Anesthetic effects on intraoperative neurophysiological monitoring JongHae Kim Department of Anesthesiology and Pain Medicine School of Medicine, Catholic University of Daegu Contents • Effects on Sensory evoked potentials Somatosensory evoked potential Brainstem auditory evoked potential Visual evoked potential • Effects on Motor evoked potential Pharmacologic Effects of Anesthetics on Sensory Evoked Potentials Somatosensory Evoked Potentials Brainstem Auditory Evoked Potentials Visual Evoked Potentials Volatile Anesthetics General Characteristics • Synaptic transmission > axonal conduction • Polysynaptic pathway (cortical recordings) > Oligosynaptic pathway (spinal cord and subcortical recording) • VEP (cortical activity) > BAEP (brainstem and subcortical activities) General Characteristics • ↑SSEP latency, • ↑central conduction time, • ↓amplitude • Dose dependent • Minimally affect the subcortical waveform Effect of Inhaled Anesthetics on SSEP (Anesthesiology 2003;99:716-37) Morphologic changes of waveforms c) Isoflurane ANESTHESIOLOGY 1986;65:35–40 Sevoflurane and Desflurane • Like isoflurane • Permit higher concentrations • At ≤1.5 MAC, ↑cortical latency, ↓amplitude, minimal effects on subcortical SSEP Nitrous Oxide General Characteristics • 60-70% of N2O: ↓ cortical SSEP amplitude 50% • Cortical latency and subcortical waves unaffected • Potentiates effect of volatile and IV anesthetics Morphologic changes of waveforms and compounding effects of N2O c) Isoflurane ANESTHESIOLOGY 1986;65:35–40 Compounding effects of N2O on early cortical waveform Anesthetic Early cortical Waveform Drug/Concentration Latency Amplitude Halothane 1.5 MAC + 60% N2O 10-15% ↑ ~80% ↓ 1.5 MAC (alone) 10-15% ↑ ~70% ↓ Enflurane 1.5 MAC + 60% N2O Not recordable Not recordable 1.5 MAC (alone) > 25% ↑ ~85% ↓ Desflurane 1.5 MAC + 65% N2O Complete loss of waveform Complete loss of waveform 1.5 MAC ≤ 10% ↑ < 50% ↓ Compounding effects of N2O on early cortical waveform Anesthetic Early cortical Waveform Subcortical Waveform Drug/Concentration Latency Amplitude Isoflurane 0.5 MAC + 60% N2O < 10% ↑ 50-70% ↓ Negligible 0.5 MAC (alone) < 15% ↑ < 30% ↑ Negligible 1.0 MAC + 60% N2O 10-15% ↑ 50-75% ↓ Negligible 1.0 MAC (alone) 15% ↑ ≒50% ↓ Negligible 1.5 MAC + 60% N2O > 15% ↑ > 75% ↓ 5% ↑ in latency 1.6 MAC (alone) 15-20% ↑ 60-70% ↓ 5% ↑ in latency 20% ↓ in amplitude Intravenous Anesthetics General Characteristics • Affect SSEP less than inhaled anesthetics • Low doses: minimal effects • High doses: slight-moderate ↓amplitude and ↑latency • Subcortical potentials unaffected Barbiturates • Dose-dependent ↑in latency, ↓in amplitude in early cortical SSEP • Cortical waves are affected more than subcortical, midlatency waveforms • Synaptic transmission > axonal conduction (≒ volatile anesthetics) • Thiopental (5 mg/kg): latency 10-20% ↑, amplitude 20-30% ↓ (less than 10 min) • Barbiturate coma allows recording cortical SSEPs Barbiturates Drug/Dose Early Cortical Subcortical Waveform Waveform Latency Amplitude Thiopental 2.5-5.0 mg/kg <10% ↑ 5-30% ↓ Negligible 75 mg/kg 15% ↑ 60% ↓ Negligible Pentobarbital Up to 20 mg/kg ≒ 10% ↑ 45% ↓ None (latency) 20% ↓(amplitude) Etomidate • Increases