Fluids and nutrition

Published on 03/06/2015 by admin

Filed under Neonatal - Perinatal Medicine

Last modified 22/04/2025

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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.
• If hypocalcaemia is to be treated, the standard parenteral dose is 3 mL of 10% calcium gluconate/kg/day. Be very wary about giving this via a peripheral IV line, as nasty extravasation burns with necrosis occur if the line infiltrates. It is satisfactory to give this as an infusion into an umbilical artery, but boluses should be given very slowly and carefully.
• Never give Ca2+ into a peripheral arterial line. Never give calcium in the same line as bicarbonate or parenteral nutrition.
• Peripheral arterial line fluid is normally 1/2N or Nsaline, given at 0.5–2 mL/hr. It should contain 2 units/mL of heparin. Such an infusion is best administered by syringe pump. No drugs are ever administered via a peripheral arterial line.

Parenteral nutrition (PN)

The provision of parenteral nutrition varies considerably from unit to unit. This variation is in: the solutions used and their composition; how the solutions are delivered; and when they are started and stopped.

• Parenteral nutrition usually consists of the infusion of a solution of amino acids (such as Vamin or Primene) and carbohydrate (dextrose), plus a solution of fat (Intralipid). Vitamins are also given, both water-soluble and fat-soluble.
• Some neonatal units make up individual prescriptions for PN with varying concentrations of dextrose and electrolytes on a base amino-acid solution. In practice this process is unwieldy and unnecessary. The baby is much smarter than you are at sorting out minor electrolyte problems.

A suggested procedure

• Use standard solutions with set concentrations of amino acids, dextrose, Na, K, Ca, Mg, Cl, P, Zn and Cu:

— Our starting solution (Solution 1) has 1% amino acids and 10% dextrose plus electrolytes and trace elements.
— Solution 2 has higher concentrations of amino acids (1.75%) and dextrose (12.5%).

• Start with Solution 1, at the same rate as the baby’s maintenance fluids were being given, increasing by 20 mL/kg/day daily if fluid balance permits.
• Change the PN to Solution 2 after a couple of days (at the next line change). Total fluids are then usually increased (again if fluid balance permits) daily by 20 mL/kg/day up to 180 mL/kg/day. If the dextrose is not tolerated, insulin at 0.05–0.2 units/kg/hr can be infused

— see page 83.

• Add water-soluble vitamins (e.g. Soluvit N) to the burette at the beginning of the first infusion after each line change (3 days per week — M,W,F), 2 mL/kg each time. The vitamins are thus given to the baby over the first few hours — minimising the vitamins’ exposure to light.
• Give lipid as a 20% solution of Intralipid at a dose of 2 g/kg/day (10 mL/kg/day). Fat-soluble vitamins can be added as Vitalipid N — 4 mL/kg/day to a maximum of 10 mL/day. Other neonatal units will start at 1 g/kg/day and increase over a few days up to as high as 3 g/kg/day, with monitoring of triglyceride levels. Some stop lipid infusion if the baby has septicaemia; we don’t.

General

• Parenteral nutrition should be administered via a central line or umbilical venous line.
• Parenteral nutrition given via peripheral lines must be very carefully managed as extravasation burns occur at sites of infiltration, and evidence from randomised controlled trials demonstrates that where peripheral IV lines are used the babies get much less nutrition because of the time spent without IV access.
• PN solutions and infusion lines are changed every 2–3 days. Remember, all central line changes must be done as full sterile procedures.

Enteral feeding

Feeding of breast milk is encouraged for all babies. Don’t automatically stop breast milk feeds if the mother is on medications. Get advice from pharmacy or a Drug Information Centre first.

Preterm babies have four main difficulties with respect to feeding:

1. weak and incoordinated deglutition <34 weeks gestational age
2. small initial gastric capacity of ∼3 mL/kg
3. poor gastrointestinal activity
4. poor digestive capacity (rarely of practical consequence).

Feeding therefore presents the need for:

1. intragastric tube feeding for babies <34 weeks gestational age
2. small initial feeds of 3 mL/kg usually
3. adequate spacing between feeds for gastric emptying — we use 3 hours to start with
4. graded increases in volume and frequency as tolerated.

If a baby is not being enterally fed, always ask yourself why not?

Babies who might not be started on enteral feeds from soon after birth include:

• ELBW babies
• babies who were growth-restricted in utero and who had absent/reverse diastolic flow in the umbilical artery
• babies with gut obstruction
• babies with significant respiratory distress
• those whose mothers wish to exclusively breast feed but have not produced any expressed breast milk (EBM) yet.

Feeding regimes

• In a well baby, who is able to tolerate full enteral feeds from day 1, feeds can be started at 60 mL/kg/day and increased to 180 mL/kg/day by day 7.
• A baby that is not fed and who is only getting a dextrose/saline solution (i.e. no feeds or PN) doesn’t need more than 120–140 mL/kg/day of total fluids.

Standard feeds contain 67 Cal/100 mL.

Low birth weight (LBW) infant formulas contain 81 Cal/100 mL.

There are many ways to start small feeds and build them up slowly: from continuous to every 1, 2, 3 or 6 hours! Here is one scheme:

• Start all feeds as bolus feeds given every 3 hours, usually at around 2–3 mL/kg.

— For babies weighing <1500 g, the aim is to change to feeds every 2 hours once tolerance of reasonable volumes 3-hourly has been established.
— For babies weighing <1000 g, feeds can be increased by 1 mL per feed every 24 hours until 5 mL every 3 hours is reached. Then change to 4 mL every 2 hours, and increase by 1 or 2 mL every 12 to 24 hours as tolerated. Very rarely, hourly bolus or continuous feeds need to be given.
— For babies weighing 1000–1500 g, the increases can be of larger volumes each feed, and undertaken every 12 to 24 hours.

• Feeds are usually changed back to being given every 3 hours when the baby’s weight reaches 1600 g and if the feeds are well tolerated.

Supplementation of breast milk

Babies of <1500 g who are fed breast milk need supplementation with phosphate (to prevent osteopaenia of prematurity) and iron (to aid recovery from anaemia of prematurity). A baby’s iron stores run out when it reaches twice its birth weight.

• For phosphate, either:

— use Sandoz Phosphate tablet; dissolve 1 tablet into 16 mL of water (gives ∼1 mmol/mL). Give 1 mL/kg/day of this mixture in 4 divided doses; this will give 1 mmol/kg/day of PO43−; or
— fortify feeds — use EBM with fortifier or an LBW formula.

• For iron, use iron solution, such as Fergon; give 2–6 mg/kg/day from 14 days of age, usually given twice a day (we recommend 0.5 mL twice a day).

Infants weighing <1500 g may benefit from ‘fortification’ of breast milk with a breast milk fortifier such as FM85 or Wyeth Breast Milk Fortifier. These fortifiers add protein, some starch calories, phosphate and calcium.

We do not recommend fortifying until feeds are well tolerated at good volumes. For artificially fed infants a LBW infant formula may be used. If the ‘ready-to-feed’ preparation is used, iron content is 8 mg/L (most normal, non-fortified baby formulas have 8–12 mg/L) and iron supplementation is not needed. Fortification is normally ceased when the baby weighs 2000–2500 g.

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