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Official reprint from UpToDate www.uptodate.com ©2011 UpToDate®
Maintenance and replacement fluid therapy in adults
Author Richard H Sterns, MD Section Editor Michael Emmett, MD Deputy Editor John P Forman, MD, MSc
Last literature review version 19.1: enero 2011 | This topic last updated: junio 13, 2010INTRODUCTION — A critical role of the kidneys is to maintain the effective circulating volume and plasma osmolality within relatively narrow limits, as well as to maintain electrolyte homeostasis. The normal homeostatic mechanisms that maintain the effective circulating volume and plasma osmolality are discussed elsewhere. (See "Physiologic regulation of effective arterial blood volume and plasmaosmolality".) Under normal circumstances, the kidneys can adjust to wide variations in dietary intake by appropriate variations in water and electrolyte excretion , which is particularly important when discussing maintenance fluid requirements. Water balance — Water losses lead to an increase in serum sodium and osmolality, resulting in stimulation of thirst and increased release of antidiuretic hormone(ADH). In normal individuals, these changes will lead to increased water intake and reduced water excretion, which will restore normal water balance. Thus, patients who are alert, have an intact thirst mechanism, and access to water will not become hypernatremic. (See "Causes of hypernatremia".) On a normal diet, the minimum water intake is estimated at 500 mL/day (assuming there are no increasedlosses). This value is based upon the balance of total water intake and production and the minimum rate of urinary loss. Individuals who can concentrate their urine to 1200 mosmol/kg who excrete 600 mosmol of solute (sodium and potassium salts and urea) per day will have a minimum urine output of 500 mL (600 mosmol ÷ 1200 mosmol/kg). There are two other sources of water in addition to fluidingestion: the water content of food (fruits and vegetables are almost 100 percent water by weight) and the water generated by oxidation of carbohydrates. There are also other sources of water loss in addition to the urine output: insensible losses and sweat. Normal adults are considered to have a minimal obligatory water intake or generation of approximately 1600 mL per day, composed of the following:• Ingested water — 500 mL • Water in food — 800 mL • Water from oxidation — 300 mL The sources of obligatory water output in normal adults are composed of the following: • • • • Urine — 500 mL Skin — 500 mL Respiratory tract — 400 mL Stool — 200 mL
Maintenance and replacement fluid therapy inadults
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However, the water of oxidation and much of the water lost from the lungs during respiration are linked . The metabolic production of CO2 and water occur in a 1:1 proportion during the oxidation of carbohydrates and fatty acids and, if the arterial pCO2 is close to 40 mmHg, these two end-products are eliminated together in alveolar air in a 1:1 proportion. Water and CO2are eliminated in parallel because the partial pressures of water vapor (47 mmHg) and CO2 (40 mmHg) are virtually equal in alveolar air, and because both CO2 and water are nearly absent in inspired air. Thus, the water of oxidation and most of the water normally lost from the lungs during respiration can probably be removed from estimates of water balance . In most patients, only the small amountof water evaporation from the upper respiratory tract results in a negative water balance. This does not apply to patients who are hyperventilating (which increases alveolar water losses) or are on a ventilator and inspiring humidified air warmed to body temperature (which decreases alveolar water losses). Evaporation of water from the skin as sweat (which usually has a sodium concentration...