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Neurological monitoring(1)
The document discusses various techniques for intraoperative neurological monitoring during surgery, including EEG, SSEP, MEP, transcranial Doppler, and cerebral oximetry. EEG measures electrical brain activity and can detect changes related to ischemia, anesthesia effects, or other insults. SSEP uses electrical nerve stimulation to measure sensory pathway function from peripheral nerves to the brain. MEP assesses motor pathways by recording responses to transcranial electrical stimulation. Transcranial Doppler noninvasively measures cerebral blood flow velocity. Cerebral oximetry monitors tissue oxygen saturation in the brain. These techniques provide different but complementary information and are useful for detecting adverse neurological events during surgery.
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Manu Jacob, Perfusionist at KMCT MCH, introduces the presentation.
Key techniques explained: EEG, SSEP, MEP, Transcranial Doppler, and Cerebral Oximetry.
EEG measures brain electrical activity, blood flow, and anesthetic effects. Monitors ischemia but should avoid critical periods.
Types of brain waves: Alpha (8-13 Hz), Beta (>13 Hz), Theta (4-7.5 Hz), Delta (1-3.5 Hz), indicating different mental states.
EEG changes during cardiopulmonary bypass are difficult to interpret due to factors like CO2 tension and hypothermia.
Definition of evoked potentials. Sensory response measured after stimulus. SSEP monitors neural pathways to identify interruptions.
MEP involves stimulating motor cortex for response. Sensitive to anesthetics, with IV agents causing less depression.
Noninvasive CBF measurement via sound waves. Monitors flow velocity and identifies emboli and vasospasm.
Uses NIRS to measure tissue oxygen saturation in cerebral cortex, differentiating from traditional pulse oximetry.
EEG facilitates assessment of cerebral perfusion. SSEP detects subcortical events, with new monitoring techniques under evaluation.
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- 2. EEG SSEP MEP Transcranial Doppler Cerebral Oximetry
- 3. Electroencephalogram –surface recordings of the summation of excitatory and inhibitory postsynaptic potentials generated by pyramidal cells in cerebral cortex EEG: Measures electrical function of brain Indirectly measures blood flow Measures anesthetic effects
- 4. EEG usedto monitor for ischemia Avoid during critical periods of the case: Changing anesthetic technique Changing gas levels Administering boluses of medications that affect EEG
- 5. Alpha wave-- 8 – 13 Hz. Beta wave -- >13 Hz. (14 – 30 Hz.) Theta wave -- 4 – 7.5 Hz. Delta waves – 1 – 3.5 Hz.
- 7. rhythmic, 8-13Hz mostly on occipital lobe 20-200 μV normal, relaxed awake rhythm with eyes closed
- 8. irregular, 14-30Hz mostly on temporal and frontal lobe mental activity excitement
- 9. Cardiopulmonary bypass Theoretically beneficial ▪ Embolic events with cannulation ▪ Increased risk in patients with carotid disease Difficult to interpret EEG changes ▪ Alteration of arterial carbon dioxide tension ▪ Changes in blood pressure ▪ Hypothermia ▪ Hemodilution (anemia)
- 10. Definition: electrical activitygenerated in response to sensory or motor stimulus Stimulus given, then neural response is recorded at different points along pathway Sensory evoked potential Latency – time from stimulus to onset of SER Amplitude – voltage of recorded response
- 11. Sensory evokedpotentials Somatosensory (SSEP) Auditory (BAEP) Visual (VEP) SSEP – produced by electrically stimulating a cranial or peripheral nerve If peripheral n. stimulated – can record proximally along entire tract (peripheral n., spinal cord, brainstem, thalamus, cerebral cortex) As opposed to EEG, records subcortically
- 12. Stimulation withfine needle electrodes Stimulate median nerve – signal travels anterograde causing muscle twitch, also travels retrograde up sensory pathways along dorsal columns all the way to brain cortex
- 13. Can measurethe electrophysiologic response to nerve stimulation all the way up this pathway Monitor many waves (representing different nerves along pathway) and localization of where the neural pathway is interrupted is possible
- 14. Transcranial electrical MEPmonitoring Stimulating electrodes placed on scalp overlying motor cortex Application of electrical current produces MEP Stimulus propagated through descending motor pathways
- 15. Evoked responsesmay be recorded: Spinal cord, peripheral n., muscle itself
- 16. MEPs verysensitive to anesthetic agents Possibly due to anesthetic depression of anterior horn cells in spinal cord Intravenous agents produce significantly less depression
- 17. Direct, noninvasive measurementof CBF Sound waves transmitted through thin temporal bone, contact blood, are reflected, and detected Most easily monitor middle cerebral artery
- 18. Does notmeasure actual blood flow but velocity Velocity often closely related to flow but two are not equivalent Surgical field may limit probe placement and maintenance of proper position CPB Detect air or particulate emboli during cannulation, during bypass, weaning from bypass, decannulation Significant data pending Detection of vasospasm (well-established) Smaller area – increase in velocity (>120cm/s)
- 19. Measures oxygensaturation in the vascular bed of the cerebral cortex Interrogates arterial, venous, capillary blood within field Derived saturation represents a tissue oxygen saturation measured from these three compartments Unlike pulse oximetry (requires pulsatile blood), NIRS assess the hemoglobin saturation of venous blood, which along with capillary blood, composes approximately 90% of the blood volume in tissues Believed to reflect the oxygen saturation of hemoglobin in the post extraction compartment of any particular tissue Measures tissue oxygen saturation
- 20. EEG isa useful modality for measuring intraoperative cerebral perfusion SSEP offers the additional advantage of measuring subcortical adverse events New techniques for neurological monitoring are being developed which need to be further evaluated and validated