2015B10 Outline how hyperventilation may reduce intracranial pressure.

 

 

↑MV  ↑VA  ↓PaCO2  ↓CSF [H+]  Vasoconstriction  ↑CVR  ↓CBF  ↓CBV  ±↓ICP

1

·   MV = RR x VT; VA = RR x (VT – VD)

·   Hence ↑MV -> ↑VA

2

·   Alveolar ventilation equation: PACO2 = VCO2 / VA x k

·   Where PACO2 = PaCO2 if minimal dead space)

·   Voluntary hyperventilation: PaCO2 40 -> minimum ~10mmHg

3

·   CO2 crosses blood-brain barrier

·   CO2 + H2O <-> H+ + HCO3- (catalyst: carbonic anhydrase)

·   ↓H+ in CSF and ECF

·   Note normalisation of pH after ~6 hours due to ion transport

4

·   ↓H+  -> ↑activity of L-Ca2+ channel, ↓nNOs activity -> ↓NO

5

·   Poiseuille’s law: ↓radius -> ↑resistance

·   Resistance = (8 x length x viscosity) / (π x radius4)

6

·   Ohm’s law: CBF = (mAP – CVP or ICP) / CVR

·   Hence ↑CVR -> ↓CBF

·   (Ratio: +1mmHg ↓PaCO2 -> 2-3% ↓CBF)

7

·   ↓Vessel radius -> ↓volume

8

·   ↓Volume of one intracranial substance -> ↓ intracranial pressure

·   Note tearaway curve – hence hyperventilation most effective when ICP is high and compensatory responses are exhausted

 

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