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PURPOSE OF INTRAVENOUS (IV) FLUID THERAPY: WHY “MAINTENANCE”?
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Parenteral fluids are typically administered to patients whose spontaneous and/or enteral intake is insufficient to meet physiologic needs. Delivering a particular quantity of water, dextrose, and electrolytes every hour at a weight-based “maintenance rate” is intended to meet the cellular requirements for basic functionality. The widely used formula for calculating this rate (the “4-2-1 rule”) was developed in 1957 and was popularized as much for simplicity as for accuracy. Its original proponents utilized a rough estimate of patient energy requirements to extrapolate fluid needs, escalating stepwise with increases in weight, to arrive at the “4-2-1” progression. This approximation results in a “maintenance” hourly value providing the necessary intake to meet a minimum of one's basal metabolic needs, but likely not those accompanying routine physical activities. It is worth noting that this formula was developed based on data from hospitalized, bedridden patients.1
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CALCULATION OF THE MAINTENANCE FLUID RATE
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Based on the estimations of the rising energy requirement with each kilogram of body weight, a patient's maintenance IV rate is calculated as shown in Table 48-1.
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As stated earlier, calculations are done according to the “4-2-1” rule. This rate can be used solely for parental fluid administration or as a goal for the total hourly fluid intake (also taking into account continuous infusions and/or high-volume medications). Patients with normal end-organ function (e.g., those children simply nil per os [NPO] prior to surgery) may tolerate delivery of a full “maintenance” IV fluid volume in addition to other medications, infusions, and flushes without complication. Contrast this with a fluid-overloaded child with multiorgan dysfunction, who may not tolerate much “surplus” IV volume without cardiopulmonary consequences. In this instance, it may be beneficial to limit total hourly intake to the calculated maintenance rate, delivering only a portion of the total via continuous IV fluids. Fluid restriction, even to intake rates below maintenance, can be appropriate in patients with impaired renal function, pulmonary pathology, or fluid overload.
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CHOOSING IV FLUID COMPOSITION
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Choice of intravenous fluids should be made with an understanding of both the clinical situation and the respective risk–benefit profile.
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The three major components of IV fluids are as follows:
Base sodium content: One of the primary determinants of serum osmolality. All intravenous fluids use sodium and chloride as the primary contributors to solution tonicity in order to allow for safe intravenous administration. IV fluids can be isotonic (similar tonicity to plasma), hypotonic, or hypertonic, depending primarily on their respective sodium and dextrose content.
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