Intravenous (I.V.) morphine is widely given to infants in neonatal intensive care units to ease or prevent pain from invasive procedures. But in preterm, mechanically ventilated infants, morphine may be no better at relieving pain during these procedures than a placebo, a new study suggests. Possible reasons include the immaturity of opioid receptors in preterm infants or altered morphine metabolism in the immature liver.

In a clinical trial, researchers studied 42 infants, gestational age 23 to 32 weeks, who were intubated within 72 hours of birth. Half received a loading dose of morphine followed by a continuous infusion, and half received placebo (an infusion of 5% dextrose).

The infants' response to heel-stick pain was documented on two pain scales at baseline (before the loading dose), 2 to 3 hours after the loading dose, and 20 to 28 hours after the loading dose. Researchers found no significant differences in responses on either pain scale at any point in the trial.

Because pain experiences in infancy help shape how a person processes and responds to pain later in life, the findings have important clinical implications. The researchers advise clinicians to treat neonates undergoing invasive procedures with other analgesic approaches such as administration of sucrose, which has been shown to briefly produce analgesia in neonates for pain lasting a short time (2 to 3 minutes). Intravenous morphine is effective for alleviating prolonged pain, reducing stress responses to surgery in term infants, and improving ventilator synchrony and sedation in preterm infants.

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