Pressure Volume Index....small numbers are not small

Cerebral Blood volume (CBV)–PaCO2 curve is flatter than CBF-PaCO2.. (0.014 mL/100g/mm Hg) See the foto

Hence decreasing PaCO2 from 40 to 20 mm Hg will will decrease CBV to 2.8 mL/100g (a 28% change).

This translates into a 10–14 mL fall in whole-brain volume.

This look as small number ....
but in the light of Pressure Volume index it will not be considered as small??

Shapiro et al. showed that the amount of fluid that must be rapidly injected into or withdrawn from the intracranial space to change ICP tenfold (e.g., from 10 to 100 mm Hg) is normally ≈26 mL.

This is called the pressure-volume index (PVI). However, PVI values in patients with mass lesions
or closed head injuries may be as low as 5 mL.

In such situations, a change in CBV of 10–15 mL would be enormous, and it is not surprising that hyperventilation has come to occupy such an important position in acute ICP/brain-volume control.

PVI reflects the importance of hyperventilation in the acute management of brain edema...Keep in mind the overuse of Hyperventilation holds another risks...The risks of hyperventilation will be discussed later

1-Stroke 1974;5:630.
2-Kosteljanetz M. Acute head injury: Pressure- volume relations and cerebrospinal fluid dynamics.
Neurosurgery 1986;18:17–24.

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