Pain and opioids in preterm and newborns

Despite the fact that neonates and infants are not capable of expressing their subjective pain sensations, it has become clear that they do perceive nociception, as pain correlates to hormonal, metabolic, immune, and cardiovascular changes. New findings support the notion that repetitive painful stimuli result in long term psycho-physiological effects with ensuing decreased attentiveness and orientation, poor regulation of behavioral state and motor processes, increase in irritability as well as an altered pattern of feeding and sleeping. These sequelae of repetitive painful experiences with an increase in sensitization of sensory afferent input supports the view of a sufficient analgesia during all kinds of painful procedures in the preterm and neonate. In order to sufficiently diminish nociceptive afferent input during surgery opioids are the drugs of choice aside from local anesthetics. However, the use of opioids in neonates and especially preterm infants must be considered in the light of certain pharmacokinetic and pharmacodynamic differences when compared to adults: 1. There is a longer elimination rate, which may result in post-operative overhang of respiratory depression, especially when opioids are given repetitively, resulting in an accumulation and an increased duration of action. 2. There is a reduced hepatic enzyme activity, which ultimately affects clearance rate. 3. The blood-brain-barrier is not fully developed in the preterm, which results in more access of opioids to binding sites in the CNS. 4. Differentiation of opioid-binding sites into mu, delta, and kappa has not reached its peak; thus, higher doses relative to body weight are needed to establish a sufficient deep plane of analgesia. 5. Caudal parts of the CNS, especially the pons-medullar region exhibit an earlier expression of receptors than the rostral parts. Sequelae of such differences are a more pronounced respiratory depression, often due to muscular rigidity, and bradycardia after which a full analgesic effect takes place. Despite such potential drawbacks, opioids are still the best choice as they sufficiently block nociceptive afferent input and when compared to other anesthetics, they show the least cardiovascular changes. One, however, has to bear in mind that dosing is done according to effect and not to body weight while potential side effects are most prominent in the preterm infant.