METABOLIC ACIDOSIS   53A

A 43-year-old man with severe depression is brought into the emergency department after being found collapsed at home. His family states that he had been recently despondent, and they had not heard from him for 3 days. On physical examination, he is unresponsive and tachypneic with a respiratory rate of 41 breaths/min despite a normal lung examination. His chest radiograph findings are unremarkable. An arterial blood gas (ABG) analysis shows a pH of 6.93, PaCO₂ of 20 mm Hg, PaO₂ of 100 mm Hg, and a HCO₃^– of 4 mEq/L. Serum electrolyte tests show an anion gap of 35.

What are the salient features of this patient’s problem? How do you think through his problem?

Salient features: Severe depression; tachypnea without evidence of lung disease; acidemia with decreased PaCO₂ and increased anion gap

How to think through: The emergency room team has obtained the most crucial test for a nonresponsive, tachypneic patient: the ABG analysis. ABG interpretation is complex, and the test is typically obtained during inherently stressful clinical circumstances, necessitating a systematic approach. Is the patient hypoxic? (No.) Is he acidemic or alkylemic? (Acidemic.) Is the primary cause metabolic or respiratory? (Metabolic. He is hyperventilating, effectively reducing the PaCO₂ well below the normal value of 40 mm Hg.) Is there an anion gap? (Yes, the anion gap is 35.) The disorder can now be named anion-gap metabolic acidosis with compensatory respiratory alkalosis. What is the differential diagnosis of this disorder? (Diabetic or alcoholic ketoacidosis, uremia, lactic acidosis, ethylene glycol or methanol ingestion, salicylate or paraldehyde intoxication, isoniazid or iron overdose.) The history of depression raises the concern for a suicide attempt by an ingestion. Which of the above should be prioritized, and what should be the next diagnostic steps? (An osmol gap would support methanol or ethylene glycol ingestion. The serum salicylate level should also be checked.) What is the treatment for methanol or ethylene glycol ingestion? (Fomepizole, a competitive inhibitor of alcohol dehydrogenase; hemodialysis.) When ingestion of any substance is suspected, co-ingestion should be considered (e.g., the serum acetaminophen level should always be checked).

METABOLIC ACIDOSIS   53B

What are the essentials of diagnosis and general considerations regarding metabolic acidosis?

Essentials of Diagnosis

Metabolic acidosis can be classified by either an increased or normal anion gap.

Anion gap = Na⁺ – (HCO^3– + Cl^–)

The hallmark of anion gap metabolic acidosis is that the low HCO^3– is associated with either normal or increased serum Cl^– so that the anion gap increases or remains normal, respectively.

General Considerations

Calculation of the anion gap is useful in determining the cause of the metabolic acidosis.

A normochloremic (increased anion gap) metabolic acidosis generally results from addition to the blood of organic acids such as lactate, acetoacetate, β-hydroxybutyrate, and exogenous toxins (e.g., ethylene glycol, methanol, or salicylate).

Most common causes of non–anion gap acidosis are gastrointestinal (GI) HCO₃^– loss and defects in renal acidification (renal tubular acidoses).

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