Page 316 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice
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Mixed Acid-Base Disorders 307
TABLE 12-3 Chloride Contribution in Metabolic Acid-Base Disorders
Test Hyperchloremic Acidosis Normal Hypochloremic Alkalosis
[Cl ]gap < 4 mEq/L 4-4 mEq/L >4 mEq/L
þ
[Cl ]/[Na ] ratio
Dogs <0.72 0.72–0.78 >0.78
Cats <0.74 0.74–0.8 >0.8
[Na ] [Cl ] <32 mEq/L 32-40 mEq/L >40 mEq/L
þ
use the chloride/sodium ratio. 18 Reference values have or
not been adequately established for dogs and cats, but
experience with limited number of cases suggests that AG ¼ð½Na þ½K Þ ð½Cl þ½HCO 3 Þ
þ
þ
values greater than 0.78 in dogs and more than 0.80 in ¼ð½UA ½UC Þ
þ
cats are associated with hyperchloremic metabolic acido-
sis, whereas values less than 0.72 in dogs and less than Thus every time there is an increase in [Cl ]or [UA ],
0.74 in cats are associated with hypochloremic alkalosis. [HCO 3 ] decreases to maintain electroneutrality. The
Whenever sodium concentration is normal, the difference AGestimates all unmeasuredanions, making nodistinction
þ
between the sodium and chloride concentrations ([Na ] between unmeasured strong anions (e.g., lactate,
þ
[Cl ]) can be used. Normally, [Na ] [Cl ] is approx- ketoanions)thatcanchangepHandweakanions(e.g.,neg-
imately 36 mEq/L in dogs and cats. Values greater than atively-charged phosphate ions and proteins) that do not
40 mEq/L are an indication of hypochloremic alkalosis, affectpHor[HCO 3 ].Inacidosisresultingfromadecrease
whereas values less than 32 mEq/L are associated with in SID caused by an increase in [Cl ], [HCO 3 ]decreases
hyperchloremic acidosis. 15 and the difference ([UA ] [UC ]), and consequently
þ
It is always important to remember that the renal adap- the AG remain constant (hyperchloremic or normal AG
tation to respiratory disorders is accomplished by changing acidosis). When the SID decreases because of an increase
SID by varying the amount of chloride or bicarbonate that in an unmeasured strong anion (e.g., lactate), [HCO 3 ]
is reabsorbed with sodium. Thus in chronic respiratory aci- decreases, [Cl ] is unchanged, and the difference ([UA ]
dosis, there is a compensatory hypochloremic alkalosis, [UC ]) increases; thus the AG also increases
þ
whereas in chronic respiratory alkalosis, there is a compen- (normochloremic or high-AG acidosis).
satory hyperchloremic acidosis. In fact, all change in bicar- Except for some relatively uncommon circumstances,
bonate concentration can be explained by the changes in an increase in the AG implies an accumulation of organic
chloride during chronic respiratory acidosis. 3 acids in the body. 40 Unfortunately, the AG is not very
sensitive in detecting increases in unmeasured strong
Increase in Unmeasured Anions
anions, especially in lactic acidosis. In addition, the AG
Unlike chloride, most other strong anions (e.g., in normal dogs and cats is mostly a result of the net nega-
ketoanions, lactate, anions of renal failure) are not rou- tive charge of proteins and thus is heavily influenced by
tinely measured and need to be estimated. Three methods protein concentration, especially albumin. 12,36 In fact,
combining blood gas results with electrolyte and protein hypoalbuminemia probably is the only important cause
data will be considered here: anion gap (AG), BE algo- of a decrease in the AG. At plasma pH of 7.4 in dogs, each
rithm, and strong ion gap (SIG). The AG is further decrease of 1 g/dL in albumin concentration is associated
discussed in Chapters 9 and 10, whereas the BE algorithm with a decrease of 4.1 mEq/L in the AG, whereas each
and the SIG are further discussed in Chapter 13. decrease of 1 g/dL in total protein concentration is
The anion gap is a helpful tool in the differentiation associated with a decrease of 2.5 mEq/L in the AG. 12
between hyperchloremic and high-AG metabolic Similar data are not available for cats.
acidoses. Chemically, there is no AG because the law of Because many critically ill patients with increased
electroneutrality must be maintained. The AG is the dif- unmeasured strong anions also have hypoalbuminemia,
ference between the unmeasured anions (UA ) and the AG may be artificially normal because of the decrease
þ
unmeasured cations (UC ). Following the in [UA ] resulting from hypoalbuminemia. The AG can
electroneutrality law, we obtain: be corrected for changes in protein concentration in dogs
12
by using the following formulas :
þ þ þ
ð½Na þ½Na þ½UC Þ
ð½Cl þ½HCO 3 þ ½UA Þ AG Alb-adjusted ¼ AG þ 4:2 ð3:77 ½albÞ
