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Methods for controlling the depth of anesthesia

General clinical
Since the introduction of ether and chloroform into medical practice, the clinic has served as a reference point for determining the depth of anesthesia. In 1847, John Snow was the first to attempt to classify the stages of anesthesia. Based on changes in the nature of breathing, consciousness, voluntary and involuntary muscular movements, ciliary reflex, Show identified five levels of ether anesthesia.
In 1937, Guedel AE introduced its classification of signs characterizing certain stages of general anesthesia, which became the "gold standard" that defines the management of anesthesiology benefits. Examining clinical signs such as level of consciousness, muscle tone, spontaneous breathing, skin moisture, pupil reaction to pain, eyeball movement, the presence or absence of swallowing and vomiting reflexes, Guedel identified four stages of general anesthesia: stage I - analgesia, stage II - excitement, stage III (stage of surgical anesthesia) was divided into four levels, the stage IV, the author attributed respiratory paralysis and death.
In the future, many researchers tried to modify this classification. The first three stages were subjected to detailing and processing; only stage IV remained unshakable. In all schemes, it is considered toxic, ending with the death of the patient. However, I.S. Zhorov in 1959 in his classification replaced stage IV, as the "stage of intoxication and death," with the "period of awakening" of the patient, rightly noting that "... death due to anesthesia can occur in its initial stages, even before start of operation. ... The lack of an awakening stage makes all existing classifications incomplete and does not give a complete picture of the course of anesthesia from its beginning to the end. Apparently, as often happens, each of the cited authors is right in his own way.
The year 1940 marked the beginning of the use of muscle relaxants in clinical practice. Starting with the use of small doses of d-tubocurarine to induce level 2-3 stage III anesthesia according to Guedel against the background of high concentrations of inhalation anesthetics, anesthetists very quickly switched to the use of high doses of muscle relaxants in combination with mechanical ventilation, slightly lowering the concentration of inhaled anesthetics, since this significantly reduced the risk of cardiovascular and respiratory complications. With the advent of muscle relaxants and mechanical ventilation, involuntary muscle movements have ceased to be indicators of the depth of anesthesia.
Currently, the use of clinical signs to control anesthesia is based on the fact that anesthesia and sensory activation are antagonists. It should be remembered that the clinical picture of the response may vary under the influence of parallel drugs (vasodilators, 5-blockers, tranquilizers, etc.) and depend on the individual characteristics of the body. Such shortcomings do not exclude the use of indicators of central and peripheral hemodynamics (ChiPH), which reflect the effectiveness of sensory protection. However, monitoring of heart rate, blood pressure, MOS, LAD, AF, the most common combination currently used in the clinic, with all its value, has certain limitations of information content, has a certain inertia and is not always easily and correctly interpreted. The “classic” variant of the ChiPH reaction (ie, a simultaneous increase in heart rate, blood pressure, MOS, LAD, OPS, etc.) is characteristic only for the initial stages of long and traumatic operations. In conditions of massive blood loss and intensive infusion-transfusion therapy (ITT), this reaction is determined by the rate of blood loss, the nature of ITT, and not the effectiveness of the anesthesia performed. At this stage, cases of unjustifiably superficial anesthesia are not uncommon.
The study of some indicators of metabolism
Monitoring the parameters of CBS and the oxygen-transport function of blood (CTFc), as well as the dynamics of hormones - “stress indicators”, can apparently serve as the most accurate reflection of the adequacy of pain relief. However, express methods for determining these indicators that are able to work on-line have not yet been developed.
The concept of minimum alveolar concentration (MAC)
A definite step towards the creation of new methods for controlling the depth of anesthesia was the development by Eger EI in 1965 of the concept of the minimum alveolar concentration of an inhaled anesthetic (MAK). The whole concept was formed by the end of the 80s.
MAK is the minimum concentration of an inhaled anesthetic in the alveolar gas, preventing a motor reaction to a standard pain stimulus (skin incision or electrical impulse) in 50% of patients. Currently, the MAC values ​​for all inhaled anesthetics have been obtained (Table 19.1). The MAK concept clearly demonstrated that the concentration of anesthetics necessary to prevent motor reactions should be higher than to turn off consciousness or conduction sensitivity.
Medicines used in anesthesia reduce MAC, which justifies their use in order to reduce the dose of the main anesthetic. For example, nitrous oxide reduces the MAC of all currently used anesthetics (Table 19.1). The value of MAC is a very reliable quantitative criterion for the action of anesthetic, and this concept was an important step towards the development of methods for controlling the depth of anesthesia, establishing the relationship between the dose of anesthetic and its effect.
However, the theory of alveolar concentrations is not without serious flaws:
- the concentration of anesthetics in exhaled gas does not always reflect its concentration in arterial blood;
- MAK gives an idea of ​​only one point of the curve "dose - effect." An attempt to compare the effect of two anesthetics at 1.5 and 2 MAK is not always justified, since the dose-effect curves may not be parallel;
- not all anesthetics have a linear relationship between narcotic and analgesic effects;
- MAC calculation is applicable only in case of anesthesia with inhaled anesthetics.
Table 19.1
Minimum alveolar concentration (MAC) for various gas anesthetics (Barash P. et al., 1992) ________________________________
Anesthetic MAK (vol%) MAC (vol%) at 60–70% N2O
Nitrous oxide 104 -
Halotan 0.77 0.29
Enflurane 1.70 0.60



