Fluids and nutrition

CHAPTER 10 Fluids and nutrition

Mark Davies, David Cartwright, Garry Inglis

Renal function

There is a lot we do not understand about newborn renal function. Adult reference ranges of electrolytes etc may not be applicable to newborns, especially those who are preterm. At birth, serum electrolyte and creatinine values are usually similar to those of the mother.

It is normal for newborns to take up to 24 hours to first pass urine and they often have a urine output of <1 mL/kg/hr during the first couple of days. This period of relative oliguria is usually followed by a diuresis.

Average glomerular filtration rate is lower in newborns than in adults but serum creatinine levels tend to be lower also. Glomerular filtration rate may be influenced by factors such as medications (e.g. dopamine, indomethacin), hypoxic-ischaemic injury, a patent ductus arteriosus and respiratory distress.

Insensible water losses are proportionally greater in newborns than in adults. However, newborns are generally quite capable of maintaining adequate water balance over a wide range of fluid intake.

In practical terms, renal function and fluid balance can be monitored using serial weight measurements, urine output and serum sodium as guides. Serum potassium is often falsely elevated in newborns because of the method of collection (heel-prick collection causes haemolysis and/or contamination with tissue fluids). Serum urea and creatinine may be confusing and probably do not add a great deal to what can be determined by using the measurements noted above.

Parenteral fluids

General

• The standard parenteral fluid used initially in infants is 10% dextrose. This gives some caloric intake for those babies not fed for some time. Alternatively, a variety of starting fluids that may contain maintenance saline or calcium can be used.
• If given through an umbilical arterial line, heparin at a concentration of 1 unit/mL is added (500 units per 500 mL bag).
• Standard starting infusion rate is 60 mL/kg/day. For very small, preterm babies, 80–100 mL/kg/day may be started. This can then be varied according to serial weight measurements or electrolyte concentrations. The initial infusion rate does not need to be routinely increased during the first three days of life, especially in infants with lung disease.
• Do not increase fluids just because the urine output is poor. If the baby is not passing urine and the fluids are increased, you may cause fluid overload and hyponatraemia.

Electrolytes

• No standard electrolyte additives are normally necessary in the first few days. All preterm infants have an excess of extracellular fluid at birth and must undergo some electrolyte losses via urine when getting rid of this excess fluid.
• The most common fluid balance problem in the first 48 hours of life is excessive evaporative fluid loss in very small infants. This is reflected in excessive weight loss and rising serum concentrations of sodium and chloride ions, [Na+] and [Cl−]. Electrolytes may need to be measured every 8 to 12 hours in extremely low birth weight (ELBW) infants in the first 48 hours, and fluid intake adjusted accordingly. Fluids do not need to be increased until the serum [Na+] is in the high 140s (in mmol/L).
• When Na+ is added, the most convenient way to do so is to make a mixture in the burette of 80 mL 10% dextrose and 20 mL Nsaline (this is 8% dextrose/one-fifth Nsaline; Nsaline is Normal saline, 0.9%).

Some units use a ready-made 10% dextrose/one-fifth Nsaline solution, and others add NaCl to a bag of dextrose. Do not use 20% SALINE in IV fluids except under exceptional circumstances. (20% SALINE contains 3.42 mmol/mL of Na+ if you need it.)

• The addition of potassium K+ should be undertaken with great caution. If required, it should be added to a burette, with the amount ordered in mmol and in mL to avoid confusion [e.g. 10% dextrose, 98 mL; KCl, 2 mL (= 2 mmol); run at 7 mL/hr]. It is safest for all burette additives to be ordered in mL of a stated concentration of the additive.

Other

• In most preterm infants the serum calcium concentration [Ca2+] will decrease in the first few days, and the total serum [Ca2+] will commonly be 1.5 mmol/L or less. There is good evidence that the ionised [Ca2+] remains normal down to a total [Ca2+] of 1.3 mmol/L.

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Jul 18, 2016 | Posted by in PEDIATRICS | Comments Off on Fluids and nutrition

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