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Anesthesia Medication Effects on Cerebral Hemodynamics
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CBF:
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CBV:
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CBF: CBV:ICP:
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CBF: CBV:ICP:Ischemia:
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CBF: CBV:ICP:Ischemia:
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CBF: CBV:ICP:Ischemia: DECREASE INCREASE
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CBF: CBV:ICP:Ischemia: Site of autoregulation
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CBF: CBV:ICP:Ischemia: Site of autoregulation Site of medication effects
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CBF: CBV:ICP:Ischemia: Site of autoregulation Site of medication effects Difficult to measure
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CBF: CBV:ICP:Ischemia: Site of autoregulation Site of medication effects Difficult to measure Varies to a lesser degree than CBF
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CBF Autoregulation
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CBF: CBV:ICP:Ischemia: Site of autoregulation Site of medication effects Difficult to measure Varies to a lesser degree than CBF Brain Tissue, CSF, CBV
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CBF: CBV:ICP:Ischemia: Site of autoregulation Site of medication effects Difficult to measure Varies to a lesser degree than CBF Brain Tissue, CSF, CBV Focal vs global Affects all variables
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CBF: (MAP-ICP)= CPP _____________ Resistance
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CBF: MAP- ICP Directly Proportional _____________ Resistance
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CBF: MAP- ICP Directly Proportional _____________ Resistance Inversely proportional
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CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance
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CBF: Normal Values:
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CBF: Normal Values: 15-20% of CO
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CBF: Normal Values: 15-20% of CO 750 mL/min
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CBF: Normal Values: 15-20% of CO 750 mL/min 50 mL/100g/min
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Questions
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Question #1 In the normal patient, what is the largest component of the cranial vault? A. Brain Tissue B. CSF C. CBV D. MAP
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Answer Question #1: A) Brain Tissue ◦The 3 components that make up the cranial vault are brain tissue, CSF, and CBV. Brain tissue accounts for 80% of the space while CSF and CBV account for 10% each. However, when ICP is altered it is typically caused by a change in CBV.
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Question #2 Which of the following parameters is true for the normal patient? A. 35% of cardiac output to the brain B. Blood flow of 50 ml/min/100 g of brain tissue C. ICP of 17 mmHg D. Both A and B
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Answer Question #2: B) Blood flow of 50 ml/min/100 g of brain tissue ◦Normal ICP is <15 and the brain normally receives 15-20% of cardiac output. The typical blood flow to the brain is 750 ml/min or 50 ml/min/100 g of tissue. Cerebral Ischemia begins when blood flow drops below 20 ml/min/100 g and infarct begins at 6 ml/min/100 g of tissue.
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Question #3 All of the following increase CBF, except: A. Hypercarbia B. Hypoxia C. Increased cerebral vascular resistance D. They all increase CBF
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Answer Question #3: C) Increased cerebral vascular resistance ◦Both hypercarbia and hypoxia increase CBF. Hypoxia increases CBF by causing an increased metabolic demand. Hypercarbia has a direct vasodilatory effect on the cerebral vasculature. Increased cerebral vascular resistance would decrease CBF.
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CBF Autoregulation OverviewIntrinsic factorsExtrinsic factorsIschemia
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CBF Autoregulation Overview MAP 60-150
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CBF Autoregulation Overview MAP 60-150 Rapid change will still affect CBF
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CBF Autoregulation Overview MAP 60-150 Rapid change will still affect CBF Disrupted by : volatile anesthetics
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CBF Autoregulation Overview MAP 60-150 Rapid change will still affect CBF Disrupted by : volatile anesthetics HTN
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CBF Autoregulation Overview Pt’s Baseline MAP is the best indicator of their autoregulation range
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CBF Autoregulation
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Intrinsic Factors Myogenic Response
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CBF Autoregulation Extrinsic Factors PaCO2
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CBF Autoregulation Extrinsic Factors PaCO2 Metabolism
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CBF Autoregulation Extrinsic Factors PaCO2 Metabolism
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CBF Autoregulation: Extrinsic Factors PaCO2: Most potent vasodilator
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CBF Autoregulation: Extrinsic Factors PaCO2: Most potent vasodilator Direct relationship to CBF
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CBF Autoregulation: Extrinsic Factors PaCO2: Most potent vasodilator Direct relationship to CBF CO2 Responsiveness Preserved with anesthetics
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CBF Autoregulation: Extrinsic Factors Metabolism (CMRO2): Main controllable factor intraop is temperature
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CBF Autoregulation: Extrinsic Factors Metabolism (CMRO2): Main controllable factor intraop is temperature: 7% change per 1 deg C
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CBF Autoregulation Ischemia Focal vs Global
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CBF Autoregulation: Focal Ischemia Cerebral Steal:
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CBF Autoregulation: Focal Ischemia Cerebral Steal: * Focal Ischemic areas are maximally dilated
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CBF Autoregulation: Focal Ischemia Cerebral Steal: * Focal Ischemic areas are maximally dilated * Cerebral Vasodilation
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CBF Autoregulation: Focal Ischemia Cerebral Steal: * Focal Ischemic areas are maximally dilated * Cerebral Vasodilation Redirects blood flow away from ischemic areas
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CBF Autoregulation: Focal Ischemia Inverse Steal:
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CBF Autoregulation: Focal Ischemia Inverse Steal: * Focal Ischemic areas are maximally dilated
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CBF Autoregulation: Focal Ischemia Inverse Steal: * Focal Ischemic areas are maximally dilated * Cerebral Vasoconstriction
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CBF Autoregulation: Focal Ischemia Inverse Steal: * Focal Ischemic areas are maximally dilated * Cerebral Vasoconstriction directs blood flow toward ischemic areas.
