Cerebral blood flow -Varun

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  • 3. AORTIC ARCH 1.Innominate artery (IA) / Brachiocephalic trunk • Rt subclavian artery (SCA)- Right vertebral artery • Rt common carotid artery (CCA) 2.Left Common Carotid Artery (CCA) 3.Left Subclavian Artery (SCA) • Left Vertebral Artery (VA)
  • 5. INTERNAL CAROTID ARTERY • Four segments: – Cervical (extracranial) – Petrous (extracranial) – Cavernous (intracranial) – Cerebral /Supraclinoid (intracranial) • Branches (proximal to distal) – Meningohypophyseal trunk – Ophthalmic artery (OA) – Superior hypophyseal artery – Posterior communicating artery (PComA) – Anterior choroidal artery (AChA) – Anterior cerebral artery (ACA) – Middle cerebral artery (MCA) (continuation of the ICA)
  • 6. Cervical segment – From the bifurcation of CCA – Enters the skull through carotid canal in petrous part of temporal bone – No branches – Persistent embryonic vessels may give rise to ECA – ICA anastomoses
  • 7. PETROUS SEGMENT -Extends from base of skull to the petrous apex -Ascending , Genu, Horizontal -Enters cranial vault via foramen lacerum. -Branches : Carotico tympanic A & A of pterygoid canal
  • 8. CAVERNOUS SEGMENT • S- shaped course in sinus referred to as CAROTID SIPHON • Passes through cavernous sinus • In proximity to CN III, IV, V1, V2 and VI • Ascending sympathetic fibres surround artery • Branches supply posterior lobe of pituitary and adjacent meninges (Meningohypophyseal Artery)
  • 9. SUPRACLINOID SEGMENT • Begins after penetration of dura • Continues until bifurcation into ACA & MCA
  • 10. SUPRACLINOID SEGMENT • Three Branches: 1.Ophthalmic A 2.Posterior Communicating A 3.Anterior Choroidal A
  • 11. OPHTHALMIC ARTERY  1st intradural branch of ICA  Supplies globe, orbit, frontal and ethmoidal sinuses, & frontal scalp  Central retinal A, Long & Short posterior ciliary branches  Branches of Ophthalmic A anastamose with Maxillary A branches - potential for collateral flow in cases of proximal carotid occlusion
  • 12. POSTERIOR COMMUNICATING A • Branches enters the base of brain between infundibulum and optic tract • Supply anteromedial thalamus and walls of third ventricle
  • 13. ANTERIOR CHOROIDAL A • Branch of supraclinoid segment of ICA close to its terminal bifurcation • Passes backward along the optic tract and around the cerebral peduncle as far as the lateral geniculate body • Enters the inferior horn of the lateral ventricle
  • 14. ANTERIOR CHOROIDAL A • Supplies the choroid plexus of lateral ventricle • Branches to optic tract, hippocampus, tail of caudate nucleus, medial and intermediate portions of globus pallidus, posterior 2/3 of internal capsule along with retro and sublenticular part, middle third of cerebral peduncle and outer part of lateral geniculate body
  • 15. ANTERIOR CEREBRAL ARTERY Three segments:  A1 –Horizontal/Pre communicating segment  Medial lenticulostriate branches  A2-Vertical/Post communicating segment From its connection to the AComA to its bifurcation into the pericallosal and callosomarginal arteries  Recurrent artery of Heubner  Orbitofrontal  Frontopolar  A3- distal ACA / cortical branches  Pericallosal  Callosomarginal
  • 16. ACA SUPPLY • Medial and orbital surface of frontal lobe • Medial surface of the parietal lobe as far as the parietooccipital fissure • Genu and Anterior 4/5th of the corpus callosum • A longitudinal 2 cm wide strip of superior surface of the frontal & parietal lobes next to the central sulcus • Anterior parts of Basal Ganglia & Anteroinferior parts of Internal Capsule
  • 17. ACA Supply
  • 18. RECURRENT ARTERY OF HEUBNER • Also called medial striate artery • Give few branches to orbital cortex, passes through anterior perforated space to join deep branches of MCA • Supplies lower part of head of caudate, lower part of frontal pole of putamen, frontal pole of globus pallidus, frontal half of anterior limb of internal capsule, and anterior portions of external capsule and lateral ventricle
  • 19. Infarct in Heubner artery territory
  • 20. MIDDLE CEREBRAL ARTERY Four segments: • M1- horizontal / sphenoidal segment: The stem of MCA  5-15 lenticulostriate branches • M2- insular segment: Runs deep in sylvian fissure and along insula ; Superior & Inferior divisions • M3- opercular segment: Follows the curvature of operculum and ends as terminal branches of MCA • M4- cortical branches: Terminal segment as it emerges from the sylvian fissure
