16.2 PN Initiation, Advancement & Monitoring

Writing the First PN Order: Fluid Before Calories

Every PN calculation starts with the fluid allowance, not the calorie goal. For children beyond the neonatal period, maintenance fluid follows the 100/50/20 rule (100 mL/kg for the first 10 kg, 50 mL/kg for the next 10 kg, 20 mL/kg for each kilogram above 20 kg), then total PN volume is fit inside that allowance alongside medications and flushes. Neonates advance fluid separately from nutrition: roughly 60–80 mL/kg/day on day 1, increasing by 10–20 mL/kg/day toward 140–160 mL/kg/day by the end of the first week, adjusted for phototherapy, humidification, and renal function. Only after volume is set do you distribute it among the dextrose/amino acid solution and the lipid emulsion.

Advancement Schedules

Amino acids start on day 1 — 2–3 g/kg/day in preterm infants, 2.5–3 g/kg/day in term neonates — and advance over 1–3 days to goal. Dextrose starts at a GIR of 4–6 mg/kg/min (preterm) or 6–8 mg/kg/min (term) and rises 1–2 mg/kg/min daily toward about 10–12 mg/kg/min, limited by serum glucose. Lipid starts at 1 g/kg/day and advances 0.5–1 g/kg/day toward 2.5–3 g/kg/day, limited by triglycerides. Older children tolerate faster advancement but still step up over 2–4 days.

The arithmetic the CSP exam loves is the GIR conversion:

GIR (mg/kg/min) ≈ (dextrose concentration % × infusion rate in mL/kg/day) ÷ 144

Worked example: an infant on D10 at 120 mL/kg/day receives (10 × 120) ÷ 144 ≈ 8.3 mg/kg/min. Run the reverse to solve for the dextrose concentration needed at a fixed fluid restriction — a classic exam setup.

Refeeding Syndrome: The Dangerous Start

Refeeding syndrome is the potentially fatal shift of phosphorus, potassium, and magnesium into cells when carbohydrate feeding resumes after prolonged starvation, driven by a surge of insulin. Hypophosphatemia is the hallmark, and consequences include respiratory failure, cardiac arrhythmia, rhabdomyolysis, seizures, and death. At-risk children include those with severe malnutrition (weight-for-length or BMI z-score at or below −2 to −3), anorexia nervosa, oncology patients, chronic underfed neglect, and any child with minimal intake for 7–10 days. The protocol:

• Start energy at roughly 25–50% of estimated needs (often ~10–20 kcal/kg/day in the severely malnourished) and advance over 4–7 days.
• Give thiamine (100 mg in older children, weight-based in infants) before or with the first dextrose to prevent Wernicke-type depletion.
• Check phosphorus, potassium, and magnesium every 12–24 hours for the first 2–3 days and replete aggressively.
• Watch for fluid overload and edema — sodium retention is part of the syndrome.

The Monitoring Schedule

Monitoring intensity is highest at initiation and with every advancement, then relaxes in stable long-term patients:

Two interpretation traps appear repeatedly on exams. First, triglyceride thresholds: many programs reduce or hold lipid when triglycerides exceed roughly 250 mg/dL in neonates (thresholds up to 400 mg/dL are used in older children); lipemic serum also falsely elevates sodium readings with certain analyzers. Second, hyperglycemia management: the first response is to reduce the GIR or dextrose concentration, because routine insulin infusions in ELBW infants increase hypoglycemia and have not shown outcome benefit — insulin is reserved for persistent, significant hyperglycemia despite dextrose reduction.

Compatibility, Cycling, and Weaning

Calcium and phosphorus compatibility limits how much mineral a PN solution can hold; solubility depends on amino acid concentration, temperature, pH, and the order of mixing, and precipitation can be fatal. Use a 0.22-micron filter for dextrose/amino acid solutions and a 1.2-micron filter for lipid-containing admixtures.

Cyclic PN — infusing over 10–16 hours instead of 24 — is introduced once a child is metabolically stable (typically not in the first weeks of life). Benefits include freedom from the pump during the day, improved insulin sensitivity, and likely reduced hepatosteatosis. The safety rule: taper the rate up over the first 30–60 minutes and down over the last 30–60 minutes to avoid hyperinsulinemic rebound hypoglycemia when the dextrose stops, and check a glucose 15–30 minutes after discontinuation in young children.

Finally, PN should be weaned as enteral tolerance improves — a favorite exam point. A common rule is to discontinue PN when enteral intake reliably supplies roughly 50–75% of total needs with adequate growth; abrupt stopping while the child is still fully PN-dependent risks hypoglycemia, so taper the rate as feeds climb.