Cerebral Perfusion Pressure (CPP)

• The brain demands a tightly regulated, continuous blood supply to maintain its high metabolic rate.
• Although the brain only weighs 2-3% of body weight, it accounts for 25% of metabolic demands.
• Disruptions to blood flow can quickly harm the brain.
• Ischemic brain tissue stops functioning within seconds and can suffer irreversible necrosis in as little as 5 minutes without blood supply.
• Homeostatic mechanisms maintain a stable cerebral blood flow of about 50 ml/100g of brain tissue per minute.

Cerebral Perfusion Pressure (CPP)

• Blood supply to brain tissue relies on cerebral perfusion pressure (CPP)
• Mean arterial pressure (MAP) pushes blood into cerebral arteries, while intracranial pressure (ICP) opposes MAP.
• MAP represents the average arterial pressure during a single cardiac cycle, calculated as [MAP= (2DP) + (SP)/3]. MAP generally ranges from 70-100 mm Hg in adults.
• ICP is the pressure inside the cranial cavity, influenced by the brain, blood, and CSF and typically maintained between 5-15 mm Hg.
• Cerebral Perfusion Pressure = Mean Arterial Pressure - Intra Cranial Pressure, or CPP=MAP-ICP.
• CPP generally falls between 60-80 mm Hg.

Cerebral Blood Flow & Autoregulation

• Cerebral blood flow remains constant at around 50 ml/100g of brain tissue per minute, despite changes in CPP or MAP, due to homeostatic mechanisms.

• Autoregulation and redistribution of blood help maintain flow.

• Sympathetic activity can direct blood from the skin and viscera to the brain during hypovolaemia.

• Autoregulation mechanisms maintain consistent CBF within a CPP range of 50-150mmHg, and a MAP range of 60-160mmHg.

• The myogenic mechanism responds to pCO2, pH, and temperature.

• Low MAP or CPP causes cerebral arteriolar dilation to increase blood flow.

• High MAP or CPP leads to cerebral arteriolar constriction to decrease blood flow.

• High cerebral metabolism causes cerebral arteriolar dilation to increase blood flow

• Cerebral blood flow is maintained at 50ml/mi/100g at MAP 60-160mmhg by myogenic mechanisms

• When autoregulation is ineffective, cerebral blood flow is dependent on MAP, meaning that it responds to MAP in a linear manner.

• Increased MAP above the upper limit can overcome vasoconstriction, leading to cerebral oedema and brain damage.

• When CPP drops below the lower limit, maximal vasodilation cannot maintain sufficient cerebral blood flow, causing cerebral ischemia.

• There is an initial elevation in oxygen extraction from the bloodstream.

• Clinical signs of ischemia become visible when the fall in perfusion exceeds the capacity of heightened oxygen extraction

• Hypoperfusion signs like dizziness, altered mental status, and irreversible tissue damage (infarction), become apparent.

• In chronic hypertension, adaptation to sustained high blood pressures shifts the curve to the right.

• Cerebral blood flow is maintained at higher CPP/MAP levels in individuals with chronic hypertension.

• Cautious blood pressure control is needed for people with chronic hypertension, because excessive reductions may cause cerebral ischaemia.

• The autoregulation curve can be unreliable in conditions such as stroke and traumatic brain injury due to impaired feedback mechanisms or damage to the vasculature.

• In cases of damaged autoregulation, cerebral blood flow may rely more on pressure; small changes can cause significant changes in cerebral blood flow.

Arterial Supply of the Brain

• Dual blood supply to the brain is received via:

• Internal carotid system (70%) - Anterior Circulation

• Branch of the common carotid artery

• Vertebral system (30%) - Posterior Circulation

• First branch of the first part of the subclavain artery

• The hypoglossal nerve crosses the internal carotid (and external carotid A) in the neck.(At risk during carotid endarterectomy)

• Internal carotid artery crosses enters the cranial cavity via the carotid canal, then travels within the cavernous venous sinus.

• Vertebral artery passes through the transverse foramina to the foramen magnum-

• The Circle of Willis is derived from the internal carotid system of the anterior circulation and supplies most of the cerebral hemispheres.

• The VB system derives the posterior circulation and supplies the brainstem, cerebellum, and occipital cortex.

