Lecture 5: Monitoring Anesthetic Depth

Is general anesthesia a reversible process?

yes

General anesthesia should include both the loss of ____________ and ________.

consciousness and sensation

What are signs of stage 2 anesthesia?

• struggling or exaggerated reaction to stimuli

• vomiting or regurgitation can occur; salivation (esp. ruminants and cats)

• Laryngospasm (esp. cats and pigs)

• Dilated pupils; nystagmus (common in horses)

• increased or irregular respiratory pattern; breath holding

• Increased heart rate; arrhythmias due to catecholamine release

• active palpebral reflexes

• tight anal sphincter tone

• jaw tone present

• May vocalize

What are signs of stage 1 anesthesia?

• most variable stage

• breath holding

• increased HR

• pupil dilation

What stage of voluntary movement is defined as the start of the administration of induction drugs until loss of consciousness?

stage I anesthesia

What is the stage of delirium and involuntary movement and lasts from loss of consciousness to onset of regular breathing pattern?

stage II

What is the stage of surgical anesthesia due to level of unconsciousness with progressive depression of reflexes, muscle relaxation and slow, regular breathing pattern?

stage II

What anesthetic stage involves loss of jaw tone?

stage III

In what planes of stage III are the vomiting and swallowing reflexes abolished?

plane 1 = vomiting

plane 2 = swallowing

What anesthetic stage persists until eyeball movement stops and is mainly useful to note horses waking up?

stage III, plane I

What stage are most surgical procedures performed during?

stage III, plane II

What are the characteristics of stage III, plane II?

• Characterized by progressive intercostal paralysis

• Respiration and heart rate have stabilized

• Sluggish palpebral reflex

• Strong corneal reflex

• Adequate muscle relaxation and analgesia

• Abolished laryngeal reflexes

What is the deep pane of surgical anesthesia?

stage III, plane III

What are the physical signs of stage III, plane III anesthesia?

• Decrease in heart rate

• Slow, shallow abdominal breathing pattern (“diaphragmatic respiration”)

• No jaw tone

• No reaction to surgical stimulation, profound muscle relaxation

• No palpebral reflex, weak corneal reflex

• Pupil progressively dilated and central

What are the physical signs of stage III, plane IV anesthsia?

• Pupils dilated

• Hypotension progressing to shock

• Slow CRT

• Slow, irregular respirations

• Lack of muscle tone

• No anal sphincter tone

What are the physical signs of stage IV anesthsia?

• Extreme CNS depression➔ anesthetic overdose

• Pale to white mm, CRT ≥ 3 seconds

• Respiratory arrest followed by cardiac arrest

• Central, widely dilated pupils

• Absent ocular reflexes

• Flaccid muscle tone

• Death unless swift intervention

What ocular signs should you check when monitoring anesthetic depth?

• palpebral reflex

• eye position

• lacrimation

• nystagmus

• corneal reflex

What signs, other than ocular, should you be checking to monitor anesthetic depth?

• Jaw tone

• Anal reflex

• Pedal reflex; tail pinch, ear pinch or ear-flick reflex

• Overall muscle tone or degree of patient relaxation

What physical characteristics are the goal for a surgical anesthesia plane?

• Relaxed jaw tone…is it easy to open with 2 fingers?

• Mild to no palpebral reflex

• Ventral medial eye rotation

• Relaxed anal tone

• Smooth, regular respiratory pattern

• Normal heart rate and BP

• No swallowing, coughing, vomiting or laryngospasm

What are some horse specific signs to look for when monitoring anesthesia?

• horses lacrimate in light stages of anesthesia

• eyes stay central instead of ventral medial eye rotation

• nystagmus in lighter stage

• corneal reflex persists into deeper anesthesia plane

What are some cat specific signs to look for when monitoring anesthesia?

may go into laryngospasm and/or may retain cough, swallow, and vomiting reflex until they reach a deeper plane of anesthesia

What are some cattle specific signs to look for when monitoring anesthesia?

have consistent, reliable eyeball rotation

What are some rodent specific signs to look for when monitoring anesthesia?

• must observe feet, ears, and muzzle for pallor or cyanosis

• pedal reflex

What are some poultry specific signs to look for when monitoring anesthesia?

• examine comb and/or wattle for pallor

• vent reflex

What are some rabbit specific signs to look for when monitoring anesthesia?

can use color of light reflected from eye to determine circulation status

How can ketamine or propofol make anesthetic depth monitoring difficult?

ketamine: typical eye signs not produced

propofol: apnea, not a problem for intubation

What is an anesthetic gas analyzer used for?

to perform minimum alveolar concentration (MAC) studies

What is the bispectral index (BIS)?

an electroencephalogram measurement used to gauge anesthetic depth

If post-op pain is suspected, when should tx start?

BEFORE they wake up

Why are horses often sedated with an alpha2 agonist going into inhalant anesthesia recovery with minimized stimulation (quiet, dark room)?

stage II can be a difficult time due to delirium/dysphoria, especially in equine patients