Fluid and electrolyte management

Major thermal injury causes massive fluid shift from the vascular space to the interstitial and intracellular compartments, resulting in soft tissue edema and, if not replaced, hypovolemic shock. The management principles for burn shock are no different than those for any form of hypovolemic shock. However, since it is relatively simple to quantitate the severity of a burn injury and difficult to assess the intravascular volume loss visually, a number of formulas have been developed to predict the volume required to restore or maintain intravascular volume. These equations call for 2 to 4 ml3 of isotonic crystalloid per kilogram of body weight for each per cent of the body surface area burned in the first 24 h; half is given within the first 8 h following injury. The formulas provide a reasonable guide to initial resuscitation but should not be followed rigorously. Subsequent fluid replacement should be based on patient response. Patients who require significantly more than the predicted volume are often found to have either very deep burns or inhalation injury.

Fluid and protein may continue to leak from the vascular compartment for several days after injury, but the rate of loss slows after the first 24 to 48 h when soft tissue edema reaches maximal levels. Further fluid requirements will depend upon the type of resuscitation used as well as current wound care and renal function. If severe hypoproteinemia is present, spontaneous mobilization of the extravascular water will be delayed. Some patients require large quantities of free water to excrete the solute load.

Burned skin no longer forms an effective barrier to water loss into dressings or by evaporation. Such losses can be crudely estimated from the formula witcr loss(ml/h} =^25 + body surfaoc arca burned

It is important to recognize that traditional indicators of volume status, such as urine output, may be unreliable in the first 10 to 14 days postburn, when large quantities of osmotically active breakdown products from injured tissue are released into the circulation and may induce an osmotic diuresis. This period also corresponds to the increase in nutrient delivery. If urine output is used to assess intravascular volume, it must be coupled with specific gravity, osmolality, etc.

Insensible water loss can be calculated from the difference between fluid intake and output and the change in weight, and can be replaced by low-sodium fluids since the most prevalent electrolyte abnormality during this period is hypernatremia from the loss of free water from the wound. When fluid balance is critical, fluid requirements can be calculated as the insensible loss from the preceding 24 h plus the desired urine output for the next 24 h.

Other electrolyte abnormalities, including hypokalemia, hypophosphatemia, hypocalcemia, and hypomagnesemia, are common, particularly in the first 10 to 14 days postinjury. Frequent monitoring may be required with more severe burns which need more aggressive replacement. When the patient is tolerating enteral nutrition, fluid and electrolyte replacement can be administered via feeding tube and the intravenous catheters can be removed.

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