Neurologic MonitoringNeurologic Monitoring

Mani K.C Vindhya M.DMani K.C Vindhya M.D

Asst Prof of AnesthesiologyAsst Prof of Anesthesiology

Nova Southeastern UniversityNova Southeastern University

Introduction to EEG Interpretation Introduction to EEG Interpretation

Basic EEG Interpretation. Basic EEG Interpretation. You basically study an EEG for three You basically study an EEG for three things --things -- frequency, amplitude, and symmetry.frequency, amplitude, and symmetry.

1. 1. Frequency.Frequency. The electrical waveform of the EEG is divided into The electrical waveform of the EEG is divided into sinesine waves of specific frequencies (cycles/sec or Hertz = Hz). The waves of specific frequencies (cycles/sec or Hertz = Hz). The mnemonic I use to remember the frequencies is:mnemonic I use to remember the frequencies is:

DDON'TON'T T TOUCHOUCH A B A BRAIN, orRAIN, or D DON'TON'T T TOUCHOUCH A B A BAGEL – AGEL – D. T. A. B.D. T. A. B. DDelta waves = 0 to 3 Hz --elta waves = 0 to 3 Hz -- D Deep Anesthesia oreep Anesthesia or D Deep "Stage 4"eep "Stage 4" Sleep Sleep TTheta waves = 4 to 7 Hz -- general anesthesia andheta waves = 4 to 7 Hz -- general anesthesia and hyperventilation hyperventilation AAlpha waves = 8 to 13 Hz --lpha waves = 8 to 13 Hz -- A Awake but relaxed, eyes closedwake but relaxed, eyes closed BBeta waves = 14 to 30 Hz -- awake and alert (eyes open) oreta waves = 14 to 30 Hz -- awake and alert (eyes open) or

excitement phase of anesthesia excitement phase of anesthesia

Amplitude Amplitude -- measured in microvolts (uV). -- measured in microvolts (uV). Sometimes amplitude isSometimes amplitude is expressed as power, expressed as power, which is the amplitude squared, (uV)2.which is the amplitude squared, (uV)2. Low amplitude = < 20 microvolts (uV) Low amplitude = < 20 microvolts (uV) Medium amplitude = 20 to 50 uV Medium amplitude = 20 to 50 uV High amplitude = > 50 uV High amplitude = > 50 uV

Symmetry. Symmetry. Do the EEG waveforms look the Do the EEG waveforms look the same on each side? same on each side?

Summary of Factors Affecting the EEG Summary of Factors Affecting the EEG Increased frequency Increased frequency

Barbiturates, benzodiazepines, etomidate, propofol (low dose) Barbiturates, benzodiazepines, etomidate, propofol (low dose) Inhalational agents (< 1 MAC) Inhalational agents (< 1 MAC) Nitrous oxide 30-70% (acts on NMDA receptors) Nitrous oxide 30-70% (acts on NMDA receptors) Ketamine (acts on NMDA receptors) Ketamine (acts on NMDA receptors) Hypoxia (initially) Hypoxia (initially) Hypercarbia (mild) Hypercarbia (mild) Seizures Seizures

Decreased frequency/increased amplitude Decreased frequency/increased amplitude Barbiturates, etomidate, propofol (moderate dose) Barbiturates, etomidate, propofol (moderate dose) Opioids Opioids Inhalation agents (> 1 MAC) Inhalation agents (> 1 MAC) Hypoxia (mild) Hypoxia (mild) Hypocarbia (moderate to extreme) Hypocarbia (moderate to extreme) Hypothermia Hypothermia

Decreased frequency, decreased amplitude Decreased frequency, decreased amplitude Barbiturates (high dose) Barbiturates (high dose) Hypoxia (mild) Hypoxia (mild) Hypercarbia (severe) Hypercarbia (severe) Hypothermia (< 35 oC) Hypothermia (< 35 oC)

Electrical silence Electrical silence Barbiturates (coma dose) Barbiturates (coma dose) Etomidate, propofol (high dose) Etomidate, propofol (high dose) Isoflurane, desflurane, sevoflurane (2 MAC) Isoflurane, desflurane, sevoflurane (2 MAC) Hypoxia (severe) Hypoxia (severe) Hypothermia (< 15-20 oC) Hypothermia (< 15-20 oC) Brain death Brain death

