Page 253 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

244 ACID-BASE DISORDERS

12 to 24 mEq/L in dogs* and13to27 mEq/L in tion rarely equals the increment in anion gap for several
cats. 7,11–13 In one study, the anion gap was significantly reasons. For example, buffers other than HCO 3
increased in aged dogs compared with young dogs (16.7 are titrated by hydrogen ions from the organic acid; the
0.7 vs. 14.3 0.8 mEq/L). The increase in anion gap volume of distribution of the organic anion may differ

was attributed to a slight decrease in serum chloride from that of HCO 3 ; and the prevailing concentration
concentration that was balanced by an increase in the of the organic anion in ECF is affected by its urinary

net negative charge associated with plasma proteins and excretion. Furthermore, the patient’s HCO 3 concentra-
4
phosphate. Inrecentstudies,theaniongapwascalculated tion and anion gap before illness are usually not known,

to be 18.8 2.9 mEq/L (range, approximately 13 and the changes in HCO 3 concentration and anion gap
to 25 mEq/L) 16 for dogs and 24.1 3.5 mEq/L (range, must by necessity be calculated from available normal
approximately 17 to 31 mEq/L) for cats. 45 values.
In reality, there is no anion gap because the law of The anion gap may be useful in identifying mixed acid-
electroneutrality must always be satisfied. This can be base disturbances. For example, consider a mixed distur-
indicated by including terms for unmeasured cations bance characterized by metabolic alkalosis and lactic aci-
(UCs) and unmeasured anions (UAs) as follows: dosis (e.g., chronic vomiting severe enough to have
caused hypotension and impaired tissue perfusion). The
þ
þ
Na þ K þ UC ¼ Cl þ HCO þ UA pH in such a setting could be normal if HCl loss from

3
þ þ the stomach was exactly counterbalanced by accumula-
UA UC ¼ðNa þ K Þ ðCl þ HCO Þ
3
tion of lactic acid from anaerobic metabolism. A markedly
Thus, the anion gap is the difference between UAs and increased anion gap suggests the presence of the
UCs and may be affected by changes in the concentration complicating organic acidosis. The usefulness of the
of either component. However, the magnitude of change anion gap in this situation is hampered by the fact that
in the concentration of any of the UCs (e.g., calcium, alkalemia itself can cause an increase in the anion gap
19,35
magnesium) necessary to cause an appreciable change by several mechanisms. Alkalemia results in loss of
in the anion gap would probably be incompatible with protons from plasma proteins and an increase in their
life. 19 As a result, most discussions of the anion gap focus net negative charge. Hemoconcentration related to vol-
on changes in UAs. ume depletion increases the concentration of plasma
Normally, plasma proteins contribute the majority of proteins and the concentration of their net negative
UA charge in mEq/L. 35 In humans, albumin contributes charge. Finally, alkalemia increases lactic acid generation
2.0 to 2.8 mEq/L for each gram per deciliter, and by stimulating phosphofructokinase. The net effect is
globulins contribute 1.3 to 1.9 mEq/L for each gram an increase in the concentration of UAs (lactate and
per deciliter. 19 For each 0.1-U increment in pH, there anionic plasma proteins) and an increase in anion gap.
is an approximate 0.1-mEq/L increase in negative charge The utility of the anion gap concept is considered further
on plasma proteins. 19,35,69,71 In dogs, net plasma protein in Chapter 12.
charge at a pH of 7.40 is 16 mEq/L and anion gap is Acidosis resulting from administration of NH 4 Cl

approximately 19 mEq/L, and at a pH of 7.40, the anion causes a decrease in HCO 3 concentration because
gap changes 0.42 mEq/L for every 1 g/L change in hydrogen ions are released during ureagenesis. There is
albumin and 0.25 mEq/L for every 1 g/L change in total a reciprocal increase in serum chloride concentration,
plasma proteins. 16 and as a result, there is no change in the anion gap
Increases in anion gap are much more common than (so-called hyperchloremic metabolic acidosis). Gastroin-

decreases, and the concept of anion gap is usually used testinal loss of HCO 3 has the same result because the
as an aid in differentiating the causes of metabolic acidosis kidneys conserve NaCl in response to volume depletion.
(see Chapter 10). In organic acidoses (e.g., diabetic The use of the anion gap in the classification of metabolic
ketoacidosis, lactic acidosis), HCO 3 is titrated by H þ acidosis is considered further in Chapter 10.

A decreased anion gap may be observed in
ions from organic acids. Theoretically, the ECF HCO 3
concentration should decrease in reciprocal fashion with immunoglobin G (IgG) multiple myeloma because
the increase in concentration of organic acid anions, and the pI of IgG paraproteins is greater than 7.4.
the serum chloride concentration should not change (so- Hypoalbuminemia or dilution of plasma proteins by crys-
called normochloremic metabolic acidosis). The anion talloid infusion can decrease the anion gap by decreasing
gap in this setting should increase proportionately. In the concentration of the net negative charge associated

practice, however, the decrement in HCO 3 concentra- with plasma proteins. Hypoalbuminemia may be the most
common cause of a decreased anion gap, and each
1.0-g/dL decrease in albumin is associated with an
approximately 2.4- to 3.0-mEq/L decrease in the anion
*References 1, 8, 34, 36, 39, 50, 58. gap. 19,44