Chp. 10: Anesthetic Depth Monitoring and Electroencephalography

Question 1

Consciousness

Answer 1

Explicit awareness; brain is in aroused state and patient has perceptual qualities of awareness

Question 2

What portions of the CNS contribute to consciousness?

Answer 2

Brain stem, pons, thalamic nuclei, and cortex with connecting neural pathways

Question 3

Goals of GA

Answer 3

Reversible unconsciousness, amnesia, analgesia, skeletal muscle relaxation resulting in immobility, and autonomic stability to maintain physiologic homeostasis

Question 4

What part of the CNS is amnesia associated with?

Answer 4

The limbic system

Question 5

What part of the CNS is consciousness associated with?

Answer 5

Interaction between cerebral cortex and thalamus

Question 6

What part of the CNS are immobility and analgesia associated with?

Answer 6

The spinal cord

Question 7

What is anesthetic depth reliant upon?

Answer 7

Concentration of anesthetic drugs within the brain, magnitude of environmental stimulation, and concurrent CNS depressant effects such as hypothermia, hypercarbia, or hypotension

Question 8

MAC

Answer 8

Concentration of inhaled anesthetic within the alveoli at which 50% of subjects do not move in response to a noxious stimulus

Equivalent to median ED50

Question 9

What reduces MAC?

Answer 9

Hypothermia, severe hypotension, advanced age, concurrent use of other CNS depressants, pregnancy, severe hypoxemia

Question 10

What increases MAC?

Answer 10

Hyperthermia, hyperthyroidism, young age, concurrent medications that raise circulating central catecholamines

Question 11

Guedel Stage 1

Answer 11

Stage of voluntary movement lasting from initial drug administration to loss of consciousness. Some analgesia may be present in deeper phases of this stage. Strong, rapid heartbeat, increased BP, and pupillary dilation. Muscle tone remains and normal eye position is maintained as well as palpebral and corneal reflexes. Strong reaction to surgical manipulation.

Question 12

Guedel Stage 2

Answer 12

Stage of delirium or involuntary movement (“excitement phase”). Lasts from loss of consciousness to onset of regular pattern of breathing. May retain muscle tone, continued HR and BP increase. Dilated pupils and brisk ocular reflexes. Strong reaction to surgical stimulation.

Question 13

Guedel Stage 3

Answer 13

Stage of surgical anesthesia. Unconsciousness with progressive depression of reflexes. Muscle relaxation develops and ventilation becomes slow and regular. Progressive bradycardia, normalized BP. Ventromedial eye in dogs and cats, central in horses.

Plane I: Until eyeball movement ceases
Plane II: Progressive intercostal paralysis
Plane II: Diaphragmatic respiration

Question 14

Guedel Stage 4

Answer 14

Extreme CNS depression and overdose. Cardiopulmonary function fails. Hypotension, weak pulses, bradycardia or transient tachycardia, prolonged CRT, cardiac arrhythmias. Hypoventilation or apnea. Acute pupil dilation and central eye position.

Question 15

What is EEG?

Answer 15

Recording of spontaneous electrical brain activity from scalp electrodes.

Question 16

What do EEG signals represent?

Answer 16

Summation of neuronal activity within the cerebral cortex, resulting from excitatory and inhibitory postsynaptic potentials in large pyramidal neurons located in the lower layers (layer V) of the cerebral cortex.

Question 17

What generates EEG activity associated with consciousness?

Answer 17

Pacemaker neurons within the ARAS and then mediated and modulated through thalamic connections.

Question 18

EEG desynchronization

Answer 18

Shift in EEG pattern from high-voltage, slow-wave activity to low-voltage, fast-wave activity

Question 19

EEG frequency bands

Answer 19

delta 0-4Hz
omega 4-8Hz
alpha 8-13Hz
beta 13-30Hz

Question 20

How do EEG frequency bands change as anesthetic depth increases?

Answer 20

Decrease in beta activity and concomitant increase in both alpha and delta activity. With further depth, omega and delta frequency bands appear. Very deep anesthesia is characterized by “burst suppression” or flat periods interspersed with alpha and beta activity.

Question 21

Bispectral Index values for adequate GA

Answer 21

40-60
under 40 indicates deep state

Question 22

What factors influence BIS?

Answer 22

Ketamine and NO increase BIS.
Age and temperature also impact BIS (hypothermia decreases BIS).

Question 23

Is BIS effective for veterinary anesthesiology?

Answer 23

Low specificity, sensitivity, and predictive values reported. Frontal-temporal sensor positioning may provide better correlation with isoflurane depth of anesthesia in dogs.

Question 24

Spectral Entropy

Answer 24

Computed from EEG in the 0.8-32Hz range and thought to encompass mainly the hypnotic elements of the EEG

Question 25

State Entropy and Response Entropy

Answer 25

When EMG activity low, SE and RE should be identical.

With arousal and increase in facial EMG, RE should increase.

Question 26

Auditory Evoked Potentials

Answer 26

Electrical potentials evoked in the auditory pathway in response to sound stimuli. An auditory stimulus is repeatedly delivered and the EEG periods that immediately follow each stimulus are averaged.

Good correlation is iso-anesthetized dogs, not so in servo-anesthetized dogs.

Question 27

What drugs do not appear to consistently change the BIS response during propofol or inhaled anesthesia?

Answer 27

Low concentrations of opioids, NMBs, and NO