Doppler ultrasound has been used to estimate cerebral blood flow velocity
(CBFv) under a variety of conditions including orthostasis, Valsalva
maneuver, etc. It has been used in functional form to interrogate the
various branches of the Circle of Willis to localize and lateralize stroke
and has been used to demonstrate that chronic orthostatic intolerance and
POTS involve marked changes in cerebral blood flow velocity which appear
to coincide with the onset of hyperpnea and often dyspnea.
We use a pulse range-gated 2 MHz probe
focused by a plastic lens (Multigon TCD System 500M) via the anterior
transtemporal approach to insonate the left and right middle cerebral
artery (MCA). Data are synchronized with SGP, IPG, HR, and BP data through
the DATAQ A/D interface. If the diameter of the MCA is relatively
unchanged throughout an experiment, and if changes in maximum flow
velocity can be substituted for changes in average velocity (the usual
assumption which can be demonstrated for pulsatile laminar flow regimes)
then changes in flow velocity reflect relative changes in flow. Changes
in the MCA are often taken to be reasonably representative of overall
cerebral blood flow changes if changes are global. The quantity [(Mean
arterial pressure at heart level) – (.776* height of the MCA above the
right atrium)] estimates the mean arterial pressure at the MCA.
Autoregressive modeling of mean BP and CBFv following the same
principles used in obtaining any transfer function can be used to estimate
cerebral autoregulation capability.