The proposed study will focus on anesthesia and anesthesia-induced hypotension as a possible cause for postoperative fluid retention and hyponatremia, and investigate gender differences in this response.

30 healthy women and 30 healthy men, who are scheduled for surgery of the middle ear or parotic gland are included in the study. Within each gender group, the subjects are randomized to receive perioperative intravenous fluid regimes either based on body weight or lean body mass (LBM). The surgery lasts for at least 3 hours, the intervention period (fluid administration according to protocol) will last 10-12 hours, the study period will be 20-24 hours. Blood samples for analysis of plasma sodium, potassium, glucose and osmolality are collected preoperatively and the following morning together with analysis of urine produced during the study period.

Inclusion Criteria:

• ASA Physical status I-II
• Subjects scheduled for otorinolaryngoiatric surgery in general anesthesia of at least 3 hours duration

Exclusion Criteria:

• BMI less than 18

• BMI greater than 33

• Diabetes mellitus requiring medication

• Treatment with diuretics

• ACE-inhibitors

• Angiotensin II antagonists

• Cortisone

• Lithium

• Diseases of the kidney

• Females:

  • Pregnancy
  • Menopause
  • Endocrine dysfunction influencing menstruation