Page 247 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

238 ACID-BASE DISORDERS

refer specifically to the pH of ECF. In acidemia the ECF Each primary metabolic or respiratory acid-base dis-
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pH is lower than normal, and the [H ] is higher than turbance is accompanied by a secondary,or adaptive,
normal. In alkalemia the ECF pH is higher than normal, change in the opposing component of the system
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and the [H ] is lower than normal. The distinction (Table 9-4). The adaptive response involves the compo-
between these terms is important. For example, a patient nent opposite the one disturbed and returns the pH of
with chronic respiratory alkalosis may have a blood pH the system toward but not completely to normal. Over-
within the normal range because of effective renal com- compensation does not occur. For example, metabolic
pensation in this setting. Such a patient has alkalosis acidosis is accompanied by a secondary or adaptive respi-
but does not have alkalemia. Patients with mixed acid- ratory alkalosis. Respiratory acidosis is accompanied by a
base disturbances can have blood pH values within the secondary or adaptive metabolic alkalosis.
normal ranges as a result of the presence of two
counterbalancing acid-base disturbances (see the follow- SIMPLE AND MIXED
ing section on Simple and Mixed Acid-Base Disorders). ACID-BASE DISORDERS

PRIMARY ACID-BASE An acid-base disorder is said to be simple if it is limited to
DISTURBANCES the primary disorder and the expected secondary, or
adaptive, response. The magnitude of the expected
Acidosis and alkalosis can each be of metabolic or respira- responses is considered in detail in the chapters devoted
tory origin, and as a result, there are four primary to the primary acid-base disorders (see Chapters 10
acid-base disturbances: metabolic acidosis, respiratory and 11). A mixed acid-base disorder is one that is
acidosis, metabolic alkalosis, and respiratory alkalosis. characterized by the presence of at least two separate pri-
The metabolic disturbances refer to a net excess or deficit mary acid-base abnormalities occurring in the same
of nonvolatile, or fixed, acid, whereas the respiratory patient. A mixed acid-base disorder should be suspected
disturbances refer to the net excess or deficit of volatile whenever the secondary, or adaptive, response exceeds or
acid (dissolved CO 2 ). falls short of that expected. In dogs, for example, the
Metabolic acidosis is characterized by a decreased expected response to metabolic acidosis is a 0.7-mm

plasma HCO 3 concentration and decreased pH Hg decrease in PCO 2 for each 1.0-mEq/L decrement in
þ
(increased [H ]) caused by either HCO 3 loss or buffer- plasma HCO 3 concentration caused by metabolic acido-

ing of a noncarbonic (nonvolatile or fixed) acid. Meta- sis (see Chapter 10 for more details).
bolic alkalosis is characterized by an increased plasma Consider a dog with these normal blood gas values:
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HCO 3 concentration and increased pH (decreased p. 7.39, [H ] ¼ 41 nEq/L, [HCO 3 ] ¼ 21 mEq/L,
[H ]), usually caused by a disproportionate loss of chlo- and PCO 2 ¼ 36 mm Hg. This dog becomes ill and is
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ride ions from the body (i.e., loss of fluid with a chloride observed to have the following blood gas values:

concentration greater than that of ECF) or p. 7.22, [H ] ¼ 60 nEq/L, [HCO 3 ] ¼ 14 mEq/L,
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hypoalbuminemia (because albumin is a weak acid). In and PCO 2 ¼ 35 mm Hg. If the dog had a simple meta-
the absence of volume depletion or renal dysfunction, it bolic acidosis, using the rule of thumb described before,
is extremely difficult to produce metabolic alkalosis by we would have expected the following results: p. 7.27,
þ

administration of alkali. Respiratory acidosis is [H ] ¼ 53 nEq/L, [HCO 3 ] ¼ 14 mEq/L, and
characterized by increased PCO 2 (hypercapnia) caused PCO 2 ¼ 31 mm Hg. Thus, the dog has a mixed acid-base
by alveolar hypoventilation. Respiratory alkalosis is disorder characterized by both metabolic and respiratory
characterized by decreased PCO 2 caused by alveolar hyper- acidoses.
ventilation (hypocapnia). In one study, metabolic acidosis Consider a patient with the following blood gas values:
þ

was the most common acid-base disturbance encoun- p. 7.40, [H ] ¼ 40 nEq/L, [HCO 3 ] ¼ 31 mEq/L, and
17
tered in dogs. PCO 2 ¼ 51 mm Hg. This patient is neither alkalemic nor
TABLE 9-4 Characteristics of Primary Acid-Base Disturbances
Disorder pH [H ] Primary Disturbance Compensatory Response
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Metabolic acidosis # " # [HCO 3 ] # PCO 2
Metabolic alkalosis " # " [HCO 3 ] "PCO 2

Respiratory acidosis # " " PCO 2 " [HCO 3 ]

Respiratory alkalosis " # # PCO 2 # [HCO 3 ]
From Rose BD: Clinical physiology of acid-base and electrolyte disorders, ed 3, New York, 1989, McGraw-Hill, p. 470, with permission of the McGraw-Hill
Companies.