Perioperative fluid management in Infants and Children
Introduction
Perioperative infusion therapy should be adapted to the physiologic differences between newborns and adults, and fluid and electrolyte regulation during growth. Fluid requirements and maturation of the kidney set limits on the type and quantity of intraoperative fluids administered.
Requirements
The major difference between infants and adults is the fact that 40% of body water in newborns is extracellular fluid (ECF), while in adults, the ECF is 20%. This extracellular water is in the interstitial fluid volume, while plasma volume is similar at all ages. The high percentage of ECF causes a high turnover of water and electrolytes, especially sodium. A newborn child may lose 10% of its body weight if it drinks nothing for a day.
Pre- and perioperative fluid losses consist of hypotonic and isotonic components. The hypotonic fluid losses from the body are from sweat and diarrhea. This is low in sodium, i.e. 0.25 to 0.5 normal saline. Isotonic fluid losses include those that result from trauma, burns and losses from the upper GI tract, e.g. due to vomiting. Ileus and peritonitis may lead to great volume shifts and acute hypovolemia.
Fluid requirements are higher in hot rooms and in fever. For every 1 C fever about 10% more fluid is necessary. Intraoperative fluid deficits may result from exposure of large areas of tissue, as in abdominal and thoracic surgery, as well as from blood loss.
Kidney
The fetal kidney produces only amniotic fluid. Renal vascular resistance (RVR) is high, renal blood flow (RBF) and glomerular filtration rate (GFR) are low. The low GFR limits the renal function in the first 24h. Postpartum renal RVR decreases and with increasing systemic blood pressure RBF and GFR increase. By four to five days there is a marked improvement in renal function, and the ability to conserve fluid and excrete an overload. After one month the kidney is approximately 60% mature. Fluid and sodium conservation is limited in newborns and infants, while excretion of water and electrolytes is possible even in premature infants. Tubular function is especially limited for special excretion and reabsorption mechanisms. Prematures have limited ability to reabsorb sodium, and will therefore become hyponatremic without adequate replacement. The thresholds for glucose and sodium bicarbonate are lower than in adults. Hyperglycemia may lead to osmolar diuresis with consequent deficits in water and sodium. By 18 months, renal function is completely matured.
NPO Guidelines
There has been a change in the recommended time for patients to be fasted. Fat and milk slow gastric emptying time. For infants younger than one year milk should be given up to 6 h before surgery, and clear fluids up to 3 h. In the age of 1 and 2 years clear fluids are allowed up to 4h before surgery. Children older than 3 years can be handled as adults. The NPO time should be as short as possible to reduce preoperative volume deficiency the child’s discomfort.
Clinical signs of volume depletion
In young infants the fontanelles, the eyes and the tongue are good indicators of volume deficit. Fontanelles and eyes may be sunken when the volume deficit is about 5%. The tongue is small in hypovolemia and the mucous membranes are dry. Blood pressure and heart rate may remain normal even when the fluid deficit is as high as 10% of body weight.
Intraoperative Management
Intraoperative fluids must be replace both preoperative deficit, and intraoperative hypotonic and isotonic deficits. When hypotonic solutions are administered, the kidney must excrete the extra water and therefore loses sodium. Because of the ability of the kidneys to dilute urine and excrete sodium even in young infants, administering of balanced salt solutions is the better choice. In the first hour patients under 4 years should receive 25 ml/kg of balanced salt solution. In children older than 4 years, fluid replacement during the first hour should be 15 ml/kg-h. Tis volume should replace the preoperative deficit, e.g., a child who has been NNPO for 6 hours = 6 * 4 ml/kg = 24 ml/kg. In the next hour, maintenance luid (about 4 ml/kg) and trauma requirements should be replaced depending on the severity of the trauma. Mild trauma – 2 ml/kg-h, in moderate trauma 4 ml/kg-h, and in severe trauma 6 ml/kg-h should be administered.
Because of immature kidney function, hydroxyethylstarch should no be administered to children younger than two years. The large starch molecules cannot be filtered by the kidneys and therefore remain for an unpredictable length of time in the circulation. Human albumin 5% may be administered in cases where blood loss requires more than crystalloid solutions but less than blood transfusion. In acute hypovolemia, 10 – 20 ml/kg of human albumin 5% may be necessary to compensate for intravascular deficit. In infants and children, intraoperative blood pressure is a good reflection of blood volume and volume depletion.
Postoperative Fluid Problems
Acute dilutional hyponatremia may occur when the postoperative infusion is immediately changed to hypotonic saline or glucose solutions. The renal threshold for glucose is low in young children and might therefore produce an osmotic diuresis with depletion oof sodium and water. The sodium depletion due to tissue damage or postoperative vomiting may produce a further decrease in plasma sodium. The plasma in acute dilutional hyponatermia is hypotonic, thus a shift of water from the hypotonic ECF to the isotonic ICF may result. This shift results in cerebral edema with clinical symptoms of central nervous system irritability, decreasing consciousness, disorientation, vomiting, and, in severe cases, seizures.
The drug of choice is sodium bicarbonate 6% in dosages of 2 ml/kg intravenously. After this, balanced salt solutions should be administered. The intravenous sodium will increase plasma osmolarity and concurrently shift wear out of brain cells. If the sodium is still below 120 mmol/l, a further dosage of 1 mg/kg may be necessary. Seizures are treated with oxygen and antiseizure medications, such as diazepam. Physiologic sodium chloride should not be used for treatment of acute dilutional hyponatremia because of its high chloride content.
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