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79 Cards in this Set
- Front
- Back
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What is cerebral perfusion pressure?
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CPP=MAP-ICP
Normal range 80-100torr If RAP is elevated:CPP=MAP-CVP |
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What is the CPP when the dura is open to atmospheric pressure?
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ICP=atmospheric pressure, so CPP=MAP
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What are the determinants of intracranial pressure?
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80% brain
12% blood 8% CSF |
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What is the Monroe-Kellie Theory?
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Any increase in one component must be compensated by a decrease another component otherwise ICP increases.
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What are the compensatory mechanisms to control ICP?
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-CSF displaced to spinal compartment
-Increased CSF absorption -Decreased CSF production -Decreased cerebral blood volume. |
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What are the S/S of IC HTN?
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HTN, bradycardia, irregular respiration = Cushing's triad; H/A, N/V, blurred vision, unilateral pupil diatlation, papilledema, CN III & VI paralysis, ALOC, seizure
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Cranial nerve VI paralysis:
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no ABDUCTION of eye
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Cranial nerve III paralysis:
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occulomotor nerve innervates inferior rectus muscle if paralyzed no ADDUCTION
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List 8 steps to treat IC HTN:
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dehydration with mannitol lasix, hyperventilation, elevate HOB, control systemic B/P, steriods, fluid restriction, cerebral vasoconstrictors, cool patient to 34 degrees
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What is cerebral blood flow?
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CBF=CPP/CVR
CBF<50% = ischemic s/s CBF<25ml/100gmtissue/min=s/s CBF<15-20/gm/min= flatt EEG CBF<10/gm/min=irreversible brain damage |
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What 3 factors alter cerebral vascular resistance?
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Changes in: PaCO2, PaO2, & temperature
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What is the single most important determinant of CBF to the anesthetist?
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PaCO2;
CBF decreases 1ml/gm/min for each 1 torr decrease in PaCO2; CBF & CVR become less sensitive to effects of hypocapnea in time (t1/2 of PaCO2 6 hours) |
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When is CBF proportional to PaCO2?
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when PaCO2 is 20-80
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What does the brain react to CO2 or ph?
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The brain is impermeable to ions, so CO2 is what the brain is sensitive to
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What are the effects of PaO2 on CBF?
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If PaO2 <50torr then blood flow will increase substantially.
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What is cerebral steal?
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a.k.a. luxury perfusion; Causes: vasodilating drugs or increased CO2 cause blood vessels in healthy tissue to dilate which increases perfusion to healthy tissues at expense of diseased tissues; Prevention: hyperventilation & barbiturates (decrease CMRO2 of healthy tissue = decrease CBF)
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What is inverse steal?
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a.k.a. Robin hood effect; decreased CO2 triggers blood vessels to constrict in healthy tissue increasing blood flow to compromised tissues
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What are the effects of temperature on CBF?
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CBF decreases 7% for every 1 degree decrease in temp;
CMRO2 decreases 6-7% for each 1 degree drop in temp |
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What is cerbral auto regulation?
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50-150torr; disrupted by tumor, trauma, SAH
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How does chronic HTN effect cerebral autoregulation?
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shifts curve to right; may not tolerate >25% reduction from baseline
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What are two other factors that influence CBF?
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Blood viscosity-decreased Hct increases CBF
ANS-SNS stimulation = vasoconstriction |
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What are 2 types of cerebral ischemia?
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Global ischemia-d/t poor perfusion (ICP>55 or systemic hypoxia)
Focal ischemia-localized ischemia d/t stroke, trauma, ect. Associated with ICP 25-55 torr. |
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What is normal ICP?
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Less than 15 torr. (5-15torr).
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Name the 3 zones of focal ischmemia:
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Inner zone=necrotic tissue
Penumbra-can survive Normal perfused |
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What type of substances cross the BBB?
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-Lipid soluble, volatile agents
-Charged particles and large molecular weight molecules do not cross (H20 soluble, proteins) -Glucose & amino acids use protein channels to cross the BBB |
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What areas of the brain does the BBB not cover?
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Chemoreceptor trigger zone, capillaries of the chriod plexus, or posterior pituitary
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Preoperative Assess/Prep
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-speech, LOC, pupils, s/s IC HTN, electrolytes, blood glucose if on steriods, anticonvulsant levels; Okay to premedicate unless lethargic
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Induction of Anesthesia:
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-deep, no fasciculating drugs, mild hyperventilation
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Maintenance of Anesthesia:
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Isoflurane is the least potent cerbral vasodilator and most potent depressant of CMR, least likely to increase ICP with modest hypocapnia
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How do volatile agents differ in their effects on the brain?