Effects of anesthetics on the EEG:Effects of anesthetics on the EEG: 1. Can be expressed as graphs showing frequency on 1. Can be expressed as graphs showing frequency on

the x-axis and amplitude on the y-axisthe x-axis and amplitude on the y-axis 2. Awake individual -- alpha and beta activity, about 2. Awake individual -- alpha and beta activity, about

20 uV amplitude20 uV amplitude

Fentanyl and other narcotics -- classically produce high Fentanyl and other narcotics -- classically produce high amplitude, low frequency delta wave activity amplitude, low frequency delta wave activity

Inhalational anesthetics: Inhalational anesthetics: Low concentrations -- increase frequency & amplitude, Low concentrations -- increase frequency & amplitude,

or "EEG activation"or "EEG activation" Higher concentrations -- high amplitude theta acticity Higher concentrations -- high amplitude theta acticity IsoIsoflurane -- starts with "iso" because it produces anflurane -- starts with "iso" because it produces an iso isoelectricelectric

"flat" EEG at > 2 MAC (This is true for sevoflurane and desflurane, too)"flat" EEG at > 2 MAC (This is true for sevoflurane and desflurane, too) EEnflurane -- starts with an "E" because it is nflurane -- starts with an "E" because it is EEpileptogenic, especially with along pileptogenic, especially with along

with hyperventilation and hypocapnia. Seizures markedly increase frequency.with hyperventilation and hypocapnia. Seizures markedly increase frequency. Sevoflurane – can also cause “seizure” activity (> 1.5 MAC)Sevoflurane – can also cause “seizure” activity (> 1.5 MAC)

Sodium pentothal and other barbiturates produce a whole Sodium pentothal and other barbiturates produce a whole spectrum of different effects, depending on the dose:spectrum of different effects, depending on the dose:

EEG activation -- in low doses EEG activation -- in low doses Barbiturate spindles Barbiturate spindles Slow delta wave activity Slow delta wave activity Burst suppression -- defined as an isoelectric EEG with < 6 Burst suppression -- defined as an isoelectric EEG with < 6 bursts of EEG activity per minute = the desired endpoint for bursts of EEG activity per minute = the desired endpoint for

barbiturate coma (cerebral protection)barbiturate coma (cerebral protection) Isoelectric or "flat" EEGIsoelectric or "flat" EEG

Other intravenous anesthetics: • a. Many of the other IV anesthetics suppress the EEG, as does sodium pentothal:

– Propofol – Etomidate – Ketamine

• b. These IV anesthetics may show seizure-like (spike and wave) activity on the EEG.

Only Pro- Both Pro- and Mostly convulsant Anticonvulsant Anticonvulsant

Nitrous oxide Enflurane Isoflurane

Sevoflurane Desflurane

Opioids Etom idate Midazolam

Propofol Thiopental Ketam ine

M ethohexital

To avoid peri-operative drug-induced seizures in To avoid peri-operative drug-induced seizures in epileptic patients:epileptic patients: Continue anti-convulsant therapy. Continue anti-convulsant therapy. Consult with patient’s neurologist to discuss Consult with patient’s neurologist to discuss

management. management. Avoid etomidate. Avoid etomidate. Do not use sevoflurane routinely. Do not use sevoflurane routinely.

Limit maximum concentration to < 1.5 MAC. Limit maximum concentration to < 1.5 MAC.

Effects of hypoxia on the EEG:Effects of hypoxia on the EEG:

Effects of hypocarbia on the EEG (similar to mild hypoxia):Effects of hypocarbia on the EEG (similar to mild hypoxia):

Effects of Anesthetics and Hypoxia on the EEGEffects of Anesthetics and Hypoxia on the EEG Hypoxia and many anesthetics: Hypoxia and many anesthetics:

IV anesthetics (pentothal, propofol, etomidate) IV anesthetics (pentothal, propofol, etomidate) Inhaled anesthetics (isoflurane, sevoflurane, desflurane) Inhaled anesthetics (isoflurane, sevoflurane, desflurane)

Share similar effects on EEG: Share similar effects on EEG: High frequency, high amplitude High frequency, high amplitude Low frequency, high amplitude Low frequency, high amplitude EEG burst suppression EEG burst suppression Isoelectric EEGIsoelectric EEG

Types of processed EEGs Types of processed EEGs Processed EEGs use power spectrum analysis to Processed EEGs use power spectrum analysis to

break the EEG down into its component frequencies.break the EEG down into its component frequencies.

