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Occupation * Consultant Paediatric Nephrologist
Affiliation * University Hospital of Wales
This is another article on the potential risks of using hypotonic intravenous solutions and the perceived risk of promoting hyponatraemia. I am concerned that this, as with other publications on the topic, uses the term "maintenance fluids" when the fluid being prescribed is also serving the role of "resuscitation" and/or "replacement". While this study may not have seen any cases of hypernatraemia in patients receiving 0.9% saline, we have. The idea that the prescription of just one type of fluid i.e. an isotonic one, is suitable in all paediatric patients covered by this studies' selection criteria reflects laziness on the part of the doctor.
For accurate prescribing of intravenous fluid, the doctor should think of three different requirements:
1. Resuscitation fluid
2. Replacement fluid
3. Maintenance fluid
Resuscitation fluid is administered to correct a deficit in the ECF volume. I wonder if this study tries to eliminate this by excluding children with "urinary osmolarity greater than 100 mOsm/kg and a weight change >2% in the first 24 hours". Unfortunately it is not clear by how much the 48 hour study period of fluid administration overlapped with the first 24 hours of admissi...
Resuscitation fluid is administered to correct a deficit in the ECF volume. I wonder if this study tries to eliminate this by excluding children with "urinary osmolarity greater than 100 mOsm/kg and a weight change >2% in the first 24 hours". Unfortunately it is not clear by how much the 48 hour study period of fluid administration overlapped with the first 24 hours of admission. Table 1 has a figure for "Time at study enrolment" without a unit but I assume this is hours from admission. From this I take it that patients are a mean of five and six hours into their admission in the two groups. If these patients are ill then I wonder if some may have been hypovolaemic and in whom it would clearly be inappropriate to administer a hypotonic solution.
If replacement of ongoing fluid losses, in excess of those normally encountered, is required then the intravenous fluid used should match the composition and volume of the losses. Given that 80 of the 294 patients were post-surgical it might be that replacement fluid was required in some of these patients but again this is not clear from the paper. It is also not clear how many of the patients were sick enough to require intensive care.
The term "maintenance fluid" should be reserved for that administered to replicate the fluid that the patient would normally be drinking but for a variety of reasons may not be able to ingest. It is given to "maintain" euvolaemia. If reserved for this purpose then "maintenance" fluid does not need to be isotonic given that normal fluid losses do not come anywhere near a sodium concentration of 150 mmol/L. Healthy kidneys can reduce sodium excretion to virtually zero.
The concern about using hypotonic solutions centres around the notion that the syndrome of inappropriate anti-diuretic hormone secretion (SIADH) is a common phenomenon and administration of hypotonic solutions will cause hyponatraemia. However I would contend that SIADH is overdiagnosed and when ADH levels are raised it is usually appropriate i.e. ADH is being produced in response to ECF volume contraction. In that situation an isotonic solution is required. Once ECF volume has been restored ADH production will be switched off and continuing use of isotonic solutions will risk causing hypernatraemia. It mystifies me as to what evolutionary drive there can be for the body to want to produce ADH in the situations that are supposedly associated with SIADH.
In summary, prescription of intravenous fluids should only be made after a careful assessment of the patient, in particular their ECF volume status. If there is suspicion that it is reduced an isotonic solution should be administered. If there are ongoing losses they should be replaced by a solution best matching the fluid being lost. If it is not possible to establish the electrolyte content of the fluid losses an isotonic solution is advised. Once ECF volume is replete then a hypotonic solution should be given and on our ward we would use 0.45% saline + 2.5% dextrose. This is still providing more sodium than is normally recommended if giving fluids orally or in TPN. Unfortunately ECF volume status is notoriously difficult to assess and if there is doubt isotonic solutions will be safer. However if 0.9% sodium chloride continues to be administered as "maintenance" fluid we will see cases of hypernatraemia. Whatever strategy is adopted it is essential to monitor serum electrolytes at least daily in children receiving a significant (>50%) proportion of their fluids intravenously.
There is ongoing debate about the volume of "maintenance" fluid we should be giving children. Currently this is based on recommendations made by Holliday and Segar in the 1950’s, matching fluid requirement to energy expenditure. It is now recognised that these do not reflect the fluid requirements of unwell children lying in bed and as a result “maintenance” fluid volume prescriptions may be twice what they should be. The prescription of more water than is needed is potentially a contributing factor to hypotonic solutions producing hyponatraemia - the prescribing of too much water rather than too little sodium.