  • 21. Middle cerebral Artery M1- Horizontal segment •Lateral Lenticulostriate •Temporoparietal artery •Frontobasal artery M2- Insular segment M3- Opercular segment •Opercular segment branches •Operculofrontal artery M4- Cortical branches •Lat Orbitofrontal •Pre rolandic •Rolandic •Ant. parietal •Post. parietal •Angular •Temporopolar •Ant temporal •Middle temporal •Post. temporal
  • 22. MCA SUPPLY • Most of the convex surface of the brain, except the frontal (ACA)and occipital(PCA) poles & the superior rim of the convex surface(ACA) • Lenticulostriate arteries (Artery of cerebral hemorrhage) supply -All of the putamen except for its anterior pole -Upper part of head of caudate N and all of its body -Lateral part of globus pallidus -Posterior part of anterior limb, genu and anterior third of posterior limb
  • 23. Posterior cerebral artery P1Segment (Precommunicating/mesencephalic) short segment from the basilar tip to the PComA – Mesencephalic br. – Cr. Nv. Nuclei 3 - 6 – Thalamoperforating arteries - diencephalon and midbrain P2 or ambient segment runs in the ambient cistern from the PComA to the posterior aspect of the midbrain – Thalamogeniculate br. – Medial posterior choroidal arteries – Lateral posterior choroidal arteries P3 or quadrigeminal segment runs within the quadrigeminal cistern behind the brainstem – Hippocampal artery – Antetior, middle, and posterior temporal arteries – Parieto-occipital artery – Calcarine artery – Posterior pericallosal artery P4 –DISTAL SEGMENT
  • 25. PCA SUPPLY -Uncus -Medial and inferior surface of temporal lobe (Temporal pole- MCA) -Thalamus, midbrain -Cuneus and splenium of corpus callosum -Medial surface of occipital lobe including entire visual cortex
  • 26. VERTEBRAL ARTERY • V1(EXTRAOSSEOUS): Segmental cervical muscular and spinal branches - passing into spinal canal via intervertebral foramina and reinforce blood supply of spine and vertebrate. • V2 (FORAMINAL): -In foramina transversaria of C6-C2 -meningeal/muscular/spinal branches
  • 27. VERTEBRAL ARTERY • V3 (EXTRASPINAL): Posterior meningeal artery • V4(INTRADURAL): – Anterior and posterior spinal arteries(Two anterior spinal A join to supply lower medulla, cervicomedullary junction and upper spinal cord) – Perforating branches to medulla – PICA: Arises from distal Vertebral Artery, supply medulla and cerebellum
  • 28. BASILAR ARTERY • Forms at pontomedullary junction from two vertebral artery • Ends at the upper border of pons • Major 5 branches 1.The pontine arteries 2. The labyrinthine 3. The anterior inferior cerebellar artery 4. The superior cerebellar artery 5.The posterior cerebral
  • 29. BASILAR ARTERY 1.The pontine arteries 2. The labyrinthine -the internal ear. - often arises as a branch of the anterior inferior cerebellar artery. 3. The anterior inferior cerebellar artery -the anterior and inferior parts of the cerebellum - A few branches pass to the pons and the upper part of the medulla oblongata. 4. The superior cerebellar artery -arises close to the termination of the basilar artery -supply superior surface of the cerebellum, pons, the pineal gland, and the superior medullary velum. 5.The posterior cerebral
  • 30. • Medulla-Vertebral Pons – Basilar Midbrain- Basilar and proximal Posterior cerebral Artery • Two vertebral artery are rarely the same size. Left is most often dominant; Right can be smaller or completely atretic • SCP-SCA, ICP-PICA, MCP-AICA and SCA
  • 31. CIRCLE OF WILLIS -In the interpeduncular fossa at the base of the brain. -It is formed by the anastomosis between the two internal carotid arteries and the two vertebral arteries
  • 33. Anteroinferior part- medial lenticulostriate arteries Superior part of head and body of caudate nucleus, lentiform nucleus- Lateral lenticulostriate arteries Thalamus and tail of caudate- by thalamogeniculate and posterior choroidal artery
  • 34. VENOUS DRAINAGE OF THE BRAIN The characteristic features of venous drainage of the brain are: • The venous return in the brain does not follow the arterial pattern • The veins of the brain are extremely thin-walled due to absence of muscular tissue in their walls • The veins of the brain possess no valves • The veins of the brain run mainly in the subarachnoid space • The cerebral veins, generally enter obliquely into the dural venous sinuses against the flow of blood in the sinuses to avoid their possible collapse following an increased intracranial pressure as they are thin walled
