Anesthesia Medication Effects on Cerebral Hemodynamics

CBF:

CBV:

CBF: CBV:ICP:

CBF: CBV:ICP:Ischemia:

CBF: CBV:ICP:Ischemia:

CBF: CBV:ICP:Ischemia: DECREASE INCREASE

CBF: CBV:ICP:Ischemia: Site of autoregulation

CBF: CBV:ICP:Ischemia: Site of autoregulation Site of medication effects

CBF: CBV:ICP:Ischemia: Site of autoregulation Site of medication effects Difficult to measure

CBF: CBV:ICP:Ischemia: Site of autoregulation Site of medication effects Difficult to measure Varies to a lesser degree than CBF

CBF Autoregulation

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

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

CBF: (MAP-ICP)= CPP _____________ Resistance

CBF: MAP- ICP Directly Proportional _____________ Resistance

CBF: MAP- ICP Directly Proportional _____________ Resistance Inversely proportional

CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance

CBF: Normal Values:

CBF: Normal Values: 15-20% of CO

CBF: Normal Values: 15-20% of CO 750 mL/min

CBF: Normal Values: 15-20% of CO 750 mL/min 50 mL/100g/min

Questions

Question #1 In the normal patient, what is the largest component of the cranial vault? A. Brain Tissue B. CSF C. CBV D. MAP

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.

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

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.

Question #3 All of the following increase CBF, except: A. Hypercarbia B. Hypoxia C. Increased cerebral vascular resistance D. They all increase CBF

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.

CBF Autoregulation OverviewIntrinsic factorsExtrinsic factorsIschemia

CBF Autoregulation Overview MAP

CBF Autoregulation Overview MAP Rapid change will still affect CBF

CBF Autoregulation Overview MAP Rapid change will still affect CBF Disrupted by : volatile anesthetics

CBF Autoregulation Overview MAP Rapid change will still affect CBF Disrupted by : volatile anesthetics HTN

CBF Autoregulation Overview Pt’s Baseline MAP is the best indicator of their autoregulation range

CBF Autoregulation

Intrinsic Factors Myogenic Response

CBF Autoregulation Extrinsic Factors PaCO2

CBF Autoregulation Extrinsic Factors PaCO2 Metabolism

CBF Autoregulation Extrinsic Factors PaCO2 Metabolism

CBF Autoregulation: Extrinsic Factors PaCO2: Most potent vasodilator

CBF Autoregulation: Extrinsic Factors PaCO2: Most potent vasodilator Direct relationship to CBF

CBF Autoregulation: Extrinsic Factors PaCO2: Most potent vasodilator Direct relationship to CBF CO2 Responsiveness Preserved with anesthetics

CBF Autoregulation: Extrinsic Factors Metabolism (CMRO2): Main controllable factor intraop is temperature

CBF Autoregulation: Extrinsic Factors Metabolism (CMRO2): Main controllable factor intraop is temperature: 7% change per 1 deg C

CBF Autoregulation Ischemia Focal vs Global

CBF Autoregulation: Focal Ischemia Cerebral Steal:

CBF Autoregulation: Focal Ischemia Cerebral Steal: * Focal Ischemic areas are maximally dilated

CBF Autoregulation: Focal Ischemia Cerebral Steal: * Focal Ischemic areas are maximally dilated * Cerebral Vasodilation

CBF Autoregulation: Focal Ischemia Cerebral Steal: * Focal Ischemic areas are maximally dilated * Cerebral Vasodilation Redirects blood flow away from ischemic areas

CBF Autoregulation: Focal Ischemia Inverse Steal:

CBF Autoregulation: Focal Ischemia Inverse Steal: * Focal Ischemic areas are maximally dilated

CBF Autoregulation: Focal Ischemia Inverse Steal: * Focal Ischemic areas are maximally dilated * Cerebral Vasoconstriction

CBF Autoregulation: Focal Ischemia Inverse Steal: * Focal Ischemic areas are maximally dilated * Cerebral Vasoconstriction directs blood flow toward ischemic areas.