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Iso, Sevo, Des <CMR >than Ethrane or Halothane
Iso, Sevo, Des >CBF/ICP <than Ethrane or halothane |
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How does enflurane effect brain physiology?
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Ethrane >1.5MAC combined with hypocarbia produces seizure activity which increases CMR and it increases ICP by increasing CSF production.
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What effects does N2O have on neuroanesthesia?
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N2O increases CMR, CBF, & ICP, the effects are attenutated by hypocapnia
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How do induction agents effect the brain?
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STP, propofol, etomidate, and versed decrease CMR, CBF, CBV, & ICP;
Ketamine increases CBF with little/no effect on CMR |
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Which IVF are appropriate for the neurosurgical patient?
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Isotonic fluids free of glucose
*Hyperglycemia aggravates ischemic damage *NS can lead to hyperchloremic metabolic alkalosis |
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How much fluid should you administer to the patient?
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give adequate volume to assure cerebral perfusion;
2ml of NS for each 3ml of U.O.; Replace U.O. and give maintenance fluids |
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What techniques are used to induce slack brain?
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Diuretics, elevate HOB, hyperventilation, barbiturates, CSF drainage lumbar drain (no more than 50ml drainage at a time)
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What parts of the crani are painful?
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Scalp & dura are sensitive to pain
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Electroencephalogram
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EEG measures global function of the brain
EEG can be used to assess the adequacy of cortical circulation , detectionof seizure activity, evaluation of burst suppression |
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How many millivolts are genrated on the skin by the EEG signal?
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EEG potential on the skin is 0.1mV (100 microvolts)
Brain waves are generally in the range of 10-100mV |
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Which pathways are evaluated with evoked potentials?
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Spinal cord-SSEP-spinal surgery
CN VIII-brainstem auditory EP-posterior fossa surgery CN II-visual EP-pituitary surgery |
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How do volatile agents effect SSEP's?
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>1 MAC latency is increased and amplitude is decreased; seen least with halothane & most with enflurane
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Are all evoked potentials impacted by inhalation agents to the same degree?
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No, Visual evoked potentials are most affected by anesthetics; Brainstem affected the least.
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What waveforms are seen in ICP monitors?
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"A" waves reflect elevated ICP 5-20min duration
"B & C" waves lesser magnitude and r/t b/p & respiration |
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Which structures lie in which fossa in the brain?
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Frontal lobe: anterior cranial fossa
Temporal lobe: middle cranial fossa Brainstem & cerebellum: posterior cranial fossa |
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How does a lesion in the posterior fossa effect the patient?
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cranial nerves (3-12), RAS, structures vital for airway CV & respiratory control are contained in a small space in the posterior fossa
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Examples of how pressure affects cranial nerves?
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Pressure on teh trigeminal nerve can cause bradycardia & hypertension
Pressure on vagus or glossopharyngeal can cause bradycardia & hypotension & decrease gag reflex |
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Example of how pressure affects the brain stem:
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Surgery on tumors in the floor of the fourth ventricle may damage the respiratory center
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Preoperative considerations for posterior fossa:
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dysphagia, laryngeal dysfunction, increased risk of chronic aspiration; can premedicate but avoid if increased ICP or symptomatic hydrocephalus
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Benefits & risks of sitting position for posterior fossa surgery:
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Benefits: good surgical & airway access;
Risks: endobronchial intubation, swelling of face & tongue, sciatic nerve injury, hypotension, VAE |
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Preoperative considerations posterior fossa:
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CVP-axillary puncture/bacilic vein; A-line transducer level at tragus; may need higher than usual volumes of IVF; avoid drugs with chronotropic effects to monitor surgical manipulation;
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VAE:
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occurs when pressure in the open vein is subatmospheric; highest sensitivity monitor: TEE & doppler, intermediate sensitivity: PA catheter, EtCO2, PaO2; low sensitivity: PaCO2 & MAP
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What is the correct location to place a doppler to detect VAE?
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3rd to 6th ICS right sternal border, verify by injecting CO2 or heparin
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What are signs of VAE?
