9.3 Pediatric Pharmacology and Dosing Calculations
Key Takeaways
- Infants have a larger volume of distribution for water-soluble drugs due to higher total body water, requiring larger weight-based loading doses.
- The Broselow Tape is the gold standard for length-based weight estimation and pediatric resuscitation drug dosing.
- For pediatric hypoglycemia, the Rule of 50 dictates D10W at 5 mL/kg, D25W at 2 mL/kg, or D50W at 1 mL/kg, with D10W increasingly preferred.
- Vasoactive drug calculations can utilize the Rule of 6 (weight in kg times 6 equals mg of drug added to 100 mL of fluid) to make 1 mL/hr equal to 1 mcg/kg/min.
Pediatric Pharmacology and Dosing Calculations
Pharmacokinetics in Pediatric Patients
The physiological differences in pediatric patients profoundly affect drug absorption, distribution, metabolism, and elimination:
- Volume of Distribution ($V_d$): Pediatric patients, especially neonates, have a much higher percentage of total body water (TBW) compared to adults. Water-soluble medications (such as succinylcholine, rocuronium, and aminoglycosides) will distribute into this larger fluid volume. To achieve the same therapeutic serum concentration, children require higher weight-based loading doses of water-soluble drugs than adults.
- Protein Binding: Neonates and infants have lower circulating levels of serum albumin and alpha-1-acid glycoprotein. Furthermore, endogenous substances like bilirubin compete for protein-binding sites. This results in a higher fraction of free, pharmacologically active drug in the plasma, which increases both the therapeutic and toxic effects of highly protein-bound medications (e.g., phenytoin, diazepam, and ceftriaxone).
- Hepatic Metabolism: Hepatic microsomal enzymes (specifically the cytochrome P450 system) are immature at birth. Consequently, the liver's ability to metabolize drugs via conjugation or oxidation is reduced in neonates. This prolongs the half-life of medications like phenobarbital and acetaminophen. However, by toddlerhood, hepatic metabolic capacity exceeds adult levels, requiring higher doses or shorter dosing intervals for some drugs.
- Renal Elimination: The glomerular filtration rate (GFR) in a term neonate is only about 30-40% of adult levels and does not reach maturity until 9-12 months of age. Drugs cleared primarily by the kidneys (such as penicillin, gentamicin, and vancomycin) will accumulate if dosing intervals are not prolonged in neonates.
Weight Estimation and Length-Based Resuscitation
In high-stress transport environments, making rapid, error-free drug calculations is critical. The Broselow Pediatric Emergency Tape is a length-based tape used to estimate a child's weight and pre-calculate medication volumes, defibrillation settings, and equipment sizes.
- Broselow Tape Administration: Place the child supine, and measure from the top of the head ("red to head") to the heel of the foot. The color zone where the child's heel lands dictates the weight category and drug doses.
- Weight Estimation Formulas: If a length-based tape is unavailable, the flight paramedic must estimate the child's weight in kilograms based on their age:
- Infants (< 1 year):
- Young Children (1-8 years):
- Older Children (9-14 years):
| Age | Estimated Weight Formula | Example: 5-year-old |
|---|---|---|
| Infant (< 1 yr) | $(0.5 \times \text{months}) + 4$ | 6 months: $(0.5 \times 6) + 4 = 7\text{ kg}$ |
| Child (1-8 yrs) | $(\text{years} \times 2) + 8$ | 5 years: $(5 \times 2) + 8 = 18\text{ kg}$ |
| Older Child (9-14 yrs) | $\text{years} \times 3$ | 10 years: $10 \times 3 = 30\text{ kg}$ |
Critical Care Pediatric Medications and Dosages
Resuscitation Medications
- Epinephrine (Cardiac Arrest / Bradycardia):
- IV/IO Dose: 0.01 mg/kg of 0.1 mg/mL (1:10,000 solution), which is equivalent to 0.1 mL/kg. Repeat every 3 to 5 minutes.
- Endotracheal (ETT) Dose: 0.1 mg/kg of 1.0 mg/mL (1:1,000 solution), or 0.1 mL/kg. Administer only if IV/IO access cannot be established.
