Fluid and electrolytes

Notes

• Aim is to maintain baby in fluid and electrolyte balance
• Situation is complicated by the normal adaptive processes taking place after birth and the effects of the environment and any disease process
• There is a contraction of the extracellular fluid compartment with loss of water and sodium over the first days of life
• This "normal" diuresis may be delayed in sick infants and occurs at the same time as an improvement in lung function
• Excessive fluid and/or sodium intake in the first days of life may delay the normal contraction of the extracellular fluid compartment. Fluid overload has been associated with more severe lung disease, PDA and a higher incidence of BPD
• Preterm infants lose large amounts of water from the skin. This insensible loss is minimised by using humidified incubators. For infants <28 weeks gestation the insensible loss through the skin during the first week of life in 80% humidity is approximately 30-50 mls/kg/day. In 50% humidity this figure rises to 60-100 mls/kg/day (may be even higher in 22-23 weekers)

Guidelines

Reduce insensible losses

• Nurse infants under 1500g birthweight in humidified incubators (80%rH) -

Fluid supply during first days after birth

	>=750 g birthweight to 1500g	> 1500 g birthweight
Day 1	75 mls/kg/day	60 mls/kg/day
Day 2	100 mls/kg/day	80 mls/kg/day
Day 3	125 mls/kg/day	100 mls/kg/day
Day 4 - 7	Increase up to 150mls/kg/day	120 mls/kg/day

If < 750g start with 100 mls/kg/day and review fluids after 4 hours (see below). May need to increase above 150 mls/kg/day if insensible losses high.

Type of fluid

• Start with 10% glucose.
• Change to parenteral nutrition when indicated - see protocol.
• Use stock bag 10% glucose/0.18% sodium chloride after day 3 if parenteral nutrition not started.
• Start enteral feeds early and replace iv fluids as feeds increased - see protocol.
• In babies >1500g who do not need IV, give same volumes of fluid as milk.

Added electrolytes not needed in the first few days. Minimise sodium intake until after diuresis. Use 0.45% saline (with 1 unit heparin/ml) in arterial lines, at rate of 0.5 - 1ml/hour.
If extra potassium is needed then change to stock bag 10% glucose/0.18% sodium chloride/0.15% potassium chloride at the same infusion rate.  A separate potassium infusion can also be considered if hypokalaemia is severe or not responding to initial change in infusion fluid.

Monitoring

• Review after first 4 - 6 hours and then 8 hourly in sick, preterm infants.

Weight

• Weight is the best measure of fluid balance but is impractical in the sick preterm baby. In the first week babies should lose around 2-3% of birthweight/day.

Urine

• Measure fluid intake and urine output

Plasma

Electrolytes	8 hourly in the first 3 days of sick baby. Daily in all others during first week of life. Can be blood gases if in normal range.
Urea	Daily - reflects catabolism rather than renal function. Often high.
Creatinine	Daily - upper limit normal 130 micromoles/litre. Falls to around 60 micromoles/litre over first week

The basic prescription of fluid volumes may be changed if:

In the first week of life

Increase fluid intake by one step 

• High plasma sodium > 145 mmol/l. Check also that sodium intake not inadvertently high (eg from intravenous flushes using 0.9% saline).
• Osmolality > 300 mmol/kg (if this being measured)
• Low urine output (< 0.5 mls/kg/min) with concentrated urine (SG> 1015 or osmolality > 300 mmol/kg).

Decrease fluid intake by one step 

• Low plasma sodium< 135 mmol/l
• Low plasma osmolality< 270 mmol/kg (if measured)

SIADH is probably less common than believed but may result in the need for more aggressive fluid restriction.

Potassium

• Plasma potassium often high in first few days. Seek senior medical advice if > 7 mmol/l. See Hyperkalaemia guidance.
• Add supplements if plasma potassium below 3.0 mmol/l.
• If on intravenous fluids and not receiving parenteral fluid then use stock bag 10% glucose/0.18% sodium chloride/0.15% potassium chloride at the same infusion rate.  A potassium infusion can also be considered if hypokalaemia severe or not responding to initial change in infusion fluid.

Hypoglycaemia (Various cut offs see appropriate hypoglycaemia section)

• Use more concentrated glucose rather than increasing fluid intake.
• To calculate the infusion rates using 2 different glucose concentrations, use a glucose calculator if needed.

Hyperglycaemia (> 8 mmol/l)

• Risk of osmotic diuresis. Reduce glucose intake or start insulin infusion if two successive sugars >12-14, generally after discussion with attending Consultant.

After first week

• Total fluid intake usually 150 mls/kg/day of parenteral feeds or milk.
• Low plasma sodium (< 132 mmol/l) is due to renal sodium leak and should be supplemented with an additional 2-4 mmol/kg/day. Positive sodium balance is needed for growth.