Pediatric and Special Population Pharmacology

Why Special Populations Require Different Pharmacological Approaches

Drug therapy in emergency medicine is never truly one-size-fits-all. The same drug given to an adult at a standard dose may be dangerously excessive for a child, inadequate for an obese patient, or toxic for a person whose kidneys or liver cannot clear it efficiently. Paramedics and EMTs must understand the physiological differences that change how drugs move through the body—pharmacokinetics—and how the body responds to them—pharmacodynamics—across the lifespan and in the presence of organ impairment.

Pediatric Patients

Children are not small adults. Their physiology differs in ways that directly affect drug dosing and drug selection.

Weight-based dosing is the foundation of pediatric pharmacology. Most pediatric drug doses are expressed in milligrams per kilogram (mg/kg). Because an error in weight estimation leads directly to a dosing error, obtaining or estimating a child's weight accurately is a patient-safety priority.

The Broselow tape is a length-based resuscitation tool that provides color-coded weight estimates and pre-calculated drug doses for children from roughly 3 kg to approximately 36 kg. To use it correctly, lay the child supine on a flat surface, place the red end of the tape at the top of the head, and read the color zone at the child's heel. Each color zone contains weight-based drug doses, equipment sizes, and fluid volumes. Never guess; always measure when possible, and use the tape or a validated estimation tool when a scale is unavailable.

Key pediatric physiological considerations include:

• Higher body water content in neonates and infants means water-soluble drugs may require a relatively higher mg/kg dose to achieve therapeutic concentrations.
• Immature hepatic enzyme systems in neonates limit metabolism of many drugs; half-lives can be much longer than in adults.
• Higher glomerular filtration rate (GFR) per kilogram in older infants and young children may increase renal clearance of some drugs.
• Smaller functional residual capacity and higher respiratory rate make respiratory arrest progress rapidly; titrate respiratory drugs with caution.

For neonates (birth to 28 days), be especially conservative: their blood-brain barrier is more permeable, hepatic glucuronidation is immature, and protein binding is reduced, increasing the free (active) fraction of many drugs.

Geriatric Patients

Adults over 65 years old undergo predictable physiological changes that alter pharmacokinetics:

• Decreased renal function: GFR declines with age. Renally cleared drugs—digoxin, many antibiotics, some opioids—accumulate more readily, increasing toxicity risk.
• Decreased hepatic blood flow and enzyme activity: hepatic first-pass metabolism decreases; oral bioavailability may rise and drug half-lives lengthen.
• Decreased total body water and lean muscle mass with increased body fat: water-soluble drugs reach higher peak concentrations; fat-soluble drugs have a larger volume of distribution and longer duration of effect.
• Decreased albumin: less protein binding means more free drug is available; this matters particularly for highly protein-bound drugs.
• Polypharmacy: many geriatric patients take multiple medications; always consider drug-drug interactions before administering EMS medications.

In practice, a useful principle is start low, go slow—begin at the lower end of the dosing range and titrate carefully. Suspected toxicity from medications the patient was already taking (for example, digoxin toxicity presenting with bradycardia, nausea, and visual changes) should be considered in any geriatric patient with an altered presentation.

Pregnant Patients

Pregnancy creates a two-patient situation. Every drug decision must consider both maternal and fetal effects.

• Increased plasma volume dilutes drug concentrations; some medications may require higher doses to achieve therapeutic levels.
• Altered gastric motility slows oral absorption.
• Placental transfer: most drugs cross the placenta to some degree; lipid-soluble and low-molecular-weight drugs cross most readily.
• Teratogenicity and fetal harm: in the prehospital emergency setting, the priority is stabilizing the mother—an unstable mother is a greater threat to fetal survival than most emergency medications. However, avoid drugs with known harm when equally effective alternatives exist.

For anaphylaxis in pregnancy, epinephrine remains the drug of choice. Do not withhold it out of concern for the fetus; untreated anaphylaxis is far more dangerous.

Positioning matters: after approximately 20 weeks gestation, place the patient in the left lateral decubitus position or manually displace the uterus to the left to relieve aortocaval compression.

Renal Impairment

Patients with chronic kidney disease or acute renal failure cannot clear renally excreted drugs at normal rates. Drug accumulation leads to prolonged effects and toxicity. In the field, assume renal impairment in patients with known dialysis dependence, diabetes, or chronic hypertension. Reduce doses or extend dosing intervals for renally cleared drugs. Avoid nephrotoxic agents when alternatives exist.

Hepatic Impairment

The liver is the primary site of drug metabolism for most EMS medications. Patients with cirrhosis, hepatitis, or acute liver failure metabolize drugs more slowly, increasing the risk of accumulation. Drugs with high hepatic extraction ratios are particularly affected. In prehospital care, use conservative doses for analgesics, sedatives, and any hepatically metabolized medication in patients with known liver disease.

Safety Summary

• Always confirm or estimate weight before dosing a pediatric patient; use the Broselow tape.
• Document the estimation method used.
• When in doubt about a dose in any special population, contact medical direction.
• The goal is always the therapeutic window: enough drug to be effective, not so much as to cause harm.