Isoflurane 1.15 0.50
Sevoflurane 1.71 0.66
Desflurane 6.0 2.83



Additional methods
As possible additional methods for assessing the depth of anesthesia, a number of researchers have studied and are studying methods that allow, based on the analysis of indirect indicators, to evaluate the effectiveness of the anesthesia performed. The latter include:
- electromyography of the frontal muscles;
- monitoring of spontaneous and provoked contractile activity of the lower esophagus;
- analysis of photoplethysmogram;
- method of "isolated forearm";
- analysis of respiratory sinus arrhythmia;
- O2 absorption and CO2 production;
- mathematical analysis of heart rhythm;
- peripheral and central thermometry, etc. These indicators also vary depending on the depth of anesthesia, but they did not find wide application in anesthesiology or a further study revealed their lack of information. However, we allow ourselves to consider some of them, noting the originality of the approach in an attempt to solve the problem under discussion.
The method of "isolated forearm"
The technique of the method consists in applying a turnstile to the patient’s shoulder even before the introduction of muscle relaxants. Thus, neuromuscular transmission remains below the level of the turnstile. By observing the appearance of movement in the “isolated forearm” in response to speech commands or surgical stimulation, one can judge the level of anesthesia performed. The technique has not been widely used because of the danger of developing ischemia in the limb.
Frontal muscle electromyography
Monitoring the depth of anesthesia using electromyography of the frontal muscle is possible due to the peculiarities of its innervation due to visceral fibers of the facial nerve and trigeminal branches, leaving a “window” for studying the autonomic nervous system. Thus, even in conditions of total myoplegia, the frontal muscles retain the ability to respond by contraction to surgical aggression. This phenomenon, called the "symptom of frowning eyebrows", formed the basis of a number of anesthetic monitors, such as AVM, DATEX, Finland. Paloheimo M. (1989), one of the enthusiasts of this method, reports that inadequate anesthesia or awakening at the end of the operation is always accompanied by a change in the amplitude of the frontal EMG, although this is often evident from other monitoring methods. Vickers M. (1991) also expresses doubt about the absolute reliability of this method, since nothing can be said about the patient’s consciousness by the nature of the EMG curve.
Monitoring of spontaneous and provoked contractile activity of the lower esophagus
The lower esophagus includes smooth muscles, which muscle relaxants do not exert. He receives the main innervation from the vagus nerve with the presence of central control mechanisms in the brain stem.
The frequency and amplitude of contractions are recorded using an inserted probe with a spray can. Studies have shown that in the absence of side factors, such as the use of ganglion blockers, atropine or epidural anesthesia at the level of the thoracic region, with a decrease in the dosage of anesthetic, the frequency of spontaneous and the amplitude of the provoked contractions increase. It seems that such monitoring has prospects for an initial assessment of the depth of anesthesia when inhaled anesthetics are used. Its effectiveness when using various intravenous anesthetics is under study.
Variational pulsometry
Currently, there are several dozen methods for analyzing heart rate.
The starting point is the numerical sequence of the values ​​of R— R-intervals. This sequence contains information about the processes that take place not in the heart itself, but in various parts of the autonomic homeostasis control system; therefore, the study of heart rhythm variability allows one to increase the severity of adaptive processes in the body in response to one or another stressful effect.
A number of researchers note that a high correlation is observed between changes in the function of the cardiovascular and sympathoadrenal systems that occur during surgery and anesthesia.
