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Fluids and Electrolyte Therapy in the Paediatric Surgical Patient 25

before the GFR, and thus serum creatinine levels, approach the levels intake adjustments. 1
of a term infant. 1 Fluid Requirements
On a clinical note, the use of gentamicin in the perioperative surgical Fluid requirements in the newborn or older child depend upon multiple
newborn, if the levels of gentamicin are not measured due to resource factors, but the majority are determined by the insensible water loss and the
constraints, can potentially increase the amount of renal dysfunction in newborn’s metabolic rate. The evaporation of water from the skin and the
this patient population. In a recent study in India, dosing of gentamicin respiratory tract has environmental factors, such as air and incubator tem-
2
with the following neonate weights was considered safe:
perature, humidity, and air flow across the child’s body, as well as infant fac-
• 10 mg every 48 hours for the neonate weighing less than 2000 tors, such as patient position, metabolic rate, and elevation in temperature.
grams Many studies have indicated that water loss from the premature
• 10 mg every 24 hours in the neonate weighing 2000–2249 grams infant is significantly greater than from the term infant, possibly due
to decreased subcutaneous fat and increased permeability through the
• 13.5 mg every 24 hours for the neonate weighing more than 2500 skin. The combination of increased water loss in the premature infant
grams
and the use of radiant warmers without humidity, which occurs in many
Gentamicin interval errors are the most common drug error reported settings, can result in a severely dehydrated premature infant who may
in a recent neonatal intensive care unit (NICU) study from the United need resuscitation. The insensible water loss can increase 50–200%
States, and certainly the effect on renal function is amplified in a setting with the use of radiant warmers in the preterm infant. This can have an
3
where drug levels cannot be measured. Gentamicin and ampicillin are significant impact on the intraoperative course, as the patient may arrive
the two most commonly prescribed antibiotics in the NICU environment, in the operating theatre dehydrated even though receiving maintenance
and inappropriate dosing can cause clinically significant renal damage. fluids in the immediate preoperative period. The low-tech approach to
Sodium excretion, which is directly correlated to GFR and indirectly humidity can be achieved merely by keeping an open container of water
to gestational age, becomes an issue in situations where there is sodium near the newborn while under warming lights. Fluid chambers need to
load or the need to retain sodium arises. The kidney’s ability to retain be cleaned and changed routinely in an effort to decrease infection in
sodium in preterm infants will not reach the term infant’s level until the nursery unit, and the fluid level of radiant warmers, which may vary
they reach the gestational age of a term infant. Clinically, this can depending upon the manufacturer, needs to be monitored.
produce situations of appropriate release of ADH and reabsorption of Many formulas exist to determine the maintenance fluid levels
water in a setting where the patient is getting a volume of hypotonic for the neonate or small child. The formula of the “4-2-1 rule” works
fluid, such as 0.25% saline in 5% dextrose. If the sodium level goes well for determining the maintenance fluids for weight groups that
below 120 mEq/l, then the patient could show signs of neurologic are less than 30 kg. In this formula, the first 10 kg of body weight is
injury, which can be nonreversible. multiplied by 4 ml/hr; the second 10 kg is multiplied by 2 ml/hr; and
A urinalysis that shows colour (concentration, presence of bilirubin), any additional kilograms of weight are multiplied by 1 ml/hr. Table
4
red blood cells, white blood cells, protein, and glucose can help to 5.3 provides examples that apply to newborns and older children to
diagnose some renal problems. The observation of protein in the urine determine maintenance fluids. Intraoperative fluid management is
can be normal in the first few days of life and then can become expected discussed in a later section of this chapter that describes translocated
in cases of hypoxia, congenital cardiac problems, and dehydration. fluid and blood loss. Typically, if one has a 1-ml blood loss, then this
Small amounts of glycosuria can be detected secondary to a low tubular 1 ml is replaced by 3 ml of crystalloid, which could be normal saline
reabsorption with a glucose load, and the glucose load can even result or Ringer’s lactate solution. This amount of replacement allows for the
in an osmotic diuresis and dehydration. Glucose in the urine may be an intravascular volume to be maintained, even during times of decreasing
early sign of sepsis, especially in the presence of other factors. intravascular volume, which could be surgery.
Normal Fluid and Electrolyte Metabolism Table 5.3: Sample 4-2-1 rule for maintenance fluids for newborns and
older children.
Total Body Water
At birth, the infant is suddenly separated from the source of water found Child’s body weight Volume
in the in utero environment, and now is in an environment with signif-
cant water loss from the skin and respiratory tract, thus promoting a 4 × 9 = 36
potential for early dehydration. During this period of transition, water
intake and renal conservation of fluids needs to maintain a homeostatic 2 × 0 = 0
state to survive. 9 kg
Total body water compromises intracellular water (ICW) and 1 × 0 = 0
extracellular water (ECW), with the ECW having an intravascular and 36 cc/hr
interstitial component. With advancing gestational age, the amount
of total body water declines from 94% of body weight in the third 4 × 10 = 40
trimester to approximately 78% at term. In the immediate postnatal
2 × 5 = 10
period, the amount of extracellular fluid decreases and the percentage
15 kg
of intracellular fluid increases, although the newborn has a large
1 × 0 = 0
interstitial reserve volume during times of decreased fluid intake. The
term infant can compensate more than the preterm infant, but newborns 50 cc/hr
with a large surface-to-weight ratio, higher total water content, limited
renal ability to concentrate, greater insensible water loss from thin 4 × 10 = 40
skin, and high blood flow all can become clinically dehydrated in a
very short period of time. The added water loss associated with radiant 2 × 10 = 20
26 kg
warmers, which are commonly found in the treatment of the newborn
1 × 6 = 6
and especially preterm infants, can result in a rapid and progressive
level of dehydration without close observation and appropriate fluid 66 cc/hr