To the Editor:-In the recent article by Johansen et al.  describing the reduction of anesthetic requirements by esmolol, nitrous oxide is probably the primary anesthetic agent. It probably is the agent effecting the greatest contribution to the minimum alveolar concentration, or its equivalent, for intravenous anesthetic agents. This premise is supported by the reported propofol Cp50of 3.85 micro gram/ml, with nitrous oxide compared to a Cp50of 15.2 micro gram/ml for propofol as the sole anesthetic agent for skin incision in tracheally intubated patients.  Although the patient groups in these two studies are not directly comparable, the absence of nitrous oxide results in an approximately 300% increase in the Cp50for propofol.
If nitrous oxide is the primary anesthetic agent, then esmolol may affect its anesthetic action by inhibiting the sympathomimetic action of nitrous oxide. This increased sympathetic activity during nitrous oxide anesthesia has been found to antagonize both central nervous depression by isoflurane and isoflurane-induced suppression of learning. [3,4] Any augmentation of the potency of nitrous oxide by the sympatholytic effects of esmolol would explain the reduction of anesthetic requirements in the study of Johansen et al. in humans.  Perel et al in the rat,  and the efficacy of esmolol as a narcotic substitute in previous studies. [6,7]
Johansen et al. give no details of the temporal events during induction of their patients. If esmolol does not effect its anesthetic action via nitrous oxide, did the authors notice a difference in the time to loss of consciousness between the propofol only, and the propofol plus esmolol groups, in the absence of nitrous oxide?
Dmitry Baranov, M.D.
Instructor of Anesthesiology
Roger J. Bagshaw, M.D.
Associate Professor of Anesthesiology; Department of Anesthesia; Hospital of the University of Pennsylvania; 3400 Spruce Street; Philadelphia, Pennsylvania 19104–4283
(Accepted for publication April 26, 1997.)