Useful rules to remember

1 TCD velocities do not measure cerebral blood flow volume. However, changes on TCD may correlate with changes in the cerebral blood flow.

2 Cerebral blood flow and mean flow velocities decrease with age.

3 Hypertension (including chronic) increases pulsat-ility of flow and may increase the mean flow velocity.

4 Hyperventilation decreases the mean flow velocity and increases pulsatility of flow.

5 Hypercapnia increases the mean flow velocity and decreases pulsatility of flow.

6 A waveform pattern is determined by various factors including cardiac output and blood pressure, as well as the brain autoregulatory or vasomotor responses and focal arterial lesions.

7 A waveform pattern can also be determined by the downstream circulatory conditions such as loss of autoregulation or elevated intracranial pressure (ICP).

8 Cerebral blood flow and mean flow velocity are inversely proportionate to vessel radius, length and blood viscosity.

9 When homologous arterial segments are compared, normal variations of up to 30% in flow velocities and pulsatility indices can be expected.

10 Variations in the angle of insonation can account for 15% and the resistance of downstream vasculature during breathing cycles for another 15% of a normal velocity/pulsatility difference.

11 Normal variations of up to 100% in flow velocities between homologous segments such as PCA or VA can be attributed to the tortuous vessel course, angle depth are superimposed on a digital subtraction angiography image (left).

of insonation changes, anatomic variants of the circle of Willis and hypoplasia/atresia.

12 Anatomic variations of the circle of Willis are common; it is normal (symmetric, with all communicating arteries properly developed) in only about 20% of patients.

13 If an intracranial artery is not found, this finding itself does not mean that this artery is occluded.

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