Longnecker's Anesthesiology > Part 4. Managing Anesthesia Care > Section A. Monitoring the Anesthesia Patient > Monitoring and Managing Perioperative Electrolyte Abnormalities, Acid-Base Disorders, and Fluid Replacement Patrick J. Neligan, MD, and Jiri Horak, MD

Key Points

"1. Water is the single most abundant compound in the body, constituting approximately 50–70% of body weight.

2. One third of body water is extracellular, representing approximately 20% of body weight. Of this one third is located within the intravascular compartment, and two thirds is extravascular or interstitial.

3. Electrolytes are characterized by their degree 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).

4. Although osmolality is equal through each of the body's compartments, electrolyte composition varies. Sodium and chloride are principally extracellular. Potassium, phosphate, magnesium, and calcium are principally intracellular.

5. Perioperative patients undergo a predictable "stress" response during which there is significant fluid and electrolyte flux. The magnitude and timing of these changes are key to management strategies.

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

7. Depletion of serum concentration of principally intracellular ions (potassium,..."