• 40 to 60 years of age
• signs and symptoms reflecting increasing ICP (intracranial pressure)
• adult onset seizure disorder
• CT or MRI

Management of Anesthesia

• Prevent undesirable changes in CBF (cerebral blood flow) and ICP.
  
• Position depends on tumor location
  
  • Supratentorial: supine
  • Infratentorial: prone or sitting (BEWARE: greatly increases risk of venous air embolism)

Cerebral Blood Flow

• Normal: 50 ml/100 Gm/min (15% of cardiac output)
• Depends on:
  • Cerebral metabolic rate for O₂ (CMRO₂)
  • P_aCO₂
  • Cerebral perfusion pressure
  • P_aO₂
  • anesthetic agents
  • temperature
  
• CBF is proportional to CMRO₂
  • relationship unaffected by intravenous agents, but
  • may be uncoupled by inhalation agents
• P_aCO₂
  • 1 mmHg change in P_aCO₂ produces 1 ml/100 Gm/min change in CBF (in same direction) but
  • transient effect (wanes in 6-8 hours)
  • normal patients: for normal CBF maintain normal P_aCO₂ = 40 mmHg
  • CBF and thus ICP may be acutely lowered hyperventilating to lower P_aCO₂
• Cerebral perfusion pressure (CPP) and autoregulation
  • CPP is proportional to mean arterial pressure (MAP)
  • if ICP > right atrial pressure (RAP), then
    CPP = MAP - ICP
  • if RAP > ICP, then
    CPP = MAP - RAP
  • normal: autoregulation keeps CBF relatively constant as long as CPP is between 50 and 150 mmHg
  • if CPP < 50 mmHg or CPP > 150 mmHg, then CBF is proportional to CPP
  • chronic hypertension shifts the CBF-CPP curve to the right so
    • higher CPP tolerated but
    • lower CPP (< 50 mmHg) may result in too-low CBF
  • autoregulatory response
    • impaired by intracranial tumors
    • not impaired by inhalation anesthetics
• P_aO₂
  • P_aO₂ < 50 mmHg causes significant increases in CBF
• Anesthetic agents
  • inhalation agents
    • decrease CMRO₂, but
    • > 0.6 MAC (minimum alveolar concentration) leads to cerebral vasodilation and dose-dependent increases in CBF
    • vasodilation greatest with halothane >> isoflurane, desflurane, sevoflurane
  • ketamine
    • generally increases CMRO₂ and CBF
    • if P_aCO₂ maintained normal in presence of elevated ICP or cerebral trauma, then ketamine does not adversely alter CBF or ICP *
  • thiopental
    • potent cerebral vasoconstrictor
    • decreases CMRO₂, CBF, and ICP
  • propofol
    • decreases CMRO₂, CBF, and ICP
    • decreased MAP leads to decreases CPP
  • etomidate
    • decreases CMRO₂, CBF, and ICP
    • increases EEG activity
  • opoids
    • depress ventilation, increasing P_aCO₂ leading to increases in CBF and ICP
    • depress level of consciousness
    • cause miosis

Pressure-volume (elastance) curves

• elastance = (change in pressure)/(change in volume)
• when intracranial compensatory mechanisms are exhauted, small changes in volume lead to dramatic changes in ICP

Intracranial pressure (ICP)

• normal: < 10 mmHg
• to lower elevated ICP, consider:
  • elevate head (to encourage venous drainage)
  • hyperventilate (effect wanes in 6-8 hours)
    P_aCO₂ 25-30 mmHg probably safest ( <20 mmHg: too much vasoconstriction with risk of cerebral ischemia)
  • drain CSF
  • drugs:
    • osmotic diuretics
    • steroids
    • barbiturates
• inhaled anesthetic agents
  • dose-dependent increase in CBF and ICP (especially in patients with intracranial tumors)
  • hyperventilation to P_aCO₂ = 30 mitigates to some extent the elevation of ICP caused by inhaled agents

Preoperative evaluation

• Evidence of intracranial hypertension
  • nausea and vomiting
  • hypertension
  • bradycardia
  • personality change
  • altered level of consciousness
  • altered pattern of breathing
  • papilledema
  • seizures
• MRI or CT
  midline shift > 5 mm and/or encroachment on CSF cisterns suggest intracranial hypertension
• avoid preop pharmacologic sedation and ventilatory depression

Induction of Anesthesia

• thiopental, propofol, or etomidate -> prompt induction without ICP elevation
• muscle relaxant to facilitate endotracheal intubation and mechanical hyperventilation (BEWARE: coughing causes marked increases in ICP)
• succinylcholine does not significantly alter CBF or ICP in patients with neurological injury *
  
• more thiopental, opioids and/or lidocaine 1-2 minutes prior to direct larynogoscopy
• ventilate to P_aCO₂ 25-30
• BEWARE: PEEP may decrease cerebral venous return leading to elevated ICP

Maintenance of anesthesia

• N₂O, opioids, benzodiazepines and/or barbiturates
• inhaled agents - isoflurane, desflurane, sevoflurane - at < 0.6 MAC OK with P_aCO₂ 25-30
• in patients with elevated ICP, avoid vasodilators (nitroprusside, nitroglycerine, trimethophan) before the dura is opened
• muscle relaxants (muscle activity may lead to elevated ICP)
• may decide to treat cerebral swelling
  • mannitol 0.25-1 Gm/kg IV
  • furosemide 0.5-1 mg/kg IV
  • thiopental
  • head up position
• IV fluids
  • minimal, 1-3 ml/kg/hr
  • glucose NOT recommended
    • may increase cerebral edema
    • hyperglycemia worsens cellular ischemic injury
  • isotonic crystalloids
  • hetastarch OK

Monitors

• standard (including ETCO₂) plus:
• arterial line
• maybe ICP monitor
• probably bladder catheter
• sitting position: CVP catheter

Awakening

• avoid coughing, straining
• thiopental or lidocaine
• BEWARE: N₂O may cause tension pneumocephalus
• delayed return to consciousness or neurologic deterioration: CT or MRI

Venous Air Embolism (VAE)

• consider whenever head > 5 cm above heart
• transected veins in cut edge of bone or dura may not collapse
• air -> RV -> pulmonary circulation
• decreased pulmonary blood flow, pulmonary edema, bronchoconstriction, cardiovascular collapse, hypoxemia
• paradoxical (air) embolism
  • coronary or cerebral circulations
  • via patent foramen ovale (PFO) (20-30% of adults have probe-patent foramen ovale)
• detection
  • precordial Doppler ultrasound near right upper sternal border is most sensitive non-invasive monitor (detects 0.25 ml)
  • trans-esophageal echocardiography is more sensitive, but more invasive and cumbersome
  • sudden decrease in ETCO₂, incease in ETN₂
  • gasping, hypotension, dysrhythmias, cyanosis, "mill-wheel" murmur
• treatment
  • irrigate operative site with fluid
  • apply occlusive material to bone edges
  • gently compress internal jugular veins
  • head down position
  • aspirate air through right atrial catheter (best if tip is at SVC-RA junction)
  • discontinue N₂O
  • inotropes may be needed
  • BEWARE: PEEP, by reversing RA-LA pressure gradient, may lead to paradoxical air emboli via PFO

CEA Aneurysm Tumor Apnea Cord References
Front Page Residency Program Top