Cerebral autoregulation is an important mechanism whereby cerebral perfusion is normally maintained at a constant level, over a relatively wide blood pressure range. It can be assessed noninvasively by the use of Trans Cranial Doppler (TCD). This means using ultrasound probes over both sides of the head to measure changes in blood flow in one of the main brain arteries (the middle cerebral artery) in response to beat to beat changes in blood pressure dynamic cerebral autoregulation (dCA). It is established that dCA is impaired following moderate to severe stroke, acting as a key role in the development of secondary brain damage related to brain swelling and further damage related to the low blood flow. The administration of clot busting therapy (thrombolysis), one of the main approved treatments of acute ischaemic stroke (AIS), results in recanalisation of the blocked artery in only approximately 50% of patients. Therefore, as well as attempts to treat major vessel blockage, improving brain blood flow, particularly to the penumbral area, through arteries that bypass the blockage is another potential therapeutic approach in AIS.One simple way of achieving this might be to lower the head of AIS patient into a lying flat (0⁰) position. However, to date, there have been very few studies exploring this. This research will use the noninvasive technique of Trans Cranial Doppler (TCD) to see how blood flow changes in different head positions, both in healthy volunteers and AIS patient. This study will provide important data regarding blood pressure management in acute stroke, an important and common clinical dilemma.