Fluid balance in the extreme preterm infant

May-Li and Aishin have collaborated on this Podcast on fluid management in extreme preterm infants.

Mx of fluid balance in very immature neonates

Introduction

Disorders of fluid and electrolyte imbalance are amongst the most common disorders encountered in extreme preterms.

The fluid and electrolyte requirements of the neonate are unique due to fluids shifts within the first few days and weeks of life.

The extremely immature infant below 26 weeks gestation is 80–90% water (total body water). In utero, the baby is immersed in fluid, the lungs are filled with liquid, the skin is porous and lacking a keratin layer; urine output is high, and renal concentrating ability is limited.

Total body water (TBW) is made up of intracellular and extra-cellular compartments.

All babies are born with an excess of TBW, mainly ECF.

1. Adults have 60% water (20% ECF, 40% ICF);

2. Term neonates have 75% water (40% ECF, 35% ICF);

3. Preterm neonates have more water (23 wks: 90% : 60% ECF, 30% ICF)

Extra-cellular water constitutes a large proportion of the extreme preterm body composition, and reduces abruptly over the first few days after birth causing weight loss.

Extracellular fluid and insensible water losses increase as weight and gestational age decrease.

Our goal is to allow a gradual weight loss over the first week by carefully monitoring fluid balance in the immediate postnatal period, Allowing initial loss of ECF over first week (as reflected by wt loss), while maintaining normal intravascular volume and tonicity (as reflected by HR, Urine output, pH). Subsequently, maintain water and electrolyte balance, including requirements for body growth. Along the way, we should anticipate issues which may exacerbate the fluid loss and take appropriate action swiftly.

ANTICIPATE AND PREVENT – the first approach to fluid therapy is adequate monitoring and appropriate supportive care, anticipate potential issues that may lead to fluid imbalance.

1. Transepidermal water loss (TEWL) is the continuous passive diffusion of water through the stratum corneum. It decreases with rising gestation and increasing postnatal age. TEWL can be as high as 140ml/kg/d during the first few days of life in a 24-26 week neonate.

We should therefore take active steps to reduce transepidermal losses. Babies below 28 weeks gestation, nursed naked under radiant warmers, are most vulnerable, and without adequate measures to decrease losses, water lost through the skin may exceed urine volume.

1. At delivery –adopt thermoregulation measures: hat, plastic bag, transwarmer.

2. Use high ambient humidity 90% . Nursing an infant in a humid environment reduces TEWL, and a reduction from 140ml/kg/d to less than 40ml/kg/d can be achieved by keeping the incubator humidity at 90%.

3. Golden hour and care after – avoid prolonged period of opening incubator window when doing any procedures, minimal handling of baby.

4. Fluid prescribing is a matter of clinical judgment – anticipate more frequent adjustment to fluids prescription depending on baby’s needs. Monitor carefully and continuously. Take into consideration daily weight, sodium level, urea, creatinine, urine output, aim for at least 12 hourly fluid balance calculation.

Fluid balance: Input = PN + other infusions + flushes + enteral feeds

Output = urine output (measured) + stool + insensible losses (through respiratory tract = 6 – 9ml/kg/d + TEWL approx. 40ml/kg/d)

Start PN immediately once central access is obtained. Additional fluids can be 10% glucose/5% glucose with additives if required.

1. High glucose – frequent problem, will lead to glycosuria – increase fluid loss, start insulin when necessary.

MONITORING

1. Weight – Daily alteration in body weight. Rapid weight changes suggest water loss or gain. Extremely preterm infants will benefit from daily weight assessment.

2. Serum sodium – rule of thumb Hyponatraemia suggests water excess; Hypernatraemia in the first few days usually suggests dehydration. Renal function – U&E’s may need to be measured 8-12 hourly initially. Gas trend is a good way to monitor. If a baby is hypernatraemic don’t automatically stop the sodium in the fluids! The baby may still need sodium inputs and we don’t want the value to drop too quickly.

3. Fluid balance – as talked about above, should be done at least BD (strict measurement of input, output, take into consideration insensible losses)

4. Urine output – <1 ml/kg/h requires investigation (may signify dehydration or poor renal function); 2–4 ml/kg/h suggests normal hydration; 6–7 ml/kg/h (suggests impaired concentrating ability or excess fluid administration).

5. Clinical examination – Peripheral oedema may signify fluid overload, Skin turgor reduced if dehydrated, but remember that this is a late sign in the neonate.

Dos and Donts

1. Do not give furosemide routinely with transfusion of packed red cells – Transfusion of packed red cells at 3 ml/kg/h does not lead to intravascular volume overload in extremely preterm infants.

2. Do not restrict fluid intake routinely whenever signs of a patent ductus arteriosus are present – Restrict only when there is evidence of volume overload; routine fluid restriction will compromise nutrition.

3. Do ask yourself if your goal is hydration or nutrition whenever considering a change in fluid intake – If hydration is satisfactory, stepwise increments in fluid intake after birth are unnecessary unless accompanied by a clinically relevant increase in nutrition

Conclusion

Fluid management in the preterm infant is challenging, necessitating frequent clinical assessment. Many factors influence fluid balance; thus, it is vital that a careful fluid management plan is made and regularly reviewed. If in doubt, a senior colleague should be consulted.

Suggested article:

Management of fluid balance in the very immature neonate

Neena Modi

Arch Dis Child Fetal Neonatal Ed. 2004 Mar; 89(2): F108–F111.