O36	HEMODYNAMICS OF CEREBRAL VASOSPASM R. Aaslid, Berne TCD is widely used for diagnosis and monitoring of cerebral vasospasm following subarachnoid hemorrhage. However, conflicting reports on its accuracy have been published. Most of the critical reports have used only intracranial velocity measurements as a criterion for the severity of spasm. The purpose of this study was to show in which situations this simple criterion can be used, and under which circumstances more information is required for assessment of vasospasm. For this purpose it is necessary to have a comprehensive understanding of all the important factors influencing cerebral hemodynamics. A model is proposed which includes the main elements determining the overall effect of vasospasm. It includes realistic pressure-flow-velocity-diameter relationships encountered in a geometry resembling that of vasospasm of the middle cerebral artery. Furthermore, a realistic representation of the cerebral autoregulation was included.  It was found that the friction pressure loss in a typical spastic segment was 3.5 times as high as that predicted by the simple Hagen-Poiseuille formula. The reason for this discrepancy is probably the 'inlet length effect' considerably increasing the friction losses. Furthermore, including the Bernoulli kinetic pressure energy, a formula is proposed that accurately describe the experimental data. From this hemodynamic perspective, support was found for the present trend to use hypertensive/hemodilution therapy in patients with vasospasm. The results also confirmed that TCD velocity measurements in the spastic segment when taken alone might not be a good index of the degree and effect of the spasm when it becomes critical and causes reduction in CBF and clinical symptoms. The paper discusses solutions to this complex situation.
O37	Key note
O38	Has Doppler changed the way we think about the brain? Karl-Fredrik Lindegaard, Department of Neurosurgery, Rikshospitalet; and Centre for Health Administration, University of Oslo, Oslo, Norway I shall deal with Doppler ultrasound as being one (of several) representations of the brain’s circulation as an object. For more than a half century the neuroscientific community has recognised the brain circulation as being responsive, flexible and yet vulnerable; and elucidating each particular type of problem by means of those representations considered as being the most suitable to this end at the different stages or epochs. Prominent examples are autoradiography; animal cranial window; tracer dilution, accumulation and transit; and electromagnetic field deviation, to name but a few. However diverse in nature, these representations have in common a provision of platforms or arenas for different types of discourse, on topics such as technical solutions, clinical and laboratory application, and pitfalls and errors, the latter essentially pertaining to when and if a given representations can be considered as revealing some of the object’s idea, its "intrinsic truth". Occasionally, existing representation, or technique, is brought to bear on new or rejuvenated clinical / surgical challenges. In exceptional cases, a new representation emerge (TCD), elicited by keen human volition to overcome a long-standing clinical problem (vasospasm) for which established representation (intra-arterial angiography, at the time) seemed not suitable. Twenty years since the introduction of Transcranial Doppler (TCD) ultrasound it is nonetheless strange to note that we also, albeit indirectly, mark the refreshing of what was in fact discovered in the late 1930’s: that ultrasound certainly can penetrate the skull. TCD provided new insight into the problem, and was applied to a several other clinical and scientific fields. In the 1970-1980’s, neurovascular microsurgery expanded, with treatment policy for ruptured intracranial aneurysm shifting from "late" to "early" surgery. More recently, giant aneurysms and tumours of the skull-base have become the surgeon’s challenge. Thus, surgical sacrifice of an artery supplying the brain is (yet again) emerging as a viable treatment option, a small-volume activity, albeit of critical importance to the individual concerned. One lesson learned from using TCD to assess the circulatory consequences of sacrificing carotid or vertebral arteries is that significant degrees of freedom are available even in exceptional cases, which also implies that my reply to the above question is affirmative. Compared to advances in technique, advances in concept and idea come slowly, and, as I have tried to point out, even reiteratively. Let, therefore, the epoch in which we practice our craft be not just the epoch of one technique or representation, but one of idea: the epoch of cerebral haemodynamics.