C h a p t e r 8
Disorders of Fluid, Electrolyte, and Acid–Base Balance
173
assessment of the degree of fluid deficit and adequate
measures to resolve the deficit and treat the underlying
cause are essential.
Isotonic FluidVolume Excess
Isotonic fluid volume excess, which represents an iso-
tonic expansion of the ECF compartment with increases
in both interstitial and vascular volumes, usually results
from an increase in total body sodium that is accom-
panied by a proportionate increase in body water.
Although it can occur as the result of excessive sodium
intake, it is most commonly caused by a decrease in
sodium and water elimination by the kidney. Among the
causes of decreased sodium and water elimination are
disorders of renal function, heart failure, liver failure,
and corticosteroid hormone excess. A condition called
circulatory overload
results from an increase in blood
volume; it can occur during infusion of intravenous
fluids or transfusion of blood if the amount or rate of
administration is excessive.
Heart failure produces a decrease in the effective
circulating volume and renal blood flow and a com-
pensatory increase in sodium and water retention (see
Chapter 20). Persons with severe congestive heart fail-
ure maintain a precarious balance between sodium and
water intake and output. Even small increases in sodium
intake can precipitate a state of fluid volume excess and
a worsening of heart failure. Liver failure (e.g., cirrho-
sis of the liver) impairs aldosterone metabolism and
decreases effective circulating volume and renal per-
fusion, leading to increased salt and water retention.
FIGURE 8-9.
Decreased tissue turgor in a 61-year-old male with severe volume depletion. Skin was
pinched over anterior leg
(A)
and continued to be tented after 10 minutes
(B)
. (From de Vries Feyers
C. Decreased tissue turgor. Images in medicine. N Engl J Med. Jan 27, 2011;364:e6. Copyright © 2011.
Massachusetts Medical Society.)
TABLE 8-3
Manifestations of Isotonic FluidVolume Deficit and Excess
Fluid Volume Deficit
Fluid Volume Excess
AcuteWeight Loss (% body weight)
AcuteWeight Gain (% body weight)
Mild fluid volume deficit: 2%
Mild fluid volume excess: 2%
Moderate fluid volume deficit: 5%
Moderate fluid volume excess: 5%
Severe fluid volume deficit: >8%
Severe fluid volume excess: >8%
Signs of Compensatory Mechanisms
Increased thirst
Increased ADH: oliguria and high urine specific gravity
Decreased Interstitial FluidVolume
Increased Interstitial FluidVolume
Decreased skin and tissue turgor
Edema
Dry mucous membranes
Sunken and soft eyeballs
Depressed fontanel in infants
DecreasedVascular Volume
IncreasedVascular Volume
Postural hypotension
Full and bounding pulse
Weak, rapid pulse
Venous distention
Decreased vein filling
Pulmonary edema (severe fluid excess)
Hypotension and shock (severe deficit)
Shortness of breath
Crackles
Dyspnea
Cough
ADH, antidiuretic hormone.
