Stages/Planes of Anesthesia

What is stages/planes?
• Animals pass through a series of anesthetic stages and planes
• These stages correspond to changes in anesthetic depth
• As animal passes through each stage, there is a progressive loss of pain perception, motor coordination, consciousness, reflexes, muscle tone and eventually cardiopulmonary function

Stage 1
• Immediately after inhalation or injectable agent administered
• Patient conscious but disoriented
• Decreased sensitivity to pain
• All reflexes present
• Patient may struggle, urinate, defecate or show other signs of fear or anxiety

Stage 2
• Begins with loss of consciousness
• All reflexes present (may appear exaggerated)
• Animal can chew and swallow (may yawn)
• Brain starts to lose control of voluntary body functions
• Animal may exhibit involuntary excitement: rapid limb movement, vocalization, struggling
• Breathing may be irregular or animal may appear to hold its breath
• Potentially unpleasant for patient and hazardous to staff
• Epinephrine release, cardiac arrhythmias & arrest
• Important to ensure smooth passage to Stage III