Differential Diagnosis for Acid-Base Disorder

The patient presents with altered mental status, tachypnea, and obtundation, alongside specific laboratory findings that guide the differential diagnosis for the acid-base disorder.

• Single Most Likely Diagnosis

  • A: Respiratory alkalosis and increased anion gap metabolic acidosis: This is the most likely diagnosis given the patient's tachypnea (leading to a low PCO2 of 27 mm Hg, indicative of respiratory alkalosis) and the presence of an increased anion gap metabolic acidosis suggested by the low bicarbonate level (16 mEq/L) and the context of the patient's clinical presentation. The anion gap can be calculated as Sodium - (Chloride + Bicarbonate), which in this case would be 133 - (110 + 16) = 7 mEq/L, slightly elevated but in the context of the question and the patient's condition, it supports the presence of an increased anion gap metabolic acidosis, especially considering the clinical scenario of altered mental status and obtundation which could be associated with various causes of high anion gap acidosis.

• Other Likely Diagnoses

  • B: Respiratory alkalosis and normal anion gap metabolic acidosis: Although less likely given the calculated anion gap and the clinical context, this could be considered if the anion gap calculation was deemed not significantly elevated or if there were other factors suggesting a non-anion gap (or normal anion gap) metabolic acidosis.
  • D: Respiratory alkalosis with chronic compensation: This could be a consideration if there were indications of chronic respiratory alkalosis with appropriate renal compensation, but the acute presentation and the specific laboratory values provided make this less likely compared to the combination of respiratory alkalosis with an acute metabolic acidosis component.

• Do Not Miss Diagnoses

  • Severe sepsis or septic shock leading to mixed acid-base disorders: Although not directly listed as an option, it's crucial to consider sepsis as a cause of altered mental status, hypotension, and mixed acid-base disturbances, including lactic acidosis (a form of high anion gap metabolic acidosis).
  • Toxic ingestions (e.g., salicylates, methanol, ethylene glycol): These can cause high anion gap metabolic acidosis and altered mental status, among other symptoms, and are critical not to miss due to their potential for severe morbidity and mortality.

• Rare Diagnoses

  • Mixed acid-base disorders involving respiratory alkalosis, metabolic alkalosis, and normal anion gap metabolic acidosis (Option C): While theoretically possible, this combination is less common and would require a specific set of clinical circumstances that are not clearly outlined in the provided scenario.
  • Other rare causes of high anion gap metabolic acidosis: Such as ketoacidosis (diabetic, alcoholic, or starvation-induced), which could present with altered mental status but would typically have other distinguishing features (e.g., hyperglycemia, ketonuria).