  • 35. SINUSES OF THE DURA MATER . (1) Postero-superior at the upper and back part of the skull. 1 Superior Sagittal (Convex or attached margin of falx cerebri) 2 Straight sinus 3 Inferior Sagittal (free or inferior margin of falx cerebri) 4 Two Transverse. 5 Occipital (2) Antero-inferior at the base of the skull. 1 Two Cavernous 2 Two Superior Petrosal 3 Two Intercavernous 4 Two Inferior Petrosal 5 Two sphenoparietal
  • 36. VENOUS DRAINAGE OF BRAIN • Anterior cerebral vein + Deep middle cerebral Vein + Striate veins = Basal Vein of Rosenthal • Thalamostriate vein + Choroidal vein = Internal cerebral vein • Internal cerebral vein + Basal Vein of Rosenthal = Great vein of Galen • Great vein of Galen + ISS = Straight sinus
  • 37. • Superior cerebral vein drain to SSS • SSS + straight sinus + Occipital sinus = Transverse sinus(Attached margin of tentorium cerebelli) • Inferior cerebral vein drain to superficial middle cerebral vein terminates to cavernous sinus • SSS connects to superficial middle cerebral vein by Troland’s vein AND Transverse sinus to superficial middle cerebral vein by vein of Labbe’ • Cavernous sinus (drain to transverse and IJV via superior and inferior petrosal sinus. VENOUS DRAINAGE OF BRAIN
  • 39. CEREBRAL BLOOD FLOW AUTOREGULATION • Neurons produce energy (ATP) almost entirely by oxidative metabolism of substrates including glucose and ketone bodies, with very limited capacity for anaerobic metabolism. • Without oxygen, energy-dependent processes cease leading to irreversible cellular injury if blood flow is not re-established rapidly (4 to 10 minutes under most circumstances)
  • 40. REGULATION OF CEREBRAL BLOOD FLOW Cerebral blood flow (CBF) is dependent on a number of factors that can broadly be divided into: a. those affecting cerebral perfusion pressure b. those affecting the radius of cerebral blood vessels
  • 41. • CBF = 50ml/100g/min (ranging from 20ml/100g/min in white matter to 70ml/100g/min in grey matter) • Adult brain weighs 1400g or 2% of the total body weight. • But CBF is 700ml/min or 15% of the resting cardiac output
  • 42. 1) CEREBRAL PERFUSION PRESSURE • Perfusion of the brain is dependent on the pressure gradient between the arteries and the veins and this is termed the cerebral perfusion pressure (CPP) • This is the difference between the mean arterial blood pressure (MAP) and the mean cerebral venous pressure CPP = MAP – ICP
  • 43. • MAP can be estimated as equal to: diastolic blood pressure + 1/3 pulse pressure, usually around 90mmHg • ICP is much lower and is normally less than 13mmHg.
  • 44. • An increase in CPP is usually the result of an increase in MAP, the contribution made by reducing ICP is minimal, apart from in pathological states when ICP is very high • In a normal brain, despite the potential for changes in MAP (sleep, exercise etc.), CBF remains constant over a wide range of CPPs. This is achieved by a process called autoregulation
  • 45. 2) THE RADIUS OF CEREBRAL BLOOD VESSELS This is regulated by four primary factors: 1. Cerebral metabolism 2. Carbon dioxide and oxygen 3. Autoregulation 4. Other factors
  • 46. CEREBRAL METABOLISM • Local or global increases in metabolic demand are met rapidly by an increase in CBF and substrate delivery and vice versa • These changes are controlled by several vasoactive metabolic mediators including hydrogen ions, potassium, CO2, adenosine, glycolytic and phospholipid metabolites and NO
  • 47. CARBON DIOXIDE AND OXYGEN • At normotension, the relationship between partial pressure of carbon dioxide in arterial blood (PaCO2) and CBF is almost linear and at a PaCO2 80mmHg CBF is approximately doubled. • No further increase in flow is possible at this point as the arterioles are maximally dilated. • Conversely at 20mmHg flow is almost halved and again cannot fall further as the arterioles are maximally vasoconstricted
  • 48. • Arteriolar tone has an important influence on how PaCO2 affects CBF. • Moderate hypotension impairs the response of the cerebral circulation to changes in PaCO2, and severe hypotension abolishes it altogether • Oxygen has little effect on the radius of blood vessels at partial pressures used clinically
  • 49. • Blood flow increases once PaO2 drops below 50mmHg. • Hypoxia acts directly on cerebral tissue to promote the release of adenosine, and in some cases prostanoids that contribute significantly to cerebral vasodilatation. • Hypoxia also acts directly on cerebrovascular smooth muscle to produce hyperpolarisation and reduce calcium uptake, both mechanisms enhancing vasodilatation.
