Posterior Fossa Procedures (Infratentorial Craniotomy) and Neuroanesthesia Emergencies

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Posterior Fossa Procedures(Infratentorial
Craniotomy)and Neuroanesthesia Emergencies

• Mani K.C Vindhya M.D
• Asst Prof of Anesthesiology
• Nova Southeastern University

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Anesthesia for Posterior Fossa Procedures(Infrate
ntorial Craniotomy)and Neuroanesthesia
Emergencies

• I. Format approach to posterior fossa case
• Preoperative concerns
• Problem list?
• Further labs and studies?
• Optimization? (Is the patient optimal for
  surgery?)
• Consults?
• Further medical treatment?
• Intraoperative concerns
• Premedication
• Monitoring
• Induction
• Maintenance
• Intra-op complications (2 main complications?)
• Emergence
• Postoperative concerns only on the long stem
• 2 main complications?
• Post-op pain relief

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Posterior fossa considerations

• Case Presentation.
• An 18 year-old male for posterior fossa
  exploration and excision of cerebellar mass
  lesion.
• History
• headache
• nausea and vomiting
• impaired hearing
• occasional diplopia
• Physical exam
• ataxia
• severe bilateral papilledema (3/4)
• BP 120/70 to 140/80
• weight 65 kg
• CT scan
• cerebellar mass lesion
• hydrocephalus
• The neurosurgeon desires intraoperative
  monitoring of
• brain stem auditory evoked potentials (BAEP's)
• facial nerve (cranial nerve VII) function

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• Problem List
• Cerebellar mass lesion with hydrocephalus
• Increased ICP
• Positioning -- sitting, prone, lateral, or supine
  (semi-lateral)
• "Full stomach" -- nausea and vomiting, increased
  ICP
• Intraoperative monitoring
• Brainstem auditory evoked potentials
• Facial nerve function (motor testing)

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Posterior Fossa

• Is a "closed-in" space
• Bone - on sides, back, and bottom
• Tentorium - on top (Posterior fossa is
  "infratentorial.")
• Brain stem - in front
• Contains about 1/4 of intracranial contents
• Tumors in the posterior fossa
• Children
• Intracranial neoplasms the most common solid
  tumors in
• childhood (about 25 of all admissions for
  neoplastic disease).
• Posterior fossa the site of origin of 50 to 60
  of brain tumors in children.
• Adults -- uncommon site for brain tumors
• Tumors in the posterior fossa are often
• "Benign by histology"
• "Malignant by location"
• Posterior fossa tumors can compress the brain
  stem (pons and medulla).
• CSF outflow tracts
• Cardiovascular centers
• Respiratory centers -- pneumotaxic center,
  apneustic center

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Posterior Fossa
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Explanation of symptoms in case presentation
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Sitting Position

• Situations in which sitting position might be
  used are mainly
• Posterior fossa procedures
• Cervical laminectomy
• Establish which position the surgeon desires for
  posterior fossa surgery
• Sitting
• Prone
• Lateral decubitus
• Supine (semi-lateral)
• Proper positioning for a seated posterior fossa
  operation
• Knees at heart level
• Neck not hyperflexed

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Sitting Position
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Advantages of the sitting position include

• Excellent surgical access
• Comfort for the surgeon
• Facilitates hemostasis (decreased blood loss)
• Improved venous and CSF drainage
• Exposes face for monitoring response to cranial
  nerve stimulation (though this can also be done
  electronically)

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Possible sitting position complications include

• Air embolism (venous and arterial)
• Cardiovascular instability
• Hypotension
• Venous pooling
• Cardiac arrhythmias
• Neurologic complications
• Quadriplegia
• Nerve injuries (e.g. ulnar, sciatic, lateral
  peroneal)
• Pneumocephalus
• Airway obstruction
• Airway swelling (head and tongue)
• Malpractice risk. The use of the sitting
  position for posterior fossa neurosurgery is
  somewhat controversial.
• Current practice is away from operating in
  sitting position6
• No evidence that position affects outcome.
• Venous air emboli can occur in any position --
• sitting, lateral, prone, or supine

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Relative Contraindications to Sitting Position

• Known cardiac septal defect a "red flag"
• Patent foramen ovale (PFO)
• Atrial septal defect
• Ventricular septal defect
• Right atrial pressure gt left atrial pressure
• Functioning ventriculo-atrial shunt
• ?? cardiac instability
• ?? extremes of age

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Air Emboli (Venous and Arterial)

• Incidence of venous air emboli (VAE) can be as
  high as 50 in neurosurgery.
• Incidence of VAE depends on both monitoring and
  position.10
• "Bottom line" on VAE
• More frequent in the sitting position.
• BUT -- VAE can occur in any position.
• Early detection and prevention with the Doppler
  and other more sensitive
• VAE monitoring methods have
• Decreased the occurrence of clinically
  significant VAE
• Increased the reported incidence of VAE

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Relative sensitivity of techniques to monitor for
VAE
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• Comments on VAE monitors
• Bubble Doppler -- still a very sensitive and
  practical way to detect VAE
• Transesophageal ECHO (TEE)
• The most sensitive way to detect VAE.
• The only way to document intraoperative
  "paradoxical air emboli" which have crossed to
  the arterial circulation, unless the surgeon sees
  air bubbles in arteries!
• ECHO could be used to detect a patent foramen
  ovale preop.
• Incidence of patent foramen ovale about 1 in 4.
  
