Examkin

Question

Difficulty: HardNephrology

A 27-year-old woman is brought to the emergency department by her roommate due to severe weakness and four days of intractable vomiting. The patient has type 1 diabetes mellitus but has not administered her insulin for the past two days because she has been unable to tolerate oral intake. On physical examination, she appears lethargic. Her temperature is 37.1°C (98.8°F), blood pressure is 92/58 mm Hg, pulse is 122/min, and respirations are 16/min. Her mucous membranes are notably dry and skin turgor is decreased. Laboratory studies show the following:

Laboratory TestResult
Sodium142 mEq/L142 \text{ mEq/L}
Potassium3.8 mEq/L3.8 \text{ mEq/L}
Chloride94 mEq/L94 \text{ mEq/L}
Bicarbonate20 mEq/L20 \text{ mEq/L}
Blood urea nitrogen35 mg/dL35 \text{ mg/dL}
Creatinine1.4 mg/dL1.4 \text{ mg/dL}
Glucose512 mg/dL512 \text{ mg/dL}

An arterial blood gas on room air reveals:

TestResult
pH7.347.34
PaCO2PaCO_238 mm Hg38 \text{ mm Hg}
PaO2PaO_294 mm Hg94 \text{ mm Hg}

Which of the following best describes this patient's acid-base status?

  1. High anion gap metabolic acidosis and metabolic alkalosisAnswer
  2. B
    High anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    High anion gap metabolic acidosis and respiratory acidosis
  4. D
    High anion gap metabolic acidosis and normal anion gap metabolic acidosis
  5. E
    Normal anion gap metabolic acidosis and metabolic alkalosis

Answer

The patient has a mixed acid-base disorder consisting of a high anion gap metabolic acidosis (due to diabetic ketoacidosis) and a concurrent metabolic alkalosis (due to severe vomiting).
This patient presents with a high anion gap metabolic acidosis (HAGMA) driven by diabetic ketoacidosis, evidenced by an anion gap of 28 (normal ~12). Despite the severe HAGMA, her bicarbonate is only mildly depressed at 20 mEq/L. To fully evaluate a HAGMA, the delta gap must be calculated to look for mixed metabolic disorders. The delta gap here is 16 (28 - 12). If HAGMA were the sole disorder, the bicarbonate should drop by an amount equal to the delta gap, resulting in an expected bicarbonate of 8 mEq/L (24 - 16 = 8). Because the patient's actual measured bicarbonate (20 mEq/L) is much higher than 8 mEq/L, there must be a second disorder independently raising the bicarbonate. In this clinical scenario, prolonged vomiting leads to substantial hydrogen ion and chloride loss, generating a concurrent metabolic alkalosis. Finally, Winter's formula calculation (1.5×20+8=381.5 \times 20 + 8 = 38) perfectly matches the patient's measured PaCO2, confirming appropriate respiratory compensation and ruling out any superimposed respiratory disorder.

Step-by-Step Solution

1
Calculate the primary anion gap to determine the core metabolic disorder.
AG=142−(94+20)=28AG = 142 - (94 + 20) = 28.
An anion gap greater than 12 indicates the presence of a high anion gap metabolic acidosis (HAGMA), which in this patient is due to diabetic ketoacidosis.
2
Calculate the expected respiratory compensation using Winter's formula.
Expected PaCO2=(1.5×20)+8=38±2 mm HgPaCO_2 = (1.5 \times 20) + 8 = 38 \pm 2 \text{ mm Hg}. The measured PaCO2PaCO_2 is exactly 38 mm Hg38 \text{ mm Hg}.
Because the measured PaCO2PaCO_2 falls precisely within the expected range, there is no concurrent respiratory acidosis or alkalosis.
3
Calculate the delta gap (delta-delta) to identify hidden secondary metabolic disorders.
ΔAG=28−12=16\Delta AG = 28 - 12 = 16. The expected normal HCO3−HCO_3^- if only HAGMA were present is 24−16=8 mEq/L24 - 16 = 8 \text{ mEq/L}. The measured HCO3−HCO_3^- is 20 mEq/L20 \text{ mEq/L}.
Because the measured bicarbonate (20 mEq/L20 \text{ mEq/L}) is significantly higher than the expected compensated bicarbonate (8 mEq/L8 \text{ mEq/L}), a second disorder that independently raises bicarbonate must be present—specifically, a metabolic alkalosis.

Key Concept

In a patient with a high anion gap metabolic acidosis, calculating the delta gap is essential to identify hidden concurrent metabolic disorders. If the measured bicarbonate is significantly higher than the expected bicarbonate (calculated as 24 minus the delta gap), a concurrent metabolic alkalosis is present.
Rate this question

Topics

Nephrology

More questions from this topic