A 22-year-old man with a history of type 1 diabetes mellitus presents to the emergency department with 2 days of severe nausea, recurrent nonbilious vomiting, and diffuse abdominal pain. He reports missing several insulin doses due to his inability to keep food down. He appears lethargic and has severely dry mucous membranes. Vital signs show a temperature of 37.1°C (98.8°F), blood pressure of 100/60 mm Hg, heart rate of 115/min, and respiratory rate of 16/min.
Laboratory results are as follows:
| Parameter | Result | Reference Range |
|---|---|---|
| Sodium | 140 mEq/L | 136-145 mEq/L |
| Potassium | 3.4 mEq/L | 3.5-5.0 mEq/L |
| Chloride | 90 mEq/L | 95-105 mEq/L |
| Bicarbonate | 24 mEq/L | 22-28 mEq/L |
| Blood urea nitrogen | 36 mg/dL | 7-18 mg/dL |
| Creatinine | 1.5 mg/dL | 0.6-1.2 mg/dL |
| Glucose | 550 mg/dL | 70-110 mg/dL |
Arterial blood gas (room air):
| Parameter | Result | Reference Range |
|---|---|---|
| pH | 7.40 | 7.35-7.45 |
| pCO2 | 40 mm Hg | 35-45 mm Hg |
| pO2 | 95 mm Hg | 80-100 mm Hg |
Based on these laboratory findings, which of the following best describes this patient's acid-base status?
- Mixed high anion gap metabolic acidosis and metabolic alkalosisAnswer
- BMixed high anion gap metabolic acidosis and respiratory alkalosis
- CSimple high anion gap metabolic acidosis with appropriate respiratory compensation
- DSimple metabolic alkalosis with appropriate respiratory compensation
- EMixed high anion gap metabolic acidosis and normal anion gap metabolic acidosis
Answer
Mixed high anion gap metabolic acidosis and metabolic alkalosis
The correct answer is a mixed high anion gap metabolic acidosis and metabolic alkalosis. This patient has a deceptively normal pH (7.40), pCO2 (40 mm Hg), and bicarbonate (24 mEq/L). However, calculating the anion gap (AG) reveals a significant disturbance: . An AG > 12 indicates a high anion gap metabolic acidosis (HAGMA), which is driven by diabetic ketoacidosis given his severe hyperglycemia. To check for additional disorders, the delta gap is evaluated: . The expected initial bicarbonate = measured bicarbonate + . Because this expected bicarbonate is significantly higher than the normal 24 mEq/L, an independent metabolic alkalosis is concurrently present, caused by the patient's severe vomiting (loss of HCl). The net normal pH is a result of these two opposing metabolic processes precisely canceling each other out.
Step-by-Step Solution
Key Concept
When a high anion gap metabolic acidosis is present alongside a normal pH and normal bicarbonate, a delta gap calculation is required to unmask a concurrent metabolic alkalosis. The opposing acid-base disorders cancel each other out, resulting in deceptively normal basic lab values.
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