Cerebral Circulation & Auto-Regulation

8/8/2019 Cerebral Circulation & Auto-Regulation

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By: Dr. Salah Al-ShawiUKMMC24/June/2010



T he brain, though representing 2% of the totalbody weight, it receives about one fifth of theresting cardiac output. T his blood supply iscarried by the two internal carotid arteries (ICA)and the two vertebral arteries that anastomose atthe base of the brain to form the circle of Willis.Carotid arteries and their branches (referred toas the anterior circulation) supply the anteriorportion of the brain while the vertebrobasilarsystem (referred to as posterior circulation)supplies the posterior portion of the brain.



T he carotid system.T he vertebrobasilar system.T

he collateral blood supply of the brain.Venous drainage



1. Common Carotid artery (CCA): T he left CCAarises from the aortic arch while the right arisesfrom the bifurcation of the innominate artery.2. External carotid artery (ECA): It starts at theCCA bifurcation. Its branches supply the jaw, face,

neck and meninges.T

he bulk of the meningealcirculation is supplied by the middle meningealartery, the most important branch of the maxillaryartery which is one of the two terminal branches ofthe ECA (the other terminal branch is thesuperficial temporal artery). T hese two terminalbranches in addition to the occipital artery canserve as collateral channels for blood supply to thebrain in instances of obstruction of the ICA. T heascending pharyngeal artery can serve as a sourceof blood in instances of occlusion of the ICA.



1.Common Carotid A.





3. Internal carotid artery (ICA): It starts at the

carotid sinus at bifurcation of CCA at the levelof the upper border of the thyroid cartilage atthe level of the fourth cervical vertebra. Itascends just behind and lateral to thehypopharynx where it can be palpated. Itpasses up the neck without any branches to thebase of the skull where it enters the carotidcanal of the petrous bone. It then runs throughthe cavernous sinus in an S-shaped curve (the

carotid siphon), then it bifurcate into anteriorcerebral artery and the larger middle cerebralartery.





1. Vertebral artery : It arises from the proximalsubclavian artery and ascends through thetransverse foramina of the cervical vertebrae. Itthen passes posteriorly around the articularprocess of the atlas to enter the skull throughthe foramen magnum. T he two vertebralarteries join each other to form the basilarartery. T he vertebral artery gives rise toanterior and posterior spinal arteries, theposterior inferior cerebellar artery andbranches to the medulla.



1. T he Vertebral A.





2. Basilar artery : It is formed by the two

vertebral arteries joining each other in themidline. It ascends along the ventral aspect ofthe pons. It ends at the ponto-midbrain junction where it divides into two posterior

cerebral arteries. It gives rise to anterior inferiorcerebellar artery, superior cerebellar artery andnumerous paramedian, short and longcircumferential penetrators.





3. Posterior cerebral artery (PCA): T he basilarartery ends by dividing into the two posteriorcerebral arteries. T hey anastomose with theposterior communicating arteries to completethe circle of Willis. Many small perforatingarteries arise from PCA to supply the midbrain,the thalamus, hypothalamus and geniculatebodies.





It is a network of blood vessels present at thebase of the brain. T his polygon of blood vesselsis formed by the proximal parts of the twoanterior cerebral arteries connected by the Ant.Communicating A. and the proximal parts ofthe two posterior cerebral arteries connected tothe distal internal carotid arteries by theposterior communicating arteries. Howeverfifty per cent of circles have hypoplastic or

absent segments and the potential for collateralflow is not always as good as it might firstappear.





Venous blood flows peripherally via superficialcerebral veins and centrally via the deepcerebral veins into the venous sinuses (whichlie between the outer endosteal and the innermeningeal layer of the dura) which drain intothe internal jugular vein. T he cerebral veins arethin walled and have no valves. T here arenumerous venous connections betweencerebral veins and dural sinuses and venoussystems of the meninges, skull, scalp and nasalsinuses so facilitating propagation of thrombusor spread of infection between these vessels.







