Autoregulation can be conveniently measured non-invasively from the bedside using transcranial Doppler (TCD). Direct measures of middle cerebral artery flow velocities (FV) are predictive of DID; elevated mean FV (>120cms/sec) and increased ratio of mFV between MCA and extracranial internal carotid artery (ICA) (Lindegaard ratio >3) being associated with cerebral vasospasm and higher incidence of adverse events [Lindegaard 1988]. The rapid increase of FV with time is also predictive, as is the relationship between FV and spontaneous changes in blood pressure (correlation index Mx) [Czosnyka 2000].