Brain Blood Supply and Cerebral Perfusion Pressure

Brain Blood Supply

• The brain needs a tightly regulated continuous blood supply due to its very high metabolic rate.
• Although it weighs only 2-3% of body weight, the brain accounts for 25% of a person's metabolic demands.
• The brain is highly sensitive to blood flow disruptions.
• Ischemic brain tissue stops working in seconds, leading to irreversible necrosis in as little as 5 minutes.
• Homeostatic mechanisms stabilize cerebral blood flow at about 50 ml/100g of brain tissue per minute.

Cerebral Perfusion Pressure (CPP)

• Blood supply relies on CPP.
• CPP depends on Mean Arterial Pressure (MAP), which drives blood into cerebral arteries, and Intracranial Pressure (ICP), which opposes MAP.
• MAP represents the average arterial pressure during a single cardiac cycle, generally 70-100mmhg in adults. MAP= (2DP) + (SP)/3]
• ICP is the pressure inside the cranial cavity due to the presence of brain, blood, and CSF, and usually stays between 5-15mmhg.
• CPP = MAP - ICP, typically ranging from 60-80mmhg.

Cerebral Blood Flow & Autoregulation

• Under normal circumstances, cerebral blood flow is maintained at a steady 50 ml/100g of brain tissue per minute.
• This rate is maintained even with changes in CPP or MAP through multiple homeostatic mechanisms.
• Autoregulation helps maintain constant blood flow.
• Redistribution of blood during hypovolemia directs blood from skin and viscera to the brain.
• Autoregulation maintains constant CBF over a CPP range of 50-150mmHg.

Autoregulatory Mechanisms

• Myogenic mechanism responds to changes in pCO2, pH, and temperature.
• Low MAP (or CPP) causes cerebral arteriolar dilation, increasing blood flow.
• High MAP (or CPP) causes cerebral arteriolar constriction, reducing blood flow.
• High cerebral metabolism triggers cerebral arteriolar dilation, increasing blood flow.

Autoregulation Limits and Consequences

• CBF is maintained at 50ml/mi/100g at MAP 60-160mmhg by myogenic mechanisms.
• Autoregulation becomes ineffective above and below these limits, making cerebral blood flow dependent on MAP.
• When MAP increases beyond the upper limit, vasoconstriction is overcome, leading to cerebral edema and brain damage.
• When CPP drops below the autoregulation limit, even maximal vasodilation cannot maintain sufficient CBF, resulting in cerebral ischemia and brain damage.
• Initially, decreased cerebral blood flow is offset by increased oxygen extraction from the bloodstream.
• Clinical signs of ischemia only appear when decreased perfusion exceeds the capacity for increased oxygen extraction.
• Signs of hypoperfusion include dizziness, altered mental status, and irreversible tissue damage (infarction).

Hypertension and Autoregulation

• In chronic hypertension, the autoregulation curve shifts right due to adaptation to sustained high blood pressures.
• In chronic hypertensive patients, cerebral blood flow is maintained well at higher CPP/MAP.
• Blood pressure control should be cautious to avoid excessive reductions that could cause cerebral ischemia.
• The autoregulation curve can be unreliable in conditions like stroke and TBI due to impaired feedback mechanisms or damage to the vasculature.
• In such cases, cerebral blood flow may become pressure-dependent, and even minor changes in MAP can significantly affect cerebral blood flow.

Arterial Supply

• The brain receives blood from two systems which include:
  • Internal Carotid System (70%) which is also known as the Anterior Circulation.
  • Vertebral System (30%) which is known as the Posterior Circulation.
• The internal carotid in the neck is crossed by the hypoglossal nerve, posing a risk during carotid endarterectomy, which may cause ipsilateral hypoglossal nerve palsy.
• The internal carotid artery enters the cranium through the carotid canal.
• The vertebral artery passes through the transverse foramina which allow passage up and into the Foramen magnum.

Circle of Willis

• The Anterior circulation derived from Internal carotid system delivers blood to the cerebral hemispheres.
• The Posterior circulation derived from VB system delivers blood to the Brainstem, cerebellum and occipital cortex.
• Carotid and vertebrobasilar circulations are linked by the Posterior communicating artery and the 2 ACA by the Anterior communicating artery which forms the Circle of Willis.

Important Relations

• The Circle of Willis in the interpeduncular fossa encircles the Optic chiasm, Pituitary, and Mammillary bodies.
• The Anterior cerebral A. runs in the longitudinal fissure on the medial surface of the brain.
• The Middle cerebral A. runs in the Lateral fissure on the lateral surface.
• The Posterior cerebral A. wraps around the cerebral peduncle into the transverse fissure at the base of the temporal and occipital lobes.
• -Aneurysms of the Oculomotor nerve which runs between the PCA and SCA and parallel to the P Com A may result in CN III palsy.

Anterior Cerebral Artery (ACA)

• The are two segments, the A1, and A2
• A2 is outside the circle of willis
  • No collateral flow comes from within the circle itself
• Supplies medial parts of the surfaces along with peripheral parts of the frontal and parietal lobe.
• This location helps it supply a region that controls the primary motor and sensory of the limbs and even helps in frontal micturition.

Middle Cerebral Artery (MCA)

• Largest branch of the ICA
• Whole MCA sits outside the circle of willis
• M1, M2, M3 are the listed segments
  • M1 is proximal from the sylvian fissure
  • M2 is within the sylvian fissure
• Gives off cortical branches that comes from the sylvian fissure

Middle Cerebral Artery (MCA) Branches

• Anterior choroidal artery comes from M1 of MCA
• Lenticulostriate are the ones that come out of M1
  • They are central to the white matter
• Deep cortical branches that supply the cortex areas come from M3

Main Functional Areas of MCA

• Provides to the surfaces of the frontal lobe, temporal and parietal.
• Some of the areas that are supported by MCA comes to the motor/sensory ones, FEF, language, etc.

Dominance of Superiority over Inferior parts of MCA

• Frontal and most of the parietal lobe is supplied by the superior division, especially Broca's, FEF, and primary motor cortex.
• In Contrast temporal and most of the parietal lobe is supplied by the inferior divisio, with Wernicke's area being supplied.

Posterior Cerebral Artery

• Commonly PCA emerges from the vertebral artery
• Two segments include P1 and P2
  • If P2 occludes, its not supported by the circle.
• Supplies occipital cortex, temporal and midbrain

Cortex Summary

• Lateral View
  • Peripheral limb comes from the ACA.
• Lateral View
  • Occipital View from PCA
• Inferior View
  • Combination of sections

Internal Capsule

• Parts include the anterior limb, genu, and posterior limb
• Key pathway functions:
  • Corticobulbar
  • Somatic Sensory
  • Cortico Spinal
  • Auditory
  • Optic

Internal Capsule Blood Supply

• Lenticulostriate branches mainly come from the middle cerebral artery.
• The branches help supply the upper parts of the internal capsule
• Medial striate helps with the ACA on the the anterior limb whereas the lateral striate branches of MCA come to the genu
• Basal nuclei is suppled by both the ACA and MCA