Presentation on theme: "Neurologic Monitoring"— Presentation transcript:
1 Neurologic Monitoring Mani K.C Vindhya M.DAsst Prof of AnesthesiologyNova Southeastern University
2 Introduction to EEG Interpretation Basic EEG Interpretation. You basically study an EEG for three things -- frequency, amplitude, and symmetry.1. Frequency. The electrical waveform of the EEG is divided into sine waves of specific frequencies (cycles/sec or Hertz = Hz). The mnemonic I use to remember the frequencies is:DON'T TOUCH A BRAIN, or DON'T TOUCH A BAGEL –D. T. A. B.Delta waves = 0 to 3 Hz -- Deep Anesthesia or Deep "Stage 4" SleepTheta waves = 4 to 7 Hz -- general anesthesia and hyperventilationAlpha waves = 8 to 13 Hz -- Awake but relaxed, eyes closedBeta waves = 14 to 30 Hz -- awake and alert (eyes open) or excitement phase of anesthesia16 monitors on head
4 Symmetry. Do the EEG waveforms look the same on each side? Amplitude -- measured in microvolts (uV). Sometimes amplitude is expressed as power, which is the amplitude squared, (uV)2.Low amplitude = < 20 microvolts (uV)Medium amplitude = 20 to 50 uVHigh amplitude = > 50 uVSymmetry. Do the EEG waveforms look the same on each side?
5 Even on the left, odd on the right Carotid endarterectomy – uses superior cervical plexus block and deep cervical plexus local anesthesia
6 Summary of Factors Affecting the EEG Increased frequencyBarbiturates, benzodiazepines, etomidate, propofol (low dose)Inhalational agents (< 1 MAC)Nitrous oxide 30-70% (acts on NMDA receptors)Ketamine (acts on NMDA receptors)Hypoxia (initially)Hypercarbia (mild)SeizuresDecreased frequency/increased amplitudeBarbiturates, etomidate, propofol (moderate dose)OpioidsInhalation agents (> 1 MAC)Hypoxia (mild)Hypocarbia (moderate to extreme)Hypothermia
8 Effects of anesthetics on the EEG: 1. Can be expressed as graphs showing frequency on the x-axis and amplitude on the y-axis2. Awake individual -- alpha and beta activity, about 20 uV amplitude
9 Fentanyl and other narcotics -- classically produce high amplitude, low frequency delta wave activity
10 Inhalational anesthetics: Low concentrations -- increase frequency & amplitude,or "EEG activation"Higher concentrations -- high amplitude theta acticityIsoflurane -- starts with "iso" because it produces an isoelectric"flat" EEG at > 2 MAC (This is true for sevoflurane and desflurane, too)Enflurane -- starts with an "E" because it is Epileptogenic, especially with along with hyperventilation and hypocapnia. Seizures markedly increase frequency.Sevoflurane – can also cause “seizure” activity (> 1.5 MAC)
11 Sodium pentothal and other barbiturates produce a whole spectrum of different effects, depending on the dose:EEG activation -- in low dosesBarbiturate spindlesSlow delta wave activityBurst suppression -- defined as an isoelectric EEG with < 6bursts of EEG activity per minute = the desired endpoint for barbiturate coma (cerebral protection)Isoelectric or "flat" EEG
12 Other intravenous anesthetics: a. Many of the other IV anesthetics suppress the EEG, as doessodium pentothal:PropofolEtomidateKetamineb. These IV anesthetics may show seizure-like (spike and wave) activity on the EEG.
13 To avoid peri-operative drug-induced seizures in epileptic patients: Continue anti-convulsant therapy.Consult with patient’s neurologist to discuss management.Avoid etomidate.Do not use sevoflurane routinely.Limit maximum concentration to < 1.5 MAC.If unstable – check serum levels of drug concentration! Consult with neurologist.
15 Effects of hypocarbia on the EEG (similar to mild hypoxia):
16 Effects of Anesthetics and Hypoxia on the EEG Hypoxia and many anesthetics:IV anesthetics (pentothal, propofol, etomidate)Inhaled anesthetics (isoflurane, sevoflurane, desflurane)Share similar effects on EEG:High frequency, high amplitudeLow frequency, high amplitudeEEG burst suppressionIsoelectric EEG
17 Types of processed EEGs Processed EEGs use power spectrum analysis to break the EEG down into its component frequencies.
