The purpose of hyperglycaemic clamping is to raise the blood glucose level to a fixed plateau and maintain the plateau for at least one hour to observe a steady-state blood glucose concentration.

The investigators intend to perform the hyperglycaemic clamping procedure one time in all 14 patients following an overnight fast and aim for a fixed blood glucose level above 8 mmol/L.

The patient will have an arterial line placed in the radial artery, a retrograde catheterization of the jugular bulb and a venous line for infusions of glucose, potassium, and isotonic saline.

To maintain potassium levels at wanted levels, an isotonic saline solution containing potassium will be infused continuously and arterial samples of potassium and glucose will be measured at minimum every 10 minutes from the start of the intervention.

Following an overnight fast, the investigators will perform simultaneous samples from the arterial line, the venous line, and microdialysis and consider these samples as baseline values.

If the patient's blood glucose level is at 8-10 mmol/L at baseline, the investigators will not proceed with the intervention and instead evaluate if the intervention can take place the following day.

After baseline values are measured, intravenous glucose 20% (200g/1,000 ml) is infused to raise the glucose levels in both plasma and extravascular glucose compartments with approximately 3 mmol/L.

Change in) MD-glucose (mmol/L) over time as a function of (a set change in) arterial blood glucose (mmol/L)

Jugular vein glucose (mmol/L) over time as a function of arterial blood glucose (mmol/L)

Mathematic model of transport of glucose over the blood brain barrier.