Unit 2: Nutritional Requirements of the Preterm Infant

The transition from intrauterine to extrauterine environment occurs in three distinct phases, each with particular fluid and electrolyte requirements depending on gestational age and birth weight:

Phase I, the postnatal rearrangement of fluid compartments. It lasts from birth to the point of maximum weight loss (in the form of water loss), up to 10% of birth weight. 

During this phase the following occurs:

• An initial decrease in urinary output due to decreased glomerular filtration rate;
• Hypertonic urine production, with high urea, K⁺ and phosphate concentration, but no high Na⁺ and Chloride.
  
• The initial oliguria (that can last from hours to days) is followed by increased diuresis through temporal additional increase of the glomerular filtration rate.
  
• Contraction of the ECF - ~48% of total body weight in ELBW infants compared to ~31% in term infants (Fusch & Jochum 2014)
  
• Cornification of the epidermal layer of the skin reducing the evaporative water loss.
• In healthy preterm infants, this phase generally lasts from 3 to 5 days; however VLBW infants may take up to 8 days to master this fluid rearrangement, probably due to respiratory insufficiency (Fusch & Jochum 2014).

By the end of phase I, urine volume is less than 2.0 ml/kg/h, and urine osmolarity is smaller than serum osmolarity.

Phase I fluid and electrolyte management clinical goals (Fusch & Jochum 2014):  

• Allow the physiological ECF contraction, although aiming not to compromise IVF volume and cardiovascular function.
• Allow a Na⁺ negative balance (2-5 mmol Na/kg/day).
• Maintenance of normal serum electrolytes.
• Allow sufficient urine loss to excrete waste  (urea, acid equivalents, etc).
• Avoid oliguria (less than 0.5-1.0 ml of urine/kg/h) for longer than 12h.
• Provide enough fluids to allow transepidermal evaporation facilitating the body thermoregulation,
• Administration of sufficient calories to keep maintenance needs (around 40-60 kcal/kg/day).

A recent Cochrane revised review (Bell & Acarregui 2014) compared restricted versus liberal water intake in preterm infants in this phase. They concluded:

About Na⁺ administration, postnatal supplementation should be individually tailored. It seems advantageous to delay supplementation until ECF contraction onset or significant weight loss (Fusch & Jochum 2014). In VLBW infants the recommendation is to restrict Na⁺ supplementation until a weight loss of approximately 6% of the postnatal body weight (Costarino et al. 1992, Hartnoll et al. 2000, Bell & Acarregui 2014). Infants with restricted Na⁺ supplementation need less supplemental oxygen and develop less BPD, however there is an increased risk for hyponatremia. Nevertheless, after the first week of age, a higher intake of Na⁺ may be beneficial for growth and mental development (Fusch & Jochum 2014, Isemann et al. 2014).

Phase II, the intermediate phase of postnatal adaptation with the establishment of enteral feeding. Characterized by:

• Decreasing transcutaneous fluid output;
• Decreasing urine volume (less than 1 to 2 ml/kg/day);
• Low Na⁺ excretion.

Phase II fluid and electrolyte management clinical goals (Fusch & Jochum 2014):

• Electrolyte replenishment for losses that might have occurred during the Phase I, due to ECF contraction;
• Electrolyte and fluid  replenishment for losses occurring in this phase to secure homeostasis;
• Increase of enteral feedings.

Phase III, phase of stable growth. Characterized by:

• Epidermis totally cornified;
• Mature kidney function, completely adapted to the extrauterine conditions;
• Low Na⁺ excretion.

Phase III fluid and electrolyte management clinical goals (Fusch & Jochum 2014):

• Full enteral intake of fluids and electrolytes;
• It must be ensured that water and electrolytes are given in sufficient amounts to both maintain homeostasis and support tissue accretion - ideally at a rate comparable to the intrauterine growth (around 15-20 g/kg/day).

Of note, it is important to mention that most data used to calculate such fluid and electrolyte requirements were obtained from studies and clinical trials performed in the 1980s and 1990s. Compared to today's population of preterm infants, these infants were relatively larger (body weight usually above 1000g), more mature (>28 weeks GA), and they were usually also sick and treated in a more invasive way than presently. The population of preterm infants nowadays are very often younger (>23 weeks GA) and receive frequently a less invasive respiratory support (such as nCPAP, nasal Continuous Positive Airway Pressure). It might well be that VLBW and ELBW preterm infants nowadays have different needs and may tolerate different fluid and electrolyte regimes than those investigated in studies to date. New clinical trials assessing fluid and electrolytes needs are warranted for these more immature preterm infants under the current neonatal intensive care and also assessing neurodevelopmental outcomes (Fusch & Jochum 2014).