- Stage I (Analgesia Stage): This stage begins with the anesthetic administration and ends when consciousness is lost. The patient is sedated with mild analgesia, and reflexes are intact.
- Stage II (Excitement or Delirium Stage): During this stage, consciousness is lost, but the brain remains hyperactive. It is characterized by irregular breathing, involuntary movements, and heightened reflexes.
- Stage III (Surgical Anesthesia Stage): This stage is the ideal depth for surgery, where the patient is unconscious and there is minimal response to painful stimuli. Respiratory and cardiovascular functions are maintained at a stable level. The role of the anesthesia provider is to keep the patient within this stage during surgery and allow a gradual transition during emergence.
- Stage IV (Medullary Depression): This stage represents overdose and involves suppression of brainstem function, including respiratory and cardiovascular collapse. Though not a stage targeted during anesthesia, it is crucial to monitor the patient to prevent an overdose that could lead to delayed emergence or failure to emerge.
Patients typically reverse these stages during emergence, re-entering a heightened state of arousal before regaining full consciousness.
| Stage | Characteristics |
|---|---|
| I — Analgesia | Begins with anesthetic administration and ends when consciousness is lost; patient is sedated with mild analgesia and reflexes are intact |
| II — Excitement or delirium | Consciousness is lost but the brain remains hyperactive: irregular breathing, involuntary movements, heightened reflexes |
| III — Surgical anesthesia | Ideal depth for surgery — unconscious, minimal response to painful stimuli, respiratory and cardiovascular function stable; the provider keeps the patient here and allows a gradual transition during emergence |
| IV — Medullary depression | Overdose, with suppression of brainstem function including respiratory and cardiovascular collapse; not a targeted stage, and an overdose here can cause delayed emergence or failure to emerge |
