Fluid Replacement Therapy

Fluid Replacement Therapy

Fluid replacement, also known as fluid resuscitation, is the medical practice of replenishing bodily fluids lost due to sweating, bleeding, fluid shifts, or other pathological processes. Fluids can be replenished through oral administration (drinking), intravenous administration, rectally, or via hypodermoclysis, which involves the direct injection of fluid into the subcutaneous tissue. Oral and hypodermic administration of fluids results in slower absorption compared to intravenous administration.

Oral rehydration therapy (ORT) is a simple treatment for dehydration associated with diarrhea, particularly gastroenteritis caused by pathogens such as cholera or rotavirus. ORT involves administering a solution of salts and sugars orally. This therapy is crucial in the developing world, where it saves millions of children annually from death due to diarrhea, the second leading cause of death in children under five.

In cases of severe dehydration, intravenous fluid replacement is preferred and can be life-saving. It is particularly beneficial when there is depletion of fluid in both the intracellular and vascular spaces.

Fluid replacement is also necessary in situations such as hemorrhage, extensive burns, excessive sweating (as in prolonged fever), and prolonged diarrhea (such as cholera).

Additionally, during surgical procedures, fluid requirements increase due to factors such as increased evaporation, fluid shifts, and excessive urine production. Even minor surgeries can lead to a loss of approximately 4 ml/kg/hour, while major surgeries may cause a loss of approximately 8 ml/kg/hour, in addition to the basal fluid requirement.

The intravenous fluids used for fluid replacement typically belong to the class of volume expanders. Physiological saline solution, or 0.9% sodium chloride solution, is commonly used because it is isotonic and does not cause potentially dangerous fluid shifts. Normal saline is also preferred when blood transfusion is anticipated, as it is the only fluid compatible with blood administration.

Lactated Ringer's solution is another isotonic crystalloid solution designed to closely match blood plasma. When administered intravenously, isotonic crystalloid fluids are distributed to the intravascular and interstitial spaces.

A variety of blood and non-blood products, including colloid and crystalloid solutions, are used in fluid replacement. Although colloids are increasingly used, they are more expensive than crystalloids. However, 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.

It is essential to achieve a fluid status that prevents oliguria (low urine production). Oliguria is typically defined as a urine output of 0.5 mL/kg/hr in adults, indicating adequate organ perfusion. Fluid therapy may need to be adjusted based on hemodynamic values and urine output.

The speed of fluid replacement may vary depending on the procedure. For burn victims, fluid replacement planning is based on the Parkland formula, which calculates the minimum amount of fluid to be given over 24 hours. During the initial period, half of the volume is administered over the first eight hours after the burn, with the remainder given over the next 16 hours. In cases of dehydration, 2/3 of the deficit may be replaced within 4 hours, with the remainder administered over approximately 20 hours.

The initial phase of volume expansion is known as the fluid challenge, which may differ from subsequent maintenance fluid administration. During the fluid challenge, large amounts of fluids are administered over a short period under close monitoring to assess the patient's response. This procedure is reserved for hemodynamically unstable patients and is distinguished from conventional fluid administration for less acutely ill patients.