Classification of anesthesia and main mechanisms
Anesthesia affects structures of the central nervous system (CNS). Depending on the route of administration of anesthetics into the body, two main types are distinguished:
- Inhalation anesthesia — the drug enters the body through the respiratory tract;
- Non-inhalation anesthesia — the drug is administered via other routes (most often intravenously).
The main goal of anesthesia is to ensure patient safety and create comfortable conditions for the surgeon's work. To achieve this, it is necessary to attain not only loss of consciousness, but also complete analgesia and adequate muscle relaxation (myorelaxation).
Theories of the mechanism of action of anesthesia
To date, there is no single concept that comprehensively explains the mechanism of anesthesia development. Various theories examine this process at the cellular (chemical and physical reactions) and physiological levels.
- Lipid theory (H. Meyer, C. Overton). Anesthetics dissolve in the lipids of neuronal cell membranes. The higher a substance's lipid solubility, the stronger its anesthetic effect.
- Adsorption theory (Traube, O. Warburg). Drugs are adsorbed onto the surface of CNS cells, altering their physicochemical properties.
- Theory of oxidation inhibition (Verworn). Enzymes responsible for redox reactions in brain tissue are blocked.
- Coagulation theory (C. Bernard). Drugs cause reversible coagulation of the protoplasm of nerve cells, depriving them of the ability to become excited.
- Physiological theory (V.S. Galkin, P.A. Anokhin). Views anesthesia as a process of CNS depression. The structure most sensitive to anesthetics is the reticular formation of the brain.
Initial stages of anesthesia
The process of saturation of the body with an anesthetic proceeds in stages. The classic clinical picture is most clearly observed with ether anesthesia.
Stage I — Analgesia The patient is conscious, but sluggish, and answers questions with single-word responses. Superficial pain sensitivity is completely lost, whereas thermal and tactile sensitivity are preserved. This stage is brief (3–4 minutes). During this period, quick procedures can be performed, such as opening abscesses, phlegmons, or painful diagnostic procedures.
Stage II — Excitation Consciousness is lost. Depression of the cerebral cortex occurs against the background of pronounced excitation of subcortical centers. The patient exhibits motor and verbal agitation, skin hyperemia, tachycardia, and increased blood pressure. Pupils are dilated (mydriasis), but reactive to light. Increased bronchial secretion, coughing, lacrimation, and vomiting are possible. Surgery must not be performed during this stage; saturation of the body with the drug must be continued. Duration ranges from 7 to 15 minutes.
Surgical stage and emergence
Surgical stage (III) begins when the patient calms down, and hemodynamic and respiratory parameters return to baseline values. It is divided into four levels:
- First level (III₁) — light anesthesia. Eyeball movements are smooth, pupils are constricted. Muscle tone and reflexes (corneal, pharyngolaryngeal) are preserved. Abdominal surgeries are difficult to perform due to the lack of muscle relaxation.
- Second level (III₂) — moderate anesthesia. Eyeballs are fixed centrally, pupils begin to dilate, reflexes fade. Muscle tone decreases, allowing operations on abdominal organs. Anesthesia is usually maintained at levels III₁–III₂.
- Third level (III₃) — deep anesthesia. Complete relaxation of skeletal muscles (including intercostal muscles), respiration becomes diaphragmatic. Corneal reflex is absent. Pulse accelerates, blood pressure drops. Due to muscle relaxation, the lower jaw may drop and block the entrance to the larynx, causing respiratory arrest. Operating at this level is life-threatening.
- Fourth level (III₄) — overdose. Maximum pupil dilation with no reaction to light. Pulse is thready, blood pressure is undetectable. Extremely high risk of respiratory and circulatory arrest.
Emergence stage (IV) begins after the cessation of anesthetic delivery. As its concentration in the blood decreases, the patient passes through all described stages in reverse order until full awakening.
Briefly: answers to topical questions
What drugs are used for inhalation anesthesia?
Liquid (vaporizable) anesthetics and anesthetic gases are used for inhalation anesthesia.
- Diethyl ether — a liquid (vaporizable) anesthetic.
- Halothane (fluorothane) — a liquid anesthetic.
- Trichloroethylene (trilene) — a liquid anesthetic.
- Penthrane — a liquid anesthetic.
- Chloroform — a liquid anesthetic.
- Nitrous oxide (nitrous oxide) — an anesthetic gas.
- Cyclopropane — an anesthetic gas.
- Methoxyflurane — an inhalation anesthetic.
- Enflurane — an inhalation anesthetic.
- Sevoflurane (sevorane) — an inhalation anesthetic.
- Xenon — an inhalation anesthetic.
What are the routes of administration for non-inhalation anesthesia other than intravenous?
In addition to intravenous administration, drugs in non-inhalation anesthesia can be administered via several routes.
- Intramuscular route — injection of drugs into a muscle.
- Rectal route (per rectum) — administration of drugs through the rectum.
- Oral route — administration of drugs through the mouth.
Non-medicated electrostimulation methods are also classified under non-inhalation general anesthesia.
What respiratory complications can occur during anesthesia?
The following respiratory complications can occur during anesthesia:
- Respiratory depression — hypoventilation (infrequent, shallow breathing) or apnea (cessation of breathing).
- Airway obstruction due to the tongue falling back.
- Laryngospasm.
- Bronchospasm.
- Aspiration — entry of gastric contents into the trachea and bronchi during vomiting.
- Aspiration pneumonia.
- Hypoxia.
What components are included in the concept of modern balanced anesthesia?
The concept of modern balanced anesthesia includes main and special components. Main components:
- Loss of consciousness — suppression of mental perception.
- Analgesia — provision of central or peripheral pain relief.
- Neurovegetative blockade — suppression of autonomic reactions.
- Muscle relaxation — relaxation of skeletal muscles.
- Maintenance of gas exchange — ensuring adequate respiration.
- Maintenance of circulation — ensuring stable hemodynamics.
- Regulation of metabolism — maintaining metabolic processes.
Special components include hypothermia (including craniocerebral), cardiopulmonary bypass, cardioplegia, and artificial arterial hypotension. To achieve these effects, anesthetic agents are combined with muscle relaxants, analgesics, and ganglion blockers.
To memorize the sequence of anesthesia stages, you can use the mnemonic: «Anesthesiologist Introduces Surgical Awakening» (Analgesia — Excitement — Surgical stage — Awakening).
Class and exam questions
Can surgery begin if the patient is in the excitement stage?
No, surgical procedures must not be performed under any circumstances. It is necessary to continue the administration of the anesthetic to deepen anesthesia to the surgical stage.
At which level of the surgical stage are surgeries typically performed?
In most cases, anesthesia is maintained at the first (III₁) or second (III₂) level of the surgical stage, which ensures adequate analgesia and patient safety.
What is the main danger of deep anesthesia (III₃)?
Deep anesthesia causes complete muscle relaxation. Consequently, the lower jaw may drop and block the laryngeal inlet, leading to respiratory arrest.
What else to study on this topic
- Features and differences between inhalational and non-inhalational anesthesia
- P.A. Anokhin's physiological theory: the role of the reticular formation
- Differential diagnosis of surgical stage levels based on ocular signs
- Causes, prevention, and treatment of respiratory arrest during anesthesia
- Dynamics of blood pressure and heart rate changes across different stages of anesthesia
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Other topics in the section "CHAPTER 3. ANESTHESIA"
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