Anesthesia for Neurosurgery
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Hypercarbia causes vaso_____
What is the benefit to cerebral blood flow with hyperventilation?
Overall decrease in cerebral blood flow, ischemic areas are able to dilate to increase blood flow
With hyperventilation or decreased metabolism, the ischemic areas dilate to redirect blood flow to the areas that need it
What effect does decreased metabolism have on cerebral blood flow?
Decreased blood flow
____ and ____ are coupled and both cause decreased cerebral blood flow
Hyperventilation, decreased metabolism
With hyperventilation, how does the PaCO2 respond? How long does this last for?
- Decreased PaCO2
- Lasts for several days until the kidneys compensate
With hyperventilation, how do CSF pH and CBF change?
CBF decreases and CSF pH increases, but only for 6-8 hours.
Why does the increase in CSF pH last only 6-8 hours?
Due to active transport of bicarb ions out of the CSF (this is much faster than renal compensation)
What 3 factors affect CBF?
hypoxia, BP, and CO2
CBF is maintained btw a MAP of 50-150 mmHg
How does PaCO2 level affect CBF?
Linearly, CBF increases by 1-2 ml/ 100 g brain tissue for each 1 mmHg increase in PaCO2
How does PaO2 affect CBF?
No effect on CBF until PaO2<60 mmHg, then CBF increases
ICP > 20 mmHg
In what 3 areas of the brain can we decrease volume to decrease ICP?
- 1) CSF / ECF
- 2) brain tissue
- 3) blood
With a noncompliant brain, how will increases in intracranial volume affect ICP?
It will increase ICP
How can the tissue compartment be manipulated to control ICP?
Hematoma evacuation or tumor resection
How can the venous blood compartment be manipulated to control ICP?
Elevate HOB to facilitate jugular venous drainage, avoid increases in intrathoracic pressures as this inhibits venous drainage (PEEP, coughing)
How can the arterial blood compartment be manipulated to control ICP?
- Avoid high PaCO2 and hypoxia (increased blood flow)
- Avoid volatiles
- -Both increase CBF and thus increase ICP
- Prevent seizures (increased metabolism and thus increased blood flow)
How can the fluid compartment (CSF and ECF) be manipulated to control ICP?
- Drain CSF (lumbar drain)
How do the volatiles affect CBF? How long does their effect last for?
- VD and thus increase CBF
- 3 hours after exposure
Which volatile agent causes the most increase in CBF?
Halothane- 200% increase
Rank the volatiles in order of VD potency
- halo> enf> iso> des > sevo
- however the differences between iso, des, and sevo are likely not clinically significant
- all are dose dependent
How do the volatiles affect CMR? Is this good or bad?
- All decrease CMR
- Decreased metabolic rate means decreased blood flow and counteracts the effect of VD
Which volatile agent decreases CMR the least?
Halo, thus the worst choice for a pt with increased ICP
How does N20 affect CBF? Is it safe to use for a pt with increased ICP?
- N20 increases CBF due to its effect of stimulating the SNS and increasing CMR
- Can be safely used as part of a balanced anesthetic
What anesthetic technique is safest for a pt with high ICP?
S/sx increased ICP
H/A, CNS depression, confusion, somnulence, N/V, CT may show midline shift
Overall goal in terms of drug selection for a pt with a "tight brain"
With this in mind, what drugs should be avoided?
- Prevent increases in CMR
- Avoid ketamine, enflurane (lowers the seizure threshold), N20 alone
Why is propofol an ideal drug for a pt with a "tight brain"?
- Decreases CMR and thus decreases CBF and ICP
- Etomidate does the same thing
- Barbiturates and benzos are also helpful
Can succ be used for a pt with increased ICP?
Yes, but need to defasiculate first (fasiculations cause a transient increase in ICP)
What factors can contribute to an increased metabolic rate and thus increased CBF?
Pain, hyperthermia, seizures
Supratentorial vs. infratentorial
- Supra- front of the brain, not the brainstem
- Infra- aka posterior fossa, near the brainstem
Supra-tentorial tumor management
- Control intracranial volume by:
- avoiding coughing
- prompt intubation
- HOB elevated
- euvolemia (possible diuretics)
Concerns related to pre-medication for a pt with increased ICP?
- Sedation may cause hypoventilation and increased PaCO2 and thus increased CBF
- Want to give enough to prevent anxiety and increased HR and metabolism
-type of diuretic
- osmotic diuretic
- 0.25 - 1 g / kg given over at least 10 minutes
Goal PaCO2 level for an intubated pt with increased ICP
What is one of the most stimulating parts of a craniotomy? How should this part of the surgery be managed?
- Pin placement
- Give fentanyl (CV stable)
Crani anesthésia maintenance
- Avoid hypervolemia
- -Usually just replacing NPO deficit
- -Isotonic fluids
Why should D5W be avoided in crani pts?
