Anesthetic Depth

Question 1

Goal of Depth Assessment

Answer 1

assure lack of awareness, recall, pain, movement while avoiding excess levels ax/assoc problems

Question 2

Determination of Depth

Answer 2

o Amt of ax drugs in brain
o Magnitude of surgical or environmental stimulation
o Underlying conditions that have synergistic CNS depressant effects

Question 3

Guedel’s Stages of Ax - Use

Answer 3

o Modern techniques seldom only use inhalants  less reliance on Guedel’s classification
 BIS monitoring vs Guedel’s classic signs for depth in humans anesthetized with diethyl ether: relatively good correlation
o Incorporation of other drugs into balanced anesthetic techniques greatly influence reflexive, autonomic responses
o Greater reliance on monitoring patient physiologic parameters become more common

Question 4

Guedel’s Stages of Ax - Definitions

Answer 4

I. voluntary awareness
II. Delirium, involuntary movement
III. Sx Ax
IV: Extreme CNS Depression

Question 5

Stage I: voluntary awareness

Answer 5

 Awake state of awareness, all levels of obtundation to loss of consciousness
 Initial administration to loss of consciousness
 Epinephrine release: increased HR, pupillary dilation
 +/- salivation, urination, defecation
 Progressive ataxia, lose ability to stand, assume lateral recumbency

Question 6

Stage II: delirium, involuntary movement

Answer 6

 Loss of consciousness to onset of regular pattern of breathing
 Excitement stage heralded by spontaneous muscle movement
 CNS depressed –> patients lose all voluntary control
 Reflexes more primitive, exaggerated
 Patients react to external stimuli by violent reflex, struggling, breath holding, tachypnea, hyperventilation
 Continued catecholamine release: fast, strong heartbeat +/- cardiac arrhythmias, wide pupil dilation, vocalization
 Salivation excessive, esp cats, ruminants
 Dogs, cats, goats – vomiting may be evoked
 Larynx of pigs, cats very sensitive – stimulation = spasms

Question 7

Stage III: Surgical Anesthesia

Answer 7

 Marked by cessation of spontaneous m movement, onset of regular breathing pattern – 4 planes
 Unconsciousness, progressive depression of reflexes
 Muscular relaxation; ventilation slows, becomes regular
 Vomiting, swallowing reflexes lost
 Planes 1-4 vs light (cessation of eyeball movement) medium (light plane sx ax stage III, plane 2), deep (III, plane 3)

Question 8

Stage III, light plane (1)

Answer 8

• PLR, central eye, palpebral, m tone, +/- reflex movement to stimulus, physiologic response to nociception, med to large pupil size, moist cornea
• Likely too responsive for sx

Question 9

Stage III, light to medium plane (2)

Answer 9

GOAL FOR SX AX

VM rotated eye, PLR/no palpebral, small to med pupil size, some m tone, no reflex movement, +/- physiologic response to nociception, moist cornea

Question 10

Stage III, deep to medium (3)

Answer 10

VM rotated eye, no PLR or palpebral, medium to large pupil size, little m tone, no reflex movement or physiologic response to nociception, semi dry cornea

Question 11

Stage III, Deep (4)

Answer 11

• Central eye, no PLR or palpebral, large pupil size, no m tone, no reflex movement or physiologic response to nociception, dry cornea

Question 12

Stage IV: extreme CNS depression, death

Answer 12

 Respirations cease
 Heart continues to beat only for short time
 BP at shock level, CRT of visible MM markedly delayed, pupils widely dilated
 Death quickly unless immediate intervention

Question 13

MACawake

Answer 13

MAC to prevent verbal command in 50% of population

Halothane: 0.4
Iso: 0.39
Sevo: 1.3

Question 14

MAC(BAR)

Answer 14

MAC to block autonomic/SNS response to sx stimulation

Halothane: 1.1

Question 15

Mac(incision)

Answer 15

MAC to prevent muscular movement IRT surgical stimulus

Halothane: 0.9
Isoflurane 0.61
Sevo 2.0

Question 16

Other Features of Depth

Answer 16

o Breathing becomes shallower, more predominant abdominal component at deeper levels
o Young animals never exhibit much m tone, LA always exhibit a lot
o Swine, ruminants: chewing = light

Question 17

Gag, swallow reflex in all species

Answer 17

Indicator that patient is light

Question 18

Nystagmus

Answer 18

does not occur in anesthetized SA, light levels of ax in ruminants/swine
 Horses: slow ‘roving’ eyeball consistent with medium plane, fast nystagmus = light
o Globe positioning inconsistent indicator of depth in horses

Question 19

Ketamine?

Answer 19

central eye position, maintenance of palpebral

Question 20

Palpebral?

Answer 20

can become fatigued with repeated testing, never exhibited by some individuals

Question 21

PLR, Dazzle

Answer 21

(Dazzle = blink IRT bright light) – reliable indicators of light to medium ax

Question 22

Lacrimation

Answer 22

decreases, stops at deeper levels of anesthesia
 Horses: light level of anesthesia