A proper understanding of the terms acidosis, alkalosis, acidemia, and alkalemia is necessary to differentiate simple from
mixed acid base disorders. Acidosis and alkalosis refer to the pathophysiologic processes that cause net accumulation of acid
or alkali in the body, whereas acidemia and alkalemia refer specifically to the pH of extracellular fluid. In acidemia, the
extracellular fluid pH is less than normal and the [H+ ] is higher than normal. In alkalemia, the extracellular fluid pH is higher than normal and the [H+ ] is lower than normal. Due to the effectiveness of compensatory mechanisms, animals can have acidosis or alkalosis but not
acidemia or alkalemia. For example, a dog with chronic respiratory alkalosis may have a blood pH that is within the normal
range. Such a patient has alkalosis, but does not have alkalemia. The primary acid base disorders are divided into metabolic
and respiratory disturbances: metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis. The
Henderson-Hasselbach equation in its clinically relevant form emphasizes the relationship between the metabolic and respiratory
systems in determining extracellular fluid pH:
pH = pKa + log([HCO3
-]/.03pCO2)
pH = 6.1 + log([HCO3
-]/.03pCO2)
Traditionally, the kidneys have been considered responsible for regulation of the metabolic component (blood bicarbonate concentration,
[HCO3-]) and the lungs for regulation of the respiratory component (partial pressure of CO2, [pCO2]). In this form, the Henderson-Hasselbach equation makes it clear that the pH of extracellular fluid is determined by the
ratio of the bicarbonate concentration and pCO2.
Table 1: Compensatory responses for acid base disturbances in dogs
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Each primary (metabolic or respiratory) acid base disturbance is accompanied by a secondary (opposing) response in the other
system (respiratory or metabolic). Blood pH is returned nearly, but not completely, to normal, and overcompensation does not
occur. Understanding these principles is central to recognition of mixed acid base disturbances. A simple acid base disorder
is defined as a primary disturbance and the expected adaptive (or secondary) response in the opposite system. A mixed acid
base disturbance results from two (or rarely three) separate primary disturbances present simultaneously in the same individual.
For example, the independent occurrence of metabolic acidosis, metabolic alkalosis, and a primary respiratory disturbance
is referred to as a "triple disorder." Four disorders cannot coexist in one individual because hyperventilation (respiratory
alkalosis) and hypoventilation (respiratory acidosis) cannot occur at the same time in one patient. The key to interpreting
mixed acid base disturbances is to know the expected adaptive (compensatory) responses. Unfortunately, the magnitude of expected
compensation in a given clinical situation is cannot be known with certainty, and data in dogs have been derived mainly from
experiments using normal animals. These compensatory rules should be applied to cats with extreme caution because values have
been derived from a limited number of normal cats with experimentally induced acid-base disorders. The expected compensatory
responses for dogs are shown in Table 1.
