A 42-year-old man is found unconscious in a subway station. He is brought to the emergency department where cardiopulmonary resuscitation is performed and intravenous glucose and thiamine are administered. His laboratory profile shows: Which of the following is the best next step in the diagnosis of this patient's acid-base status?

Question

A 42-year-old man is found unconscious in a subway station.  He is brought to the emergency department where cardiopulmonary resuscitation is performed and intravenous glucose and thiamine are administered.  His laboratory profile shows:  Which of the following is the best next step in the diagnosis of this patient's acid-base status?

Quizplus · Accepted Answer

Explanation: This patient has metabolic acidosis (pH <7.35, primary decrease in HCO₃^-). To correctly identify his acid-base status, the best next step is calculation of the anion gap (AG). The AG represents the unmeasured anions in the body. It can be calculated with the following formula: ₃^- AG = Na − (HCO₃^- + Cl) ₃^- The normal AG is between 10 and 14 mEq/L. An increase in the AG indicates the presence of nonchloride acids that contain inorganic, organic, exogenous or unidentified anions. Finding an increased anion gap clinically narrows the differential diagnosis of metabolic acidosis to a few common causes. Some of the most common causes of anion gap metabolic acidosis are: Lactic acidosis: Hypoxia, poor tissue perfusion, mitochondrial dysfunction Ketoacidosis: Type I diabetes, starvation, or alcohol Methanol ingestion: Formic acid accumulation Ethylene glycol ingestion: Glycolic and oxalic acid accumulation Salicylate poisoning: Salicylate accumulation (also causes concomitant respiratory alkalosis) Uremia (eg, end-stage renal disease): Failure to excrete H⁺ as NH₄⁺ (Choice B) The osmolal gap should be calculated in selected cases of increased anion gap acidosis, such as when ethanol, methanol, or ethylene glycol toxicity is suspected. It is calculated by determining the difference between the measured and calculated serum osmolality. (Choice C) The urine anion gap (urine Na + urine K − urine Cl) is calculated when there is a normal anion gap metabolic acidosis. The urine anion gap helps determine if such acidosis is due to the renal or intestinal bicarbonate losses. Renal losses of bicarbonate occur in distal renal tubular acidosis (positive urine anion gap) or carbonic anhydrase inhibitor use. Gastrointestinal loss of bicarbonate occurs classically in diarrhea (negative urine anion gap). (Choice D) Calculation of the urine osmolar gap will not help determine the acid-base status of this patient. (Choice E) A venous blood gas should differ little from an arterial blood gas except for the decrease in O₂ levels seen in venous blood compared to arterial blood. Venous blood that contains a similar oxygen content as arterial blood may indicate a disorder of respiratory metabolism such as cyanide poisoning. ₂ Educational objective: The anion gap must always be calculated in patients with metabolic acidosis in order to narrow the differential diagnosis.

A 42-Year-Old Man Is Found Unconscious in a Subway Station