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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 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.

In severe dehydration, intravenous fluid replacement is preferred, and may be lifesaving. Physiologic normal saline, 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 is the only approved fluid replacement capable of carrying oxygen. Lactated Ringers is another isotonic crystalloid solution and it is designed to match most closely blood plasma. If given intravenously, isotonic crystalloid fluids will be distributed among remains the intravascular and interstitial spaces.

Fluid shifts occur when the body's fluids move from the intracellular space into the intravascular space, or interstitial space, or vice versa. Physiologically, this occurs because of an osmotic gradient. Water will move from one chamber into the next passively across a semi permeable membrane until the osmolality is equal. Many medical conditions can cause fluid shifts. When fluid moves out of the intravascular space (the blood vessels), blood pressure can drop to dangerously low levels, endangering critical organs such as the brain, heart and kidneys. When fluid shifts out of the cells (the intracellular space), cellular processes slow down or cease from intracellular dehydration. Fluid shifts into the brain cells can cause increased cranial pressure.

The third space where bodily fluid resides is the interstitial space, or the space filled with interstitial fluid between the cells within the tissues. Depletion of this space is not life threatening, and usually accompanies depletion of the other spaces. Extensive tissue swelling occurs when the third space fills with excess fluid, known as edema. An example of severe third spacing is ascites, as seen in severe livere failure as a result of a low albumin level in the circulating blood.

Third space is also a term used to refer to, for example, the bowel with an ileus, and the collection of fluid therein, usually post-operatively. The interstitial space could be considered to be a sub-section of the extracellular compartment. The distinction one could make is that fluid in the interstitium is still readily available to either the intravascular or the intracellular compartments, in response to ionic changes, whereas that which is pooled in the bowel is not so readily available. Hopefully someone can clarify this point.

Hemorrhage, extensive burns and excessive sweating (as from a prolonged fever), and prolonged diarrhea (cholera) deplete both the intracellular and the vascular spaces. Immediate swift replenishment of fluid via an intravenous line (or several) is required. Initial treatment of trauma and burn victims places high priority on aggressive fluid replacement to maintain organ perfusion. The planning of fluid replacement for burn victims is based on the Parkland formula (4mL Lactated Ringers/kg/% TBSA burned). The parkland formula gives the miniumum amount to be given in 24 hours. Half of the value is given over the first 8 hours after the time of the burn (not from time of admission to ED) and the other half over the next 16 hours. It is important to note that in adults an urine output of 0.5mL/kg/hr is considered adequate and suggests adequate organ perfusion (1.0mL/kg/hr in children). The parkland formula is not perfect and fluid therapy will need to be titrated to hemodynamic values and urine output.

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