• A paradoxical air embolus can occur even if a
  person does not have a patent foramen ovale. Air
  can traverse the pulmonary circulation.
• Why don't we get pre-op ECHO's for all sitting
  position cases?
• Expensive
• The number of complications as a result of
  paradoxical air emboli is actually small

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• Pulmonary artery pressures (PAP)
• CVP catheters
• Have two uses regarding VAE
• VAE detection (much less sensitive than Doppler
  or TEE)
• VAE aspiration (potentially life-saving in some
  situations)
• How do you confirm CVP catheter placement in the
  right atrium
• (to aspirate air lock if VAE
  occurs)?
• Length of catheter (but could be in jugular vein)
  
• Rapid saline bolus through CVP (listen to
  Doppler)
• PAC's (imply atrial stimulation)
• PVC's (imply ventricular stimulation)
• Right ventricular pressure wave on CVP (Pull
  catheter back into right atrium.)
• Chest X-ray
• Biphasic P-wave on intracardiac electrocardiogram

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Biphasic P-wave on intracardiac electrocardiogram
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• Intraoperative Concerns
• Premedication?
• Thorough pre-op interview to allay anxiety.
• None ("light" sedation with benzodiazepine if at
  all).
• Avoid narcotics pre-op (respiratory depression,
  nausea and vomiting).
• Monitoring Checklist -- use "routine" monitors as
  reminders
• Stethoscope -- esophageal
• Precordial Doppler -- to detect venous air emboli
  (VAE)
• EKG -- ST segments
• Non-invasive blood pressure
• Radial A-line -- BP, paO2, paCO2 (25-30), HH,
  K
• CVP -- follow volume status, aspirate VAE
• Temperature -- esophageal probe
• Blood warmer, Bird or Bear humidifer
• Warming/cooling blanket

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• Oxygen monitor ( volume monitor and PIP)
• Adjust ventilation according to paCO2
• Pulse oximeter
• End-tidal CO2 (and infrared gas analyzer)
• Useful to trend paCO2 (End-tidal CO2 lt paCO2)
  
• Detecting VAE (decreased end-tidal CO2, increased
  ET N2)
• Restraints ( twitch monitor -- muscle relaxants)
  
• Intake and output
• Foley catheter (furosemide, mannitol)
• Maintain even IO (Don't "run em' dry.")
• Avoid dextrose-containing solutions in IV's
• Position injuries
• Ulnar or sciatic nerves (in sitting position)
• Neck not hyper-flexed or -extended

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• Special monitors
• 1. Facial nerve (C.N. VII) function
• Neurosurgeon directly stimulates facial nerve in
  operative field.
• Facial muscle movement is observed
• Directly by the anesthesiologist (under drapes)
• Indirectly by electrode and monitor
• Anesthetic implication -- neuromuscular blockade
  must wear off or be reversed at time of
  stimulation.
• 2. Brainstem auditory evoked potentials or
  responses
• (BAEP's or BAERs)
• Specialized form of EEG monitoring
• Background EEG activity is electronically
  subtracted out.
• The EEG waveform evoked by auditory stimulus
  (clicking in ear) remains.

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• Shape of a typical BAEP seven peaks1
• Latency time to first peak (usually 2 msec)
• Amplitude height of the peaks

c. The seven peaks of the BAEP are believed to
correspond to passage of a stimulus through
"generators" in the auditory nerve, brainstem
and cortex.
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The seven peaks of the BAEP
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BAEP
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• What do we look for during surgery?
• Mainly two things
• Increase in latency (gt 10)
• Decrease in amplitude (lt50)
• These two changes could be indicative of
  impending injury or ischemia in the BAEP pathway.
  
• BAER's are barely affected by anesthetics
• No anesthetic drug produces a change in BAERs
  that
• could be mistaken for a surgically induced
  change.
• 2) Etomidate decreases amplitude and increases
  latency (but this is not clinically significant).

