Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 17 Fluid, Electrolyte, and Acid-Base Balance
and decreased p CO 2 may be associated with compensated
respiratory alkalosis as well as compensated metabolic
acidosis. Compensating responses for chronic respiratory
alkalosis lasting several weeks may actually be sufficient to
return pH to normal. In dogs, anticipated renal compensation
for a chronic respiratory alkalosis results in a decrease
of bicarbonate of 0.55 mEq/l for each mmHg decrease in
p CO 2 ( de Morais, 1992a, 1992b ).
E . Mixed Acid-Base Imbalances
Mixed acid-base disorders occur when several primary acidbase
imbalances coexist ( de Morais, 1992a ). Metabolic acidosis
and alkalosis can coexist and either or sometimes both of
these metabolic abnormalities may occur with either respiratory
acidosis or alkalosis ( Nairns and Emmett, 1980 ; Wilson
and Green, 1985 ). Evaluation of mixed acid-base abnormalities
requires an understanding of the anion gap, the relationship
between the change in serum sodium and chloride
concentration, and the limits of compensation for the primary
acid-base imbalances ( Saxton and Seldin, 1986 ; Wilson and
Green, 1985 ). Clinical findings and history are also necessary
to define the factors that may contribute to the development of
mixed acid-base disorders. The following are important considerations
in evaluating possible mixed acid-base disorders:
1. Compensating responses to primary acid-base
disturbances do not result in overcompensation.
2. With the possible exception of chronic respiratory
acidosis, compensating responses for primary acid-base
disturbances rarely correct pH to normal. In patients
with acid-base imbalances, a normal pH indicates a
mixed acid-base disturbance.
3. A change in pH in the opposite direction to that
predicted for a known primary disorder indicates a
mixed disturbance.
4. With primary acid-base disturbances, bicarbonate and
p CO 2 always deviate in the same direction. If these
parameters deviate in opposite directions, a mixed
abnormality exists.
Although mixed acid-base abnormalities undoubtedly
occur in animals and have been documented in the veterinary
literature, they are often overlooked ( Wilson and Green,
1985 ). An appreciation of the potential for the development
of mixed abnormalities is essential for the correct interpretation
of clinical and clinicopathological data, which would
otherwise be quite confusing. Care should be taken when
evaluating suspected mixed acid-base abnormalities that
sufficient time has elapsed so that anticipated compensating
responses could have occurred ( de Morais, 1992a ).
F . Anion Gap
The anion gap can be calculated as the difference between the
major cation (sodium) and the measured anions (chloride
TABLE 17-3 Causes of Alterations in Anion Gap
Decreased anion gap:
Increased cationic protein
Polyclonal gammopathy (IgG)
Hypoalbuminemia
Hyperchloremic acidosis
Altered protein anionic equivalents
Laboratory error
Increased anion gap:
Metabolic acidosis
Organic acids (lactic, keto acids)
Hypovolemic shock
Anaerobic exercise
Diabetes
Grain overload
Ketosis
Nonmetabolizable acids
Inorganic acids (sulfate, phosphate)
Uremic acidosis
Intoxication or poisoning
Salicylate
Paraldehyde
Metaldehyde
Methanol
Ethylene glycol
Laboratory error
bicarbonate) ( Emmett and Narins, 1977 ). Some investigators
prefer to use the following formula:
anion gap (sodium potassium)
(chloride
bicarbonate) (17-11)
The addition of potassium to the equation, however, adds
little to the diagnostic utility of this calculation ( Emmett
and Narins, 1977 ; Epstein, 1984 ; Oh and Carroll, 1977 );
the anion gap calculated with the inclusion of potassium
concentration will be about 4 mEq/l higher. Because most
of the published data on the anion gap in animal species
are the result of calculations using the second equation (Eq.
17-11), this form will be used in Table 17-3 . Provided the
component determinations are valid, the calculated anion
gap provides an approximation of the so-called “ unmeasured
anions. ” Normally, these unmeasured anions consist
primarily of negatively charged plasma proteins because
the charges of the unmeasured cations (potassium, calcium,
and magnesium) tend to balance out the charges of the
unmeasured anions (phosphate, sulfate, and organic ions).
The anion gap is most useful in situations where the concentrations
of phosphate and plasma proteins, particularly
albumin, are within the normal range (see Section V.I). The
anion gap for most species of domestic animals appears to
be similar to that defined for human subjects (i.e., approximately
10 to 20 mEq/l) (10 to 20 mmol/l). However, there