Neurosurgery notes/CSF/CSF and ICP/subitem 1

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CSF and ICP
  • General
    • CSF formation = absorption + storage
    • CSF pressure = Rout*CSFformation + Pss + Vascular component?
    • Brain does not float in CSF (volume brain : CSF = 10:1)
    • CSF equalizes pressure in brain compartments
  • Mechanism
      • Hydrocephalus is far more complex than disorder of CSF circulation
        • Which are reasons and which are effects?
        • What are weights?
        • Does this naive scheme depend on time? Age? Aetiology?
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  • Marmarou’s model
      • Remained unchanged over time
      • Reference: A theoretical model and experimental evaluation of the cerebrospinal fluid system. Thesis, Drexel University, Philadelphia, PA, 1973
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    • Different equation describing Marmarou’s model
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      • Taking into account the expression in the equation we can find the differential equation, in which ICPₛₛ, difficult to measure, is replaced by the baseline pressure ICPb
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      • It can be solved analytically for infusion test
        • After introducing the initial condition and the expression into equation the relation describing the change of ICP during infusion can be presented as
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  • CSF circulation
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    • Resistance to CSF outflow
      • Component
          • The resistance of the CSF outflow is the sum of total resistances, which include at the foramen of Munro, the aqueduct, the foramena of Luschke and Magendie, the subarachnoid spaces of the infra- and supra-tentorial regions, the tentorial notch, and the arachnoid granulations of the venous sinuses. Multiple resistances at the various regions of the CSF pathway may cause hydrocephalus. In addition, the degree and the location of a resistance increase may determine the treatments associated with the different types of hydrocephalus.
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      • Davson’s equation
          • Pb = Pss + Rcsf * If
          • Normal value: 5-10 mmHg/(ml/min)
          • Elevated values: > 13 mmHg/(ml/min)
          • Too low values: < 4 mmHG/(ml/min)
          • Pb is baseline pressure
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  • Brain compliance
    • Concept
        • E- elasticity [1/ml]
        • Normal values <0.18 [1/ml]
        • Compliance depends on pressure. Higher pressure lower compliance
         
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    • Pressure-volume curve
        • Greater elasticity - steeper the curve
        • At low pressures compliance is constant
        • But at above Poptimum the compliance changes exponentially with pressure
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    • Relationship between Pulse Amplitude and mean ICP
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