Bispectral index (BIS) Bispectral index (BIS) is generated in part by power spectrum is generated in part by power spectrum analysis.analysis.

Compressed Spectral Array (CSA) Compressed Spectral Array (CSA) expresses EEG as “hills and expresses EEG as “hills and valleys.”valleys.”

Frequency – on the x-axis Frequency – on the x-axis Amplitude – shown as height of the hill, or y-axis Amplitude – shown as height of the hill, or y-axis Time. The “hills and valleys” are stacked over time on a 3-D z-Time. The “hills and valleys” are stacked over time on a 3-D z-

axis. axis.

Example of effect of carotid occlusion on CSA: Example of effect of carotid occlusion on CSA:

Arrow marks time of left carotid artery occlusion

Density Spectral Array (DSA) Density Spectral Array (DSA) Similar to CSA Similar to CSA Expresses “hills and valleys” as “dark and light.” Expresses “hills and valleys” as “dark and light.”

Lifescan (Periodic Analysis) Lifescan (Periodic Analysis) – expressed EEG activity as – expressed EEG activity as “telephone poles” “telephone poles”

Introduction to Evoked Potentials Introduction to Evoked Potentials

– Sensory evoked potentials (or responses)

• Brainstem auditory evoked responses

• Somatosensory evoked potentials

• Visual evoked responses

EVOKED LATENCY ANESTHETIC

POTENTIAL INTERACTION

BAER’s 2 msec Barely affected

SSEP’s (median 20 msec Somewhat affected

nerve)

SSEP’s (posterior 40 msec Somewhat affected

tibial nerve)

VER’s 70-100 msec Very inhibited

Brainstem auditory evoked potentials or responses Brainstem auditory evoked potentials or responses (BAEP's or BAER’s)(BAEP's or BAER’s)

Specialized form of EEG monitoring Specialized form of EEG monitoring Background EEG activity is electronically subtracted out. Background EEG activity is electronically subtracted out. The EEG waveform evoked by auditory stimulus (clicking The EEG waveform evoked by auditory stimulus (clicking

in ear) remains.in ear) remains.

Shape of a typical BAEP = seven peaks Shape of a typical BAEP = seven peaks Latency = time to first peak (usually 2 msec) Latency = time to first peak (usually 2 msec) Amplitude = height of the peaks Amplitude = height of the peaks

The seven peaks of the BAEP The seven peaks of the BAEP are believed to are believed to correspond to passage of acorrespond to passage of a stimulus through "generators" stimulus through "generators" in the auditory nerve, brainstem and cortex.in the auditory nerve, brainstem and cortex.

What do we look for during surgery? What do we look for during surgery? Mainly two things: Mainly two things:

Increase in latency (> 10%) Increase in latency (> 10%) Decrease in amplitude (<50%) Decrease in amplitude (<50%)

These two changes could be indicative of impendingThese two changes could be indicative of impending

injury or ischemia in the BAEP pathway injury or ischemia in the BAEP pathway

BAER's are barely affected by anesthetics:BAER's are barely affected by anesthetics: 1. No anesthetic drug produces a change in BAER’s 1. No anesthetic drug produces a change in BAER’s

that could be mistaken for a surgically induced that could be mistaken for a surgically induced change.change.

2. Etomidate decreases amplitude and increases 2. Etomidate decreases amplitude and increases latency (but this is not clinically significant).latency (but this is not clinically significant).

Somatosensory evoked potentials Somatosensory evoked potentials Upper extremityUpper extremity:: MMedian nerve SSEP has “edian nerve SSEP has “MM” ”

shape shape

Lower extremity: Lower extremity: Posterior tibial nerve SSEP has Posterior tibial nerve SSEP has “W” shape “W” shape

What do we look for during surgery? What do we look for during surgery? Decrease in SSEP amplitude Decrease in SSEP amplitude Increase in latency (time to first peak or dip) Increase in latency (time to first peak or dip)

Six I’s that inhibit SSEP’s: Six I’s that inhibit SSEP’s: Inhaled anesthetics, including isoflurane Inhaled anesthetics, including isoflurane

N2O doesn’t decrease amplitude alone, but has a N2O doesn’t decrease amplitude alone, but has a synergistic effect with volatile agents.synergistic effect with volatile agents.