However, along with reports on the successful use of variational pulsometry to assess the severity of operational stress in the literature, there are works in which the use of this method led to the production of uninterpreted data. This is due to the presence of “abnormal R – R intervals corresponding to impaired cardiac conduction, since even a relatively small number of them leads to very significant deviations from the actual values ​​in the calculations. Therefore, this type of monitoring is not applicable in patients with persistent forms of rhythm disturbance.
Reduces the value and efficiency of the method, the need to set a certain number of cardio intervals, the decrease of which leads to an increase in the degree of error in the calculation, and the increase is irrational for reasons of efficiency of data acquisition.
Neurophysiological control methods
The central nervous system is the main target for anesthetics. Therefore, it should be recognized that the most accurate and informative would be direct control of the central nervous system as an integrative system for responding to the ineffectiveness of anesthetic protection. Modern methods of monitoring neurological functions are trying to get away from the disadvantages associated with the use of clinical signs. In this case, an assessment of spontaneous and induced EEG activity is used.
Electroencephalography
The frequency spectrum of the EEG extends from 0.5 to 100 Hz, although most of its power (99%) is concentrated in the range from 1 to 30 Hz. In visual analysis, an EEG looks like a complex aperiodic wave process. The appearance of signals fluctuates during the day and cyclically changes during sleep. There are 4-5 main classes of EEG frequencies. They were established on the basis of the application of the first instrumental methods and are in the following ranges: 8 (0.5–3 Hz), t (4–7 Hz), a (8–13 Hz), in (14–17 Hz) and B2 (over 18 Hz).
The transition from wakefulness to sleep, the use of drugs, some pathological conditions are accompanied by changes in the EEG (table. 19.2).
Gibbs FA first described the change in the pattern of EEG during induction in anesthesia in 1937. Further studies on the determination of the stages of general anesthesia based on EEG were performed for: ether, cyclopropane, methoxyflurane and barbiturates.
Intravenous and inhaled anesthetics do not have the same effect on EEG, and their equipotential concentrations produce very different EEG frequencies (Table 19.3). Nevertheless, the general rule for changing the EEG pattern under their action, proposed by Faulconer AJ and Bickford RG (1990), which manifests itself as "a slowdown in frequency and an initial rise, followed by a decrease in the EEG amplitude depending on the clinical range of the depth of anesthesia", can be applied for most anesthetics used today.
Table 19.2
EEG changes occurring with the introduction of certain drugs and pathological conditions (Barash P. et al .. 1992) ___
EEG Characterization Medications and homeostasis conditions
Increasing frequency Barbiturates (low doses) Benzodiazepines (low doses) Etomidate (low doses)



Nitrous oxide (30–70%) Inhalation anesthetics (<1 MAK) Calipsol
Arterial hypoxemia (mild) Hypercapnia (moderate) Epileptic seizure
Decrease in frequency / increase in amplitude Barbiturates (moderate doses) Etomidate (moderate doses)
Opiates
Inhalation anesthetics (1 MAK) Arterial hypoxemia (moderate) Hypercapnia (moderate to severe) Hypothermia
Decrease in frequency / decrease in amplitude Barbiturates (high doses)
Arterial hypoxemia (moderate) Hypercapnia (severe) Hypothermia (<35 ° C)
Isoelectric line (electrical silence) Barbiturate coma Etomidate (high doses) Isoflurane (2 MAK)
Arterial hypoxemia (severe) Hypothermia (<20 ° C)
Brain death



Subsequent advances in automated processing of EEGs made it possible to supplement the visual analysis of EEGs with frequency and periodiograms that facilitate and accelerate the quantitative assessment of changes in the frequency-amplitude characteristics of EEGs.
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