Intracranial Pressure (ICP)

ICP = cranial (closed space) pressure on brain, CSF, & blood components

•  Brain components: Brain mass/cells (80%); blood (10%); CSF (10%)

•  Normal range: 0–10 mm Hg; CSF: 150 mL normal volume; 450 mL/d

•  Increased ICP can lead to herniation & severe neurologic sequelae

• Acute ↑—shunting of CSF to spinal canal; ventricular compression

• Further ↑—compress brain tissue, mass effect, neuro deterioration

• Severe ↑—Cushing’s triad (↑↑ BP + ↓↓ HR + irreg. resp.)
Herniation—pupil asymmetry, ocular paresis, obtundation, nausea, decerebrate posturing, hemiplegia

Figure 20-1. Relationship between cerebral blood flow in response to changes in PaCO₂ and PaO₂.

CRANIOTOMY

Anesthetic Management

General Features

•  Mayfield pins & horseshoe headrest both commonly used

• Pinning is highly stimulating

• Anesthesiologist must anticipate (rather than react to) stimulation

• Best to place invasive arterial monitoring before pinning

• May inject pin sites with local anesthetic before pinning

• May need to deepen anesthesia/control BP in anticipation of pinning (may give IV, propofol, nicardipine, or opioid 30 sec prior)

•  Airway may be rotated away from anesthesiologist

•  Soft bite block should be placed immediately after induction between molars

• Tissue edema from tongue biting can form during surgery

• Can prevent tube kinking from biting or positioning

•  Eye protection

• Secure taping of the closed eyelids

• Careful application of ophthalmic ointment recommended to avoid physical & chemical injury of the cornea

•  Plan early for IV access (arms out or tucked)

• Decide early on need & site for intraop infusions

• Infuse via visible IV (if possible) to ↓ unnoticed infiltration

•  Main times of stimulation: Intubation/pinning/incision/drilling/dural incision

•  Avoid coughing/bucking/movement at any time

•  Rapid blood loss is possible during any craniotomy

•  Immediate postop assessment of neurologic function is crucial

• Inability to perform neuro exam → immediate postop CT imaging

• Avoid routine use of sedative medications (confound postop assessment)

•  Opioids: Often used as part of a balanced-anesthesia technique

• Aggressive use of narcotics may lead to delayed emergence

• Give opioid dose early (i.e., induction, pinning, incision)

• Fentanyl = short-acting & titratable (5–10 mcg/kg total dose)

• Hydromorphone = longer-acting, give for postop analgesia

• Morphine = sedating, slow emergence; avoid in intracranial proc.

(Morphine metabolites may accumulate in renal failure)

• Remifentanil (0.1–0.5 mcg/kg/min) & sufentanil (0.1–0.2 mcg/kg/hr can be used as infusions

• Consider intravenous acetaminophen (1 g/6 hrs) for adjunctive analgesia

•  Tight control of BP via invasive monitoring

•  Fluid resuscitation

• Normal saline is usually the crystalloid solution of choice (can ↑↑ CI → metabolic acidosis)

• Avoid hypotonic or glucose containing fluids (can inc. brain swelling/injury)

• Colloid used as indicated by clinical context

•  Periop antibiotics usually indicated

•  Anticonvulsant therapy

• Indications for anticonvulsant & steroids vary with each pt

• May potentiate neuromuscular blockers (acute) or antagonize NMB (chronic)

• Anticonvulsant often used if contact with cortical tissue anticipated

• IV phenytoin (dilantin) loading dose: 18 mg/kg in 250 mL NSS at 25 mg/min)

Caution: Rapid bolus dosing of phenytoin may cause significant cardiac arrhythmia, hypotension, & cardiovascular collapse

• Fosphenytoin (a phenytoin prodrug with ↓ side effects) loading dose: 15–20 phenytoin equivalent mg/kg in 250 mL NSS @ 50–100 PE mg/min

• IV levetiracetam (keppra) loading dose: 1000 mg in 100 mL of normal saline over 15–30 min

Underlying renal failure, metabolic syndrome, & baseline meds should be considered prior to anticonvulsant admin

•  IV dexamethasone (10 mg bolus, repeat 4 mg q6h) for edema when indicated

• Usually reserved for cases with intracranial lesion, elevated ICP, edema

Induction

•  Controlled induction with hemodynamic stability

•  Moderate hyperventilation in setting of ↑ ICP

•  Combination of propofol 1–2 mg/kg, fentanyl 2–4 mcg/kg & nondepolarizing muscle relaxant is one approach

•  Rapid, short-acting anti-HTN agents (esmolol, nicardipine) should be available

•  Succinylcholine associated muscle fasciculation may result in transient ↑ ICP

•  Coughing, bucking, or sympathetic surge during laryngoscopy with ↑ BP may cause sudden & untoward ↑ ICP (can use lidocaine 1 mg/kg to prevent)

•  Physiologic manifestations of light anesthesia may be devastating in pts with intracranial aneurysm susceptible to rupture

