tion of blood flow can be quantified. However, absolute CBF cannot be derived. This technique is relatively non-invasive, involves less radiation than a CT scan, and is useful in the assessment of cerebral ischemia secondary to vasospasm.

Transcranial Doppler The transcranial Doppler (TCD) was introduced by Rune Aaslid in 1982. Using a 2 MHz pulsed Doppler, flow velocities of the basal cerebral arteries can be measured in a non-invasive manner. Although actual CBF cannot be derived from the velocities, valuable information can nevertheless be obtained that can aid patient management. When the diameter of the insonated vessel stays constant, changes in flow velocity reflect corresponding change in flow. Under normal conditions, the basal cerebral arteries, being conductance vessels, vary little in diameter with physiological vasodilation or vasoconstriction, conditions that depend on change in resistance vessels. On the other hand, pathological constriction of the conductance vessel will lead to a dichotomy, with increase in flow velocity paradoxically reflecting a decrease in flow, as in the case of vasospasm following subarachnoid hemorrhage. TCD has been found to be useful in the management of vasospasm, allowing early diagnosis and assessment of therapy.

Vasospasm vs Hyperemia Increase in flow velocity as diagnosed by TCD can be secondary to development of vasospasm or hyperemia, with obvious different clinical implications. This is particularly relevant in patients with traumatic subarachnoid hemorrhage (SAH), since 20-40% may develop vasospasm. To distinguish vasospasm from hyperemia, the ratio of intracranial flow velocity to extracranial internal carotid flow velocity (Lindegard index) is frequently used. Hyperemia is considered to be present when the index is less than 3. Mild vasospasm occurs when the index is greater than 3, moderate vasospasm when the index is 5-7, and severe vasospasm when the index is greater than 7.

Non-invasive Assessment of ICP Significant elevation of ICP compromising cerebral perfusion results in a characteristic flow pattern on TCD with low diastolic flow velocities. Increasing ICP will result in correspondingly decreasing diastolic velocity, culminating

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