Page 248 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Introduction to Acid-Base Disorders 239

acidemic because blood pH is 7.40; however, based on MEASUREMENT OF BLOOD

the PCO 2 and [HCO 3 ], the patient is not normal. This GASES
patient has a mixed disorder characterized by metabolic
alkalosis and respiratory acidosis. The two disorders have Most blood gas analyzers measure pH and PCO 2 . The
counterbalancing effects, resulting in a normal pH. HCO 3 concentration is calculated. Total CO 2 content

Mixed acid-base disorders are considered in detail in is determined by adding a strong acid to plasma or serum
Chapter 12. and measuring the amount of CO 2 produced according
to the following reaction:
COMPENSATORY RESPONSES H þ HCO 3 ⇄ H 2 CO 3 ⇄ CO 2 þ H 2 O
þ
FOR PRIMARY ACID-BASE
DISTURBANCES The term total CO 2 content refers to the fact that this

method includes both dissolved CO 2 and HCO 3 pres-
ent in the sample. As a result, total CO 2 content is greater
The guidelines for secondary or adaptive responses are
concentration in normal individuals by
listed in Table 9-5 for reference. Note that there are single than HCO 3
approximately 1 to 2 mEq/L:
rules of thumb for each of the metabolic acid-base
disorders but two rules of thumb (one each for acute
CO 2diss þ HCO ¼ 0:0301 PCO 2 þ HCO
and chronic disorders) for the respiratory acid-base 3 3
disorders. This is a consequence of the fact that the adap- ¼ 0:0301ð40Þþ 24
tive respiratory response to metabolic disorders begins ¼ 25:2mEq=L
immediately and is complete within hours. Conversely,
the response to respiratory disorders occurs in two If a sample to be analyzed for total CO 2 content is han-
phases. In the first phase, there is immediate titration of dled aerobically, the dissolved CO 2 is released to the
predominantly intracellular nonbicarbonate buffers, atmosphere, and the value obtained is approximately

resulting in an initial change in plasma HCO 3 concen- equal to the HCO 3 concentration.
tration. The second phase is carried out by the kidneys Total CO 2 concentrations determined by automated
and is characterized by alterations in net acid excretion chemistry analysis may differ substantially from those
and bicarbonate reabsorption. This response begins obtained by standard blood gas analysis. In one study
within hours but takes 2 to 5 days to achieve maximal of normal dogs and cats, factors implicated in this discrep-
effectiveness. Thus, there are two expected compensatory ancy included underfilling of blood collection tubes,
responses: acute (<24 hours) and chronic (>48 hours). delays between sampling and analysis, and freshness of
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One caution about rules of thumb is that they define laboratory reagents. According to the results of this
the average response and not 95% confidence intervals. study, values for total CO 2 obtained by routine blood
Acid-base maps depict 95% confidence intervals and, gas analysis may be up to 5 mmol/L higher than those
although more awkward to use, allow the clinician to con- obtained by automated analysis. Another study compar-
sider normal variation in response (Fig. 9-4). Thus, a ing total CO 2 measurement by three different methods
patient should be considered to have a mixed disorder (radiometer blood gas analyzer, Coulter DACOS ana-
only when the blood gas value in question deviates con- lyzer [Beckman Coulter, Fullerton, Calif.], and Kodak
siderably from the calculated expected value. Guidelines Ektachem DTE analyzer [Eastman Kodak, Rochester,
for establishing a diagnosis of mixed acid-base disorder N.Y.]) found lower than expected agreement among
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are discussed in Chapter 12. the different methods of analysis. In this study, sample

TABLE 9-5 Expected Renal and Respiratory Compensations to Primary Acid-Base
Disorders in Dogs
Disorder Primary Change Compensatory Response

Metabolic acidosis # [HCO 3 ] 1.0-mm Hg decrement in PCO 2 for each 1-mEq/L decrement in [HCO 3 ]

Metabolic alkalosis " [HCO 3 ] 0.7-mm Hg increment in PCO 2 for each 1-mEq/L increment in [HCO 3 ]

Acute respiratory acidosis " PCO 2 1.5-mEq/L increment in [HCO 3 ] for each 10-mm Hg increment in PCO 2
Chronic respiratory acidosis " PCO 2 3.5-mEq/L increment in [HCO 3 ] for each 10-mm Hg increment in PCO 2

Acute respiratory alkalosis # PCO 2 2.5-mEq/L decrement in [HCO 3 ] for each 10-mm Hg decrement in PCO 2
Chronic respiratory alkalosis # PCO 2 5.5-mEq/L decrement in [HCO 3 ] for each 10-mm Hg decrement in PCO 2