Fluids, Electrolytes and Nutrition

At birth infants have excess extracelluar fluids.  The excess is inversely related to the degree of prematurity: a 28-week infant has relatively more excess fluid than a 32-week infant and so on.  An average weight loss of 1-2% each day for several days reflects appropriate mobilization of excess fluid.  In the preterm infant at risk for RDS and PDA, the excess fluid is a risk factor for increased ventilator and oxygen requirements and BPD.

However, the skin of severely premature infants has no epithelium (shiny skins) so that there is massive evaporative water loss.  These infants may need hundreds of cc/kg/day to avoid dehydration.  However, skin epithelializes (not shiny) rapidly over the first 36-72 hours.  If the fluids are not rapidly and sharply cut back, they are again at risk for fluid excess, PDA and BPD.

Prior to about 12 hours of age, many laboratory values reflect the mother.  When the electrolytes are normal at 12 hours, add maintenance electrolytes.  Glucose starts immediately with the first IV fluids (usually D₁₀W).

The following guidelines are subject to modification based on individual cases:

• Blood loss and hypovolemia:  Heart rate and capillary refill time are rapid/early indicators.  Hypotension is a poor/late indicator due to the infant's excellent sympathetic response to hypovolemia.  Also "normal" blood pressure in ELBW infants is not well established.  Perfusion/heart rate are good physiologic indicators.
• Previous fluid overload or iv excess requires correction in subsequent fluids
• Dehydration, hyperbilirubinemia, polycythemia, hypernatremia require increased hydration.
• Indomethacin causes decreased urine output by decreasing renal blood flow.  Treatment is to decrease fluids, and follow the sodium closely.  Do not give the next dose until the urine output recovers.
• Phototherapy is over-rated as a cause of dehydration, but does add to fluid loss; not only by radiative heating, but by increased fluid loss through the stool.
• Open warmers, which lack the humidity of a closed isolette, raise fluid losses
• When dealing with RDS/PDA it may be appropriate for sodium to drift up to 140-145 to avoid excess fluid in the lung and help keep the PDA closed.  Sodium greater than 145 is not desirable.
• With chronic lung disease, it may be appropriate for the sodium to drift downward to 130-135 to avoid excessive lung fluid.

Magnesium:  Hypermagnesemia may cause respiratory depression and decrease GI motility.  Stools and bowel sounds are good evidence of GI mobility.

Calories

At birth, infants are supposed to lose water weight.  For normal term infants the loss does not mean that they cannot begin to be well nourished soon.  For ill or preterm infants, the limits of the immature gastroenteral function and fluid volume limits make immediate optimal nutrition more difficult in the first several days of life. Usually, TPN can be started by 2-4 days, and tiny, slowly advancing feeds can be started within a week.  The schedule may be delayed by severe illness, electrolyte imbalances requiring frequent adjustments, and insults to the GI tract such as infection or asphyxia, potentially leading to NEC.

Subsequently, the need to gain weight is again the primary guideline.  Other modifiers include:

• Severity of illness often increases the calories required to gain weight
• Any stimulus to the basal metabolic rate increases the caloric requirements
• Repair or recovery increases the requirement
• Infants with BPD sometimes need enormous caloric support to gain weight.  Weight gain is related to survival.
• Failure to thrive despite your best efforts is unusual.
• Available routes of entry and volume limitations may be a challenge to your effort to provide enough calories to attain weight again:
• peripheral ivs are difficult to maintain above 10-12.5% dextrose
• central lines are at increased risk of infection above 20-25% dextrose
• a single line of access which must also take meds, transfusion, etc., limits nutritional access
• GI tract hypermobility or immobility may limit the volume that can be given enterally
• GI absorptive function may limit the calories and volume that can be delivered.