Since the moment of origin of anesthesiology, the practicing doctors and researchers made various attempts of control of depth of anesthesia. In the beginning it pursued the aim not to allow excessively deep anesthetic plane as one inhalation anesthetic by means of which all components of the general anesthesia were reached was used usually: consciousness switching off, analgesia, neurovegetative blockade and muscular relaxation. The anesthetic overdose in this case led to oppression of functions of breath and blood circulation at the patient and quite could end with his death. For this purpose the doctor who is carrying out anesthesia sought not to allow such succession of events and controlled its level on clinical signs: to the pupil size, movement of eyeglobes, oppression of a lid reflex, degree of a muscular relaxation, arterial blood pressure and pulse.
Further, in process of development of specialty, components of the general anesthesia began to be reached not by one, and at once several drugs. In this regard there was an opportunity not to use high doses of inhalation anesthetic, and to reach an analgesia use of opiates, and a muscular relaxation – by means of kurarepodobny means. It made anesthesia in general safer, but the return problem – its insufficient level appeared.
It is considered that intranarkozny awakening happens to frequency up to 4% among all general anesteziya. At the same time, techniques of hypnosis allow to establish existence of memories of the undergone operation already at 20–30% of patients. The problem of intranarkozny awakening when carrying out total intravenous anesthesia is especially relevant and also during the work with critical patients, in a heart surgery and obstetrics. Considering that clinically adequately it is impossible to control anesthesia depth in modern conditions because of a large number of the used drugs, to the forefront there are tool methods.
The number of tool techniques of control of depth of anesthesia was offered considerably: electromyography of frontal muscles, monitoring of sokratitelny activity of the lower esophageal sphincter, photoplethysmogram assessment, mathematical analysis of a warm rhythm, metabolic researches, etc. But all these methods are rather bulky and of little use for broad use.
One of control methods of depth of anesthesia is the analysis of the electroencephalogram registered at various stages of an anesthesiology grant. But the problem was here that interpretation of this indicator is inaccessible to a wide range of doctors because of the complexity in interpretation. Therefore in 1994 researchers offered the concept of the bispektralny index – the integrated indicator received from the machine analysis of the electroencephalogram which is expressed in numbers from 0 to 100 and reflects a brain degree of activity. It was established that the bispektralny index (BIS) is the integrated indicator which is not depending on the applied anesthetic (or their combinations). In clinical practice BIS monitors began to be used.
Now BIS monitoring is the most widespread way of control of depth of anesthesia. For example, in the USA its use reaches more than 60% of all carried-out general anesteziya. In Switzerland this method is used everywhere. In Russia this technique also gains the increasing popularity. An analog of BIS monitoring is the monitoring of entropy realized in the Datex Ohmeda monitors and measuring practically the same indicators.
As it was already mentioned, the bispektralny index is expressed in numbers from 0 to 100 where 0 – total absence of activity of a cerebral cortex, and 100 – clear consciousness at the patient. The recommended target indicators of BIS during the general anesthesia are values from 45 to 60.
Registration of the bispektralny index is carried out by imposing on area of the head of the patient of disposable or reusable electrodes which register indicators of the electroencephalogram and transfer them to the monitor for further processing. Electrodes are imposed prior to the beginning of anesthesia, at the same time it is important to observe a technique of their imposing strictly.
Considering rather high cost of monitoring of a such type, it should be noted that when carrying out the general anesthesia without the accompanying pathology it is possible to achieve the guaranteed consciousness switching off from patients by use of the adequate doses of inhalation anesthetic which are usually expressed in MAC (minimum alveolar concentration). Use of IAC from 1.3 and above almost completely guarantees switching off of consciousness at the patient. However use of this concept not in all clinical situations is feasible and here monitoring of depth of anesthesia comes to the rescue. Areas in which it is strongly recommended to use this type of monitoring: neuroanesthesiology, cardioanesthesiology, obstetric anesthesiology, carrying out anesthesia at critical patients and control of sedation of patients in intensive care units.