General Anaesthesia - Stages of Anaesthesia

Stages of Anaesthesia

The Guedel's classification by Arthur Ernest Guedel described four stages of anaesthesia in 1937. Despite newer anaesthetic agents and delivery techniques, which have led to more rapid onset and recovery from anaesthesia, with greater safety margins, the principles remain.

Stage 1

Stage 1 anaesthesia, also known as the "induction", is the period between the initial administration of the induction agents and loss of consciousness. During this stage, the patient progresses from analgesia without amnesia to analgesia with amnesia. Patients can carry on a conversation at this time.

Stage 2

Stage 2 anaesthesia, also known as the "excitement stage", is the period following loss of consciousness and marked by excited and delirious activity. During this stage, respirations and heart rate may become irregular. In addition, there may be uncontrolled movements, vomiting, breath holding, and pupillary dilation. Since the combination of spastic movements, vomiting, and irregular respirations may lead to airway compromise, rapidly acting drugs are used to minimize time in this stage and reach stage 3 as fast as possible.

Stage 3

Stage 3, "surgical anaesthesia". During this stage, the skeletal muscles relax, vomiting stops, and respiratory depression occurs . Eye movements slow, then stop, the patient is unconscious and ready for surgery. It has been divided into 4 planes:

1. eyes initially rolling, then becoming fixed
2. loss of corneal and laryngeal reflexes
3. pupils dilate and loss of light reflex
4. intercostal paralysis, shallow abdominal respiration

Stage 4

Stage 4 anaesthesia, also known as "overdose", is the stage where too much medication has been given relative to the amount of surgical stimulation and the patient has severe brain stem or medullary depression. This results in a cessation of respiration and potential cardiovascular collapse. This stage is lethal without cardiovascular and respiratory support.