Stages of General Anesthesia

Neurophysiological mechanisms of staging

The formation of distinct phases during general anesthetic administration is based on the sensitivity gradient of various structures within the central nervous system. CNS regions exhibit unequal susceptibility to drugs.

• First, synaptic transmission in the structures of the cerebral cortex and the reticular formation is suppressed.
• The substantia gelatinosa of the spinal cord (substantia gelatinosa), which is responsible for pain impulse transmission, also possesses exceptionally high sensitivity. This is precisely what ensures early analgesia.
• The vital centers of the medulla oblongata—the respiratory and vasomotor centers—are the last to be suppressed.

Subcortical centers are generally more resistant to drug effects compared to the cortex. Due to this, during the surgical stage, when consciousness is already lost, basic regulation of circulation and respiration is preserved. It is this precise sequence of shutting down the regions of the brain and spinal cord that determines the occurrence of predictably succeeding stages.

First and second stages of anesthesia

The classic clinical picture (described using diethyl ether as an example) includes four consecutive phases.

Stage I: Analgesia The term itself originates from the Greek words an (negation) and algos (pain).

• Consciousness: preserved, the patient is contactable, although gradual depression is noted.
• Effects: pain sensitivity is noticeably reduced. Pronounced analgesia and anterograde amnesia develop (events are not retained in memory).
• Vital functions: pulse, blood pressure, and respiratory rhythm remain unchanged.

Stage II: Excitement

• Consciousness: completely lost.
• Motor sphere: the patient exhibits verbal and motor excitement, making unmotivated movements. Increased muscle tone is observed.
• Reflexes: spinal reflexes are enhanced. The risk of vomiting and aspiration increases significantly due to heightened gag and cough reflexes.
• Autonomic status: pupils dilate (mydriasis), breathing becomes irregular, and tachycardia is noted.

The mechanism behind this phase is the "disinhibition" phenomenon. Suppression of the cortex removes its inhibitory influence on subcortical structures (primarily the midbrain centers). Deprived of control, they exhibit chaotic activity.

Third stage: Surgical anesthesia

In this phase, signs of excitement disappear completely. The patient is ready for surgery.

General characteristics:

• Consciousness and pain sensitivity are completely absent.
• Unconditioned reflexes are suppressed, and muscle tone is significantly reduced.
• Respiration normalizes and becomes regular.
• Autonomic status stabilizes (blood pressure is normal), and pupils are constricted.

As anesthesia deepens within the third stage, four consecutive levels are distinguished:

1. Light superficial anesthesia ($III_1$).
2. Light anesthesia ($III_2$).
3. Deep anesthesia ($III_3$).
4. Ultradeep anesthesia ($III_4$).

If anesthesia continues to deepen excessively, complications may arise: bradycardia (slow pulse), cardiac arrhythmias, blood pressure drop, and gradual depression of respiratory function.

Termination dynamics and risks

The termination of anesthesia and potential critical states are also subject to strict physiological rules.

• Recovery stage: begins after the cessation of anesthetic administration. The logic of this process is strictly orderly—CNS functions return in the reverse order of their suppression. What was turned off last recovers first.
• Agonal stage: a critical state occurring during drug overdose. Its pathogenesis is caused by excessive depression of the central nervous system, leading to paralysis of the vital centers of the medulla oblongata (vasomotor and respiratory).

Class and exam questions

What else to study on this topic

• Detailed clinical characteristics of the 4 levels of the surgical stage ($III_1$–$III_4$)
• Pharmacological effect of anesthetics on the gelatinous substance of the spinal cord
• Features of the development of anterograde amnesia in the first stage of anesthesia
• Neurophysiology of the reticular formation and its role in maintaining consciousness

Other topics in the "Special Pharmacology" section

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