Half an hour after a 30-year-old woman had had some custard cake, she experienced lancinating abdominal pain, nausea, vomiting. Objectively: body temperature - 36,0°C, pale skin, breathing rate - 20/min, Ps- l0o/min. AP- 95/65 mm Hg, loud cardiac sounds. Dry tongue. Abdomen was painful in its epigastrial part, there were no signs of peritoneum irritation. What is the first measure to be taken?