18 Bispectral index (BIS) is generated in part by power spectrum analysis. Compressed Spectral Array (CSA) expresses EEG as “hills and valleys.”Frequency – on the x-axisAmplitude – shown as height of the hill, or y-axisTime. The “hills and valleys” are stacked over time on a 3-D z- axis.
19 Arrow marks time of left carotid artery occlusion Example of effect of carotid occlusion on CSA:
20 Density Spectral Array (DSA) Similar to CSAExpresses “hills and valleys” as “dark and light.”
23 Brainstem auditory evoked potentials or responses (BAEP's or BAER’s) Specialized form of EEG monitoringBackground EEG activity is electronically subtracted out.The EEG waveform evoked by auditory stimulus (clickingin ear) remains.
24 Shape of a typical BAEP = seven peaks Latency = time to first peak (usually 2 msec)Amplitude = height of the peaks
25 The seven peaks of the BAEP are believed to correspond to passage of a stimulus through "generators" in the auditory nerve, brainstem and cortex.
26 What do we look for during surgery? Mainly two things:Increase in latency (> 10%)Decrease in amplitude (<50%)These two changes could be indicative of impendinginjury or ischemia in the BAEP pathway
27 BAER's are barely affected by anesthetics: 1. No anesthetic drug produces a change in BAER’s that could be mistaken for a surgically induced change.2. Etomidate decreases amplitude and increases latency (but this is not clinically significant).
28 Somatosensory evoked potentials Upper extremity: Median nerve SSEP has “M” shape
29 Lower extremity: Posterior tibial nerve SSEP has “W” shape
30 What do we look for during surgery? Decrease in SSEP amplitude Increase in latency (time to first peak or dip)Six I’s that inhibit SSEP’s:Inhaled anesthetics, including isofluraneN2O doesn’t decrease amplitude alone, but has a synergistic effect with volatile agents.IV agents, but to a lesser extent than inhaled anestheticsEtomidate = the exception; it increases SSEP amplitudeIschemia/hypoxiaInjury, to the spinal cord or anywhere in SSEP pathway“Ice cold” temperatures (< oC)Incompetence (observer foul-ups)Anemia can also have an effect on SSEP so maintain hematocrit.Good practice is to use 0.5 MAC and TIVA
31 Summary of effects of anesthetics on SSEP’s: Inhaled anesthetics – dose-related decrease in amplitudeand increase in latencyUse less than 1 MAC volatile agentNitrous oxide – profound depressant effect on SSEPs, especially when used in combination with volatile agent
32 a. Propofol and thiopental Intravenous agentsa. Propofol and thiopentalSmall decrease in amplitude and increase in latencyPropofol is commonly used for TIVA(total IV anesthetic technique)Opioids – negligible effects on SSEPsKetamine and etomidate – increase SSEP amplitude(Etomidate is exceptional. It increases SSEP amplitude but decreases BAEP amplitude.)Anesthesia for SSEP’s (progression from routine to desperate)Volatile agent / N2O / narcoticVolatile agent / narcoticVolatile agent / propofol / narcoticTIVA: total IV anesthetic with propofol and narcoticEtomidate / narcotic (really desperate)Increases amplitudeAdrenocortical suppression with etomidateOpioids – Use sufentanil, remifentanil, or alfentanilEtomidate has a steroid structure. If you give this, then adrenal activity will be suppressed. This will affect blood pressure maintenance. If patient does not take steroids, then we have to give stress dose steroids.
33 Other Evoked Potentials Visual evoked potentialsM shapeLong latencySensitive to all anesthetics except opiates
34 Motor evoked potentials Evoked by transcranial electrical or magnetic stimulationEffects of anesthetics are profound.Same anesthetic progression as with SSEP’s
35 Why bother to test motor function? SSEP’s mainly test for dorsal (posterior) spinal cord function.Motor EP’s mainly test for ventral (anterior) spinal cord function.