Replacement fluid therapy
The initial goal of treating dehydration is to restore intravascular volume. The simplest approach is to replace dehydration losses with 0.9% saline. This ensures that the administered fluid remains in the extracellular (intravascular) compartment, where it will do the most good to support blood pressure and peripheral perfusion.
Therapy may be started with a rapid bolus of 0.9% saline to combat incipient shock. But correction of dehydration has to be accompanied by provision of maintenance fluid- after all, the child is breathing, losing free water through the skin, and is urinating! As discussed earlier, maintenance fluid is provided as D5 0.18% or D5 0.3% saline. The combination of 0.9% saline (dehydration correction) and 0.18% saline (maintenance fluid) averages to approximately 0.45% (half-normal) saline. This approximation is acceptable because the kidneys will sort out what to keep and what to excrete.
A typical sequence of events in the management of a child with 10% dehydration AND A NORMAL SERUM Na LEVEL is given below. Management of children with a serum Na level of < 135 or> 145 mEq/L is beyond the scope of this discussion.
Step 1: In the ER, the child is estimated is having 10% dehydration. The blood pressure is low and the heart rate is very high. This child is in incipient shock. The goal is to rapidly stabilize the vital signs; maintenance fluid is not a consideration at this time.
The child is given a 20 ml/kg bolus of 0.9% saline over 10-30 minutes. The vital signs stabilize (the bolus can be repeated if necessary).
Step 2: the patient is transferred to the inpatient unit. By this time, serum electrolytes levels are available and the serum sodium concentration is within the normal range. Subsequent fluid therapy is calculated as follows:
This child's total fluid loss was 10% of 10 kg, or 1000 ml. Of this, 200 ml has already been infused in the ER, so the remaining deficit is 800 ml.
Typically, half the total deficit is replaced in the first eight hours after admission and the remaining fluid is given over the next 16 hours. So, this child needs 300 ml of 0.9% saline in the next eight hours (for a total of 500 ml) and another 500 ml in the next 16 hours.
However, maintenance fluid also has to be administered. The volume of maintenance fluid for 24 hours is 1000 ml (100 ml/kg X 10 kg). This needs to be given as D5 0.18% saline (important- see note #3 below).
Now the fluid calculation looks like this:
|0-8 hours||9-24 hours|
|Deficit||300 ml of 0.9% saline||500 ml of 0.9% saline|
|Maintenance||333 ml of D5 0.18% saline||666 ml of D 5 0.18% saline|
|Averaged total||663 ml of D5 0.45 normal saline||1166 ml of D5 0.45 normal saline|
Note #1: Once the child has started urinating, KCl should be added to the intravenous fluids at a concentration of 20 mEq/L.
Note #2: If the child continues to vomit or have significant diarrhea, the volume of ongoing fluid loss should be estimated and added to the deficit every few hours as 0.9% saline. Ideally, the diapers should be weighed. If this is not possible, then a volume of 50-100 ml should be used for each stool in an infant and 100-200 ml for the older child.
Note #3: The dehydration component of fluid replacement MUST be provided as 0.9% saline. NEVER use a hypotonic saline, such as D5 0.18% (fifth-normal saline), D5 0.3% (third-normal saline) or even D5 0.45% (half-normal saline) to correct dehydration. Dehydration and hypovolemia result in secretion of anti-diuretic hormone, which causes retention of free water, and provision of hypotonic replacement fluid can lead to potentially life-threatening hyponatremia.
Step 3: Suppose the child is well hydrated by the second hospital day, but is still feeling queasy and does not want to drink. Maintenance fluids can now be continued as D5 0.18% saline with 20 mEq/L of KCl.
The moral of the story:
- If you are correcting only dehydration (as when giving a bolus in the ER), use 0.9% saline.
- If you are correcting dehydration and providing maintenance fluids at the same time, add both volumes and use D5 0.45% saline.
- If you are providing maintenance fluid only, use D5 .18% saline. Some hospitals only have D5 0.33% saline available; this is also acceptable.
- Once the child starts urinating, add KCl at a concentration of 20 mEq/L.
- Estimate and replace ongoing losses, if significant.
Some more words of caution:
In the past decade, there have been a number of case reports of patients developing dangerous hyponatremia during intravenous fluid therapy. To avoid this,
- As discussed above, use ONLY normal saline for volume replacement. Never use hypotonic saline; these patients are secreting ADH which can lead to water retention. The appropriate volume of normal saline can be combined with the hypotonic saline being used for provision of maintenance fluid requirements so that the final solution is D5 0.45 normal saline.
- NEVER use excessive volumes of hypotonic saline as a maintenance fluid. Calculate the requirement, and don't exceed it!
- If the serum sodium is dropping below 138 mEq/L, switch to normal saline for rehydration and maintenance.
- If a patient is suspected to have the syndrome of inappropriate secretion of ADH (SIADH), use only normal saline for rehydration and maintenance.
- Post-operative patients have a tendency for SAIDH. These patients should receive only normal saline, even for maintenance.
Over the past four decades, oral rehydration has been demonstrated to be quite effective in replacing diarrheal fluid losses. This therapy is best reserved for the child with mild or moderate dehydration who is not vomiting.
Commonly available oral rehydration solutions such as Pedialyte(r) contain 2.5 G/dl of carbohydrate and 45 mEq Na, 35 mEq Cl, 20 mEq K and 30 mEq/L of base. The WHO/UNICEF oral rehydration solution contains 2 G/dl of carbohydrate, and 90 mEq Na, 80 mEq Cl, 20 mEq K and 30 mEq/L of base.
The mildly dehydrated child needs approximately 50 ml/kg of the oral rehydration solution over the first four hours. Children with moderate dehydration should be given approximately 100 ml/kg of the solution in the first four hours. Following that, an attempt should be made to reinstitute breast or formula feeding.
Apple juice, de-fizzed cola, ginger ale, ice tea etc. contain inadequate electrolyte and are NOT recommended for rehydration.
More info (a lot more!): Oral rehydration