A variety of electrolyte disorders are common postoperatively. These result from fluid losses, intravenous fluid administration, and intrinsic regulatory mechanisms that come into play at this time. During surgery, large volumes of isotonic fluids as well as blood products may be given to compensate for the blood loss and third spacing that occurs with the surgical procedure. In the postoperative period patients must remain fasting until bowel function has returned. Frequently, nasogastric, chest, or biliary tubes are present and drain fluids and maintenance intravenous fluids are continued. Parenteral nutrition often complicates fluid balance. There may be shifts in acid–base status such as metabolic acidosis from lactic acidosis secondary to tissue ischemia, or respiratory alkalosis from hyperventilation as a result of pain or mechanical ventilation. Anesthetics, diuretics, steroids, insulin, cardiac medications, and non-steroidal anti-inflammatory drugs administered during this time period may alter plasma electrolytes and further complicate the picture.

Sodium disorders

Abnormalities of serum sodium are generally disorders of water balance. Hyponatremia occurs when there is too much free water, which may be retained appropriately or inappropriately. Hypernatremia occurs in patients who have an impaired thirst mechanism or have limited access to water and are not repleted appropriately. Of the two, hyponatremia is generally more common as a postoperative problem.

Hyponatremia

In order to understand sodium and water disorders one must understand the relationship between plasma sodium concentration and serum osmolality.