Pediatric Physiology, Pharmacology, and Anesthetic Agents
Developmental physiology
Age changes every pharmacokinetic parameter. Neonates have higher total body water and lower fat, a larger volume of distribution for water-soluble drugs, lower albumin and alpha-1-acid glycoprotein (more free drug), immature CYP enzymes and glomerular filtration, and reduced hepatic blood flow. The result: prolonged elimination of many drugs, greater sensitivity to respiratory depressants, and a narrower therapeutic window.
Pharmacologic principles
- Inhalational agents: lower MAC in neonates and infants than older children; MAC rises through infancy and peaks around 6–12 months, then declines. Uptake is faster in infants because of higher alveolar ventilation relative to FRC and greater cardiac output to vessel-rich organs.
- Propofol: larger weight-based induction dose in infants; watch for hypotension and apnea.
- Opioids: neonates are more sensitive to respiratory depression; remifentanil clearance is high even in neonates.
- Muscle relaxants: succinylcholine is relatively contraindicated in children with undiagnosed myopathy (hyperkalemia, rhabdomyolysis); vecuronium/rocuronium duration is prolonged in neonates.
- Local anesthetics: maximum weight-based doses are lower in infants due to reduced clearance; bupivacaine has a lower ceiling in neonates.
Traps
A common trap is assuming a "standard" mg/kg dose is safe in a neonate when clearance is immature. Another is confusing MAC values across ages. Questions may test that neonates feel pain and require analgesia - the old myth of immature pain pathways is false.
Rule of thumb
For any drug, ask: is clearance mature? If the patient is under 3 months or premature, reduce dose or interval and monitor longer.
Worked example
A 2-month-old (4 kg) for hernia repair. Sevoflurane MAC is lower than in a 6-month-old; induction is rapid because of high alveolar ventilation/FRC ratio. Use a lower propofol induction dose, avoid succinylcholine if myopathy is possible, and plan for prolonged monitoring given immature clearance.