1 What is a metabolic acidosis?

A metabolic acidosis is an acid-base disorder that results from either the loss of bicarbonate-rich fluids from the body or the accumulation of fixed acids. Accumulation of acids can be the result of increased metabolic production of acids (e.g., ketoacidosis) or decreased excretion of acids (e.g., renal failure).

2 How is metabolic acidosis identified on a blood gas profile?

As discussed in Chapter 24, metabolic acidosis is usually recognized when a patient has a decreased blood pH (acidosis) along with a concurrent decreased plasma bicarbonate concentration ([HCO₃⁻], metabolic component). Metabolic acidosis may also be identified as part of a mixed acid-base abnormality in a patient with respiratory alkalosis, if the patient’s [HCO₃⁻] is lower than would be expected as part of a compensatory response to the alkalosis.

3 How common is metabolic acidosis?

Metabolic acidosis is probably the most common acid-base disorder seen in companion animals. Many common clinical conditions may produce this abnormality.

4 What are two general mechanisms by which metabolic acidosis may develop?

Metabolic acidosis develops by one of two main mechanisms:

5 Are there clinical signs that would suggest a patient has a metabolic acidosis?

Generally, the clinical signs present would reflect the underlying disease process rather than the metabolic acidosis itself. Humans have a deep, rhythmic respiratory pattern called Kussmaul’s respiration, but this has not been described in veterinary species.

6 In the absence of a blood gas analysis, what findings on a routine chemistry profile might suggest the presence of a metabolic acidosis?

The most specific indicator of a metabolic acidosis on a routine chemistry profile would be a reduced total carbon dioxide (TCO₂). TCO₂ is a measurement of all forms of CO₂ in the blood. Dissolved CO₂ (which is the PCO₂ from a blood gas profile) is a minor portion of the total CO₂ content of the blood. The majority (∼95%) of the CO₂ in the blood is in the form of bicarbonate, so TCO₂ is a reflection of [HCO₃⁻].

Hyperchloremia with a normal serum sodium concentration would also suggest the presence of metabolic acidosis. Normally, changes in chloride concentration parallel changes in sodium concentration. However, when an animal has a metabolic acidosis caused by loss of bicarbonate (a negatively charge ion) from the body, an independent rise in chloride concentration will occur to maintain electroneutrality.

7 Define cation and anion.

A cation is a positively charged electrolyte, such as sodium, potassium, calcium, or magnesium. An anion is a negatively charged electrolyte, such as chloride, bicarbonate, or phosphate.

8 What is meant by electroneutrality?

In any body fluid, there must be an equal number of cations and anions. To maintain electroneutrality, if a cation is removed from a particular fluid component, either an anion must be removed along with it or a different cation must be added back to replace it. This is an important concept in understanding renal handling of ions and understanding electrolyte imbalances.

9 What is the anion gap, and what is the formula used for its calculation?

The anion gap is the difference between the amount of the commonly measured cations (e.g., sodium, potassium) and the commonly measured anions (e.g., chloride, bicarbonate) in the blood. Although many of these electrolytes are measured, the most common formula used is:

This calculation yields a positive number, usually between 15 and 25 in dogs and cats, indicating that there are fewer measured anions in the blood than there are measured cations.

10 What does the anion gap represent?

According to the concept of electroneutrality, there are equal numbers of total anions and total cations in the blood. The fact that there are less measured anions in the blood than measured cations indicates that there are more unmeasured anions in the blood than unmeasured cations (and the amount of this excess is identical to the amount of the excess measured cations). Therefore the magnitude of the difference between the measured cations and anions in the blood provides an indication of the relative amounts of unmeasured anions and cations in the blood.

If the measured anion gap increases to greater than normal, it means that the difference between unmeasured anions and unmeasured cations has also increased. This indicates either an increase in unmeasured anions or a decrease in unmeasured cations. In reality, the number of unmeasured cations is small and relatively constant. Therefore, increases in anion gap reflect an increase in unmeasured anions, most often organic acids of some type.

11 What is the clinical use of calculating an anion gap?

In a patient with a metabolic acidosis, calculating the anion gap can differentiate those conditions caused by excess production of organic acids (these have an increased anion gap) from those caused by other mechanisms (these have a normal anion gap).

12 What are some common causes of a metabolic acidosis with increased anion gap?

13 What exogenous toxins cause a high anion gap acidosis?

The most common endogenous toxin in veterinary medicine is ethylene glycol. In humans, salicylate intoxication and methanol poisoning also cause a high anion gap acidosis, but these toxins are not typically seen in companion animals.

14 What is the mechanism of acidosis in ethylene glycol poisoning?

In the liver, ethylene glycol is metabolized by the enzyme alcohol dehydrogenase. The initial metabolite is glycoaldehyde. Subsequent metabolism produces glycolic acid and glyoxylic acid. Glycolic acid is the metabolite responsible for the severe metabolic acidosis.

15 What symptoms associated with ethylene glycol poisoning would suggest this toxicity?

There are three clinical phases of ethylene glycol toxicity, caused by the different metabolites, as follows:

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