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Fluid Compartments

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Example: 70-kg man (1 L = 1 kg)

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Total Body Water:

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42,000 mL (60% of BW)

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  • Intracellular: 28,000 mL (40% of BW)
  • Extracellular: 14,000 mL (20% of BW)
  • Plasma: 3500 mL (5% of BW)
  • Interstitial: 10,500 mL (15% of BW in a 70-kg man)

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Total Blood Volume

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Total blood volume = 5600 mL (8% of BW in a 70-kg man)

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Red Blood Cell Mass

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• Man, 20–36 mL/kg (1.15–1.21 L/m2) • Woman, 19–31 mL/kg (0.95–1.0 L/m2)

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Water Balance

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• 70-kg man

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The minimum obligate water requirement to maintain homeostasis (if temperature and renal-concentrating ability are normal and solute [urea, salt] excretion is minimal) is about 800 mL/d, which would yield 500 mL of urine.

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“Normal” Intake:

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2500 mL/d (about 35 mL/kg/d baseline)

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  • Oral liquids: 1500 mL
  • Oral solids: 700 mL
  • Metabolic (endogenous): 300 mL

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“Normal” Output:

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1400–2300 mL/d

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  • Urine: 800–1500 mL
  • Stool: 250 mL
  • Insensible loss: 600–900 mL (lungs and skin). (With fever, each degree above 98.6°F [37°C] adds 2.5 mL/kg/d to insensible losses; insensible losses are decreased if a patient is undergoing mechanical ventilation; free water gain can occur from humidified ventilation.)

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Baseline Fluid Requirement

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Afebrile 70-kg Adult:

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35 mL/kg/24 h

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If Not a 70-kg Adult:

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Calculate the water requirement according to the following kg method:

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  • For the first 10 kg of body weight: 100 mL/kg/d plus
  • For the second 10 kg of body weight: 50 mL/kg/d plus
  • For the weight above 20 kg: 20 mL/kg/d

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Electrolyte Requirements

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• 70-kg adult, unless otherwise specified

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Sodium (as NaCl):

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80–120 mEq/d (children, 3–4 mEq/kg/24 h)

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Chloride:

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80–120 mEq/d as NaCl

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Potassium:

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50–100 mEq/d (children, 2–3 mEq/kg/24 h). In the absence of hypokalemia and with normal renal function, most of this K is excreted in the urine. Of the total amount of K, 98% is intracellular, and 2% is extracellular.

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If the serum K level is normal, about 4.5 mEq/L, the total extracellular pool of K+ = 4.5 × 14 L = 63 mEq. K is easily interchanged between intracellular and extracellular stores under conditions such as acidemia or alkalemia. K demands increase with diuresis and building of new body tissues (anabolic states).

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Calcium:

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1–3 g/d, most of which is secreted by the GI tract. Routine administration is not needed in the absence of specific ...

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