It is a "proof of concept" study, aimed to evaluate whether the "optimal CPP", defined by the best PRx, corresponds to the acceptable CBF values in patients affected by CBF disfunction caused by TBI or SAH.

Cerebral blood flow (CBF) disturbances are common following TBI and SAH. The occurrence of CBF derangements is detrimental for the neurological outcome in both settings, but the management of neurologically critically ill patients does not involve CBF measure routinely. Cerebrovascular autoregulation, can be assessed by the cerebrovascular pressure-reactivity index (PRx) that point out the response of ICP to spontaneous changes in arterial blood pressure (ABP). Autoregulation has been proven to be a powerful protective mechanism. Adding together the information on CBF and autoregulation, might drive clinical strategy in exceptionally noteworthy and innovative way. Currently, a novel Thermal Diffusion (TDP) microprobe has been introduced for the continuous bedside monitoring of regional CBF: TDP is a promising technique in the reliable detection of flow derangements at the patient's bedside.

It is a "proof of concept" study, aimed to evaluate whether the "optimal CPP", defined by the best PRx, corresponds to the acceptable CBF values.

Patients admitted with the diagnosis of TBI and SAH in for whom ICP and CPP needs to be monitored on clinical ground will be also monitored with a TD probe and routinely tested for cerebral autoregulation, thus obtaining the CBF corresponding at a given the "best CPP" and autoregulation status.

Continuous CBF measures and PRx monitoring may allow more accurate identification and early detection of adverse cerebral conditions. This approach may bring us a step closer to the goal of outcome improvements in patients suffering from intracranial insult.

Inclusion criteria:

• Patients admitted with the diagnosis of SAH and requiring intensive monitoring, and ICP probe.
• Patients admitted diagnosis of severe TBI and requiring intensive monitoring, and ICP probe.

Exclusion criteria:

• Age < 16 years
• Previous SAH, brain surgery, stroke, brain trauma

Patients admitted with the diagnosis of TBI and SAH in for whom ICP and CPP needs to be monitored on clinical ground will be also monitored with a TD probe and routinely tested for cerebral autoregulation, thus obtaining the CBF corresponding at a given the "best CPP" and autoregulation status. Continuous CBF measures and PRx monitoring may allow more accurate identification and early detection of adverse cerebral conditions. This approach may bring us a step closer to the goal of outcome improvements in patients suffering from intracranial insult.