NEUROLOGICAL SYSTEM PART IV

1. NEUROLOGICAL SYSTEMPART IV DENNIS STEVENS CRNA, MSN, ARNP SEPTEMBER 2007 FLORIDA INTERNATIONAL UNIVERSITY ADVANCED BIOSCIENCE IN ANESTHESIOLOGY II NGR 6145

2. CEREBRAL BLOOD FLOWANDBLOOD-BRAIN BARRIER OBJECTIVES • Discuss significant aspects of cerebral vascularization. • Explain arterial circulation of the brain including the circle of Willis. • Describe venous drainage of the brain and corresponding structures. • Discuss cerebral perfusion pressure and autoregulation associated with cerebral blood flow. • Explain significant aspects of the blood-brain barrier including those factors that may disrupt it

3. CEREBRAL BLOOD FLOWANDBLOOD-BRAIN BARRIER CEREBRAL VASCULARIZATION • Introduction: • Approximately 18% of total blood volume circulates in the brain • Brain is responsible for 20% of total body oxygen consumption • Constant flow of oxygen must be maintained: • Loss of consciousness • Irreparable damage • Cerebrovascular disease occurs secondary to vascular compromise or hemorrhage in the CNS

4. CEREBRAL BLOOD FLOWANDBLOOD-BRAIN BARRIER ARTERIAL SUPPLY OF THE BRAIN • Extra cerebral vessels; R carotid artery arises from R subclavian, L carotid artery arises from aortic arch • Intracranial cerebral vessels; internal carotid artery divides into anterior cerebral and middle cerebral arteries • Two vertebral arteries (arising from the subclavian arteries) join to form the basilar artery which gives rise to the posterior cerebral artery; supplying occipital lobes and brain stem

5. CEREBRAL BLOOD FLOWANDBLOOD-BRAIN BARRIER CIRCLE OF WILLIS • Circle of Willis is a confluence of vessels that gives rise to all major cerebral arteries • It is fed by the paired internal carotid arteries and the basilar artery • When the circle is complete, it contains a posterior communicating artery on each side and an anterior communicating artery • Each major artery supplies a certain territory • Sudden occlusion affects its territory immediately, sometimes irreversibly

6. CEREBRAL BLOOD FLOWAND BLOOD-BRAIN BARRIER VENOUS DRAINAGE • Venous drainage of the brain and coverings includes veins of the brain itself, dural venous sinuses, dura’s meningeal veins, and diploic veins • Eventual cerebral venous drainage is the internal jugular vein • Cerebral veins contain no valves

7. CEREBRAL BLOOD FLOWANDBLOOD-BRAIN BARRIER REGULATION OF CEREBRAL BLOOD FLOW • Cerebral perfusion pressure (CPP) is the difference between mean arterial pressure (MAP) and intracranial pressure (ICP) • CPP = MAP – ICP • CPP is normally 80 – 100 mmHg • Moderate to severe increases in ICP can significantly compromise CPP and CBF • Decreased CPP associated with changing EEG patterns • Sustained CPP less than 25 mmHg results in irreversible damage

8. CEREBRAL BLOOD FLOWANDBLOOD-BRAIN BARRIER CEREBRAL AUTOREGULATION • Cerebral blood flow remains nearly constant between MAP of 60 – 160 mmHg • Beyond these limits blood flow becomes pressure dependent • Cerebral autoregulation curve is shifted to the right in patients with chronic arterial hypertension • Long-term antihypertensive therapy…! • Pressures greater than 150 – 160 mmHg can disrupt the blood-brain barrier: • Cerebral edema • Hemorrhage

9. CEREBRAL BLOOD FLOWANDBLOOD-BRAIN BARRIER CEREBRAL AUTOREGULATION • Extrinsic factors affecting autoregulation: • Respiratory gas tensions: • CBF directly proportionate to PaCO2 • Temperature: • CBF changes 5 – 7% per 1° C • Viscosity • Autonomic influences

10. CEREBRAL BLOOD FLOWANDBLOOD-BRAIN BARRIER BLOOD-BRAIN BARRIER • Cerebral blood vessels are unique in that the junctions between vascular endothelial cells are nearly fused. Paucity of pores responsible for the blood-brain barrier • Effectively isolates the brain and spinal cord extracellular compartment from the intravascular compartment • A number of midline brain structures receive neurosecretory products from the blood and therefore lack a blood-brain barrier

11. CEREBRAL BLOOD FLOWANDBLOOD-BRAIN BARRIER BLOOD-BRAIN BARRIER • Allows passage of lipid-soluble substances but restricts movement of those that are ionized or have large molecular weights • Movement of a given substance across the blood-brain barrier is governed simultaneously by… • Carbon dioxide, oxygen, and lipid-soluble substances freely enter the brain, whereas most ions, proteins, and large substances penetrate poorly • CNS endothelial cells lack transport mechanisms

12. CEREBRAL BLOOD FLOWANDBLOOD-BRAIN BARRIER BLOOD-BRAIN BARRIER • Rapid changes in plasma electrolyte-concentration produce a transient osmotic gradient between plasma and the brain • Acute hypertonicity of plasma results in net movement of water out of the brain, while acute hypotonicity of plasma results in a net movement of water into the brain • Marked abnormalities in serum sodium or glucose concentrations should generally be corrected slowly

13. CEREBRAL BLOOD FLOWANDBLOOD-BRAIN BARRIER BLOOD-BRAIN BARRIER • Blood-brain barrier may be disrupted by: • Severe HTN • Tumors • Trauma • Strokes • Infections • Marked hypercapnia • Hypoxia • Sustained seizure activity • Hypothermia stabilizes the blood-brain barrier

14. CEREBRAL BLOOD FLOWAND BLOOD-BRAIN BARRIER REFERENCES Morgan, G.E., Mikhail, M.S., and Murray, M.J. (2006). Clinical Anesthesiology. (4th Ed.) New York, NY: McGraw-Hill. Nagelhout, J.J. and Zaglaniczny, K.L. (2005). Nurse Anesthesia. (3rd Ed.) St. Louis, MO: Elsevier- Saunders. Waxman, S.G. (2000). Correlative Neuroanatomy (24th ed.). New York, NY:McGraw-Hill.