Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.52 no.3 Campinas May/June 2002
LETTERS TO THE EDITOR
Rogean Rodrigues Nunes, TSA, M.D.
In relation to published articles proposing the bispectral index (BIS) to monitor anesthetic dept or hypnosis intensity during anesthesia, I would like to ask some questions and make some constructive comments.
In all papers involving BIS, especially with the A-1000 monitor, the authors have described the version of the devices (ex. 1.0, 2.0, 3.0, 3.3 etc). These devices were approved in the USA by the FDA to be used with specific electrodes (Zipprep) are adequate for impedance matching tests with for channels 1.2 and reference (Fz) and which, according to the manufacturer, should show values below 7.5 KW, even to start readings, in all versions (updates).
Recently, Thøgersen et al. 1 have compared hemodynamic parameters to validate BIS values found with model A-1000 (version 3.12). These authors have used common ECG electrodes to replace Zipprep electrodes in 12 cases. Their conclusion was that common ECG electrodes may replace Zipprep electrodes. However, their use was not recommended in research projects maybe because the balance among electrodes has showed very high impedance variations when both channels were compared. In addition, the authors have not mentioned individual impedance values in channels 1, 2 and Fz, which are an integral part of the test.
The database collected to create and validate BIS was built based on alertness-sedation scales, as well as on memory research and not on hemodynamic variables. These may or not correspond to BIS variations since clinical signs (BP, HR, sweating and tearing) are affected by diseases (hyper or hypothyroidism), drugs (anti-hypertensive, anti-colinergic etc) 2 and patients volume replacement status. Cause and effects are different things. In addition, there has been neither anesthetic technique standardization to compare results nor exclusion criteria, such as hypertensive history or drug therapy, being even included patients with thyroid diseases.
In our department, when using ECG electrodes to replace Zipprep in 350 patients, although adequately preparing the skin and asking patients to remain with closed eyes (avoiding the light interference and the moving of the eyes doing after the signals) we have observed very discrepants impedances among all the tested points, and that would make amplifiers common rejection mode (CRM) extremely critical. Johansen et al. 3, in a recent study, have stated that false BIS increase could be seen with high impedance electrodes.
Through personal mail (e-mail) to Dr. Ira Rampil 4, I have asked if the use of common ECG electrodes to replace Zipprep electrodes was correct. His answer was: no problem, as long as the contact impedance is low and balanced among the electrodes, and you have a method of attaching them to the machine.
Finally, I believe that for a better index proposal in any research, strict and universally reproducible criteria are needed (techniques used, number of patients, explanations with equipment utilization description, etc) so that in the future such indices are not disbelieved due to mere methodological failures.
01. Thøgersen B, Ørding H - Bispectral index monitoring: comparison of two types of electrode. Anaesthesia, 2000;55:242-246.
02. Ghoneim MM - Awareness During Anesthesia. 1st Ed, Oxford, Butterworth Heinemann, 2001;69-91.
03. Johansen JW, Sebel PS - Development and clinical application of electroencephalographic bispectrum monitoring. Anesthesiology, 2000;93:1336-1344.
04. Rampil IJ - A primer for EEG signal processing in anesthesia. Anesthesiology; 1998;89:980-1002.