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Questions
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Question #1 In the normal person, cerebral autoregulation maintains a constant CBF between what MAP values? A. 45-100 mmHg B. 50-115 mmHg C. 55-125 mmHg D. 60-150 mmHg
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Answer Question #1: D) 60-150 mmHg ◦While some sources have varying values of cerebral autoregulation, most have the low value of between 50 and 60 in the healthy patient.
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Question #2 What is the most potent cerebral vasodilator? A. PaO2 B. PaCO2 C. Isoflurane D. Nitrous Oxide
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Answer Question #2: B) PaCO2 ◦While isoflurane and nitrous oxide are cerebral vasodilators, the most potent cerebral vasodilator is PaCO2. High PaO2 is a vasoconstrictor.
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Question #3 In the case of focal ischemia, which of the following would be a desirable effect to reduce damage? A. Vasodilation of non-ischemic tissue B. Vasoconstriction of ischemic tissue C. Vasoconstriction of non-ischemic tissue D. None of the above
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Answer Question #3: C) Vasoconstriction of non- ischemic brain tissue ◦In focal ischemia a small area of brain tissue is receiving too little blood flow. The process of inverse steal involves vasoconstriction of non- ischemic tissue and vasodilation of ischemic tissue.
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CBF and Anesthetics Inhaled AnestheticsIV General Anesthetics
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CBF and Anesthetics: Inhaled Anesthetics
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Decrease CMRO2 (except N2O)
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CBF and Anesthetics: Inhaled Anesthetics Decrease CMRO2 which CBF
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CBF and Anesthetics: Inhaled Anesthetics Are direct cerebral vasodilators which CBF
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CBF and Anesthetics: Inhaled Anesthetics Net effect determined by MAC
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CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance
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CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance
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CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance
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CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance
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CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance
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CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance
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CBF and Anesthetics: Inhaled Anesthetics Over 1 MAC, cerebral vasodilation greatly increases
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CBF and Anesthetics: Inhaled Anesthetics Over 1 MAC: cerebral vasodilation greatly increases & autoregulation is impaired
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Autoregulation & Volatiles
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CBF and Anesthetics: Inhaled Anesthetics Over 1 MAC: cerebral vasodilation greatly increases & autoregulation is impaired
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CBF: CBV:ICP:Ischemia: Site of autoregulation Site of medication effects Difficult to measure Varies to a lesser degree than CBF Brain Tissue, CSF, CBV Focal vs global Affects all above variables
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CBF and Anesthetics: Inhaled Anesthetics Inhaled Anesthetics in normal patients with normal PaCO2 will cause minimal ICP changes
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CBF and Anesthetics: Inhaled Anesthetics Inhaled Anesthetics in patients with reduced intracranial compliance can greatly increase CBV and ICP.
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Questions
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Question #1 A trauma patient comes into the OR with suspected head injury. Which volatile anesthetic would be least appropriate for anesthesia maintenance? A. Sevoflurane B. Isoflurane C. Enflurane D. Halothane
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Answer Question #1: D) Halothane ◦All volatile anesthetics have the potential of increasing ICP, but halothane increases CBF the most. Other volatile anesthetics would be more appropriate in this scenario.
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Question #2 Which volatile anesthetic would increase CSF absorption? A. Desflurane B. Sevoflurane C. Isoflurane D. Volatile anesthetics do not increase CSF absorption
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Answer Question #2: C) Isoflurane ◦Isoflurane is unique in that it is the only volatile agent that facilitates the absorption of CSF and has a favorable effect on CSF dynamics.
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Question #3 Which of the following inhalation agents have been shown to increase ICP? A. Nitrous oxide B. Sevoflurane C. Isoflurane D. All of the above
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Answer Question #3: D) All of the above ◦Nitrous has the potential to increase ICP substantially. The other volatile anesthetics increase ICP as well, but not to the same degree as nitrous.
