Fluid and Blood Administration

Water Requirements

Water maintenance rates in children are based on studies that demonstrated a direct association between metabolic rate and water requirements. In general, water requirements on a per-kilogram basis increase with decreasing size of the child. This is secondary to the higher metabolic rate with decreasing size and the relatively greater evaporative loss from body surfaces in smaller children because of a higher ratio of body surface area to weight. Prematurely born infants have an even greater rate of evaporative loss because of thinner, more permeable, vascularized skin. In general, the amount of evaporative water loss per kilogram is inversely proportional to the gestational age.

In a very influential paper published in 1957, Holliday and Segar developed an easily remembered formula for calculating caloric requirements of the “average” hospitalized child (>2 weeks of age) based on bodyweight. They also demonstrated that the water requirement in milliliters was equal to the total energy expended (i.e., 1000 mL of water is required for every 1000 kcal expended). Thus the formula for caloric requirements can easily be used to calculate daily water requirements using the “4–2–1 rule” ( Table 14.1 ).

Subsequent studies during pediatric surgery demonstrated that intraoperative caloric and fluid requirements are less than those calculated by Holliday and Segar. Nevertheless, for the vast majority of healthy children undergoing surgery, these formulae will consistently prevent perioperative fluid and electrolyte abnormalities, and thus, are still in use today.

Electrolyte Requirements

For each 100 calories metabolized in 24 hours, the average child requires 3 mEq of sodium, 2 mEq of potassium, and 2 mEq of chloride. These recommendations are based on the approximate electrolyte concentration of human breast and cow’s milk, and the resulting normal urine osmolality after milk ingestion. In most otherwise healthy hospitalized children, these electrolyte requirements will be met by administering a solution of 0.2% sodium chloride with 20 mEq of added KCl per liter at the normal maintenance rate ( Table 14.2 ).

In the perioperative period, it is necessary to replace surgical losses in addition to providing maintenance fluids and electrolytes. Therefore we commonly use solutions that contain a greater percentage of sodium. This better approximates the fluid that is associated with losses during surgery and avoids hyponatremia that would result from large infusions of a hypotonic fluid, especially in the face of an increase in antidiuretic hormone release caused by the stress of surgery. Lactated Ringer’s (LR) solution (or similar) meets these needs for the majority of surgical procedures in children. Normal saline contains 154 mEq/L of sodium and is preferred when replacing large amounts of isotonic fluid because LR is somewhat hypotonic.

Replacement of Preoperative Deficit

Normally, children presenting for elective surgical procedures have incurred a fluid deficit during the preoperative fasting interval. Although ingestion of clear liquids is often encouraged up to 1 or 2 hours before the scheduled procedure time, depending on institutional guidelines, many children have fasted for a longer time. Many pediatric anesthesiologists feel that this deficit should be replaced with isotonic fluid to compensate for anesthetic-induced vasodilatation and unexpected intraoperative or postoperative fluid and blood loss. Preoperative fasting deficits are calculated by multiplying the hourly maintenance rate by the number of hours the child fasted. Traditionally, 50% of this deficit is replaced in the first hour of intravenous hydration and 25% in each of the next 2 hours. For relatively short, minor surgeries, a volume of 4 mL/kg of isotonic solution has been suggested as an appropriate replacement volume that covers the preoperative deficit and intraoperative maintenance requirements.

Children presenting for emergency surgery may have increased fluid losses secondary to fever, vomiting, edema, and blood loss. Therefore these children should receive earlier and more aggressive volume replacement, until establishment of normal urine production (1–2 mL/kg/h). Keep in mind that aggressive fluid deficit replacement is associated with a decrease in core body temperature.

Glucose Administration

Because of the normal hyperglycemic response to surgery, healthy infants over about 1 month of age do not require addition of glucose to intraoperative maintenance fluids. Children at risk for intraoperative hypoglycemia include those less than a month of age, weight for age <5th percentile, ASA status ≥III, having a gastric or jejunal tube, poor feeding, and abdominal surgery. These children should have intermittent glucose checks during anesthesia. A 2% dextrose solution appears to provide the best balance between the risk for hypoglycemia and hyperglycemia, which may cause an osmotic diuresis.

Hospitalized neonates, especially those born prematurely, will often be receiving increased amounts of glucose to compensate for limited glycogen reserves. Because most of these patients will demonstrate a hyperglycemic response to the stress of surgery, this author’s practice is to initially administer the same solution at half its original rate, check blood glucose values at least hourly, and readjust the rate if necessary. Normal saline or LR solution is then used to replace any deficit or intraoperative isotonic fluid loss.

Children who received clonidine premedication or neuraxial blockade before the surgical incision may not develop the intraoperative hyperglycemic stress response. These patients should receive a maintenance fluid that contains dextrose or should undergo intraoperative blood glucose monitoring at regular intervals.

Replacement of Intraoperative Fluid Losses

In addition to insensible losses and ongoing metabolic needs, fluid is lost intraoperatively as a result of evaporation from exposed tissues, “third-spacing,” and surgical blood loss. Third-spacing is the transfer of relatively isotonic fluid from the extracellular volume space to a nonfunctional interstitial compartment, and is triggered by surgical trauma, infection, burns, and other mechanisms of tissue injury. The amount of volume lost to the third space can be estimated based on experience with the particular type of surgery, observation of the surgical field, and the clinical response to volume replacement.

Insensible fluid losses during minor procedures will average less than 3 mL/kg/h. This value will increase based on the location and extent of the surgical injury. For example, a neonate undergoing an exploratory laparotomy for necrotizing enterocolitis and gangrenous bowel may require 50 to 100 mL/kg/h to maintain euvolemia. Most thoracic and neurosurgical procedures require 5 to 10 mL/kg/h.

Intraoperative fluid losses are replaced with an isotonic, nonglucose-containing solution such as LR, NS, or Plasmalyte. When using crystalloid to replace surgical blood loss, three times as much crystalloid solution should be administered as the amount of estimated blood lost. End points of intraoperative volume replacement include an appropriate blood pressure and heart rate, and adequate tissue perfusion as evidenced by a urine output near or above 1 mL/kg/h.

Massive amounts of replacement with each solution carry unique disadvantages. Because of its slightly hypotonic nature, large amounts of administered LR solution are associated with a decreased serum osmolality and development of edema. Large amounts of administered NS are associated with development of dilutional or hyperchloremic acidosis.