NICU Clinical Formulas & Neonatal Physiology Reference Sheet

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Educational-Use Warning

Do not use formula output alone for clinical orders; verify with institutional NICU protocol, patient condition, gestational age, postnatal age, renal function, and medication references.

This should appear above the calculator because NICU formulas are educational tools and bedside decisions require clinical judgment, local policy, and patient-specific review.

How to read this page

Every formula below is shown in display-ready form with explicit units, neonatal caveats, and the clinical reasoning that makes the number meaningful. Ambiguous constants are spelled out, units are standardized, and the formulas are grouped by the specialty they belong to so a fluid question, a ventilator question and a pharmacokinetic question do not sit in one undifferentiated list.

Read each row in the same order the columns are arranged: the formula, then its assumptions, then when not to trust it. Many of these equations are mathematically correct but clinically unsafe if applied without gestational age, day of life, urine output, renal function, hemodynamics, or blood-gas context. Neonatal interpretation is emphasized throughout because many generic pediatric formulas behave differently in preterm infants, during the first postnatal days, and under respiratory support.

Primary neonatal fluid and nutrition background comes from RCH, NIDDK, NCBI Bookshelf, Health Canada, and AAP-family material.

Assumptions and constants used across formulas

Common defaults: FiO₂ as a decimal; P_b = local barometric pressure (sea level ≈ 760 mmHg); P_H2O = 47 mmHg at 37°C; respiratory quotient R ≈ 0.8; glucose D% means g/100 mL; for monovalent ions, mmol = mEq.

These constants keep the formulas internally consistent. The most important bedside clarification is GIR: D10 contains 10 g/100 mL = 100 mg/mL, so the familiar bedside constant 0.167 is simply 10/60 when using D% and mL/hr. For altitude, the rigorous method is not a crude “altitude factor”; it is substituting the local barometric pressure into the alveolar gas equation.

Keep these assumptions in view next to the formula. Hidden assumptions are how % glucose gets mixed with mg/mL, FiO₂ in percent with FiO₂ as a decimal, and mmHg with kPa.

Typical neonatal targets

GIR: term 4–6 mg/kg/min; preterm 6–8 mg/kg/min.
Urine output: about 1–5 mL/kg/hr in many NICU contexts.
Early TFI example: day 1 about 60 mL/kg/day, day 2 about 80, day 3 about 100, day 4–5 about 120 — adjusted for gestational age, insensible water loss, phototherapy, PDA, renal status, and disease.
Growth: weight gain 15–20 g/kg/day for many preterm infants.

These are reference targets, not prescriptions. The same numerical result can be appropriate, low, or unsafe depending on whether the infant is a 24-week ELBW infant under a radiant warmer, a term infant with HIE under fluid restriction, or a recovering feeder-grower. Interpret them with gestational age, day of life, weight, urine output, serum electrolytes, and glucose.

Never present these as universal “normal values.” RCH explicitly notes that neonatal fluid needs depend on birth weight, gestation, corrected age, illness, and insensible losses; Health Canada and international expert tables emphasize individualized preterm nutrition needs.