cortical SSEP amplitude (400%) Not related to myoclonus1 d/t altered balance btw inhibitory and excitatory influences at the cerebral cortex level2 • Decreases subcortical amplitude (50%) 1. Kochs E, Treede RD, Schulte J: Increase in somatosensory evoked potentials during anesthesia induction with etomidate. Anaesthesist 1986; 35:359–64 2. Samra SK, Sorkin LS: Enhancement of somatosensory evoked potentials by etomidate in cats: An investigation of its site of action. ANESTHESIOLOGY 1991; 74:499–503 Ketamine • Increases cortical SSEP amplitude (max. effect within 2-10 min of bolus) ≒etomidate • No effect on cortical latency or subcortical waveforms Etomidate and Ketamine Drug/Dose Early Cortical Waveform Subcortical Latency Amplitude Waveform Etomidate 0.3-0.4 mg/kg + 2 mg/kg/h <10% ↑ 40-180% ↑ None (latency) 50% ↓(amplitude) 1 mg/kg 10% ↑ 150% ↓ Negligible Ketamine 0.5 mg/kg No effect No effect No effect 2-3 mg/kg + 2 mg/kg/h No effect 0-30% ↑ Negligible Propofol • ≒ Barbiturates • Rapid emergence for timely postop. neurologic assessment • 2.5 mg/kg: no changes in cortical and subcortical amplitudes, increased cortical latency (8%) and CCT (20%) • Cortical SSEP is best preserved (> N2O, midazolam, sevoflurane) Propofol Drug/Dose Early Cortical Waveform Subcortical Latency Amplitude Waveform 2.5 mg/kg <10% ↑ No change Negligible 2.5 mg/kg, then 10 mg/kg/h 10-15% ↑ 50% NA + sufentanil 0.5 mcg, then 0.25 mcg/kg/h Benzodiazepine • Diazepam (0.1-0.25 mg/kg): mild↓ in N- 20 amplitude, moderate ↓ in later wave cortical amplitude, abolished very long latency peaks (200-400 ms) • Midazolam (0.2-0.3 mg/kg): modest ↓ in amplitude, slight ↑of latency Benzodiazepine Drug/Dose Early Cortical Waveform Subcortical Latency Amplitude Waveform Midazolam 0.1-0.3 mg/kg <5% ↑ 25-40% ↓ Negligible Diazepam 0.1-0.25 mg/kg Minimal ↓ NA Opioids • Unimportant changes in latency and amplitude • No significant effect on SSEP (up to fentanyl 130 mcg/kg), Bolus > Infusion • Alfentanil: modest amplitude depression • Remifentanil < fentanyl Opioids • Subarachnoid meperidine blocks voltage- dependent Na+ channels: 60%↓amplitude, 10%↑latency • Subarachoid fentanyl (25 mcg), morphine (20 mcg/kg) + sufentanil (50 mcg), morphine (15 mcg/kg): no significant chagnes Opioids Drug/Dose Early Cortical Waveform Subcortical Latency Amplitude Waveform Morphine 0.25 mg/kg < 10% ↑ ≒ 20% ↓ NA Fentanyl 2.5 mcg/kg + N2O 5-10% ↑ Variable No change 25-100 mcg/kg < 10% ↑ 10-30% ↓ Negligible Sufentanil Sufentanil + N2O + 5-10% ↑ ≒ 50% ↓ No change 0.5%isoflurane/1 mcg/kg + infusion 5 mcg/kg Sufentanil (alone) ≒ 5% ↑ ≒ 40% ↓ No change (latency) Amplitude: 40% ↓ 1 mcg/kg + Sufentanil 5-10% ↑ No change NA propofol Opioids Drug/Dose Early Cortical Waveform Subcortical Latency Amplitude Waveform Remifentanil (with 0.4 MAC isoflurane) 1 mcg/kg + 0.2 NA 15-30% ↓ NA mcg/kg/min 2.5 mcg/kg + 0.5 30-40% ↓ mcg/kg/min 5.0 mcg/kg + 1.0 ≒ 40% ↓ mcg/kg/min Alfenanil 10 mcg/kg alone NA 50% ↓ NA 100 mcg/kg + 2 with N2O No effect 40% ↓ Clonidine and Dexmedetomidine • Clonidine: no change in latency and amplitude • Dexmedetomidine: affects amplitude minimally • During isoflurane anesthesia, dexmedetomidine blunts isoflurane’s effect on SSEP amplitude1. 