  • 50. • In adults under normal circumstances (ICP <10mmHg), CPP and MAP are very similar and CBF remains constant with a CPP of 60-160mmHg • The higher the ICP the more CPP deviates from MAP. • Autoregulation is thought to be a myogenic mechanism, whereby vascular smooth muscle constricts in response to an increase in wall tension and to relax to a decrease in wall tension AUTOREGULATION
  • 51. • At the lower limit of autoregulation, cerebral vasodilation is maximal, and below this level the vessels collapse and CBF falls passively with falls in MAP. • At the upper limit, vasoconstriction is maximal and beyond this the elevated intraluminal pressure may force the vessels to dilate, leading to an increase in CBF and damage to the blood-brain- barrier.
  • 52. OTHER FACTORS • Blood viscosity: As viscosity falls, CBF increases. However, there will also be a reduction in oxygen- carrying capacity of the blood • Temperature: CMRO2 decreases by 7% for each 1°C fall in body temperature and is paralleled by a similar reduction in CBF. • Drugs: Cerebral metabolism can be manipulated (reduced) and consequently CBF ,cerebral blood volume and ICP is reduced.
  • 53. Clinical implications • Hyperventilation reduces the PaCO2 and causes vasoconstriction of the cerebral vessels and therefore reduces cerebral blood volume and ICP in patients with raised intracranial pressure, for example after traumatic brain injury. • However if PaCO2 is reduced too much, it may reduce CBF to the point of causing or worsening cerebral ischaemia • PaCO2 is therefore best maintained at level of 35- 40mmHg to prevent raising ICP. This reactivity may be lost in areas of the brain that are injured.
  • 54. Clinical implications • Pressure autoregulation can be impaired in many pathological conditions including patients with a brain tumour, subarachnoid haemorrhage, stroke, or head injury. • The reduction in CMRO2 with decrease in temperature is the factor that allows patients to withstand prolonged periods of reduced CBF without ischemic damage for example during cardiopulmonary bypass (Hypothermia helps to reduce excitatory neurotransmitter release, important to central nervous system protection)
  • 55. Clinical implications • Infusions of the barbiturate thiopentone are used to reduce cerebral metabolic rate and so decrease high ICP after head injury • Anaesthetic drugs (volatile agents) cause a reduction in the tension of cerebral vascular smooth muscle resulting in vasodilatation and an increase in CBF(minimal with isoflurane)
  • 56. •Arterial insufficiency due to thromboembolism, hemodynamic compromise, or a combination of these factors may lead to the recruitment of collaterals •The arterial anatomy of the collateral circulation includes extracranial sources of cerebral blood flow and intracranial routes of ancillary perfusion COLLATERALS IN CEREBRAL CIRCULATION
  • 57. • It is commonly divided into primary or secondary collateral pathways • Primary collaterals include the arterial segments of the circle of Willis, whereas, the ophthalmic artery and leptomeningeal vessels constitute secondary collaterals
  • 58. • Interhemispheric blood flow across the anterior communicating artery and reversal of flow in the proximal anterior cerebral artery provide collateral support in the anterior portion of the circle of Willis • Additional interhemispheric collaterals include the proximal posterior cerebral arteries at the posterior aspect of the circle of Willis. • The posterior communicating arteries may supply collateral blood flow in either direction between the anterior and posterior circulations
  • 59. • Anatomic studies note absence of the anterior communicating artery in 1% of subjects, absence or hypoplasia of the proximal anterior cerebral artery in 10%, and absence or hypoplasia of either posterior communicating artery in 30% • The number and size of these anastomotic vessels are greatest between anterior and middle cerebral arteries, with smaller and fewer connections between middle and posterior cerebral arteries and even less prominent terminal anastomoses between posterior and anterior cerebral arteries.
  • 60. • Distal branches of the major cerebellar arteries similarly provide collateral links across the vertebral and basilar segments of the posterior circulation • Leptomeningeal and dural arteriolar anastomoses with cortical vessels further enhance the collateral circulation. • Other collateral routes less commonly encountered in acute stroke are tectal plexus joining supratentorial branches of the posterior cerebral artery with infratentorial branches of the superior cerebellar artery And the orbital plexus linking the ophthalmic artery with facial, middle meningeal, maxillary, and ethmoidal arteries
  • 61. Extracranial arterial collateral circulation. Anastomoses from the facial (a), maxillary (b), and middle meningeal (c) arteries to the ophthalmic artery and dural arteriolar anastomoses from the middle meningeal artery (d) and occipital artery through the mastoid foramen (e) and parietal foramen (f)
  • 62. Intracranial arterial collateral circulation in lateral (A) and frontal (B) views. Posterior communicating artery (a); leptomeningeal anastomoses between anterior an
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