Treatment of a water deficit
Stop any ongoing water loss
If this is the result of ADH deficiency then administer ADH.
If the cause is an osmotic diuresis then remove source and address any sodium or potassium deficit.
Replace the deficit slowly, if possible by the oral route
If hypernatraemia is acute or there are serious CNS symptoms, then initial reduction of plasma sodium may have to be rapid. However aim to replace total water deficit over 2-3 days.
Oral replacement is best, unless unable to administer fluids orally. Can give water.
If need intravenous fluid replacement - what to use?
| 5% Dextrose |
Isosmotic Glucose metabolised leaving EFW. |
| Half normal saline |
Not appropriate if
poluria and sodium concentration in urine is less than 75 mmol/l as there will be no net
gain of EFW. May be appropriate if need to expand ECF volume. |
| Sterile water |
Will lead to haemolysis. Can be given via central line in severe cases where no alternative available eg hyperglycaemia prevents use of 5% dextrose. |
Calculation of the water deficit in the ICF
Assume total number of intracellular osmoles does not change.
Total number of effective intracellular osmoles = Normal ICF volume x 2 x [Na+] as the concentration of intracellular and extracellular osmoles must be equal in steady state.
If the normal ICF volume is 30 litres and the [Na+] rises to 160 mmol/l from a normal of 140 mmol/l, then the new ICF volume =
30L x 280 / 320 = 26.25L , a deficit of 3.75 litres.
Calculation of the ECF water deficit
The plasma [Na+] does not give any information about ECF volume. Assessment of ECF volume is based on clinical assessment and is likely to be imprecise.
Sodium content of the ECF equals the product of the plasma [Na+] and ECF volume. Using the estimated ECF volume it is possible to estimate whether there has been a gain or loss of total body sodium.
Replace ECF deficit rapidly if patient hypotensive.
Correct ICF deficit slowly unless serious symptoms.