Maintenance Fluid Requirements in Disease
Maintenance fluid calculations assume that fluid loss from sensible and insensible 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 consider patients with kidney disease who have anuria, oliguria, or polyuria. Maintenance IV fluids for these patients will not be written with the standard formula because their urinary losses are not taking place at a normal rate. Maintenance fluids using the standard formula would be too much for an anuric child with no urinary losses and too little for those with a concentrating defect in their kidneys causing polyuria.
Important: 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.
What is the 24-hour fluid requirement for a 10 kg child who has a fever of 40 degrees C. and a respiratory rate of 70/min? (Presuming the child is not receiving humidified O2)
| 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 |
What volume of maintenance fluid would you order for the 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 |
