VPT 321: Veterinary Neuropharmacology (2+1) copy 1

STAGES OF ANAESTHESIA

• The stages of anaesthesia are divided into distinct packages, each correlating with a particular set of physiological responses or reflexes.
• The organisational scheme includes four stages of anaesthesia.
• Stage I
  • This stage is also known as the stage of analgesia or stage of voluntary excitement or stage of analgesia and amnesia.
  • This period begins from the beginning of induction to the loss of consciousness.
  • The patient exhibits voluntary resistance to restraint and to anaesthetic vapours.
  • Release of adrenaline due to excitement and fear causes increase in blood pressure and pulse
• Stage II
  • This stage is also known as the stage of delirium or stage of involuntary excitement or stage of uninhibited action.
  • This period begins from the loss of consciousness to onset of automatic respiration. There will be apparent excitement. Patient may show struggle, hold the breath, muscle tone increases, jaws are tightly closed, breathing is jerky, vomiting, involuntary micturition and defecation noticed. Pupillary dilatation, increase in heart rate and blood pressure due to sympathetic stimulation are noticed.
• Stage III
  • This stage is also known as the stage of surgical anaesthesia. It is divided into four planes as Plane 1, Plane 2, Plane 3 and Plane 4.
  • This period begins from the onset of automatic respiration to respiratory arrest.
    • Plane 1 – Light surgical anaesthesia – roving eye balls and plane ends with eyes becoming fixed.
    • Plane 2 – Moderate surgical anaesthesia – loss of corneal and laryngeal reflex.
    • Plane 3 – Deep surgical anaesthesia – Pupil starts dilating and light reflex is lost.
    • Plane 4 – Excessive surgical anaesthesia – intercostal paralysis, shallow abdominal breathing.
• Stage IV
  • This stage is also known as the stage of medullary paralysis or respiratory paralysis.
  • This period is between respiratory arrest and cardiac arrest.
• Not all anaesthetics display all the stages of anaesthsia.
• Ether : induces all stages
• Ultra short acting barbiturates : induces stages I, III and IV
• Dissociative anaesthetics : induces stages I and II
• Halothane, Isoflurane : induces stages I, III and IV . View for the progress of anaesthesia