Thyroid status was characterized in very preterm infants (gestational age < or =32 wk; n = 61) from birth through d 14, and in infants who died within 16 d after delivery (n = 10), where it was also correlated with metabolism of iodothyronines in peripheral tissues (brain, liver, kidney, skeletal muscle, and adipose tissue). At 3 d of life, mean plasma levels of thyroxine, triiodothyronine, and TSH started to decrease, being lower in the critically ill compared with healthy premature neonates. Activities of the three iodothyronine deiodinases enzymes (type I, II, and III, respectively) were detected in all postmortem tissue samples, except for absence of the type II activity in kidney. All activities were the highest in liver and differed in other tissues. Lack of correlation between the type I activity in liver (and kidney), and plasma levels of thyroid hormones suggested that the thyroid was the primary source of circulating triiodothyronine. On the other hand, namely in brain, correlations between activity of the deiodinases and plasma hormone levels were found which suggested a complex control by thyroid hormones of their own metabolism. High activity of type III in liver, adipose tissue, and skeletal muscle demonstrated a role of these tissues in thyroid hormones degradation. Results support the view that peripheral tissues of very preterm infants are engaged in local generation of triiodothyronine, and inactivation of thyroid hormones, but do not represent a major source of circulating triiodothyronine.

Body weight was shown to be the main PK covariate of phenobarbital disposition. Subsequent dosing nomograms are provided for phenobarbital dosing during ECMO.

Therapeutic hypothermia (HT) is frequently used in neonates with hypoxic-ischemic encephalopathy and young infants during cardiopulmonary bypass (CPB). Hypothermia and CPB result in physiological changes contributing to pharmacokinetic (PK) and pharmacodynamic (PD) changes. Changes in the absorption, the volume of distribution (Vd) and the total body clearance (CL) of drugs used during hypothermia and CPB might lead to the interindividual PK variability resulting in either insufficient or toxic plasma concentrations and have an impact on the biodisposition and action of drugs. Both under- or overdosing of medicines in these critically ill patients may contribute to a worse overall outcome. Overall, hypothermia decreases CL but may decrease or increase Vd by changing intravascular blood volume, organ perfusion and enzymatic metabolic processes. In addition, maturational as well as organ specific changes may occur during hypothermia superimposed on the underlying disease and/or procedures such as extracorporeal membrane oxygenation (ECMO) or CPB. This paper will provide an overview of variables and potential covariates (e.g., asphyxia, sepsis, multiorgan dysfunction syndrome, cardiac arrest) determining the PK of frequently used drugs. In addition, the effects of hypothermia on individual drugs are described as well as alternative ways for future study designs such as the use of population PK-PD and opportunistic sampling. Ultimately, these investigations are warranted to obtain specific dosing nomograms of medicines for use in clinical practice and to improve the treatment results of this vulnerable group of pediatric patients.

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