism is automatically activated, when the plasma target concentration is reduced. As it was mentioned, propofol anaesthesia was induced rapidly by achieving a plasma target of 6 µg ml–1. It is known from the literature that this results in adequate hypnosis after about 60 s. I guess a lower target plasma concentration was used after intubation; thereby, the automatic lockout interval was activated.
We agree with Dr D'Mello that most probably measuring the cerebral pharmacodynamics of propofol by the Narcotrend (or even any other device using the electroencephalogram) would have resulted in indicating a low level of hypnosis during this time. However, it is known that especially during the transition from consciousness to unconsciousness, the range of the Narcotrend level differs markedly inter-individually.2 Therefore, it is difficult to define a threshold level differentiating the conscious and the unconscious state precisely in the individual patient. This holds true for all the other monitors of ‘depth of anaesthesia’, too.3 Therefore, it remains a challenge to clearly identify periods of wakefulness during general anaesthesia. However, I am sure using an EEG monitor, using TCI devices which include the effect-site concentration rather than the plasma concentration and careful clinical observation of the patient together will help to avoid periods of awareness.4
References
1 Rundshagen I, Hardt T, Cortina K, Pragst F, Fritzsche T, Spies C. Narcotrend-assisted propofol/remifentanil anaesthesia vs clinical practice: does it make a difference? Br J Anaesth (2007) 99:686–93.
2 Schneider G, Kochs EF, Horn B, Kreuzer M, Ningler M. Narcotrend does not adequately detect the transition between awareness and unconsciousness in surgical patients. Anaesthesiology (2004) 101:1105–11.[CrossRef][Web of Science][Medline]
3 Bruhn J, Myles PS, Sneyd R, Struys MMRF. Depth of anaesthesia monitoring: what's available, what's validated and what's next? Br J Anaesth (2006) 97:85–94.
4 Triem JG, Röhm KD, Boldt J, Piper SN. Comparison of a propfol-based anaesthesia regimen using optimated-target-controlled-infusion (OTCI) and manually-controlled infusion (MCI) technique. Anaesthesiol Intensivmed Notfallmed Schmerzther (2006) 41:150–5.[CrossRef]
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