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millwheel murmur, low EtCO2, Low O2, presence of end tidal nitrogen, dysrhythmia, hypotension, return of spont. ventilation
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How is VAE treated?
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Flood surgical field with NS, stop N2O, give 100% O2, bone wax to bone edges, admin high volume IVF, admin vasopressors, compress jugular veins, place in Durant's (LLD with HOB down), aspirate CVP
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How to position CVP?
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catheter passed thru basilic vein, place CVP tip 3cm below the RA/SVC junction-will have isoelectric P Wave
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Emergence from posterior fossa surgery:
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dysphagia, speech problems, hypoventilation, hypertension, facial/tongue edema
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What is the incidnece of intracranial aneurysm?
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2% of population has cerebral aneurysm
Less than 0.05% experience a SAH |
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What are common locations for aneurysms?
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Circle of Willis: MCA, anterior communicating, anterior cerebral artery, posterior communicating artery
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What is the purpose of the circle of Willis?
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To provide collateral blood flow to the brian if a mjor artery is non-functional
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When are aneurysms clipped?
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Aneurysms can be clipped before rupture >7mm
Early clipping decreases risk of rebleeding and vasospasm Aneurysms are usually clipped w/in 72hours of SAH |
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What are the risks of SAH?
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vasospasm, hydrocephalus, increased ICP
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How is vasospasm treated?
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Triple "H": hypervolemia CVP>10 or PAOP 12-20; hemodilution Hct 33%; HTN SBP 120-150 (not clipped) 160-200 clipped or MAP 20-30torr above baseline; in vasospasm the vascular bed is not autoregulated so increased MAP increases CBF
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Preoperative concerns with aneurysm:
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SIADH-serum sodium, H20 restriction (fluid overload is issue)
Sodium wasting-cerebral salt wasting syndrome, pt has hyponatremia, high urine Na, & hypovolemia |
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What are the anesthetic goals for aneurysm surgery?
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avoid acute htn, provide brain relaxation, keep MAP high normal to maintain adequate CBF, possible manipulation of B/P
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ECG changes common with SAH:
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t wave inversion, nonspecific TW changes, QT prolongation, ST segment depression and U waves
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What are your concerns on induction?
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Smooth, if patient ruptures on induction: hyperventilation with 100% O2, control b/p, give STP
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What is brain protection?
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nonspecific term taken to reduce injury; decrease CMR, optimize CPP; deepen anesthetic plane, provide mild hypothermia
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Is brain relaxation indicated in aneurysm surgery?
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some feel that reducing brain volume rapidly can lead to a reverse tamponading effect
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What should the b/p be in controlled hypotension?
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MAP in aneurysm should be 60-70torr and decreased to 50 torr for clipping;
Isoflurane is an appropriate agent to use for deliberate hypotension; Deliberate hypotension can lead to intracerebral steal |
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Is controlled hypotension appropriate for aneurysm surgery?
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appropriate for aneurysm and tumor surgery; hypotension decreases transmural pressure and may improve surgeon view
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How should you respond to an intraoperative rupture?
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reduce MAP to 40-50torr
compress carotid artery once bleeding controlled give 15-30mg/kg STP over 30min in doses of 100-200mg |
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How should fluids be managed after aneurysm clipping?
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Give isotonic crystalloids and replace intraoperative fluids and give additional volume
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What are the properties of CSF?
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-specific gravity 1.005
-CSF compared to plasma: 7% more Na+, 30% less glucose, more chloride, 40% less K+, lower pH |
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Where is CSF formed?
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choroid plexus of all 4 ventricles, especially in lateral ventricles
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At what rate is CSF formed?
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formed at rate of 21mL/hr or 500-700mL/day; normal CSF volume at any one time is ~150ml
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What is the main concern in the patient with hydrocephalus?
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The expansion of ventricles can lead to increased ICP
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What are the two types of hydrocephalus?
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Communicating-CSF flows normally but not reabsorbed leading to a net increase in CSF volume;
Non-communicating-obstruction of CSF flow leads to dilation of the ventricles |
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What is the most common site of CSF obstruction?
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Aqueduct of Sylvius
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What is the anesthetic technique for V.P. shunt surgery?
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Too much hyperventilation can move ventricles making them difficult for the surgeon to pass catheter; tunneling can raise B/P; b/p can drop suddenly as brain stem pressure is reduced
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