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INTRODUCTION

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Prior to discussing the controversial topic of whether crystalloids or colloids are superior, one must first understand the distribution of body fluid compartments as well as the control of fluid distribution between these compartments. The first concept is total body water (TBW), which is described and broken down as follows:

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  • 600 cc/kg—varies with age, gender, and adiposity

  • 60% body weight in average adult man

  • 50% body weight in average adult woman

  • 80% body weight in neonate

  • 65% body weight in 12-month-old infant.

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Total body water is distributed into multiple compartments within the body as follows:

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  • Intracellular fluid (ICF) = 400-450 cc/kg

  • Extracellular fluid (ECF) = 150-200 cc/kg (20%-30% TBW)

    • Interstitial compartment

    • Transcellular compartment

    • Intravascular fluid compartment

  • Intravascular blood volume = 65-70 cc/kg in adults, 90 cc/kg in a full term newborn, 100 cc/kg in a premature newborn, and 75-80 cc/kg in an infant.

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CONTROL OF FLUID DISTRIBUTION

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Cell membranes exist between the intracellular and extracellular fluid compartments, while capillary membranes divide the extracellular fluid compartment into the interstitial and intravascular compartments. These membranes are semipermeable and contain Na/K ATPase pumps which work to create concentration gradients, extruding Na+ out of the cell and keeping K+ in. Water passes freely down the concentration gradient, but larger molecules cannot. During ischemia or trauma, these membranes become leaky allowing water and large molecules to pass freely between compartments.

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The Starling equation describes the passage of fluids between the capillaries and the tissues, and is given as: Jv = Kf [(PmvPt) − r(COPmv − COPt)]. The variables are defined as follows:

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  • Jv = transcapillary fluid filtration rate

  • Kf = filtration coefficient determined by capillary surface area and permeability

  • Pmv = capillary pressure

  • Pt = tissue pressure

  • r = reflection coefficient (1.0, no molecular passage; 0, free molecular passage)

  • COPmv = colloid oncotic pressure of capillary

  • COPt = tissue colloid oncotic pressure

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Osmolality is defined as the number of osmoles per liter solution. It can be calculated using the equation 1.86(Na+) + glucose/18 + BUN/2.8. Osmolarity is defined as the number of osmoles per 100 g solvent. This value is equivalent to osmolality in dilute solutions (human body) and normally ranges from 285 to 295 mOsm/L (approximately 2 × Na+).

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Fluid Therapy

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The goals of fluid replacement therapy are to replace preoperative deficits, maintenance fluids, insensible fluid losses, electrolyte losses, and blood loss. Insensible losses come from evaporation of H2O from respiratory tract, sweat, feces, and urinary excretion. Maintenance fluid, therefore, is about 2 mL/kg/h, usually in the form of a crystalloid solution. Preoperative deficits are determined by multiplying the number of hours NPO × maintenance fluid requirements and are ...

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