Fluid replacement

Fluid replacement or fluid resuscitation is the medical practice of replenishing bodily fluid lost through sweating, bleeding, fluid shifts or other pathologic processes. Fluids can be replaced via oral administration (drinking), intravenous administration, rectally, or by hypodermoclysis, the direct injection of fluid into the subcutaneous tissue. Fluids administered by the oral and hypodermic routes are absorbed more slowly than those given intravenously.

Oral
Oral rehydration therapy (ORT) is a simple treatment for dehydration associated with diarrhea, particularly gastroenteritis/gastroenteropathy, such as that caused by cholera or rotavirus. ORT consists of a solution of salts and sugars which is taken by mouth. It is used around the world, but is most important in the developing world, where it saves millions of children a year from death due to diarrhea—the second leading cause of death in children under five.

Indications
In severe dehydration, intravenous fluid replacement is preferred, and may be lifesaving. It is especially useful where there is depletion of fluid both in the intracellular space and the vascular spaces.

Fluid replacement is also indicated in fluid depletion due to hemorrhage, extensive burns and excessive sweating (as from a prolonged fever), and prolonged diarrhea (cholera).

The table to the right shows daily requirements for some major fluid components. If these cannot be given parenterally, they may need to be given entirely intravenously. If continued long-term (more than approx. 2 days), a more complete regimen of total parenteral nutrition may be required.

In addition, during e.g. surgical procedures, fluid requirement increases by e.g. increased evaporation, fluid shifts and/or excessive urine production. Even a small surgery may cause a loss of approx. 4 ml/kg/hour, and a large surgery approximately 8 ml/kg/hour, in addition to the basal fluid requirement.

Types of fluids used
The types of intravenous fluids used in fluid replacement are generally within the class of volume expanders. Physiologic saline solution, or 0.9% sodium chloride solution, is often used because it is isotonic, and therefore will not cause potentially dangerous fluid shifts. Also, if it is anticipated that blood will be given, normal saline is used because it is the only fluid compatible with blood administration.

Blood transfusion is the only approved fluid replacement capable of carrying oxygen; some oxygen-carrying blood substitutes are under development.

Lactated Ringer's solution is another isotonic crystalloid solution and it is designed to match most closely blood plasma. If given intravenously, isotonic crystalloid fluids will be distributed to the intravascular and interstitial spaces.

Blood products, non-blood products and combinations are used in fluid replacement, including colloid and crystalloid solutions. Colloids are increasingly used but they are more expensive than crystalloids. A systematic review found no evidence that resuscitation with colloids, instead of crystalloids, reduces the risk of death in patients with trauma, burns or following surgery.

Procedure
It is important to achieve a fluid status that is good enough to avoid oliguria (low urine production). Oliguria has various limits, but a urine output of 0.5 mL/kg/h in adults is usually considered adequate and suggests adequate organ perfusion. The parkland formula is not perfect and fluid therapy will need to be titrated to hemodynamic values and urine output.

The speed of fluid replacement may differ between procedures. The planning of fluid replacement for burn victims is based on the Parkland formula (4mL Lactated Ringers X wt.in kg X % TBSA burned= Amount of fluid ( in ml) to give over 24 hours). The parkland formula gives the minimum amount to be given in 24 hours. Half of the volume is given over the first eight hours after the time of the burn (not from time of admission to ED) and the other half over the next 16 hours. In dehydration, 2/3 of the deficit may be given in 4 hours, and the rest during approx. 20 hours.

The initial volume expansion period is called the fluid challenge, and may be distinguished from succeeding maintenance administration of fluids. During the fluid challenge, large amounts of fluids may be administered over a short period of time under close monitoring to evaluate the patient’s response. Fluid challenge, as the procedure of giving large amounts of fluid in a short time, may be reserved for hemodynamically unstable patients, distinguished from conventional fluid administration for patients who are not acutely ill, who receive fluids as part of a diagnostic study, or for less acutely ill patients in whom fluid administration can be guided by fluid intake and output recordings.