Stages of Anesthesia

• An irregular descending CNS depression.
• In brain: 
  • Higher functions lost first —> progressively lower brain area involved. 
  • Vital centers located in medulla paralyzed at last as depth of anesthesia increases.
• In spinal cord: 
  • Lower segments affected earlier than upper segments. 

• Guedel described 4 stages with reference as ether.
  • Note: IIIrd stage divided into 4 planes.
• Only observed in slow acting GA.
  • With faster-acting inducing agents, no clear cut stages observed with GA’s. 

Stage 1:

• Stage of analgesia.
• Extends from beginning of anesthetic inhalation to loss of conciousness.

Stage 2:

• Stage of delirium or excitement.
• Extends from loss of conciousness to beginning of regular respiration.
• Features:
  • Presence of roving eye ball (maximum movement of eye).
  • Pupil is partially dilated.
  • Loss of eyelash reflex -> 1st reflex to be lost.
  • No loss of eyelid reflex.

Stage 3:

• Stage of surgical anesthesia. 
• Extends from beginning of regular respiration to cessation of spontaneous breathing.
• Divided into 4 planes.
• Plane 1:
  • From beginning of regular respiration to cessation of eye movement.
  • Presence of roving eye ball.
  • Loss of eyelid reflex.
  • Plane ends with eye-ball fixation.
  • Normal pupil size.
• Plane 2:
  • From cessation of eye movement to respiratory paresis.
  • Fixed eye ball.
  • Loss of corneal reflex.
  • Pupil starts dilating (1/2 dilated).
  • Lacrimation present.
• Plane 3:
  • From respiratory paresis to respiratory paralysis.
  • Pupil 3/4 dilated.
  • Swallowing & laryngeal reflexes are lost.
  • Lacrimation present
• Plane 4:
  • Intercostal paralysis.
  • Only abdominal respiration.
  • Fully dilated pupil. 
  • Lost carinal (cough reflex) –> Final reflex to be lost.

Stage 4:

• Stage of medullary paralysis.
• Presence of respiratory arrest & apnea.
• Fully dilated & fixed pupil.

• Return of reflexes is in opposite sequence.
  • i.e., 1st – Carinal reflex. 
  • Last – Eyelash reflex.
• Note: Cough should come first but swallowing comes first.
  • Because coughing requires diaphragm & respiratory muscles effort.

Monitoring the depth of anesthesia:

• Done mainly clinically:
  • Tachycardia
  • Hypertension.
  • Lacrimation.
  • Perspiration (sweating).
  • Eye movement.
  • Preserved reflexes (coughing).
  • Movement on painful stimuli.
  • Laryngospasm / Bronchospasm.
  • Tachypnea.
  • 3 waves on EEG.
  • Evoked responses.
  • Bispectral index.