Acid-Base Disorders Board Review Questions, Free CME Quick Quiz

Mixed metabolic acidosis/respiratory alkalosis; salicylate ingestion

Educational Objective:

Discuss the acid-base findings of aspirin overdose.

Key Points:

This patient has a classic presentation of salicylate intoxication.

Explanation:

The patient has a mixed disorder. This can be determined because the patient is tachypneic, which would be expected to raise the patient's blood pH; in fact, the patient's blood pH is mildly elevated. However, it is not as elevated as would be expected. A decrease in pCO2 level of 15 (normal pCO2 = 40) would be expected to result in an increase in blood pH level by 0.15 (change in CO2 by 10 points results in a change in pH by approximately 0.1 points). Thus, the expected blood pH value from pure respiratory alkalosis is 7.40 + 0.15 = 7.55.

Because there is not this expected change--and, in fact, the blood is more acidic than expected--secondary metabolic acidosis is present. This could represent a response to metabolic acidosis with respiratory compensation.

But, using both the Winter formula, pCO2 = 1.5(HCO3) + 8 ± 2, and the rule of thumb that the last 2 digits of the blood pH should match the pCO2 in cases of metabolic acidosis with respiratory compensation, the interpretation of metabolic acidosis with respiratory compensation cannot be correct.

A patient cannot have both respiratory alkalosis and acidosis at the same time; therefore, this additional acidotic process must be metabolic acidosis, which is contributing to this less-than-expected blood pH value.

This patient has a classic presentation of salicylate intoxication.

The patient is exhibiting symptoms immediately following an argument (a common inciting event for impetuous ingestions), is complaining of a change in hearing (while tinnitus is most commonly described with salicylate toxicity, other changes in hearing such as rushing sensation or decreased acuity are also described), has an elevation in her respiratory rate (it is important to double-check the respiratory rate because the first reading may be incorrect), and a minimally elevated temperature.

In addition, the patient has a mixed acid-base disturbance on blood gas. Thus, both metabolic acidosis and respiratory alkalosis are present. Similarly, the patient has an elevated anion gap. This, combined with the mixed acid-base disturbance, is nearly pathognomonic for salicylate toxicity.

The patient's normal mental status argues against methanol intoxication because, by the time an acid-base disturbance results following methanol ingestion, the methanol would be expected to produce a decrease in level of her consciousness or awareness.

The patient's blood sugar level is normal, thus arguing against diabetic ketoacidosis as the cause of metabolic acidosis. In addition, diabetic ketoacidosis would not be expected to cause secondary respiratory alkalosis.

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