ABSTRACT
A number of physiological factors have the ability to alter global and regional CBF.
Cerebral blood flow
Most methods described are based on applications of the Fick principle (see Chapter 39). Kety and Schmidt used this principle in 1945, and are credited with the first quantitative measurements of CBF in humans. At steady state they used a form of the equation:
In the Kety-Schmidt method, nitrous oxide (N2O) is used as the diffusible tracer (15% in air is breathed for 10-15 min), and its wash-in is followed by taking frequent paired peripheral arterial and jugular venous bulb samples to measure the N2O. These are then plotted and the speed at which the two curves equilibrate measures the rate at which N2O is being delivered to the brain. At equilibrium it is assumed that the brain and venous concentrations are equal. The disadvantages of this method are that it:
? is invasive; ? is slow (i.e. it takes at least 10-15min); ? provides an averaged value only; ? measures only global flows (and only flows to the perfused brain).