Maintenance

•  May use amnestic dose (0.5–1.0 MAC) of inhalational agent to ↓ brain volume

•  Narcotic bolus as needed up to a predetermined estimate of total dose

•  Consider TIVA if monitoring evoked potentials or if need to ↓ brain volume

• Consider using a processed EEG monitor to guide TIVA dose

•  Common strategy

• Propofol infusion for amnesia & brain relaxation

(Stop infusion once 1° resection complete to expedite emergence)

• Narcotic infusion for analgesia & immobility

(If muscle relaxation contraindicated because of monitoring)

•  Provided an appropriate anesthetic depth has been reached, HTN should be liberally treated with labetatol or nicardipine to decrease likelihood for emergence hypertension

•  Closely monitor urine output & resuscitate with normal saline

•  Monitor glucose & treat glucose >160 mg/dL with insulin

• Hyperglycemia may predispose to or exacerbate neurologic injury

•  If not contraindicated by monitoring requirements, maintain muscle relaxation to one twitch with bolus or infusion of nondepolarizing muscle relaxant

•  Maintain MAP within 20% of baseline

•  N₂O may be used with several caveats:

• May complicate EEG or potential monitoring if level not constant

• May ↑ cerebral blood flow & contribute to brain swelling/↑ ICP

• May contribute to pneumocephalus (particularly in posterior fossa proc.) or pneumothorax expansion (esp. in trauma)

• May have deleterious effects on neuronal cells (under investigation)

Emergence

•  BP control is critical

• HTN episodes at emergence & postop may cause ↑ bleeding or edema

• Treat with labetalol or nicardipine

•  Prophylaxis for nausea/vomiting recommended (ondansetron)

• Avoid promethazine, droperidol, diphenhydramine (can be sedating)

•  Emergence should begin after Mayfield pins have been removed

•  Time should be allowed (5 min) for head wrapping before extubation

•  Extubation: Reaches baseline mental status

•  Small boluses of propofol (10–40 mg) or remifentanil (0.2–0.5 mcg/kg) can smooth emergence as can a dexmedetomidine infusion

•  Continuous vital sign monitoring during transport

Special Features of Operations Involving the Posterior Fossa

•  ↑ ICP commonly a concern (due to obstruction by a posterior fossa mass)

• Consider aggressive treatment of ICP prior to induction

• In severe cases, consider ventriculostomy under local anesthesia

• ↑ ICP may occur if head down during positioning/surgical prep

•  Prone, head-up position—concern for air embolus

•  Potential for postop, compressive pneumocephalus

•  Kinking of ETT with neck flexion in pins (MUST place bite block)

• Postextubation macroglossia, tongue ischemia/injury

•  Proximity to critical neurologic structures involving regulatory centers

• Brainstem/medulla/pons regulatory centers

• Bradycardia, apnea, rapid swings in BP may occur

• Possibility for new postop deficits

•  Small operative window may lead to inc. brain swelling

• Jugular venous drainage/head position

Awake Craniotomy

•  Used for resection of tumors (motor or speech cortex) & epileptic foci

•  Team approach with emphasis on communication, patience, & experience

•  Critical to set pt & surgeon expectations

•  Awake with variable sedation versus “asleep → awake → asleep” with LMA/ETT

• Preference varies by center & experience

• Simplest method is to avoid airway instrumentation

• Only period of intense stimulation is opening/drilling/dural incision

•  Patient comfort

• Positioning with padding/pillows for optimal comfort

• Vasoactive substances (nitroglycerin) may cause profound headache

• Warming blanket as needed for patient comfort

• Placement of Mayo stand over head to lift drapes off of the awake patient

•  Preparation

• IV access, monitors, & arterial line prior to blocks

• Consider bilateral nasal trumpets (28–34 Fr) connected to O₂ for airway management

• Topical anesthetic & vasoconstrictors to nares prior to placement

•  Consider peripheral nerve blocks

• CNV1—supraorbital, supratrochlear n.

• CNV₂—auriculotemporal, zygomaticotemporal n.

• Cervical branches—posterior auricular, greater & lesser occipital n.

• Remifentanil infusion for analgesia during block placement; PONV prophylaxis at start

• Choice of local: Use long-acting ropivicaine 0.375% with 1:200,000 epi

• Bupivicaine in large volume may have increased risk of cardiac toxicity

• Allow for additional local to be infiltrated by surgeons

• Metoprolol prior to block to blunt tachycardia

•  Maintenance

• Deeper sedation only during drilling & opening of bone flap

• Minimize sedation just before dural incision

• Monitor CO₂—hypercarbia can contribute to brain swelling

• Propofol/remifentanil with spontaneous ventilation

• Dexmedetomidine infusion is a useful adjunct

• Minimal resp. depression, can cause hypotension/bradycardia

• Treat with glycopyrrolate (0.2 mg) if bradycardia is clinically significant

• Load 1 mcg/kg over 15 min & infuse 0.3–1 mcg/kg/hr

• Neuromonitoring

Map functional cortex—usually speech area

Requires continuous patient feedback, communication, contact