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 %|
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|
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.
|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|