T he cerebral blood flow, represent 15-20% of thecardiac output. T his is equal to 750ml/min or50ml/100g/min.Although, the adult brain weighs only 1.4 kg

but, it·s uses 20% of the basal oxygenconsumption of the body. T his is equal to 50 mlsO2/min or 3-3.5 mls/100g/min.Cerebral Blood Flow (CBF) = Cerebral Perfusion

Pressure (CPP) divided by the Cerebral VascularResistance (CVR).CBF=CPP / CVR



CPP:- Is the pressure driving the flow of bloodthrough the brain.CPP= MAP ² ( T he greater of the ICP or CerebralVenous Pressure). T his type of situation referredto as ´Starling Resistor mechanismµ.In normal conditions the ICP is higher than theCerebral Venous Pressure). So

CPP = MAP ² ICP.





Principal mechanisms responsible for the controlof cerebral vascular resistance (CVR):

M yogenic hypothesis.M etabolic hypothesis.Chemical factors.N eurogenic Regulation.



T he myogenic hypothesis states that whenvascular smooth muscle is stretched, it contracts.T hus, if arterial pressure is suddenly increased,the arterioles are stretched and the vascularsmooth muscle in their walls contracts in responseto this stretch.Contraction of arteriolar vascular smooth musclecauses constriction (i.e., increased resistance),thereby maintaining a constant flow in the face ofincreased pressure. Remember the hydrulicequation of ohm·s law Q = P/R.



Response begin within seconds of a change in CPP and

is complete between 10s and 2 minutes.



In chronic hypertensive patients, the curve will be

shifted to the right.T

he main point to know herethat the lower limit of the autoregulation is alsoshifted to the right.



In neonates, the pressure autoregulation is left-shifted in comparison to the adult curve.T he plateau is narrower (from 30-90 mmHg),and slops upward to the right.T he neonates have a lower BP so it·sphysiologically appropriate that the curve isleft-shifted.T he decrease in the upper limit means that theneonate is less able to tolerate high BP.



It refers to the ability of a tissue to adjust it·sblood supply so that it receives sufficient bloodflow to carry out it·s functions.Increase metabolic activity in the brain is

associated with an appropriate increase inperfusion to supply increased oxygen andother metabolite and to remove wastes.T his relationship between flow and metabolismis known as Flow-Metabolism Coupling.Metabolic regulation is important for minute tominute regulation of regional CBF even thoughglobal CBF may remain relatively constant.





As a result of metabolic activity, the tissuesproduce various v asodilator metabolites (e.g.,CO 2 , H + , K+ lactate , and adenosine). T hegreater the level of metabolic activity, thegreater the production of vasodilatormetabolites.T hese metabolites produce v asodilation of

arterioles ,

which decreases resistance and,therefore, increases flow to meet the increaseddemand for O 2.



T he cerebral circulation is most sensitive to Pa CO2.

Co2 is very potent cerebral vasodilator.



T he curve is almost linear between an arterial Pco2from 20-80mmHg.T he normal Pco2 is at about the midpoint of thesteep part of the curve allowing maximal sensitivityin both direction.





T he reason that the CBF does not increase untilthe arterial Po2 falls down below 50 mmHg is

related to the shape of the ODC (O2 dissociationcurve) for hemoglobin.T he blood oxygen content is not much altered,while we are still on the flat upper part of the

ODC.As Pao2 falls below 50 mmHg, the o2 content ofthe blood falls rapidly.So, there is no much decrease in cerebral O2supply until this level is reached.Cerebral arterioles start to vasodilate and CBFincreases to maintain adequate O2 delivery.



T he sympathetic nervous system is not very important inthe cerebral circulation for causing changes in CVR.Changes in the CBF due to autonomic stimulation can atbest alter CVR about (5-10%).Despite this, the nervous system is extremely importantin CBF regulation, because of it·s effect on the CPP.T he carotid baroreceptors monitor the MAP, if thepressure falls, the signal firing from the carotid sinusdecrease, and this will result in powerful sympatheticstimulation to the heart (increase HR & contractility) andto peripheral circulation (vasoconstriction) in attempt tomaintain MAP sufficient for brain and other tissue.





L inda S. Costanzo physiology 3 rd edition.Kerry Brandis, T he Physiology Viva.Medical Physiology, Guyton 11 th edition.http://www.doctorslounge.comhttp://www.anaesthetist.comhttp://www.medscape.com

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Cerebral Circulation & Auto-Regulation