Ekhard E. Ziegler, MD
Peer Review Status: Internally Peer Reviewed
Most neonatologists now embrace the idea that a nutritional insult (starvation) is unlikely to have beneficial effects in an infant already under intense stress. Efforts at minimizing the duration and severity of starvation must, of necessity, rely heavily on the parenteral provision of nutrients. The prevailing hormonal milieu, which accounts, among other things, for glucose intolerance, places limitations on our ability to provide nutritional support. But, within these limitations, nutritional intake should be maximized -- and the earlier, the better. Enteral nutrition should be pursued all the while, but with a view toward nourishing the gut rather than the whole baby.
Indication and time of intiation
The smaller the infant, the greater the need for parenteral nutrition and the greater the urgency to initiate it. Thus, infants with birth weights less than 1500 g should, with few exceptions, receive parenteral nutrition as a matter of routine. These infants should be on TPN by 48 hours of age at the latest. There is no rationale for withholding TPN in these infants for a period longer than is technically required to order and start TPN. Postponing the initiation of TPN simply means that greater nutrient deficits will accrue and that it will take more time later on to make up for the deficits.
On the other hand, larger infants require parenteral nutrition only when enteral feedings are not possible for periods of more than a few days. Because larger infants have greater nutrient reserves, the urgency to start nutrition support is much less than in smaller infants.
Prescribing parenteral nutrition
Three neonatal venous nutrition (NVN) solutions are available. Their main components are listed in Table 1.
|Standard||High Amino Acid||High Amino Acid electrolyte-free|
1 All solutions also provide (per liter): 2 mg zinc, 0.4 mg copper, 0.2 mg manganese, 4 µg chromium, 10 µg selenium;
2 Trophamine or Aminosyn PF; cysteine is added at 14 mg/g amino acids
3 Higher when potassium is added (e.g., 30 mEq when K is 20 mEq)
The standard and high-amino acid solutions differ only in their amino acid content. We retain the designation "standard" for the solution providing 1.4% amino acids, although high amino acid solutions are now used at least as frequently as the standard solution. The concept behind the standard solution is that in 100 ml/kg/day it provides 1.4 g amino acids per kg/day, the presumed maintenance requirement. If one of the high amino acid solutions is prescribed at 60-70 ml/kg/day, that same amino acid intake is achieved, albeit in a smaller volume. In patients with labile electrolyte and/or blood glucose levels, the remainder of the daily fluid volume can be provided from glucose-electrolyte solutions that can be changed readily in response to changing needs. Potassium, when it is needed after the first few days, has to be prescribed as a separate item. The electrolyte-free solution is free of sodium, potassium and chloride. It is intended for the small preterm infant during the first few days of life and provides maximum flexibility in working around the common fluid-electrolyte problems of small infants. It goes without saying that, once the electrolyte disturbance has been resolved that prompted the use of an electrolyte-free solution, supplemental electrolytes must be provided or an electrolyte-containing solution used.
Vitamins (MVI Pediatric) must be prescribed separately. The dosage is 2.0 ml/kg/day for babies weighing up to 2.5 kg. Babies weighing >2.5 kg receive the maximum dose of 5.0 ml/day.
Dosage of amino acids
There is no rational basis for intakes less than 1.4 g/kg/day (i.e., maintenance, Table 2) at any time, even on the first day that TPN is given. Whenever energy intakes exceed 40 kcal/kg/day, intakes of amino acids should be increased beyond 1.4 g/kg/day. As a rough guideline, an amino acid/energy ratio of approxiamately 3.5 g/100 kcal should be maintained. In this way it is ensured that the infant receives sufficient amino acids at all times, especially if and when growth occurs. In larger infants, a lower ratio, e.g., 3.0, should be used.
|Maintenance and growth||3.2||3.0||3.0||2.8|
When higher than usual intakes of calcium and phosphorus are desired, e.g., in case of marked osteopenia, or simply to prevent osteopenia, increased concentrations of these minerals can be given. The permissible concentrations depend on the amino acid and glucose concentrations in the TPN solution. Consult the dietitian and/or pharmacist regarding prescribing information.
If additional acetate is desired for the management of metabolic acidosis, it can be added as the Na or K salt. The choice of salt(s) will depend on serum electrolyte levels.
The primary reason for providing parenteral lipds remains the provision of essential fatty acids. That objective is achieved with a lipid intake of 0.5 g/kg/day. There are good reasons for using lipid also as source of fuel, although it appears that a good portion of lipids goes into storage rather than being oxidized as fuel. Intakes of up to 2.5 g/kg/day are commonly used in preterm infants and appear to be safe, as long as they are given slowly. Lipid emulsions are available as 10% and 20% emulsions, with some reports suggesting more favorable metabolic effects with 20% emulsions than 10% emulsions.
Certain rules must be followed. Lipids should be given as slowly as possible, i.e., spread out over 20 hrs each day whenever possible, leaving 4 hrs for administration of intravenous medications. Triglyceride levels should be monitored if rates greater than 150 mg/kg/hr are used. If visible lipemia is noticed, the lipid infusion should be stopped and a serum triglyceride level measured.
Because blood glucose and electrolytes are already being closely monitored in preterm infants, no routine monitoring is required specifically for infants receiving parenteral nutrition, with one exception. Because electrolyte-free TPN is also phosphate-free, serum phosphorus must be monitored if such a solution is used for more than 2 days. Whatever the BUN is, a small rise of it is to be expected when TPN is started or when the amino acid intake is increased.
An important rule in monitoring is never to draw the blood sample from a line that contains the substance to be monitored. No amount of flushing can guarantee that you are not obtaining a falsely high value!