When prescribing medications for patients it is not unusual to need to consider, and balance, multiple competing requirements and limitations. Within the context of critically unwell children, there may be the need to provide fluids and energy intravenously, but certain medical conditions may impose limits that need to be adhered to.
Particularly with children being treated for congenital cardiac abnormalities there may be the need to restrict the amount of fluid that can be administered to preserve cardiac function by controlling factors such as load on the heart. For children unable to receive nutrition orally it is often the case that the limited fluid allowance is the only medium in which sources of energy can be provided, though excessive energy provision can lead to hypercapnia, hyperventilation, or lipaemia (Mesotten et al, 2018).
In the case we will consider here we are looking after an unwell child that is nil-by-mouth and fluid restricted. The consultant has decided to limit fluid provision to 60 ml/kg/day. Because they are unwell energy will be provided solely from peripheral glucose infusions, with a target of 6-8 mg/kg/minute of glucose to provide sufficient, but not excessive, carbohydrate-based energy (Mesotten, 2018).
QUESTION 1
Show that 10% w/v glucose will be unable to provide the target glucose infusion rate with this limited amount of fluid available.
5% glucose is considered to be isotonic, with an osmolarity similar to that of plasma, which is around 290 mOsm/L (Labtestsonline, 2021). As the osmolarity increases the chance of a solution causing thrombophlebitis of peripheral vessels increases, such that a limit of 600-900 mOsm/L is often considered to be the safe range concentration of solutes in solutions to be administered peripherally (Gorski et al, 2011; Hartman et al, 2018). Concentrations higher than this need to be given centrally into large vessels with fast-flowing blood to avoid irritating the vessel walls.
QUESTION 2
Given that the molecular mass of glucose is 180.18 g/mol, and 1 mole/litre of a non-dissociating solute (such as glucose) will produce a solution with an osmolarity of 1 Osm/L, calculate the osmolarity of 10% glucose.
QUESTION 3
Since we only have peripheral access for this child, what is the maximum concentration of glucose we could consider running peripherally if we choose a conservative limit of 600 mOsm/L for the osmolarity?
QUESTION 4
If we were to fit the minimum desirable glucose delivery rate into the maximum available volume, what would be the concentration and osmolarity of the required solution?