Awake

Alex_Grey-Insomnia

The Awake Craniotomy

Notes from Gupta/Gelb ch 20 “Perioperative Management of Awake Craniotomy” and Oxford Handbook of Anesthesia

 

Indications

-Tumor or lesion near eloquent areas of brain

-steroetactic and DBS for Parkinson’s

-epilepsy surgery

 

Background

-either local + sedation, or GA with i/o wake up, or combination of sedation + awake

 

Preop

-good rapport with patient essential

-determine if patient will tolerate, mentally, lying flat for long period (especially challenging in children)

-obesity, reflux and highly vascular tumors may also caution against

 

Cortical Mapping

-Cortical mapping = electrical stim to recognize sensitive areas, can ID motor, sensory, speech

-despite mapping, many surgeons prefer fully awake to ensure no neurodeficits post op

-intraop ECoG (electrocorticography) may be used to ID epileptogenic focus

 

I/O Management

-routine a-line, foley (if long)

-positioning in comfort is key, head best positioned with 3 pin fixator under local (minimize movement, max airway control)

-always have access to airway

-Multiple techniques: local, local + sedation, asleep awake asleep with GA and I/O wake up  (for more complex/extensive)

-Skin, scalp, pericranium and periosteum of skull –> extensive sensory innervation; use field block or scalp block (see prior post).  Dura has extensive innervation, so anesthestize nerve trunk near middle meningeal and edges of cranium (see scalp block post).

-Precedex and Remi, both good choices for short-acting pain control and sedation without profound respiratory depression (especially precedex).

-Anti-emetics important –> especially for temporal lobe epilepsy surgery –> highly emotogenic.

 

Airway Control

-many techniques, from spon vent without devices to ETT and LMA.

-Risk of coughing during removal of LMA or ETT when dura open!

-ETT/LMA allow more precise monitroing of ETCO2

-Can be difficult to reintubate, and ETT makes assessing verbal responses difficult/impossible

-Alternative: Soft nosopharyngeal airway (with 25% cocaine paste as per Gupta/Gelb), can remain in place throughout procedure and monitor ETCO2, oxygen through other nostril. Risky in obese patients.

 

I/O Problems

– Uncooperative patient, cardiovascular (htn, hypotension, tachy), excessive sedation–>increased PaCO2–>Edema, resp depression, loss of airway, brain swelling, seizures(tx with propofol 20-40mg, or iced saline spray to cortex), pain, Local toxicity

-Patients with mass effect tumors -> most at risk for PaCO2 related brain swelling

 

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Neuroanesthesia Notes

Cerebral Salt Wasting Syndrome
triad: hypoNa, volume contraction (low CVP), and high urine Na
a/w SAH –>bnp –>salt waste

SIADH is your differential, but SIADH is 2/2 renal retention of free water, so 24hr urine Na is normal in SIADH, normal CVP

DI and primary hyperaldosteronism –> elevated Na

Ref. Miller 8th ed p 2177

Intracranial HTN >15mmHg

SSEPs
-stimulation of peripheral nerves
-ascends ipsilateral dorsal column
-recorded on c/l somatosensory cortex

Hyperventilation lowers ICP
-reduces CBV and CBF
-duration wanes in 6-10hrs
-CBF decreases 2% for every mmHg decrease in PaCO2
-1ml/100g/min CBF increase per 1mmHg increase in Paco2 so since normal CBF is 50ml/100g/min then this is a 2% change

 

 

Mannitol

Non-reabsorbable sugar -> osmotic diuretic

inhibits water reabsportion in proximal convuluted tubule (mainly), as well as thing descending loop and collecting ducts

Water loss>>electrolyte loss

Clinical uses:

  • early oliguria
  • early brain edema
  • postischemic acute renal failure
  • Neurosurgical anesthesia for good operating conditions

Retention of mannitol –> further volume expansion –> pulmonary edema in CHF patients.

Half life 100 minutes, with 15 minute onset to ICP reduction.

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Intra-operative Use

I/O use for rapid reduction of brain volume.

Dose = 0.25g/kg (some evidence this works but doesn’t last as long) to infusion of 100g for all. 1g/kg also used.

Infusion always, if given too fast –> hyperosmolarity –> vasodilatory effect –> brain engorgement –> increase ICP

Concerns regarding entering the parenchyma, or whether it is effective only if BBB is preserved, make some reluctant to use – best to give a test dose. If it decreases ICP / improves surgical field, then repeat.

Theoretical ‘acceptable’ upper limit of osmolarity 320 mOsm/L, but evidence is “soft” and incrimental doses of 12.5g used until no longer see a clinical response.

From Miller’s Anesthesia 2015 ch70, Anesthesia for Neurosurgery; and Pharmacology and Physiology for Anesthesia: Foundations and Clinical Application 2013.