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Inhaled anaesthetics to blunt arousal, haemodynamic responses to intubation after i.v. induction
- Benno Rehberg, Ingrid Rundshagen, Daniel Hadzidiakos, and Jan H. Baars (15 December 2006)
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Benno Rehberg Dept. of Anaesthesiology, Charité-Universitaetsmedizin Berlin, CCM, Ingrid Rundshagen, Daniel Hadzidiakos, and Jan H. Baars
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Dear Editor, With great interest we have read the recent publication by Coppens et al. (1) entitled “Do we need inhaled anaesthetics to blunt arousal, haemodynamic responses to intubation after i.v. induction with propofol, remifentanil, rocuronium?” in which the authors studied the addition of desflurane and sevoflurane during rapid sequence induction with propofol, remifentanil and rocuronium to blunt hemodynamic responses. They conclude that neither sevoflurane nor desfluran are able to do so. We would like to propose a possible explanation for this result, which has not been mentioned in the discussion of the paper. Even with the overpressure technique, there is a time delay for any volatile anaesthetic to equilibrate the alveolar concentration with that at the molecular structures where the anaesthetic effects take place. Although concentrations at these structures cannot be measured, they can be estimated by observing the effect and introducing a hypothetical “effect compartment” in the pharmacokinetic-pharmacodynamic model describing the relation between concentration, time and effect (the authors actually reported effect compartment concentrations for propofol in their study). The time delay between a change in the alveolar concentration and the observed effect is commonly described in terms of an equilibration constant ke0 or the corresponding time constant t½ke0. For sevoflurane, this time constant is approximately 2.4 minutes when the effect is measured using the spectral edge frequency of the EEG, the comparable value for desflurane is 1.1 minutes (2) . Since the t½ke0 will include not only physiological, but also computational delays, the time constant may overestimate the true delay of concentration equilibration. However, when using the cardiocirculatory response to intubation as the endpoint, the physiologic delay may be even longer as the one observed when measuring EEG effects. For propofol at least, the t½ke0 –values have been found to be much larger for cardiocurculatory responses than for EEG effects (3). Thus, when applying a concentration of 1 MAC of sevoflurane at loss of consciousness, the concentration in the effect compartment 90 seconds later (the time of intubation in the study of Coppens et al.) can be calculated using the equation by which the time constant ke0 is defined: dCeff/dt= (Cet-Ceff)*keo Using the EEG-derived ke0, this calculation would yield an effect compartment concentration of less than 0.6 vol% sevoflurane and approximately 3.6 vol% desflurane at time of intubation, if a concentration of 1 MAC of each drug was present as end-tidal concentration for 90 seconds. Since concentrations of volatile anaesthetics needed to blunt the hemodynamic response to a painful stimulus are even higher as the MAC-values measured by the movement response (4), these concentrations are unlikely to obliterate the cardiovascular effect of intubation. Table of References 1. Coppens MJ, Versichelen LF, Mortier EP, Struys MM. Do we need inhaled anaesthetics to blunt arousal, haemodynamic responses to intubation after i.v. induction with propofol, remifentanil, rocuronium? Br J Anaesth 2006; 97: 835-41 2. Rehberg B, Bouillon T, Zinserling J, Hoeft A. Comparative pharmacodynamic modeling of the electroencephalography-slowing effect of isoflurane, sevoflurane, and desflurane. Anesthesiology 1999; 91: 397-405 3. Kazama T, Ikeda K, Morita K, et al. Comparison of the effect-site k(eO)s of propofol for blood pressure and EEG bispectral index in elderly and younger patients. Anesthesiology 1999; 90: 1517-27 4. Roizen MF, Horrigan RW, Frazer BM. Anesthetic doses blocking adrenergic (stress) and cardiovascular responses to incision--MAC BAR. Anesthesiology 1981; 54: 390-8 Conflict of Interest:None declared |
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