- 1) glucose is deleterious and further neuro injury
- 2) glucose gets metabolized to free water which is hypotonic
Usual position for a crani of the posterior fossa? Major risk associated with this position
Advantages to use of the sitting position
Better exposure, decreased bleeding
T or F, sitting position is associated with increased M+M?
Why is there a risk of VAE with the sitting position?
- Surgical site is above the heart (risk anytime the surgical site > 5 cm above RA)
- Venous sinuses are attached to the dura (open venous sinuses in the surgical site above the heart)
HD effects of the sitting position
Paradoxical air embolism
-what is it?
-what increases the risk?
- Air travels from right side of heart to LA
- PEEP slightly increases the risk
Most sensitive method for detection of VAE?
Most commonly used method?
- Most sensitive= TEE
- Most used= precordial doppler (2nd most sensitive method)
Heard with precordial doppler when air enters the heart
How does ETCO2 change with a VAE?
How does ETN2 change?
- ETCO2 will decrease due to air in pulmonary vessels
- ETN2 will increase (dead space)
Actions to take upon recognition of a VAE
- Prevent more air from entering
- -Tell the surgeon (s/he'll will pack and flood the surgical field)
- -Apply jugular compression
- -Lower HOB
- Treat the intravascular air
- -Aspirate CVP catheter to pull air out
- -D/C N20
- -FiO2 to 100%
- -Pressors and CPR if needed
How will BP change with a VAE
Decreased BP due to decreased preload (air not blood is going to RA)
T or F, M+M is increased for a pt in sitting position for a crani?
Complications of posterior fossa surgeries
- Macroglossia and airway obstruction
- Extreme neck flexion can damage the SC
- CN disturbances affecting respiratory and CV
- Air entrained in the brain
- Most often associated with posterior fossa cranis
N20 use and posterior fossa craniotomy
- Ok when cranium is open
- Contraindicated when dura is being closed,
- but due to post-op complication of tension pneumocephalus post-op (unrelated to N20 use), should just avoid N20
T or F, the same principles for management of increased ICP apply to management of a posterior fossa crani?
Hormones produced by the anterior pituitary?
Hormones produced by the posterior pituitary?
How are pituitary tumors typically dx?
- Ammenorhea in F
- Decreased libido in M
- Both due to increased prolactin levels
When is the transphenoidal approach appropriate for removal of a pituitary tumor?
Tumor < 10 mm
T or F, transphenoidal approach for removal of a pituitary tumor is not an intracranial procedure?
Transphenoidal approach for removal of a pituitary tumor infection risk
- High due to entering the surgical site via the nose
- Abx very important
Hunt and Hess classification
- Used to assess degree of SAH and assess prognosis
- Used for cerebral aneurysm pts
T or F, considerations for pts with a cerebral aneurysm are similar to those for pts with increased ICP
Why are pre-op CVP and A line essential for a pt with a cerebral aneurysm?
- BP control and IV volume
- Major goal is to reduce IC volume
BP control during aneurysmal clipping?
What about during aneurysmal resection?
- Clipping- high normal
- Resection- low normal (if clip were to slip need to rapidly reduce BP so surgeon can visualize field and stop bleeding)
Cerebral aneurysm rupture actions during rupture
- Reduce blood flow
- Ipsilateral carotid artery compression
- Replace blood loss
- 100% FiO2
Cerebral aneurysm rupture actions after controlling hemorrhage
- Cerebral preservationElevate BP
- Reduce edema
Control ventilation and monitor ICP
- R/t cerebral aneurysm repair
- Prevented by triple H therapy
- -hypervolemia (keep vessels full to prevent vasospasm)
- -borderline high CO2 also helps to keep vessels full and prevent vasospasm
Relative contraindications to induced hypotension
- Ischemic CVA
- Severe HTN
- Extremes of age
Complications of induced hypotension
- Cerebral thrombosis or ischemia
- Coronary artery thrombosis
- Renal insufficiency
- Hepatic failure
- Increased bleeding at post-op site
- Rebound hypertension
For what procedure might induced hypotension be needed?
Cerebral aneurysm rupture
Pre-anesthesia assessment of the head injured pt
- Airway (c-spine)
- Associated injuries
- Pre-existing chronic illness
- Injury circumstances- timing, drugs or ETOH, duration of unconsciousness
Clearing a c-spine
X-ray and intact neuro exam
Inline neck stabilization
- Used for unstable c-spine to secure advanced airway
- 3 people needed
- 1- hold neck neutral
- 2- cricoid pressure
- 3- instrument the airway
GCS 3 categories
- Eye opening (max score of 4)
- Verbal response (max score of 5)
- Motor response (max score of 6)
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