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• Induction -- "typical" anesthetic regimen for
  intracranial procedures, assuming airway meets
  good criteria (i.e. Mallampati classification)
• 1. Method "Modified" rapid sequence induction
  and intubation
• (with cricoid pressure)
• Preoxygenate and denitrogenate (100 O2 by mask
  with head in good "sniffing" position)
• Cricoid pressure (NV with increased ICP)
• Hyperventilate to decrease paCO2 prior to
  intubation
• Typical induction agents
• Propofol, etomidate, or thiopental
• suitable I.V. induction agents
• Fentanyl or sufentanil
• as narcotic analgesics to
  supplement
• Lidocaine IV to blunt hypertensive and ICP
  response to intubation d.
• Neuromuscular junction blockers rocuronium,
  vecuronium, or succinylcholine (with prior
  defasciculating dose of non-depolarizing NMJ
  blocker)

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• Maintenance
• 1. Reasonable Maintenance Regimens for
  Intracranial Neuroanesthesia (going from routine
  to desperate).
• a. N2O isoflurane (½) fentanyl
• N2O the first agent to go if theres
  brain swelling or venous air emboli or ischemia
  danger (i.e. aneurysm or head trauma)
• MAC equivalents of sevoflurane or
  desflurane might also be
  substituted for isoflurane.
• Sufentanil could be substituted for
  fentanyl.
• b. Isoflurane (1) fentanyl
• c. Isoflurane (½) propofol fentanyl
• ! Volatile agents are next to go if high ICP or
  brain swelling
• Total IV anesthetic Propofol fentanyl
• Barbiturate coma -- for intractible brain
  swelling or cerebral
• protection during aneurysm clipping (titrated to
  EEG burst suppression)
• Thiopental
• Pentobarbital

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• Non-depolarizing neuromuscular junction blocker
  (must wear off or be reversible by time of CN VII
  testing).
• Vecuronium
• Rocuronium
• Pancuronium increases HR
• Cis-atracurium
• Additional maneuvers to decrease ICP

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• Intraoperative Complications -- "Emergencies in
  Neuroanesthesia"
• Intraoperative Air Embolus
• What should I do immediately?
• Tell the surgeon, who should flood the field.
• Discontinue N2O and give 100 O2
• Aspirate air from the CVP line
• Light neck compression
• Call for help
• Supportive measures (i.e., treat hypotension,
  arrhythmias)
• Try to position patient for CPR
• What else can I do? Other considerations
• Dopamine or dobutamine? (Suggested to increase
  right ventricular contractility)
• Raise CVP by fluid loading, not PEEP. PEEP would
  raise
• CVP, but at the risk
  of sending a paradoxical (arterial) air
  embolus through a probe-patent foramen ovale.
• Avoid the Valsalva maneuverThis increases CVP,
  but
• could cause paradoxical air embolus
• causes severe hypotension

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• "Tight Brain" -- Think through things we can do
  to decrease intracranial volume and ICP.
• Airway disaster? In any anesthetic emergency, the
  first things to think of are HYPOXIA
  HYPERCARBIA! (Is this a respiratory disaster?)
• Furosemide and mannitol?
• Dexamethasone?
• Open the spinal drain or insert a
  ventriculostomy?
• Can I hyperventilate more (to paCO2 of 25)?
• Anesthetic choice?
• Muscle relaxant?
• DC N2O, switch to 100 O2?
• TIVA (total IV anesthetic) technique?
• Barbiturate coma as a last resort
• Improve venous return?
• Reverse Trendelenberg position? Elevate head of
  bed?
• Reposition the head? Is there venous occlusion
  secondary to positioning?

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• Sudden Cardiovascular Changes
• a. With any sudden change in vital signs
  (hyper- or hypotension,
• tachy- or bradycardia, cardiac
  arrhythmias, etc.), the first three things to
  think of in this situation are
• HYPOXIA HYPERCARBIA! (Always first)
• SEVERE AIR EMBOLUS (Close second)
• Artifact - has BP transducer or table moved up or
  down?
• b. Bradycardia
• 1)With hypertension
• a) Most likely cause Cushing response
  2ndary to
• ICP (closed skull)
• surgical retraction (open skull)
• brain stem stimulation
• traction on trigeminal nerve (CN V)

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Cushing Response
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• Potential treatments
• Tell surgeon.
• Propofol or pentothal bolus
• Deepen anesthesia
• Other possible causes
• Impending brain stem herniation
• (turn off spinal drain?)
• Inadvertent phenylephrine bolus
• Bradycardia With hypotension
• Most likely cause vagal stimulation
• Potential treatments
• Tell surgeon.
• Ephedrine
• Atropine (or glycopyrrolate)

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• Tachycardia
• With hypertension
• Most likely causes light anesthesia or
  sympathetic stimulation
• Another cause inadvertent Ephedrine bolus
• Potential treatments deepen anesthesia (1st),
  antihypertensives (2nd)
• With hypotension
• Most likely cause hypovolemia
• Another cause hemodynamic instability
• due to sitting position
• Potential treatments
• Replace IO cc per cc
• Phenylephrine

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Premature Ventricular Contractions (PVC's)
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• Emergence.
• Hypertension is frequently a problem.
• Avoid "coughing and bucking" while head is
  secured. Two agents to treat coughing and bucking
  are
• Propofol
• Lidocaine
• It's desirable for the patient to awaken quickly
  so neurosurgeons can obtain a neurologic
  assessment.

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• Postoperative Concerns.
• Many potential postoperative problems are related
  to the location of the surgery on or near the
  brain stem.
• Cardiovascular centers - hypertension
• Respiratory centers - respiratory
  depression, apnea
• Lower cranial nerves