[Value of transcranial Doppler sonography].

Transcranial Doppler sonography (TCD) has gained increasing relevance since its introduction by Aaslid in 1982. Basal cerebral arteries are insonated with a 2-MHz ultrasound beam. The backscattered ultrasound is analyzed by Fast-Fourier transformation and permits continuous and noninvasive determination of flow velocities (v) and pulsatility index (PI). Cerebral vasospasm after subarachnoid hemorrhage is characterized by steadily increasing flow velocities and decreasing PI. Increasing cerebrovascular resistance due to hyperventilation or intracranial pressure results in high PIs. Since the diameters of basal cerebral arteries remain constant under CO2 variations, altered flow velocities indicate altered cerebral blood flow (CBF) in this situation. Thus TCD is a useful tool for analysis of CO2 reactivity, which is relevant for patients with brain injuries or carotid lesions. Intraoperatively, TCD has been used for detection of emboli during heart operations or carotid endarterectomies. Carotid cross-clamping induces proportional reductions of flow velocities and CBF; reduction of flow velocities to less than 40% of the preclamp value should result in shunt insertion. Although often pretended, constancy of basal cerebral arteries remains questionable in the perioperative period: arterial blood pressure, intracranial pressure, sympathetic tone, and vasodilating agents like nitroglycerin, halothane and isoflurane are likely to influence large cerebral arteries. Therefore intraoperative TCD results should be interpreted cautiously. TCD is a fascinating monitoring device with many advantages. It can be useful for many patients and physicians, if its limitations and the clinical situation are kept in mind. Further studies have to define the role of TCD in neuroanaesthesia.