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Maintenance fluid requirements in disease

Maintenance fluid calculations assume that fluid loss from sensible and sensible routes is taking place at a normal rate. But a febrile infant will be having a much greater transcutaneous evaporative water loss than one with a normal body temperature. Similarly, a child with tachypnea will lose excess water from the lungs- unless she is receiving humidified oxygen, in which case she will lose none!

Also, would you write standard maintenance fluids for an anuric child? Of course not- he has no urinary loss. Similarly, a child whose kidneys have a concentrating defect will exhibit polyuria and need more than the standard volume of maintenance fluids.

Moral: Before using a standard formula for calculating maintenance fluids, ensure that the child is not having higher or lower losses than usual!

As a rule of thumb, total water loss can normally be broken down as follows:

Respiratory loss 20 %
Transcutaneous loss: 30%
Urine 50%
Total 100%

For a 10 kg child (water loss: 1000 ml/day), loss from the different routes for a 24-hour period would be:

Respiratory loss 200 ml
Transcutaneous loss: 300 ml
Urine 500 ml
Total 1000 ml

When we prescribe maintenance fluid for a 10 kg child for 24 hours as 1000 ml, we are assuming that loss from the various routes is occurring at a normal rate. However, adjustments are sometimes necessary:

  • Tachypnea: Add 20-50% to the respiratory replacement. Note: If a person is receiving humidified oxygen, respiratory water loss is nil.
  • Fever: Add 10% to the transcutaneous loss replacement for every degree temperature above 38o C.
  • Anuria: Exclude urinary loss from replacement.
  • Oliguria: Measure actual urine output every 12 hours and add it to the insensible loss for the next 12 hours.
  • Polyuria: Measure actual urine output every 1-2 hours and add it to the insensible loss for the next 1-2 hours.

So- what is the 24-hour fluid requirement for a 10 kg child with pneumonia who has a fever of 40 degrees C. and a respiratory rate of 70/min?

We need to compensate for both fever and tachypnea.

Normal Patient
Respiratory loss: 200 ml (increase by 50%) 300 ml
Transcutaneous loss: 300 ml (increase by 20%) 360 ml
Urine: 500 ml (unchanged) 500 ml
Total: 1000 ml 1160 ml

If the patient's clinical condition suggests that skin or lung loss is excessive, the allocation for either or both can be increased by up to 50%. If urinary losses are higher or lower than expected, take out the urinary component; then measure the actual urinary output periodically and add it to the maintenance for the next similar period. (Don't panic- examples follow!)

What volume of maintenance fluid would you write for next 12 hours for a 10 kg child with oliguria whose measured urine output in the previous 12 hours has been 50 ml?

Normal for 12 hours Patient
Respiratory loss: 100 ml (unchanged) 100 ml
Transcutaneous loss: 150 ml (unchanged) 150 ml
Urine: 250 ml 50 ml (measured)
Total: 500 ml 300 ml