- Amiodarone (Refractory VF / Pulseless VT): 5 mg/kg IV/IO bolus. May repeat up to two times for refractory shockable rhythms (maximum single dose 300 mg; maximum cumulative dose 15 mg/kg).
- Lidocaine: 1 mg/kg IV/IO bolus, followed by an infusion of 20-50 mcg/kg/min.
- Atropine (Bradycardia): 0.02 mg/kg IV/IO. Minimum dose is 0.1 mg (to prevent paradoxical bradycardia that can occur with sub-therapeutic doses). Maximum single dose is 0.5 mg for a child and 1.0 mg for an adolescent.
- Sodium Bicarbonate: 1 - 2 mEq/kg IV/IO slow push. Indicated for prolonged cardiac arrest, severe metabolic acidosis, or hyperkalemia. Ensure adequate ventilation before administration, as bicarbonate breaks down into carbon dioxide ($CO_2$), which must be cleared by the lungs.
Hypoglycemia Management (The "Rule of 50")
To easily calculate the volume and concentration of dextrose to administer for pediatric hypoglycemia (blood glucose $< 60\text{ mg/dL}$ in children, or $< 40\text{ mg/dL}$ in neonates), use the Rule of 50: the percentage of dextrose multiplied by the volume in mL/kg must equal 50.
- Neonates (< 28 days): Use D10W (10% Dextrose) at 5 mL/kg ($10 \times 5 = 50$). Neonates have fragile cerebral vasculature and are highly susceptible to intraventricular hemorrhage from hyperosmolar solutions. D50W must never be given to neonates.
- Infants and Young Children: Use D25W (25% Dextrose) at 2 mL/kg ($25 \times 2 = 50$). If D25W is unavailable, dilute D50W 1:1 with sterile water or normal saline to create D25W.
- Adolescents: Use D50W (50% Dextrose) at 1 mL/kg ($50 \times 1 = 50$).
- Note: Modern clinical guidelines increasingly favor utilizing D10W at 5 mL/kg for all pediatric age groups because it is less hypertonic, easier on peripheral veins, and reduces the risk of tissue necrosis in the event of extravasation.
Vasoactive Infusion Calculations
Vasoactive medications are titrated to maintain perfusion.
- Dopamine: 2 - 20 mcg/kg/min IV/IO.
- 2 - 5 mcg/kg/min: Dopaminergic effects (renal perfusion).
- 5 - 10 mcg/kg/min: Beta-1 adrenergic effects (increased inotropy and chronotropy).
- 10 - 20 mcg/kg/min: Alpha-1 adrenergic effects (systemic vasoconstriction).
- Epinephrine Infusion: 0.1 - 1.0 mcg/kg/min IV/IO. Indicated for cold shock (bradycardia, poor perfusion, low systemic vascular resistance).
- Norepinephrine Infusion: 0.1 - 1.0 mcg/kg/min IV/IO. Indicated for warm shock (distributive shock with vasodilation and normal/high cardiac output).
Infusion Calculation: The "Rule of 6"
Before the widespread adoption of smart pumps, flight crews used the Rule of 6 to calculate the amount of medication to add to a bag of IV fluid so that the infusion rate in mL/hr corresponds to the dosage in mcg/kg/min. Once mixed, an infusion rate of 1 mL/hr will deliver exactly 1 mcg/kg/min. Example: For a 10 kg child: $10 \times 6 = 60\text{ mg}$ of drug is added to 100 mL of fluid. Running this at 5 mL/hr will deliver 5 mcg/kg/min.
For modern transport pumps, use the standard mathematical formula:
A flight paramedic is preparing to administer dextrose to a 3-week-old neonate who has a blood glucose reading of 32 mg/dL and is exhibiting jitteriness. According to the 'Rule of 50' and neonatal resuscitation guidelines, what is the most appropriate dextrose concentration and dose to administer?
The flight paramedic is calculating an epinephrine infusion for a 15 kg pediatric patient in septic shock. The desired dose is 0.2 mcg/kg/min. If the concentration of the epinephrine bag is 16 mcg/mL, what is the correct infusion rate in mL/hr?