The swedish observational study on neonatal thiopentone pharmacokinetics provides some interesting data. The authors rightly point out that the duration of anaesthesia following a thiopentone bolus is determined by redistribution rather than metabolism. It is interesting that blood levels in the mother at six minutes following induction with a dose of 5.5mg/kg were three times the average level in the neonate five minutes after a dose of 3mg/kg. This is likely due to the higher blood volume per kg in neonates, as well as their relatively large brains, so leading to a higher initial volume of distribution.

It is unfortunate that umbilical arterial thiopentone levels were not measured in infants delivered by caesarean section. The venous values only confirm that placental transfer is very efficient. The total dose the infant receives (and their corresponding blood level) will also be affected by the umbilical blood flow. Without this data, how can the author's claim that one should wait 4 hours before giving thiopentone to an infant already exposed via its mother?

After redistribution has occurred, thiopentone, in common with other barbiturates and alcohol, is metabolized at a constant rate. The half life will vary with blood level. This may explain the discrepancy between the author's calculations and earlier studies using higher doses of thiopentone for neuroprotection. It is meaningless to calculate the half life of a drug which is metabolized by zero order kinetics. It is better to simply state the rate at which blood levels fall.

Conflict of Interest:

None declared