Sevoflurane Added to a Total Intravenous Anesthetic During Evoked Potential Monitoring
A 47-year-old woman is having a two-level lumbar decompression. Anesthesia is total intravenous with propofol 120 microg/kg/min and remifentanil, no volatile agent, BIS 45. Baseline median SSEP N20 amplitude is 2.4 microV and transcranial MEPs are robust from abductor pollicis brevis bilaterally. During exposure the patient moves, so the anesthesiologist adds sevoflurane at 0.8% end-tidal; rocuronium 10 mg is also given to settle the field. Ten minutes later, N20 amplitude in both arms is 1.1 microV with latency increased 1.2 ms, and MEPs from both hands are unobtainable at 400 V. Posterior tibial SSEP amplitude has fallen by a similar proportion. TOF shows 2 of 4 twitches. No surgical manipulation has occurred at the neural elements.
1. Account for the change. Rank the plausible causes and justify your ranking.
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The temporal sequence and the symmetry are the two strongest clues. Every limb changed by a similar proportion, and the change began after two pharmacologic interventions and before any surgical contact with neural tissue. A structural cause at one level of the neuraxis does not produce proportionate, simultaneous decline in arms and legs.
Ranked:
- Combined volatile agent and partial neuromuscular blockade. Volatile anesthetics produce dose-dependent, reversible reduction of cortical SSEP amplitude with latency prolongation; at 0.8% end-tidal sevoflurane (roughly 0.4 MAC) a 30-50% amplitude drop is expected. MEPs are far more sensitive than SSEPs to volatile agents and, critically, to nondepolarizing blockade - a train-of-four of 2 of 4 is enough to abolish myogenic MEPs while SSEPs remain recordable.
- Physiologic confounders: hypocapnia from settling the patient, transient hypotension with the bolus, or a fall in temperature. These are symmetric too and should be checked.
- Technical: stimulator output, a shifted corkscrew, increased impedance. Symmetry across four limbs argues against a single-channel fault.
- Surgical: essentially excluded by the timing.
A useful discriminator is that the SSEPs degraded but did not disappear, which fits graded cortical suppression, whereas myogenic MEPs disappear abruptly once blockade or volatile concentration crosses threshold.
2. What do you recommend, and what is the correct handling of the baseline?
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Recommend reversing the controllable factors rather than accepting the new signals. Concretely: turn the sevoflurane off or reduce to 0.2-0.3 MAC, compensate depth with propofol and remifentanil, and restore train-of-four to 4 of 4 (sugammadex or time, depending on what the surgeon will accept). Confirm normocapnia, normothermia, and pressure. Then wait for equilibration - end-tidal and effect-site concentrations need ten to fifteen minutes before the evoked responses stabilize.
Do not re-baseline silently to the suppressed values. Re-baselining at a lower amplitude compresses the dynamic range and makes a later genuine 50% loss look like nothing. If a new baseline must be set because the anesthetic plan genuinely changed, establish it only after twenty to thirty minutes of steady state at the new conditions, and record both: the original values, the reason for the change, and the new reference. Everyone reading the record later - surgeon, interpreting physician, reviewer - must be able to see that the reference moved for an anesthetic reason.
In parallel, tell the surgeon that during this interval the monitor's sensitivity is reduced, and agree on a stricter alarm threshold relative to the new baseline (for example, any further 50% amplitude drop, or a latency increase of 1 ms).
3. The surgeon insists on keeping 0.8% sevoflurane because the field is moving. How do you proceed and what gets documented?
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State the trade-off plainly and let the surgeon make an informed decision rather than silently accepting a degraded monitor: at 0.8% end-tidal with 2 of 4 twitches, myogenic MEPs cannot be interpreted, and the SSEPs are attenuated enough that a further ischemic or mechanical insult would be detected later and less reliably. Offer the compromise that preserves both goals - deepen propofol and remifentanil (which suppress movement and are far less suppressive to MEPs than volatile agent), use a remifentanil bolus for stimulation runs, or restrict the volatile to the periods between monitoring runs and re-check after washout.
If the decision is to continue with the volatile, document: the concentrations and TOF, the time of the change, the evoked potential values before and after, the fact that the loss of MEPs and the SSEP attenuation were attributed to anesthetic effect on the basis of symmetry and timing, and that the surgeon was informed of the reduced sensitivity and elected to proceed. Then make the monitoring plan for that setting explicit - for example, SSEPs only, with periodic MEP attempts to test for recovery of the response if the agent is lightened.
This protects the patient and the record. The failure mode to avoid is a chart that shows disappearing waveforms with no explanation, which is indistinguishable from a missed intraoperative injury.