Anesthesia Medication Effects on Cerebral Hemodynamics

Anesthesia Medication Effects on Cerebral

Hemodynamics

CBF:

CBF:

CBV:

CBF:

CBV:

ICP:

CBF:

CBV:

ICP:

Ischemia:

CBF:

CBV:

ICP:

Ischemia:

CBF:

CBV:

ICP:

Ischemia:

DECR

EASE IN

CREA

SE

CBF:

CBV:

ICP:

Ischemia:

Site of autoregulation

CBF:

CBV:

ICP:

Ischemia:

Site of autoregulationSite of medication effects

CBF:

CBV:

ICP:

Ischemia:


Difficult to measure

CBF:

CBV:

ICP:

Ischemia:


Difficult to measureVaries to a lesser degree than CBF

CBF

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CBF

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CBF:

CBV:

ICP:

Ischemia:



Brain Tissue, CSF, CBV

CBF:

CBV:

ICP:

Ischemia:




Focal vs globalAffects 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 CO750 mL/min

CBF:

Normal Values: 15-20% of CO750 mL/min50 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

Overview

Intrinsic factors

Extrinsic factors

Ischemia

CBF Autoregulation

Overview

MAP 60-150

CBF Autoregulation

Overview

MAP 60-150Rapid change will still affect CBF

CBF Autoregulation

Overview


Disrupted by :volatile anesthetics

CBF Autoregulation

Overview


Disrupted by :volatile anestheticsHTN

CBF Autoregulation

Overview

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

CBF

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CBF

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CBF

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CBF Autoregulation

Intrinsic Factors

Myogenic Response

CBF Autoregulation

Extrinsic Factors

PaCO2

CBF Autoregulation

Extrinsic Factors

PaCO2Metabolism

CBF Autoregulation

Extrinsic Factors

PaCO2Metabolism

CBF Autoregulation: Extrinsic Factors

PaCO2:Most potent vasodilator



Direct relationship to CBF



Direct relationship to CBFCO2 Responsiveness

Preservedwith anesthetics


Metabolism (CMRO2):Main controllable factor intraop is temperature


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:


Cerebral Steal:* Focal Ischemic areas are

maximally dilated



maximally dilated* Cerebral Vasodilation



maximally dilated* Cerebral Vasodilation

Redirects blood flow away from ischemic areas


Inverse Steal:


Inverse Steal:* Focal Ischemic areas are

maximally dilated



maximally dilated* Cerebral Vasoconstriction



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. 45-100 mmHg B. 50-115 mmHg C. 55-125 mmHg D. 60-150 mmHg

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.

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 Anesthetics

IV General Anesthetics

CBF and Anesthetics: Inhaled Anesthetics


Decrease CMRO2 (except N2O)


Decrease CMRO2 which CBF


Are direct cerebral vasodilatorswhich CBF


Net effect determined by MAC


Resistance


Resistance


Resistance


Resistance


Resistance


Resistance


Over 1 MAC, cerebral vasodilation greatly increases


Over 1 MAC: cerebral vasodilation greatly increases & autoregulation is impaired

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Over 1 MAC: cerebral vasodilation greatly increases & autoregulation is impaired

CBF:

CBV:

ICP:

Ischemia:




Focal vs globalAffects all above variables


Inhaled Anesthetics in normal patients with normal PaCO2 will

cause minimal ICP changes


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 Anesthetics

IV General Anesthetics

CBF and Anesthetics: IV General Anesthetics

CO2 responsiveness is preserved


CO2 responsiveness is preservedUnlike Inhaled Anesthetics,

autoregulation is also preserved

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All Gen Anesthetics except Ketamine reduce CMRO2, which

reduces CBF


Many also have direct cerebral vasoconstriction


Many also have direct cerebral vasoconstriction

Global/ Near Global: Barbs, Propofol

Regional (Not Uniform): Etomidate


Resistance


Resistance


Resistance


Resistance


Resistance


Resistance


Net Effect: Substantial Reduction in CBF (30-

60%)…

CBF:

CBV:

ICP:

Ischemia:



CSF, CBV, Brain Tissue

Focal vs globalAffects all above variables


Net Effect: Reduction in CBF (30-60%) leads to smaller reduction in

ICP (10-20%)


CBF 30-60% CBV 20-40%ICP 10-20%


CBF 30-60% CBV 20-40%ICP 10-20%

with Hypocapnia


But, in Normocapnia, if MAP falls below autoregulation range,

cerebral vasodilation will increase.

CBF

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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 4th Ed. McGraw-Hill.

Stoelting, R. K., and Miller, R. D. (2007). Basics of Anesthesia 5th Ed. Elsevier: Philadelphia.

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Anesthesia Medication Effects on Cerebral Hemodynamics

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