• The anastomosis between carotid and vertebrobasillar circulation is by the Posterior communicating artery, as well as the two ACA, by the Anterior communicating artery; form the Circle of Willis.

• The Circle of Willis encircles the Optic chiasm, Pituitary and Mammillary bodies.

• The anterior cerebral A. runs in the longitudinal fissure (medial surface of the brain) around the corpus callosum.

• Lateral fissure represents the middle cerebral A. and goes to the lateral surface of brain outwards via the Sylvian fissure.

• The cerebral peduncle is surrounded by the posterior cerebral A., into the transverse fissure on the base of the temporal and occipital lobes.

• The oculomotor nerve runs parallel to the P Com A. and PCA and runs between PCA and SCA.

• Aneurysms of the oculomotor nerve present with CN III palsy.

• Anterior cerebral artery has two segments: A1, A2

• A2 sits outside the circle

• There is no collateral flow from the circle

• A2 divides into multiple cortical branches.

• Medial rolandic A is motor and sensory primary and supply medial surface.

• ACA supplies medial surface & peripheral rim of lateral surface of frontal & parietal cortex, most of the corpus callosum; this territory contains both the primary motor and sensory cortices of the lower limb, and the frontal micturition centre.

• The middle cerebral artery (MCA) is the largest branch of the ICA.

• All of the MCA rests outside the Circle of Willis and has no collateral flow from the circle.

• Runs laterally into the Sylvian fissure; there are three segments, M1, M2 and M3.

• M1 is a proximal part from the origin to the Sylvian fissure

• M2 is within the Sylvian fissure

• M3 represent the cortical branches coming out through Sylvian fissure from the superior and inferior divisions

• The anterior choroidal arteries rise from M1 of MCA, and typically are a branch of the ICA.

• The multiple small-diameter (~1mm) branches originate from the M1.

• Lenticulostriate or Lateral striate arteries

• Supply the central white matter, like the internal capsule and basal nuclei.

• The MCA supplies most of the lateral surface of Frontal, Parietal & Temporal cortex, and Internal capsule.

• Divisions include primary motor & sensory cortices of UL and Face, Language areas, FEF, Posterior MMAA etc.

• Frontal and most of the parietal lobe is supplied by a dominant superior division.

• Broca's, FEF is always supplied by the superior division.

• Temporal and a larger portion of the parietal lobe is the division.

• Whereas the dominant inferior division supplies the Wernicke's area.

Posterior Cerebral Artery

• PCA often derives from the vertebral artery.
• There are 2 segments: P1, P2.
• Occlusion of P2 is not supported by the circle
• The circle commonly relates to the cerebral peduncle.
• These supplies the Occipital cortex, inferior & Medial temporal cortex & Midbrain
• This means that the midbrain and visual cortex are important.

Summary - Blood Supply of the Cerebral Cortex

• The medial brain of the ACA, and inferior view of the brain are: posterior by PCA and posterior temporal lobe

Internal Capsule Review

• Subdivisions include anterior limb, posterior limb and genu (bend)

Blood Supply of the Internal Capsule

• Mainly from the lenticulostriate branches of the middle cerebral artery (MCA)
• The top part of the internal capsule, including the anterior limb, genu, and posterior limb is supplied by the lenticulostriate branches
• In the lower area of the internal capsule, the medial striate branches of the ACA provide the anterior limb, the lateral striate branches of the MCA provide the genu, and the anterior choroidal artery provides the posterior limb.
• Strate branches off both the ACA and MCA supply the basal nuclei.

Blood Supply of the Visual Pathway

• Runs inside IC and subcortically to reach cortex.
• Posterior limb of IC: MCA/Anterior choroidal A
• Meyer’s loop: MCA (Inf. division)
• Baum’s loop: MCA (Inf or Sup. division)
• Optic radiation: MCA (Inf or Sup. division)
• 1ry Visual cortex : PCA (Macular area PCA+MCA)

Blood Supply of the Brainstem

• It is from the vertebro-basilar system
• Paramedian derive supply to the medial brainstem
• Circumferential Brs derive supply to the lateral and posterior structures
• Cerebellar blood supply is derived from the 2 cerebellar arteries of the vertebrobasilar system
• Anterior: anterior inferior cerebelar artery
• Posterior: posterior inferior cerebelar artery
• Both lateral aspect and pons are also medulla