Flow of Cerebrospinal fluid

[Flow of cerebrospinal fluid]

Blood brain barrier

1. Histological / physiological barrier
2. Formed by tight junctions between endothelial cells + astrocyte foot processes
3. Substances able to pass: Lipids / lipid permeables (opiates, GAs, respiratory gases, glucose)
4. Disrupted by: infections, tumours, trauma, iscahemia

Areas outside BBB: Hypothalamus, posterior pituitary

Cerebral blood flow

1. 500ml / kg // ~750ml/min (15% cardiac output)
2. Autoregulated between 50-150mmHg
   • Myogenic response: rise in pressure in artery causes reflex contraction increasing vascular resistance, keeping flow constant
   • Vasodilator "washout": locally produced vasodilators washed out leading to vascular resistance
3. CO2: hypercarbia increases blood flow (by vasodilation)
4. Hypoxia: produces vasodilation (less pronouced)

Cerebral Perfusion Pressure = Mean Arterial Pressure - Intracranial pressure
Must be >70mmHg to maintain adequate brain perfusion

Cushing Reflex

1. Elevated ICP
2. Leads to hypertension
3. Reflex bradycardia

Monroe-Kellie Doctrine

1. Cranial cavity considered to be rigid sphere with non-compressible contents
2. Increased ICP - of once compartment means shift in others
   • Brain: Tumours, cerebral oedema, BIH
   • blood: ICB - subdural, extradural, SAH, intracerebral
   • CSF: hydrocephalus

Signs of raised ICP

1. Headache, nausea, vomiting
2. LOC
3. Papilloedema
4. Brain herniation: leading to herniation, coma, respiratory failure, death
   • Subfalcine: cingulate gyrus herniates beneath falx
   • Foramen magnum: displacement of medulla and cerebellar tonsils
   • Transtentorial: Uncus of temporal lobes passes through tenttorial hiatus

[Attachment of falx / tentorium]

Management of Head Injury

1. Determine GCS
2. Imaging
   • Indications: persisting neurology, persisting headache/vomiting; reduced level of consciousness
   • Suspected penetrating injury
   • Suspected base of skull injury
3. Consider transfer to neurosurgical centre
4. Monitoring
   • CVP
   • Arterial pressure
   • Intracranial pressure monitoring
5. Support:
   • Temperature regulation
   • Careful fluid balance
6. Management of Raised intracranial pressure - 3Ps

   Brain

   • Sedation / antiepileptics / barbiturates
   • Mannitol - reduce brain oedema
   • Fluid restriction
   • ??Steroids

   Blood - SBP

   • Gelofusin / fluid bolusing - maintain MAP
   • Inotropic support
   • PCO2 control - ventilator settings
   • Evacuate haematoma

   CSF

   • VP Shunt
   • External ventricular drainage
   • Improve venous drainage - remove obstructions around neck, nurse upright, reduce ventilatory PEEP