Neurosurgery notes/Trauma/Head Trauma general/Normal head trauma/Pressure and viscosity

Pressure and viscosity

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Normal head trauma
  • The Hagen-Poiseuille law of laminar flow in a cylindrical tube
    • Is utilized to describe cerebral blood flow in relation to
      • Cerebral perfusion pressure (CPP),
      • Vessel diameter,
      • Blood viscosity.
    • CBF= k[CPP × d^4]/ (8 × l × v),
      • k is a constant,
      • d is artery diameter,
      • l is artery length,
      • v is blood viscosity.
    • CBF
      • Adult 50 ml/ 100 g/ min
        • To facilitate constant metabolic supply to the brain, provided that CPP is in the range of 60– 160 mmHg (Cipolla, 2009).
      • CPP = MAP - ICP.
      • CBF is also maintained via autoregulation
  • Normal brain
    • Cerebral vasculature possesses myogenic capacity to dilate or contract to counteract deviations in wall tension (that reflect the CPP) to maintain constant flow.
    • Viscosity
      • Changes in whole blood viscosity, largely determined by haematocrit and serum fibrinogen, affect CBF and induce an autoregulatory response under normal physiological circumstances.
      • Increase in viscosity → decreases metabolic supply to the brain → arterial dilation
      • Decrease in viscosity → increases metabolic supply → causes arterial constriction
  • Most severe TBI
    • Brain autoregulation is lost → brain becomes vulnerable to systemic pressure disturbance → secondary insults (e.g. ischaemia from reduced CBF or swelling from excess CBF).
      • Outside the normal CPP 60– 160 mmHg, vessels become either maximally dilated or experience forced dilation (i.e. ‘pressure breakthrough’).
    • Following acute cerebral infarction, increased haematocrit and fibrinogen are associated with reduced CBF, and that changes in blood viscosity may have important hemodynamic effects on cerebral circulation as well during acute stress.