1. Bloom M, Beric A, Bekker A: Dexmedetomidine infusion and somatosensory evoked potentials. J Neurosurg Anesthesiol 2001; 13:320–2 Clonidine and Dexmedetomidine Drug/Dose Early Cortical Waveform Subcortical Latency Amplitude Waveform Clonidine 2-10 mcg/kg No effect No effect 10% Amplitude ↓ No effect (latency) Dexmedetomidine Low sedative dose NA ≒ 10% ↓ ≒ 20% Amplitude ↓ High sedative dose NA ≒ 30% ↓ ≒ 10% Amplitude ↓ Neuromuscular Blocking Drugs • No effects on SSEP, BAEP, or VEP • Improve waveform quality through elimination of the EMG artifact Regional Administration • Local infiltration & subarachnoid block: abolish SSEPs • Epidural block: depends on dose and dermatome • IV lidocaine: unlikely to interfere with intraop. monitoring Drug/Dose Early Cortical Waveform Subcortical Latency Amplitude Waveform Lidocaine 1.5 mg/kg, then 3 mg/kg/h 5% ↑ 25-30% ↓ Negligible Implications for Perioperative Monitoring • Volatile anesthetics: 1.0 MAC alone • Desflurane or sevoflurane: 1.5-1.75 MAC • IV anesthetics > volatile anesthetics • Propofol-sufentanil reduce amplitude significantly1 1. Borrissov B, Langeron O, Lille F, Gomola A, Saillant G, Riou B, Viars P: Combination of propofol-sufentanil on somatosensory evoked potentials in surgery of the spine. Ann Francaises d Anesth et de Reanimation 1995; 14:326–30 Implications for Perioperative Monitoring • Preserve amplitude ! • Low baseline amplitude: > 50 yr, congenital scoliosis, paralytic scoliosis, spinal stenosis, spinal tumor, other preexisting neurologic deficits Strategies to Enhance the amplitude and reproducibility of SSEPs • High-pass 30-Hz digital filtering1 • Substitution of propofol for N2O • Eliminating N2O • Substitution of remifentanil for fentanyl & N2O • If N2O is necessary, combine it with midazolam2 • Adjuncts(dexmedetomidine, clonidine, neuroaxial opioids) reduce MAC 1. Kalkman CJ, ten Brink SA, Been HD, Bovill JG: Variability of somatosensory cortical evoked potentials during spinal surgery: Effects of anesthetic techniques and high-pass digital filtering. Spine 1991; 16:924–9 2. Koht A, Schutz W, Schmidt G, Schramm J, Watanabe E: Effects of etomidate, midazolam, and thiopental on median nerve somatosensory evoked potentials and the additive effects of fentanyl and nitrous oxide. Anesth Analg 1988;67:435–41 Strategies to Enhance the amplitude and reproducibility of SSEPs • Ketamine • Etomidate: bolus 0.5-1 mg/kg + infusion 20-30 mcg/kg/min1 • Low concentrations of volatile anesthetics + etomidate or propofol (anesthetic depth) + vasodilator and β- blocker (control BP and myocaridal stress) • Sevoflurane permits faster SSEP recovery2 1. Sloan TB, Ronai AK, Toleikis JR, Koht A: Improvement of intraoperativesomatosensory evoked potentials by etomidate. Anesth Analg 1988; 67:582–5 2. Ku ASW, Irwin MG, Chow B, Gunawardene S, Tan EE, Luk KDK: Effect of sevoflurane/nitrous oxide versus propofol anaesthesia on somatosensory evoked potential monitoring of the spinal cord during surgery to correct scoliosis. Br J Anaesth 2002; 88:502–7 Summary (Periop. Implications) • Volatile anesthetics: 0.5 MAC with N2O or 1.0 MAC without N2O • Desflurane, Sevoflurane: