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Rev. Bras. Anestesiol. vol.57 no.4 Campinas July/Aug. 2007
LETTER TO THE EDITOR
Consequences of the addition of nitrous oxide to anesthesia during pneumoperitoneum in laparoscopic surgeries
To the Editor,
I read carefully the study published in volume 57, number 1, January 2007: "Consequences of the Addition of Nitrous Oxide to Anesthesia during Pneumoperitoneum in Laparoscopic Surgeries" and I would like to make some considerations.
When one uses a new technology based on stochastic signals1, such as the electroencephalogram (EEG) and electromyography (EMG), that cannot be separated because their frequency and amplitude are similar, one faces a serious signal validation problem when using neuromuscular blockers (NMB) in doses higher than 1ED 95%. It is even worse when one uses muscle relaxants without monitoring of the neuromuscular blockade and considers that the bispectral index is within the ranges specified by the manufacturer. There are several reports on the literature demonstrating that NMB change BIS values 2-4.
Besides, we presented a study at the 52nd Brazilian Congress of Anesthesiology in which neuromuscular blockers were used in 20 volunteers, without any type of sedation, and both BIS and state and response entropies reached values considered compatible with surgical anesthesia, and BIS values returned to values statistically similar to basal levels when T1 was above 1.9% in the recovery phase. Therefore, we believe that studies involving such monitoring without observing this limiting factor, invalidates any results obtained, especially with the dose of cisatracurium used (3ED 95%) 5. The author herself mentioned that profound neuromuscular blockade with improper hypnosis is one of the main causes of altered BIS, which invalidates the results achieved.
Thus, it would be interesting that studies involving new technologies were better evaluated and, perhaps, approved only after it is approved by the ethics commission of the institution, since they can even harm the patient. This happens because BIS can vary from 60, in reality, representing the use of NMB, to higher values, which, besides the syndrome of posttraumatic stress, can affect implicit and explicitly memory 6. However, I realize that a few errors in method or discussion may happen, and they should not compromise the results reported, which is the case of: PEEP 5 cmH2O, on page 2, 9th line, 2nd column. Besides, in the discussion one can find the following sentence: "This potent analgesic component of nitrous oxide had already been observed through electroencephalographic parameters when administered to humans." I would like to mention that this quote is related with a study we published in the Brazilian Journal of Anesthesiology 7 and that was not exactly our conclusion, but the following: "To conclude, under the conditions of this study, 30% and 50% N2O had a weak sedative action, which was not enough to abolish consciousness. The variations in BIS were compatible with the changes in EAS, which did not happen with SEF1 and SEF2 that did not prove to be valid quantitative indexes of the degree of hypnosis for the technique used. However, they can reflect a possible analgesic component of nitrous oxide." The conclusion of our study could not reflect unequivocally an analgesic component because we did not apply nociceptive stimuli to the patients.
Another important matter is the definition of SEF 95% (spectral edge frequency). The study states: "SEF 95%, Spectral Edge Frequency, reflects the frequency of the predominant spectral power, i.e., the moment that 95% of existing frequencies are below that value", and quotes the following reference: Heier T, Steen PA Assessment of anesthesia depth. Acta Anesthesiol Scand, 1996;40:1087-1100, which has the following definition: "The spectral edge (SE, frequency below which 95% of the EEG power resides) and median frequency (MF, frequency below which 50% of the EEG power resides) are also " Therefore, the definition of the study is incorrect and does not reflect the reference quoted. What is written is not supported by engineering, neurology, or physics. SEF 95% is, in reality, the spectral analysis of an original signal in the domain of time that was transformed into the domain of frequency, and this signal is treat mathematically through the Fourier rapid transform, resulting in an analysis of the energy plane (potency) in function of the frequency. Thus, the cut of 95% of potency reflects the following: SEF 95% is the frequency below which we observe 95% of the whole potency (or energy) of the original signal 8. Another question, equally important, regards electrode positioning. In the present study, the description of electrode placement states: "To evaluate the bispectral index, SEF 95%, and suppression rate, BIS electrodes were placed in the frontal region (BIS Sensor Xp, Aspect Medical System, EUA)." In reality, the set has four electrodes and the placement is referential, i.e., one of the electrodes is the reference and is located in the frontal region (FPz) and the other is the explorer, located between the external angle of the eye and the hair line (FT9) 9, and captures the cortical activity corresponding to the anterior temporal region, which was not specified in the study.
Dr. Rogean R. Nunes,
Professor in Surgery
Electronic Engeneering UNIFOR
Member of the Sociedade Brasileira de Engenharia Biomédica
Member of the Sociedade Brasileira de Matemática Aplicada e Computacional
Dra. Sara Lúcia Cavalcante, TSA
Doctor of Anesthesiology
Associated Professor of the Faculdade de Medicina da UFC
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