Questions

Question #1 In the normal person, cerebral autoregulation maintains a constant CBF between what MAP values? A mmHg B mmHg C mmHg D mmHg

Answer Question #1: D) mmHg ◦While some sources have varying values of cerebral autoregulation, most have the low value of between 50 and 60 in the healthy patient.

Question #2 What is the most potent cerebral vasodilator? A. PaO2 B. PaCO2 C. Isoflurane D. Nitrous Oxide

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.

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

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.

CBF and Anesthetics Inhaled AnestheticsIV General Anesthetics

CBF and Anesthetics: Inhaled Anesthetics

Decrease CMRO2 (except N2O)

CBF and Anesthetics: Inhaled Anesthetics Decrease CMRO2 which CBF

CBF and Anesthetics: Inhaled Anesthetics Are direct cerebral vasodilators which CBF

CBF and Anesthetics: Inhaled Anesthetics Net effect determined by MAC

CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance

CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance

CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance

CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance

CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance

CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance

CBF and Anesthetics: Inhaled Anesthetics Over 1 MAC, cerebral vasodilation greatly increases

CBF and Anesthetics: Inhaled Anesthetics Over 1 MAC: cerebral vasodilation greatly increases & autoregulation is impaired

Autoregulation & Volatiles

CBF and Anesthetics: Inhaled Anesthetics Over 1 MAC: cerebral vasodilation greatly increases & autoregulation is impaired

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

CBF and Anesthetics: Inhaled Anesthetics Inhaled Anesthetics in normal patients with normal PaCO2 will cause minimal ICP changes

CBF and Anesthetics: Inhaled Anesthetics Inhaled Anesthetics in patients with reduced intracranial compliance can greatly increase CBV and ICP.

Questions

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

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.

Question #2 Which volatile anesthetic would increase CSF absorption? A. Desflurane B. Sevoflurane C. Isoflurane D. Volatile anesthetics do not increase CSF absorption

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.

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

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.

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

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.

Question #5 Which anesthetic drug would decrease CMRO2 the most? A. Isoflurane B. Sevoflurane C. Fentanyl D. Meperidine

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.

CBF and Anesthetics Inhaled AnestheticsIV General Anesthetics

CBF and Anesthetics: IV General Anesthetics CO2 responsiveness is preserved

CBF and Anesthetics: IV General Anesthetics CO2 responsiveness is preserved Unlike Inhaled Anesthetics, autoregulation is also preserved

Autoregulation & Volatiles

CBF and Anesthetics: IV General Anesthetics All Gen Anesthetics except Ketamine reduce CMRO2, which reduces CBF

CBF and Anesthetics: IV General Anesthetics Many also have direct cerebral vasoconstriction

CBF and Anesthetics: IV General Anesthetics Many also have direct cerebral vasoconstriction Global/ Near Global: Barbs, Propofol Regional (Not Uniform): Etomidate

CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance

CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance

CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance

CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance

CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance

CBF: MAP- ICP Metabolism _____________(CMRO2) Resistance

CBF and Anesthetics: IV General Anesthetics Net Effect: Substantial Reduction in CBF (30- 60%)…

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

CBF and Anesthetics: IV General Anesthetics Net Effect: Reduction in CBF (30- 60%) leads to smaller reduction in ICP (10-20%)

CBF and Anesthetics: IV General Anesthetics CBF 30-60% CBV 20-40% ICP 10-20%

CBF and Anesthetics: IV General Anesthetics CBF 30-60% CBV 20-40% ICP 10-20% with Hypocapnia

CBF and Anesthetics: IV General Anesthetics But, in Normocapnia, if MAP falls below autoregulation range, cerebral vasodilation will increase.

CBF Autoregulation

Questions

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

Answer Question #1: C) Sevoflurane ◦All volatile anesthetics impair cerebral autoregulation at anesthetic levels. Barbiturates and propofol preserve it.

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

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.

Question #3 Ketamine increases which of the following? A. CBF B. CMRO2 C. ICP D. All of the above

Answer Question #3: D) All of the above ◦Ketamine increases CBF, CMRO2, and ICP.

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

Answer Question #4: A) Barbiturates have a more global reduction in CBF and CMRO2 than etomidate.

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

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.

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.