IV agents, but to a lesser extent than inhaled anesthetics IV agents, but to a lesser extent than inhaled anesthetics Etomidate = the exception; it increases SSEP amplitude Etomidate = the exception; it increases SSEP amplitude

Ischemia/hypoxia Ischemia/hypoxia Injury, to the spinal cord or anywhere in SSEP pathway Injury, to the spinal cord or anywhere in SSEP pathway ““Ice cold” temperatures (< 34.5 oC) Ice cold” temperatures (< 34.5 oC) Incompetence (observer foul-ups) Incompetence (observer foul-ups)

Summary of effects of anesthetics on SSEP’s: Summary of effects of anesthetics on SSEP’s: Inhaled anesthetics – dose-related decrease in amplitude Inhaled anesthetics – dose-related decrease in amplitude

and increase in latencyand increase in latency Use less than 1 MAC volatile agent Use less than 1 MAC volatile agent Nitrous oxide – profound depressant effect on SSEPs, Nitrous oxide – profound depressant effect on SSEPs,

especially when used in combination with volatile agent especially when used in combination with volatile agent

Intravenous agents Intravenous agents a. Propofol and thiopental a. Propofol and thiopental

Small decrease in amplitude and increase in latency Small decrease in amplitude and increase in latency Propofol is commonly used for TIVA Propofol is commonly used for TIVA

(total IV anesthetic technique)(total IV anesthetic technique) Opioids – negligible effects on SSEPs Opioids – negligible effects on SSEPs Ketamine and etomidate – increase SSEP amplitude Ketamine and etomidate – increase SSEP amplitude

(Etomidate is exceptional. It increases SSEP amplitude but decreases BAEP (Etomidate is exceptional. It increases SSEP amplitude but decreases BAEP amplitude.) amplitude.)

Anesthesia for SSEP’s (progression from routine to desperate) Anesthesia for SSEP’s (progression from routine to desperate) Volatile agent / N2O / narcotic Volatile agent / N2O / narcotic Volatile agent / narcotic Volatile agent / narcotic Volatile agent / propofol / narcotic Volatile agent / propofol / narcotic TIVA: total IV anesthetic with propofol and narcotic TIVA: total IV anesthetic with propofol and narcotic Etomidate / narcotic (really desperate) Etomidate / narcotic (really desperate)

Increases amplitude Increases amplitude Adrenocortical suppression with etomidate Adrenocortical suppression with etomidate

Other Evoked PotentialsOther Evoked Potentials Visual evoked potentials Visual evoked potentials

M shape M shape Long latency Long latency Sensitive to all anesthetics except opiates Sensitive to all anesthetics except opiates

Motor evoked potentials Motor evoked potentials

Evoked by transcranial electrical or magnetic Evoked by transcranial electrical or magnetic stimulation stimulation

Effects of anesthetics are profound. Effects of anesthetics are profound. Same anesthetic progression as with SSEP’s Same anesthetic progression as with SSEP’s

Why bother to test motor function? Why bother to test motor function? SSEP’s mainly test for dorsal (posterior) spinal SSEP’s mainly test for dorsal (posterior) spinal

cord function. cord function. Motor EP’s mainly test for ventral (anterior) Motor EP’s mainly test for ventral (anterior)

spinal cord function. spinal cord function.

Anterior Spinal Artery SyndromeAnterior Spinal Artery Syndrome

Transcranial Doppler (TCD)Transcranial Doppler (TCD) A. Carotid endarterectomy A. Carotid endarterectomy -- might be used during -- might be used during

CEA cross-clamping toCEA cross-clamping to detect emboli or decreased detect emboli or decreased cerebral blood flow velocitycerebral blood flow velocity

Vasospasm Vasospasm TCD can be used to detect vasospasm after aneurysmal TCD can be used to detect vasospasm after aneurysmal

SAH. SAH. Vasospasm causes an increase in cerebral blood flow Vasospasm causes an increase in cerebral blood flow

velocity. velocity.

Thank youThank you