Since its clinical introduction at the beginning of the eighties, TCD technology has begun to gain increasing acceptance as a noninvasive CBF monitor and since then its indications were continuously extended.
Transcranial Doppler ultrasonography today is of established value in (a) detecting severe stenosis in the major basal intracranial arteries, (b) assessing patterns and extend of collateral circulation with known regions of severe stenosis or occlusion, (c) evaluating and following patients with cerebral vasoconstriction of any cause, especially after SAH, (d) detecting arteriovenous malformations and studying their supply arteries and flow patterns, and (e) assessing patients with suspected brain death.
Uses of TCD ultrasonography as monitor during cerebrovascular and cardiovascular interventions such as cerebral endarterectomy, cardiopulmonary bypass, or testing physiologically and pharmacologically induced responses of the cerebral circulation has only recently been applied. The interest from neuroanesthetists and neurointensivists in this technique derives from its unique capacity to provide real-time information about intracranial hemodynamics. A variety of methods have previously been examined as a mean to assess cerebral perfusion in anesthesia and intensive care settings, however, as summarized previously (vide 3.) most of these methods are limited by various difficulties. Moreover, prior to the development of TCD ultrasonography, most studies measured cerebrovascular dynamics only under relatively steady-state conditions. Transcranial Doppler ultrasonography, however, allows an on-line and continuous monitoring of cerebrovascular dynamics with a time resolution within seconds. This technique, therefore, enables us to obtain improved insight into fast-acting components of homeostatic cerebrovascular mechanisms such as autoregulation and CO2 reactivity, something that could never be done using traditional methods.
While TCD ultrasonography is not new, it had a difficult time achieving acceptance as a valid estimate of CBF in comparison with older, time-tested methods. Today it is clear that TCD ultrasonography cannot provide absolute measures of CBF. However, several studies have demonstrated that TCD-monitored changes in CBFV correlate closely with changes in cerebrovascular hemodynamics induced by physiologic and/or pharmacologic challenges. It is apparent that, if used in an appropriate fashion, this technique can provide a reasonable accurate assessment of relative changes in CBF.