0071643192.pdf

TABLE 7.1. Comparison of Intracellular and Interstitial Fluid

% TBW % WEIGHT Na+ Cl- HCO 3 - K+ Ca^2 + Mg^2 + INCLUDES

Intracellular 2/3 40% 14 4 10 150 < 1 30 Proteins

Interstitial 1/3 20% 140 113 27 5 9 3 Plasma

FLUID AND HYDRATION

Basic Definitions

■ Solvent: Solution or medium, usually H 2 O
■ Solute: Crystalloid (cation or anion) or colloid (plasma protein)
■ Osmosis: Movement of solvent (usually H 2 O) across a membrane through
an osmolar gradient
■ Osmolality: Pressure exerted by the number of particles in the solution

Total Body Water and Distribution

■ Total body water =about 60% of total body weight
■ Two-thirds of TBW is intracellular =40% of total body weight (see Table 7.1).

Physiology

■ Cell membranes are semipermeable and allow free passage of solvent but
not solute, hence water moves freely between intracellular, interstitial, and
plasma compartments to maintain equilibrium.
■ Normal serum osmolality is around 285 mOsm/L.
■ Osmolality can be calculated using this formula:

2[Na+]+Glucose+BUN=calculated osmolality

18 2.8

■ Hyperosmolar states include:
■ Alcohol
■ Hypernatremia
■ Hyperosmolar hyperglycemic nonketotic coma
■ Ketoacidosis
■ Uremia
■ Hypo-osmolar states are usually caused by hyponatremia.
■ The osmolal gap is the difference between the calculated and measured
osmolality and should be <10 mOsm/L.
■ High osmolal gaps are caused by increases in measured (but not calcu-
lated) serum osmolality.
■ Account for ethanol’s contribution to the osmolar gap by dividing by the
blood ethanol in mg/dL by 4.6.
■ Causes of high osmolal gaps include:
■ Ethanol—most common
■ Isopropyl alcohol
■ Mannitol
■ Sorbitol
■ Glycerol
■ Acetone
■ Methanol
■ Ethylene glycol

ENDOCRINE, METABOLIC, FLUID, AND

ELECTROLYTE DISORDERS

Total blood volume in adults is

70 mL/kg or about 5 L in a

70-kg person. Total blood

volume is 80–90 mL/kg in

children.

In patients with altered mental

status and an unexplained

increased anion gap, use the

osmolal gap as a screening

test for methanol or ethylene

glycol toxicity.

An increased osmolar gap

that is explained by an

elevated serum ethanol level

is normal in a drunk patient.