Neuroanesthesia for supratentorial surgery involves a thorough understanding of the physiopathology of intracranial pressure, cerebral homeostasis and regulation of cerebral perfusion pressure as well as the effects of anesthesia and surgery on these elements.

The main objective of anesthesia during neurosurgery is to preserve the integrity of the brain by maintaining cerebral homeostasis, and assuring cerebral protection using normovolemia, normotension, normoglycemia, moderate hyperoxia and hypocapnia and hyperosmolality with the administration of mannitol.

During surgery, the use of surgical retractors must be limited to avoid possible ischemia of the brain tissue. Surgical retractors can be replaced by chemical retractors. The concept of chemical retraction involves a reduction of cerebral blood flow, maintaining cerebral perfusion pressure, moderate hyperventilation, drainage of cerebrospinal fluid and osmotherapy.

Mannitol, an osmotic agent, has been widely used to reduce the volume of the brain, the intracranial pressure and to facilitate the surgical approach in reducing the risk of cortical lesions during the opening of the skull.

Mannitol 20% is usually given intravenously in bolus doses of 0.5-1g/kg over 30 minutes. However, over the last few years, the concept of a dose-response relationship has emerged. Some recent studies tend to demonstrate that higher doses of mannitol could reduce intracranial pressure significantly without any important side effects.

The main objective of the present study is to compare two doses of mannitol (0.7 and 1.4 g/kg) on brain relaxation during supratentorial craniotomies.

80 patients will be divided in two equal groups (Group 1: to receive 20% mannitol 0.7 g/kg or Group 2: 1.4 g/kg).

The anesthetic technique and monitoring will be standardized. The administration of mannitol will start following the induction of general anesthesia. The infusion will be given intravenously over 30 minutes.

Brain relaxation will be assessed by a senior surgeon at the opening of the dura mater on a scale from 1 to 4 (1= perfectly relaxed, 2= satisfactorily relaxed, 3= firm brain, 4=bulging brain)

If needed, in case of significant cerebral edema, a rescue dose of 20% mannitol 0.25 g/kg will be administered.

Hemodynamic variables (MAP, heart rate), temperature, urine output, perioperative fluid balance, blood loss and laboratory values (blood gases, electrolytes, osmolality, hematocrit, glycemia, lactates) will be collected immediately prior to the infusion of mannitol and at 30, 60, 180 minutes following the administration of mannitol.

The type of cerebral lesion, its location and size (in 3 dimensions) will be noted. The presence of a median-line shift will also be assessed.

Inclusion Criteria:

• Patients aged 18 or older
• Patients who are to undergo an elective supratentorial craniotomy
• Physical status ASA I to IV inclusive.

Exclusion Criteria:

• Pregnancy
• Severe congestive heart failure
• Severe chronic renal failure
• Recent use (less than 24 hours before surgery)of mannitol or other hypertonic solution