Chloride-rich isotonic crystalloid solutions have been shown to exert electrolyte imbalance and metabolic acidosis in critically ill patients. The aim of this investigation is to compare the influence of 0.9% saline and plasmalyte solution on acid-base balance and electrolytes in healthy patients undergoing major spine surgeries.

Inclusion Criteria:

1.50 patients (aged 20-65) undergoing cervical or lumbar spine surgery who were diagnosed spinal stenosis involving more than tow spinal levels.

Exclusion Criteria:

1. ASA class III or IV
2. pregnancy
3. breast feeding
4. lack of mental weakness(including disabled)
5. using diuretics
6. respiratory insufficiency
7. metabolic acidosis or alkalosis
8. coagulation disorder
9. over hydtration
10. renal failure or serum creatinine > 1.4mg/dl
11. anemia (Hemoglobin 9.0<mg/dl)
12. hypernatremia (Na > 145mEq/L)
13. Hyperkalemia (K>5.5mEq/L)
14. psychological medication or any drugs that influences renal clearance