Patient Encounter 1 Body Fluid Compartments

Calculate the total body water, ICF, and extracellular fluid in a 70-kg male.

To maintain fluid balance, the total amount of fluid gained throughout the day (input, or "ins") must equal the total amount of fluid lost (output, or "outs"). Although most forms of the body's input and output can be measured, several cannot. For a normal adult on an average diet, ingested fluids are easily measured and average 1,400 mL/day. Other fluid inputs, such as those from ingested foods and the water by-product of oxidation, are not directly measurable. Fluid outputs such as urinary and stool losses are also easily measured and are referred to as sensible losses. Other sources of fluid loss, such as evaporation of fluid through the skin and/or lungs, are not readily measured and are called insensible losses. Table 27-1 shows the estimated ins and outs (I&Os) for a healthy 68-kg (150-lb) man.6 The measurable I&Os are routinely measured in hospitalized patients and are used to estimate total fluid balance for each 24-hour period. It is important to realize that in hospitalized patients, multiple other forms of fluid loss must be considered. These include losses from enteric suctioning (most commonly, nasogastric [NG] tubes), from surgical drains (e.g., chest tubes, nephrostomy tubes, and pancreatic drains), via fistulous tracts, and enhanced evaporative losses (burns and fever).

TBW depletion (often referred to as "dehydration") is typically a gradual, chronic problem. Because TBW depletion represents a loss of hypotonic fluid (proportionally more water is lost than sodium) from all body compartments, a primary disturbance of osmolality is usually seen. The signs and symptoms of TBW depletion include CNS disturbances (mental status changes, seizures, and coma), excessive thirst, dry mucous membranes, decreased skin turgor, elevated serum sodium, increased plasma osmolality, concentrated urine, and acute weight loss. Common causes of TBW de pletion include insufficient oral intake, excessive insensible losses, diabetes insipidus, excessive osmotic diuresis, and impaired renal concentrating mechanisms. Long-term care residents are frequently admitted to the acute care hospital with TBW depletion secondary to lack of adequate oral intake, often with concurrent excessive insensible losses.

Table 27-1 Approximate I&Os for a Healthy 68-kg (150-lb) Man

input

mL/ctay

Output

mL/day

Ingested fluid0

1r400

Urine"

1,500

Fluid in food

350

Sktn losses

500

Water of oxidation

350

Respiratory Tract

400

losses

Stool

200

Total

?r6ÜÜ

Total

2;60Q

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