Page 311 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

CHAPTER • 12

Mixed Acid-Base Disorders

Helio Autran de Morais and Andrew L. Leisewitz

“A patient will have as many diseases as he/she pleases.”
Anonymous

A mixed acid-base disturbance is characterized by acidosis and respiratory acidosis), and allows detection of
the presence of two or more separate primary acid-base complications associated with therapy (e.g., a patient with
abnormalities occurring in the same patient. An acid-base chronic respiratory acidosis that develops metabolic alka-
disturbance is said to be simple if it is limited to losis after treatment with diuretics experiences further
the primary disturbance and the expected compensatory compromise of ventilation by the metabolic process).
response. Box 12-1 shows a classification of mixed Patients with long-standing conditions may have a
acid-base disorders. simple acid-base disorders. Those patients are at
Recognition of a mixed acid-base disorder is important higher risk of developing a mixed acid-base disturbance
from a diagnostic and a therapeutic point of view. when the disease progresses, when they develop
It permits early detection of complications (e.g., the pres- complications, or when they are treated with drugs that
ence of metabolic acidosis and respiratory alkalosis in a interfere with acid-base status (e.g., furosemide).
dog with parvovirus gastroenteritis may indicate sepsis), Box 12-2 shows examples of potential causes of such
provides orientation for treatment (e.g., NaHCO 3 is preexisting conditions.
contraindicated in the majority of patients with metabolic In approaching mixed acid-base disturbances, a proper
understanding of the terms acidosis, alkalosis,
acidemia, and alkalemia is crucial. Acidosis and alkalo-
BOX 12-1 Classification of Mixed sis refer to the pathophysiologic processes that cause net
Acid-Base Disorders 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
Disorders with Neutralizing Effects on pH pH is less than normal and the [H ] is higher than nor-
þ
Mixed Respiratory-Metabolic Disorders mal. In alkalemia, the extracellular fluid pH is higher than
Respiratory acidosis and metabolic alkalosis normal and the [H ] is lower than normal. For example,
þ
Respiratory alkalosis and metabolic acidosis a patient with chronic respiratory alkalosis may have a
Mixed Metabolic Disorders blood pH value that is within the normal range. Such a
Metabolic acidosis and metabolic alkalosis patient has alkalosis but does not have alkalemia.
Disorders with Additive Effects on pH
Mixed Respiratory-Metabolic Disorders COMPENSATION
Respiratory acidosis and metabolic acidosis
Respiratory alkalosis and metabolic alkalosis The definition of a simple acid-base disturbance includes
both the primary process causing changes in PCO 2 or
Mixed Metabolic Disorders [HCO 3 ] and the compensatory mechanisms affecting

Normal plus high-anion gap metabolic acidosis these measurements. A primary increase or decrease in
Mixed high-anion gap metabolic acidosis
one component (e.g., PCO 2 or [HCO 3 ]) is associated
Mixed normal-anion gap metabolic acidosis
with a predictable compensatory change in the same
Triple Disorders direction in the other component (Table 12-1). Lack of
Metabolic acidosis, metabolic alkalosis, and respiratory appropriate compensation is evidence of a mixed acid-
Acidosis base disorder. Unfortunately, the magnitude of expected
compensation in a given clinical situation is not known

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