Osmotherapy has been used as the medical treatment for brain edema and intracranial hypertension in critically brain injured patients. Measurement of serum osmolality during osmotherapy is of clinical importance to determine clinical efficacy, adjust dosage and avoid side effect. Serum osmolality is often measured in laboratory by cryoscopic technique as the reference method. However, in clinical setting, routine measurement of serum osmolality is not feasible at bedside, either in intensive care unit (ICU) or neurosurgical ward. Therefore, clinicians usually estimate serum osmolality by using equations derived from serum osmoles that can be measured by bedside blood gas analysis or routine laboratory chemical analysis, such as sodium, potassium, urea, and glucose. In present study, mannitol or hypertonic saline will be used in patients after craniotomy, and serum osmolality will be measured before and during drug infusion. Investigators hypothesize that the correlation of measured and calculated serum osmolality is better during infusion of hypertonic saline than mannitol.

Inclusion Criteria:

• adult patients after elective craniotomy, requiring hyperosmolar agents for prevention or treatment of brain edema

Exclusion Criteria:

• age younger than 18 yr or older than 65 yr
• history of diabetes
• unstable hemodynamic condition
• presence of renal failure