Pain Assessment in Infants & Children

Special Considerations in Treating Infants and Children

When treating pain in infants it is important to understand that although most of the major organ systems are anatomically well developed at birth, their functional maturity is often delayed. In the first months of life in both preterm and full-term newborns, these systems rapidly mature, most approaching a functional level similar to adults before 3 months of age. General principles of newborn physiology and its effects on the pharmacology of opioids and local anesthetics are described in the following text:

• Most analgesics (including opioids and local anesthetics) are conjugated in the liver. Newborns, and especially premature infants have delayed maturation of the enzyme systems involved in drug conjugation, including sulfation, glucuronidation, and oxidation. Several of these hepatic enzyme systems, including cytochrome P450 subtypes, and the mixed-function oxidases, mature at varying rates over the first 1 to 6 months of life.¹⁸
• Glomerular filtration rates are diminished in the first week of life, especially in premature infants, but generally are sufficiently mature to clear medications and metabolites by 2 weeks of age.¹⁹
• Newborns have a higher percentage of body weight as water and less as fat compared with older patients. Water soluble drugs, therefore, often have larger volumes of distribution.
• Newborns have reduced plasma concentrations of both albumin and alpha-1 acid glycoprotein than older children and adults. For some drugs, this may lead to higher concentrations of unbound drug (active), and thereby greater drug effect or drug toxicity.
• Newborns, and especially premature infants, have diminished ventilatory responses to hypoxemia and hypercarbia.^20,21 These ventilatory responses can be further impaired by CNS depressant drugs such as opioids and benzodiazepines.