CNIM Anesthesia Effects on Neurophysiological Monitoring 1 — Questions and Answers
Question 1: Which inhaled anesthetic causes the GREATEST suppression of cortical evoked potentials at equivalent concentrations?
- Nitrous oxide alone
- Isoflurane (Correct answer)
- Desflurane
- Sevoflurane
Correct answer: Isoflurane
Isoflurane causes greater cortical evoked potential suppression than desflurane or sevoflurane at equivalent MAC values due to its profound effect on cortical synaptic transmission.
Question 2: Nitrous oxide specifically affects evoked potentials by:
- Selectively suppressing subcortical potentials while sparing cortical ones
- Reducing amplitude of cortical responses, particularly affecting MEPs significantly (Correct answer)
- Having no effect on any evoked potential modality
- Selectively abolishing BAEPs
Correct answer: Reducing amplitude of cortical responses, particularly affecting MEPs significantly
Nitrous oxide, particularly when combined with volatile agents, reduces cortical response amplitudes and has a pronounced suppressive effect on MEPs even at low concentrations.
Question 3: The optimal anesthetic protocol for intraoperative neurophysiological monitoring (IONM) when MEP monitoring is required is:
- High-dose volatile agent with neuromuscular blockade
- Total intravenous anesthesia (TIVA) with propofol and remifentanil, avoiding NMB (Correct answer)
- Ketamine infusion with 70% nitrous oxide
- Halothane at 0.5 MAC with pancuronium
Correct answer: Total intravenous anesthesia (TIVA) with propofol and remifentanil, avoiding NMB
TIVA with propofol and remifentanil minimizes cortical and MEP suppression while avoiding NMB that would prevent muscle CMAP recording, making it the gold standard for IONM.
Question 4: How do opioids affect intraoperative evoked potentials?
- They cause profound suppression of all evoked potential modalities
- They have minimal effect on SSEPs, MEPs, and BAEPs at clinical doses (Correct answer)
- They selectively abolish MEPs
- They increase SSEP latencies by more than 10 ms
Correct answer: They have minimal effect on SSEPs, MEPs, and BAEPs at clinical doses
Opioids at clinical doses have minimal effects on SSEPs, MEPs, and BAEPs, making them compatible with IONM and suitable as part of TIVA protocols for monitored cases.
Question 5: Etomidate transiently increases SSEP amplitudes because it:
- Decreases synaptic inhibition in the somatosensory cortex (Correct answer)
- Blocks GABA receptors in the spinal cord
- Activates glutamate receptors
- Increases cerebral blood flow
Correct answer: Decreases synaptic inhibition in the somatosensory cortex
Etomidate decreases cortical inhibitory activity (GABA modulation), temporarily enhancing cortical excitability and increasing SSEP amplitudes, which can be exploited therapeutically.
Question 6: Ketamine's effect on intraoperative MEP monitoring is characterized by:
- Severe MEP suppression requiring dose reduction
- Relative MEP preservation or enhancement due to NMDA receptor antagonism (Correct answer)
- Complete abolition of corticospinal responses
- Selective enhancement of BAEPs only
Correct answer: Relative MEP preservation or enhancement due to NMDA receptor antagonism
Ketamine's NMDA receptor antagonism tends to preserve or even enhance MEPs, making it useful as an adjuvant anesthetic when MEP monitoring is difficult.
CNIM Anesthesia Effects on Neurophysiological Monitoring 1
Which inhaled anesthetic causes the GREATEST suppression of cortical evoked potentials at equivalent concentrations?