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Rev. Bras. Anestesiol. vol.57 no.4 Campinas July/Aug. 2007
LETTER TO THE EDITOR
To the Editor,
The considerations made demonstrate careful reading of the study. We would like to thank the authors of the letter for the observations regarding BIS parameters, an important feedback from someone who feels comfortable in the arid field of electronic engineering. We also would like to thank the opportunity to discuss the study once more.
Regarding the statement that the results are not valid due to the concomitant use of neuromuscular blockers (NMB), we would like to remind you that the pharmacodynamics of cisatracurium have been studied extensively and are well characterized in the literature. After 25 minutes, using doses of up 4ED 95%, even in patients with liver disease 1, patients had a satisfactory recuperation pattern, with an index T4/T1 > 0.7. In the study mentioned here, as explained in the results, the pneumoperitoneum was performed 36.6 ± 11.2 minutes after induction, i.e., after the administration of the muscle relaxant. As described, the administration of nitrous oxide was initiated only 5 minutes after the end of the insufflation of the pneumoperitoneum, respecting the 10-minute interval to allow the stabilization between inspired and expired concentrations, and only then the data was gathered. The muscle relaxant was not repeated in this period; therefore, the data was gathered at the time of the pneumoperitoneum, when the neuromuscular blockade was not so severe. In the discussion, we described the possible interference in BIS values by deep neuromuscular blockade with improper hypnosis. That is, if it was discussed and we were aware of it, how could we have not thought of this possibility?
Regarding BIS values, we would like to explain that it was not maintained at 60, but between 40-60; therefore, 60 was the trigger to change the inspired fraction of sevoflurane. Besides, expired concentrations of sevoflurane above 1.4% 2 are described as hypnotics by themselves, without the addition of nitrous oxide. The lower inspired concentration of sevoflurane was of 1.3% when associated with 60% nitrous oxide, which is known to increase the effects of the halogenated drug to the point that the risk of waking up or even implicit memory is, at best, remote. And, although it was not mentioned in the results, because it is routine in our institution, every patient was followed closely during postanesthetic recovery, and none mentioned having intraoperative memories, and every patient was discharged in the first postoperative day without any complaints or intercurrences.
The analgesic action of nitrous oxide is unquestionable, and has been demonstrated by other monitoring methods 3 in clinical 4 and experimental 5 trials. The complementary information provided by our study is that it is safe to be used during the pneumoperitoneum.
It seems superfluous to mention, but we should emphasize that in our Institution we follow rigorously the requirements of CNS 196/96 resolution and its complements, and we only start a study after the approval by the Ethics Committee in Research, and after the patient has signed the informed consent, which was mentioned in the first paragraph of Methods. BIS is a monitor that has been registered at ANVISA for many years, and it can be easily purchase by anyone. Therefore, it is not an experimental device that deserves special attention from the Ethics Committee when it evaluates a protocol that uses this device. And I would like to mention one should be very careful when making this type of statement, questioning the ethics of a scientific protocol, because it concerns not only the honesty of the researchers and the Institution, but also the Ethics Committee of the Institution (Universidade Federal do Ceará), whose work is based on unquestionable integrity and respect.
Regarding the placement of the electrodes, we understand that it is public knowledge, even among those who do not use BIS regularly, that the brand of electrodes used in the study (BIS Sensor Xp, Aspect Medical System, USA) are sold as a set and, therefore, it would be impossible to gather data without the electrode located between the external angle of the eye and hair line. But, nonetheless, we would like to thank the authors for the observation.
We would like to thank you once more for the opportunity to discuss this subject once more and clear any doubts you might still have.
Lenilson Marinho Souza Filho
Josenília Maria Alves Gomes, TSA
Erick Leite Messias
Rodrigo Dornfeld Escalante
01. DeWolf AM, Freeman JA, Scott VL et al. Pharmacokinetics and phamacodynamics of cisatracurium in patients with end-stage liver disease undergoing liver transplantation. Brit J Anaesth, 1996;76:624-628.
02. Ekman A, Brundin L, Sandin R A comparison of bispectral index and rapidly extracted auditory evoked potentials index responses to noxious stimulation during sevoflurane anesthesia. Anesth Analg, 2004;99:1141-1146.
03. Hans P, Dewandre PY, Brichant JF et al. Effects of nitrous oxide on spectral entropy of the EEG during surgery under balanced anaesthesia with sufentanil and sevoflurane. Acta Anaesthesiol Belg, 2005;56:37-43.
04. Manikandan R, Srirangan SJ, Brown SC et al. Nitrous oxide vs periprostatic nerve block with 1% lidocaine during transrectal ultrasound guided biopsy of the prostate: a prospective, randomized, controlled trial. J Urology, 2003;170:1881-1883.
05. Richebe P, Rivat C, Creton C et al. Nitrous oxide revisited: evidence for potent antihyperalgesic properties. Anesthesiology, 2005;103:845-854.