Molecular and neurochemical mechanisms
The action of general anesthetics is implemented at multiple levels of the central nervous system. Their effect is based on the ability to modulate the function of ion channels and receptors, shifting the balance toward the inhibition of nerve impulses.
Main receptor interaction mechanisms:
- Sensitization of inhibitory systems. Drugs enhance receptor sensitivity to major inhibitory neurotransmitters. In the brain, these are GABA (gamma-aminobutyric acid) receptors, and in the spinal cord, glycine receptors.
- Suppression of excitatory systems. Anesthetics suppress the activity of receptors that respond to glutamate, primarily NMDA receptors.
Interestingly, specific binding sites for most drugs in this group have been found directly on these receptor complexes.
In addition, inhalational anesthetics directly affect synaptic transmission by significantly reducing the release of key neurotransmitters in the CNS. Under their influence, the release of acetylcholine, dopamine, serotonin, and norepinephrine decreases, which further depresses brain activity.
Pharmacokinetics: rate of onset
For inhalational drugs, the most critical pharmacokinetic parameter is the blood/gas partition coefficient. This indicator dictates how quickly a patient will go under anesthesia and how soon recovery will occur after the drug is discontinued.
The dependence mechanism is directly related to the physical solubility of the anesthetic in the blood. If solubility is high, the drug easily and in large quantities passes from the pulmonary alveoli into the bloodstream. As a result, it begins to "accumulate" in the liquid portion of the blood, and considerable time is required to create the necessary partial pressure for the substance to pass into brain tissue.
- Good blood solubility. Leads to a relatively slow induction of anesthesia and an equally prolonged, delayed recovery. This group includes halothane, enflurane, isoflurane, and classic diethyl ether.
- Poor blood solubility. The gas rapidly saturates the blood (since it barely dissolves in it) and instantly travels to the brain. The result is a rapid onset of anesthesia and very quick recovery of consciousness after surgery. Typical examples: nitrous oxide, xenon, and sevoflurane.
Features of modern drugs
The clinical picture of modern inhalational agents is characterized by variability. Unlike older drugs, newer anesthetics have a significantly less pronounced excitation stage, making induction into anesthesia more comfortable and safer.
At the same time, the depth of achieved analgesia (pain relief) can vary greatly depending on the chosen drug, which requires careful monitoring of the blood/gas coefficient and knowledge of the receptor profile of the specific substance.
Briefly: answers to topical questions
What stages are distinguished in the clinical picture of classic ether anesthesia?
Four successive stages are distinguished in the clinical picture of classic ether anesthesia.
- Stage I — Analgesia (consciousness is preserved, eyelids closed).
- Stage II — Excitement (eyelids tightly clenched, skin color bright red).
- Stage III — Surgical (tolerant). Divided into 4 levels of depth: superficial, light, deep, and ultradeep anesthesia. Characterized by the disappearance of excitation signs, normalization of respiration, decreased muscle tone, and complete absence of consciousness.
- Stage IV — Recovery (eyelids alternately half-open or closed).
Which drugs belong to the group of non-inhalational general anesthetics?
The group of non-inhalational (intravenous and rectal) general anesthetics includes the following drugs:
- Hexobarbital (Hexenal) — a medium-duration drug.
- Sodium thiopental — a medium-duration barbiturate with rapid onset of effect.
- Propanidid (Sombrevin) — an ultra-short-acting drug.
- Propofol is a short-acting anesthetic.
- Etomidate is a short-acting agent.
- Ketamine is a short-acting agent that induces dissociative anesthesia.
- Sodium oxybate (Sodium oxybate) is a long-acting agent.
This group also includes epontol, baytinal, and viadril.
What specific side effects are characteristic of halothane?
The following specific side effects are characteristic of halothane:
- Myocardial sensitization — an increased sensitivity of the heart to catecholamines, which increases the risk of severe arrhythmias when epinephrine is administered.
- Malignant hyperthermia — a life-threatening condition characterized by a sharp rise in temperature up to 42–43 °C and skeletal muscle spasm, occurring when halothane is combined with suxamethonium iodide.
- Hypotension — a decrease in blood pressure due to depression of the vasomotor center, ganglion-blocking, and direct myotropic effects.
- Bradycardia — slowing of the pulse due to stimulation of the vagus nerve center.
Which non-inhalation anesthetic induces dissociative anesthesia via NMDA receptor blockade?
Dissociative anesthesia via NMDA receptor blockade is induced by ketamine. This drug is a phencyclidine derivative and acts as a non-competitive NMDA receptor antagonist, eliminating the excitatory effect of glutamate on CNS structures.
- Dissociative anesthesia — a state of functional dissociation characterized by the depression of certain brain structures while others remain active.
At the same time, spontaneous respiration, muscle tone, and reflexes are preserved, consciousness is only partially lost, but pronounced analgesia and amnesia are achieved. The stage of surgical anesthesia does not occur.
The WORSE an anesthetic dissolves in blood, the FASTER it enters the brain (rapid induction and rapid emergence).
Class and exam questions
Which excitatory receptors are blocked by general anesthetics?
First and foremost, they depress the activity of glutamate receptors, specifically NMDA receptors.
How does blood solubility affect the rate of emergence?
Agents with poor blood solubility (a low blood/gas partition coefficient) provide the fastest recovery from anesthesia.
Which inhibitory neurotransmitters are affected by general anesthesia?
Anesthetics enhance the sensitivity of GABA receptors in the brain and glycine receptors in the spinal cord.
What else to study on this topic
- Mechanisms of GABA receptor sensitization in the brain
- The role of spinal cord glycine receptors in the development of anesthesia
- Specific binding sites for anesthetics on NMDA complexes
- Clinical significance of reduced serotonin and dopamine release during anesthesia
- Comparative characteristics of sevoflurane and isoflurane
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