INTRAOPERATIVE MONITORING DURING CAROTID ENDARTERECTOMY (CEA)

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INTRAOPERATIVE MONITORING DURING CAROTID
ENDARTERECTOMY (CEA)

• ChePatrick, REEG,EPT,CNIM

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Introduction

• Electrophysiological monitoring during CEA has
  proven to be extremely beneficial in preventing
  permanent neurological deficits.
• Intraoperative EEG monitoring is the most common
  choice for CEA
• EEG provides a quantitative measure for the
  spontaneous electrical activity of the brain.

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CEA Facts

• CEA - introduced in 1954 as a surgical procedure
  designed to remove plaque material from an
  occluded artery in the neck.
• Plaque material is made up of cholesterol,
  calcium and fibrous tissue.
• If untreated, a blocked artery can cause a
  stroke.
• Strokes are the third leading cause of deaths in
  the United States.
• NEJM (1991) - concludes that CEA is highly
  beneficial to patients with recent TIAs and high
  grade stenosis of greater then 70 of the ICA.

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Understanding Our Arterial System

• The brain receives blood through four arteries
• a. The right and left common carotid
  arteries.
• b. The right and left vertebral arteries.
• The common carotids bifurcate into
• - external carotid artery (supplies face
  and skull
• - internal carotid artery
• (supplies the brain, no branches outside
  cranium)

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Carotid Circulation

• The circle of Willis is a band of arteries at the
  base of the brain formed by the internal and the
  basilar artery.
• These arteries form a complete ring which
  effectively act as anastomoses for each other
  during cross clamping
• In theory, if a communicating artery becomes
  blocked, blood can flow from another part of the
  circle to ensure that the brain is not
  compromised.

Basilar view of the Circle of Willis
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• Atherosclerotic plaque tends to develop at the
  bifurcation of ICA and ECA (just below the level
  of the jaw)

arrow pointing to bifurcation
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Plaque material
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Carotid Endarterectomy Surgery

• CEA was developed to prevent cerebrovascular
  accidents or TIAs
• Clamping of the ICA in the neck to remove the
  plaque from the blocked artery.
• To avoid the risk of producing ischemia of the
  ipsilateral hemisphere and a consequent deficit,
  techniques employed
• Intraoperative EEG monitoring
• Somatosensory evoked potentials
• Transcranial Doppler
• CBF measurements
• Internal carotid stump pressure
• However, EEG is rapidly becoming the standard of
  care for inpatients undergoing CEA because it is
  continuous, non-invasive, inexpensive and most
  importantly, it provides direct feedback within
  seconds after carotid clamping that the brain is
  adequately perfused and oxygenated.

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Why are we monitoring?

• EEG changes can occur 10-15 second after cross
  clamping of the ipsilateral internal carotid
  artery occur due to reduced cerebral perfusion as
  a result of insufficient collateral circulation.
• Most surgeons resort to shunting once changes are
  reported,
• After removal of clamp, slowing may persist for
  2-3 minutes until arterial blood flow has been
  restored to the carotid artery
• Prior to surgical monitoring, surgeons would
  place a bypass shunt around the atherosclerotic
  plaque, which carried blood to the brain for
  oxygenation

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Technical considerations

• Use at least 19 electrodes (international 10-20
  system) - covers major vascular territories and
  allows overlap within the montage should an
  electrode fail
• Electrode application (vary depending on
  departmental policy)
• disc electrodes with collodion, disposable
  sub-dermal needles or electrode caps.
• Patient arrives in laboratory to under go hook-up
  prior to entering the operating room.
• 14 or more channels - allows simultaneous display
  of activity from a variety of cortical areas.
• Preoperative EEG baseline obtained during relaxed
  wakefulness should be performed prior to surgery.
  Patients with an abnormal EEG are more likely
  to show changes with cross-clamping of the ICA
• Recording parameters
• HFF of 70 Hz, LFF 1.0 Hz (60 Hz notch filter
  may be necessary)
• paper speed of at least 15 mm/second

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Time To Put The Patient To Sleep
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Effects of Anesthesia on EEG

• Pharmacologic agents used for induction and
  maintenance
• Low doses rhythmic beta activity (18-25 Hz) over
  the anterior hemisphere (similar to a normal
  awake and relaxed patients with eyes closed)
• This beta increases in amplitude, widespread,
  anterior maximum and rhythmic, activity (WAR)
• In addition, intermittent delta waves may be seen
  over the anterior hemisphere.
• Other factors that can effect EEG
• - lowering of PaC02 lt 40 mm Hg
• - decreased blood pressure
• - hypothermia
• - hypoxia

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Surgical Procedure

• Obtain baselines with patient awake and eyes
  closed.
• Anesthetic that provides minimal adverse effects
  on CNS is preferable
• Communicating with the anesthetist to use agents
  that will not interfere with the EEG recording is
  very important. medications ready to use.
• Just remind the anesthesiologist Not To Bolus

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Induction
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• Post induction, the EEG can demonstrate
  generalized slowing, with a decrease in voltage,
  or a burst suppression pattern.
• Should this happen, alert the surgeon immediately
  since you will not be able be able to tell if a
  change has occurred from the patients baseline
  when clamping occurs.
• The surgeon can choose to proceed with the
  surgery or wait until the bolus has worn off.
  Keep in mind cerebral profusion can not be
  determined with barbiturates on board.

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Burst Suppression