Check your answers on page 10.12968/jprp.2023.5.8.350
The ductus arteriosus is a connection between the aorta and the pulmonary arteries, an integral part of appropriate foetal circulation. In most cases, it is not needed after a child is delivered and can cause problems, such as persistently high pulmonary blood pressures, heart failure and endocarditis, as proper adult circulation does not become fully established. However, there are certain cardiac malformations; for example, tetralogy of Fallot or transposition of the great arteries, which can benefit from the duct being kept open until a longer-term intervention can be made.
Placental prostaglandins are one of the factors that help keep the duct open, and the loss of the placenta during birth usually facilitates closure. In the absence of the placental prostaglandins, exogenous prostaglandin E2 (Pfizer, 1986) can instead be infused. This is an unlicensed indication and can involve complex manipulations as the stock solution is a higher concentration than the doses needed will require.
Case study
The following scenario considers the case of a 2 kg neonate with critical pulmonary stenosis that requires a continuous intravenous infusion of prostaglandin E2 to prevent her duct from closing. The starting point for the infusion for this child is 5 ng/kg/minute (Evelina Paediatric Formulary Committee, 2023) to be administered from a syringe made up to contain 15 mcg/kg in 50 ml.
QUESTION 1
What is the concentration of the syringe (in nanograms/ml) to be used for this child?
QUESTION 2
What is the flow rate (in ml/hour) that this child will need for this dose?
The stock solution of Prostin E2 is 0.75 mg in 0.75 ml (Pfizer, 1986).
QUESTION 3
What volume of the stock solution will need to be drawn up to add to the syringe to be used for this patient?
You will see from your answer to Question 3 that this is not an accurately measurable amount for a patient of this size, so we need to perform a trituration of the stock solution to reduce the concentration and enable us to draw up a measurable amount.
QUESTION 4
If the stock solution is diluted to a final ratio of 1:10, what volume of glucose 5% needs to be added to the stock vial?
QUESTION 5
What volume of this diluted stock solution needs to be diluted to 50 ml to make the solution needed in Question 1?
A risk with dinoprostone infusions is that if the flow rate drops below 1 ml/hour, the lack of back pressure build up delays any alarm sounding on the infusion pump; for example, in cases of line occlusion, for long enough that the duct may close and not be amenable to reopening, requiring urgent surgical intervention. For this reason, dinoprostone infusions are often diluted in cases of dose reduction to allow a larger volume of more dilute solution to infuse and allow early detection of infusion problems.
QUESTION 6
What rate will be needed for this patient if the dose is reduced to 3 ng/kg/min and the infusion concentration is halved to account for the reduced rates likely to be needed?