Physiology

Cerebral Blood Flow (CBF) = Cerebral Perfusion Pressure (CPP) / Cerebral Vascular Resistance (CVR)

CPP = Systemic BP - Intracranial Pressure (ICP)

Normal ICP = 0-10 mm Hg (0-135 mm CSF). Small increases in CSF volume may be accommodated by CSF expulsion to the lumbar theca.

Factors affecting CVR:

• chemoregulation - accumulation of metabolic by-products
  • for example a fall in arteriolar PCO2 produces a cerebral vasoconstriction; reverse for cerebral vasodilatation
    • rapid reduction of intracranial blood volume is conventionally obtained by controlled hyperventilation via the CO2-regulation of CBF
      • effect is mediated via a pH-dependent constriction of precapillary resistance vessels
        • note though that prolonged hyperventilation is probably of limited value or may even be harmful (1)
          • there is a risk of inducing focal ischaemia with pronounced hyperventilation
          • reduction of CBF and cerebral blood volume during hyperventilation is also transient in spite of preserved hypocapnia
          • in the most severely damaged patients hyperventilation is least effective in reducing CBV because of impaired cerebrovascular CO2-reactivity
• autoregulation - maintains CBF over a CPP range between 60 mm Hg and 160 mm Hg by vasoconstriction and vasodilatation respectively.

Reference:

1. Grande PO et al. Volume-targeted therapy of increased intracranial pressure: the Lund concept unifies surgical and non-surgical treatments. Acta Anaesthesiol Scand. 2002 Sep;46(8):929-41.