0071643192.pdf

ENDOCRINE, METABOLIC, FLUID, AND

ELECTROLYTE DISORDERS

Fluid Balance

WATER

■ Adults need 2000–3000 mL of water a day on average.

■ Water loss can be categorized as sensible and insensible.

■ Sensible water loss =urinary output =approximately 1500 mL/day.

■ Cellular function creates approximately 300 mL of water/day.

■ Minimum normal urine output of a hydrated adult without renal compro-

mise is approximately 0.5 mL/kg/hour.

■ Insensible water loss includes:

■ Respiratory losses of approximately 600 mL/day

■ Integumentary/evaporative skin losses of about 300 mL/day

■ GI/Feces losses of about 100 mL/day

■ The body handles water regulation with three major mechanisms:

■ Antidiuretic hormone (ADH)

■ Dehydration→decreased free water →increased serum osmolality →

stimulate ADH release →retention of free water.

■ Aldosterone and renal tubule water absorption

■ Hypovolemia →renin released by kidneys →aldosterone released by

adrenals→sodium retention in renal tubules →water retention.

■ Hypothalamic thirst center

■ Driven by increases in serum osmolality, renin-angiotensin, β-adrenergic

stimulation and drugs such as lithium, as well as by drops in intracellular

and extracellular volume

An 86-year-old male with urosepsis presents dehydrated. How much intra-

vascular fluid does 1 L of 0.9 NS provide? How about 1 L of 0.45 NS or D5W?

Following redistribution with extra vascular space, 1 L of NS provides

250 mL of intravascular fluid, while 0.45 NS provides only 125 mL of intravascular

fluid. D5W initially provides 333 mL of intravascular fluid, but only a minimal

amount of that remains once the sugar is metabolized.

FLUIDS ANDDEHYDRATION

■ Fluid tonicity

■ 0.9% NS and lactated Ringer’s are approximately isotonic to plasma.

■ D5W is approximately isotonic, but sugar is then metabolized leaving

hypotonic free water.

■ Fluid distribution

■ 1 L of 0.9 NS will distribute 250 mL intravascularly and 750 mL in the

interstitial space.

■ 1 L of 0.45 NS places only 125 mL in the intravascular space.

■ 1 L of D5W places 333 mL initially in the intravascular spaces, but after

the sugar is metabolized only one-quarter (83 mL) remains.

■ Dehydration

■ Dehydration can be broken up into three categories related to its osmo-

lality: Hypotonic, isotonic, and hypertonic (see Table 7.2).

■ For more detailed information on hypo- and hypernatremic states, see

“Sodium.”

■ In patients with hypernatremia, the total free water deficit can be cal-

culated with the equation

Water deficit (L) =0.6×wt (kg) ×(Na+/140−1)