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Question #4 During global ischemia, what is a potentially beneficial effect that can be induced by volatile anesthetics? A. An increase in CMRO2 with an increase in CBF B. An increase in CMRO2 with a decrease in CBF C. A decrease in CMRO2 with an increase in CBF D. A decrease in CMRO2 with a decrease in CBF
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Answer Question #4: C) A decrease in CMRO2 and an increase in CBF ◦Volatile anesthetics can produce what is referred to as luxury perfusion, a reduced CMRO2 with an elevated CBF. This can be particularly beneficial during global ischemia.
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Question #5 Which anesthetic drug would decrease CMRO2 the most? A. Isoflurane B. Sevoflurane C. Fentanyl D. Meperidine
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Answer Question #5: A) Isoflurane ◦The mechanisms by which iso, sevo, and des decrease CMRO2 is similar, but iso is the one that reduces CMRO2 the most.
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CBF and Anesthetics Inhaled AnestheticsIV General Anesthetics
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CBF and Anesthetics: IV General Anesthetics CO2 responsiveness is preserved
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CBF and Anesthetics: IV General Anesthetics CO2 responsiveness is preserved Unlike Inhaled Anesthetics, autoregulation is also preserved
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Autoregulation & Volatiles
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CBF and Anesthetics: IV General Anesthetics All Gen Anesthetics except Ketamine reduce CMRO2, which reduces CBF
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CBF and Anesthetics: IV General Anesthetics Many also have direct cerebral vasoconstriction
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CBF and Anesthetics: IV General Anesthetics Many also have direct cerebral vasoconstriction Global/ Near Global: Barbs, Propofol Regional (Not Uniform): Etomidate
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CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance
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CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance
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CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance
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CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance
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CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance
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CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance
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CBF and Anesthetics: IV General Anesthetics Net Effect: Substantial Reduction in CBF (30- 60%)…
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CBF: CBV:ICP:Ischemia: Site of autoregulation Site of medication effects Difficult to measure Varies to a lesser degree than CBF CSF, CBV, Brain Tissue Focal vs global Affects all above variables
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CBF and Anesthetics: IV General Anesthetics Net Effect: Reduction in CBF (30- 60%) leads to smaller reduction in ICP (10-20%)
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CBF and Anesthetics: IV General Anesthetics CBF 30-60% CBV 20-40% ICP 10-20%
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CBF and Anesthetics: IV General Anesthetics CBF 30-60% CBV 20-40% ICP 10-20% with Hypocapnia
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CBF and Anesthetics: IV General Anesthetics But, in Normocapnia, if MAP falls below autoregulation range, cerebral vasodilation will increase.
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CBF Autoregulation
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Questions
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Question #1 At anesthetic doses all of the following drugs would preserve cerebral autoregulation except: A. Propofol B. Sodium Thiopental C. Sevoflurane D. All preserve cerebral autoregulation
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Answer Question #1: C) Sevoflurane ◦All volatile anesthetics impair cerebral autoregulation at anesthetic levels. Barbiturates and propofol preserve it.
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Question #2 Which of the following agents decreases both cerebral blood flow and cerebral metabolism in the normocarbic patient? A. Halothane B. Ketamine C. Nitrous Oxide D. Propofol
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Answer Question #2: D) Propofol ◦Of the drugs on the list propofol is the only drug that decreases CBF and CMRO2. Ketamine, halothane, and nitrous oxide all increase CBF.
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Question #3 Ketamine increases which of the following? A. CBF B. CMRO2 C. ICP D. All of the above
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Answer Question #3: D) All of the above ◦Ketamine increases CBF, CMRO2, and ICP.
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Question #4 Which statement best describes the reasoning for using barbiturates over etomidate for cerebral protection in focal ischemia? A. Barbiturates have a more global reduction in CBF and CMRO2 than etomidate B. Etomidate has a more global reduction in CBF and CMRO2 than barbiturates C. Barbiturates have a more focal reduction in CBF and CMRO2 than etomidate D. Etomidate has a more focal reduction in CBF and CMRO2 than barbiturates
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Answer Question #4: A) Barbiturates have a more global reduction in CBF and CMRO2 than etomidate.
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Question #5 Barbiturates do all of the following except: A. Decrease cerebrovascular resistance B. Reduce cerebral blood flow C. Reduce cerebral metabolic rate D. Produce hypnosis
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Answer Question #5: A) Decrease cerebrovascular resistance. ◦Like most other IV anesthetics, barbiturates increase cerebrovascular resistance by direct vasoconstriction as well as by decreasing CMRO2.
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References Evers, A. S., and Maze, M. (2003). Anesthetic Pharmacology -- Physiologic Principles and Clinical Practice. Churchill Livingston: New York. Morgan, G.E., Mikhail, M.S., & Murray, M.J. (2005). Clinical Anesthesiology 4 th Ed. McGraw-Hill. Stoelting, R. K., and Miller, R. D. (2007). Basics of Anesthesia 5 th Ed. Elsevier: Philadelphia.
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