Neurophysiological Mechanisms of Staging
The distinct phases observed during general anesthesia administration are driven by the sensitivity gradient of various central nervous system structures. CNS regions exhibit varying susceptibility to pharmacological agents.
- Synaptic transmission in the cerebral cortex and reticular formation is suppressed first.
- The substantia gelatinosa of the spinal cord (substantia gelatinosa), responsible for pain impulse transmission, also exhibits exceptionally high sensitivity. This accounts for early analgesia.
- The vital centers of the medulla oblongata (respiratory and vasomotor centers) are suppressed last.
In general, subcortical centers are more resistant to drugs than the cortex. Consequently, during the surgical stage—when consciousness is already lost—basic regulation of circulation and respiration is preserved. This specific sequence of sequential shutdown of the brain and spinal cord dictates the predictable, consecutive stages of anesthesia.
Stages I and II of Anesthesia
The classic picture (described using diethyl ether as the model) includes four consecutive phases.
Stage I: Analgesia The term originates from the Greek an (negation) and algos (pain).
- Consciousness: Maintained; the patient is responsive, although progressive depression is noted.
- Effects: Pain sensitivity is markedly reduced. Pronounced analgesia and anterograde amnesia develop (events are not retained in memory).
- Vital Functions: Pulse, blood pressure, and respiratory rate remain unchanged.
Stage II: Excitement (Delirium)
- Consciousness: Completely lost.
- Motor Activity: The patient exhibits verbal and motor agitation, making unmotivated movements. Increased muscle tone is observed.
- Reflexes: Spinal reflexes are enhanced. The risk of vomiting and aspiration increases significantly due to exaggerated gag and cough reflexes.
- Autonomic System: Pupils dilate (mydriasis), breathing becomes irregular, and tachycardia is noted.
This phase develops through the "disinhibition" phenomenon. Depression of the cortex removes its inhibitory control over subcortical structures (primarily midbrain centers). Released from control, they manifest chaotic activity.
Stage III: Surgical Anesthesia
In this phase, signs of excitement disappear entirely, and the patient is ready for surgical intervention.
- General Characteristics:
- Consciousness and pain sensitivity are completely absent.
- Unconditioned reflexes are depressed, and muscle tone is significantly reduced.
- Respiration normalizes and becomes regular.
- Autonomic status stabilizes (blood pressure within normal limits), and pupils are constricted.
As anesthesia deepens within Stage III, four consecutive planes (levels) are distinguished:
- Light surgical anesthesia ($III_1$).
- Moderate surgical anesthesia ($III_2$).
- Deep surgical anesthesia ($III_3$).
- Ultra-deep surgical anesthesia ($III_4$).
If anesthesia continues to deepen excessively, complications may arise: bradycardia, cardiac arrhythmias, falling blood pressure, and progressive depression of respiratory function.
Recovery Dynamics and Risks
Emergence from anesthesia and potential critical states follow strict physiological rules.
- Recovery Stage: Begins after discontinuation of the anesthetic. The logic of this process is strictly orderly—CNS functions return in the reverse order of their depression. What was turned off last recovers first.
- Agonal Stage: A critical state resulting from drug overdose. Its pathogenesis stems from excessive central nervous system depression, leading to paralysis of vital medullary centers (vasomotor and respiratory).