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  1. Water is the single most abundant compound in the body, constituting approximately 50% to 70% of body weight. One-third of body water is extracellular; of this, one-third is located within the intravascular compartment, and two-thirds is extravascular or interstitial.

  2. Electrolytes are characterized by their extent of dissociation (strong or weak ions), the number of particles present (millimoles), the number of electrical charges per unit (milliequivalents), and the number of active molecules per unit volume (milliosmoles).

  3. Osmolality is uniform through each of the body's compartments, but electrolyte composition varies. Sodium and chloride are principally extracellular. Potassium, phosphate, magnesium, and calcium are principally intracellular.

  4. The perioperative stress response elicits significant fluid and electrolyte flux. The magnitude and timing of these changes are key to management strategies.

  5. Changes in extracellular sodium concentrations often, but not always, reflect body water composition. Hyponatremia is indicative of free water overload. Hypernatremia is indicative of dehydration.

  6. Depletion of serum concentration of principally intracellular ions (potassium, phosphate, magnesium, and calcium) reflects significant total body electrolyte depletion.

  7. All acid–base abnormalities can be explained in terms of strong ion difference (SID), weak acid concentration, Paco2, and extracellular free water.

  8. The 6 primary acid–base abnormalities are acidosis caused by increased Paco2, acidosis caused by reduced SID, acidosis caused by increased acid-buffering system (ATOT), alkalosis associated with reduced Paco2, alkalosis caused by increased SID, and alkalosis caused by reduced ATOT.

  9. A variety of tools are available for interpreting acid–base abnormalities. These include the base deficit (BD) excess, corrected anion gap (AG), strong ion gap (SIG), and the base deficit or excess (BDE) gap.

  10. Upper gastrointestinal (GI) losses should be replaced with isotonic saline. Lower GI and extracellular fluid (ECF) losses should be replaced with balanced salt solutions (BSS). Blood loss can initially be replaced with crystalloid in a 1:3 to 1:5 ratio, but as blood loss increases, the volume of crystalloid required increases geometrically.

  11. Overresuscitation with crystalloid may lead to poor perioperative outcomes. "Normal saline" solution, given in significant volume, causes metabolic acidosis, and hyperchloremia may reduce splanchnic blood flow.

  12. Colloid solutions restore circulating volume more rapidly than crystalloid with less tissue edema, but their use remains controversial.

  13. Volume replacement strategies for major surgery should not be formula based but dynamic and goal directed using volume and flow monitoring devices.

  14. Postoperative fluid restriction may be appropriate for some patients, especially those undergoing lower intestinal resection.

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The human body is an aqueous soup containing carbohydrates, protein, fat, ions, and trace elements bundled into and around cellular structures built on an exoskeleton. Fundamental to the structure and function of the body are the chemical properties of water. Patients undergoing surgery or experiencing critical illness undergo dramatic changes in the volume, distribution, and composition of body water. Patients undergoing the most minor of procedures undertake a period of fasting, rehydration with intravenous (IV) fluids, and neurohormonal changes in fluid distribution. To understand strategies and implications ...

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