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Difficulty: Very hardNephrology

A 28-year-old woman, who is 14 weeks pregnant, is brought to the emergency department after 4 days of intractable nausea, vomiting, and diffuse abdominal pain. She has a history of type 1 diabetes mellitus and has not taken her insulin for the past 48 hours due to her inability to tolerate oral intake. She reports profound weakness and lightheadedness when standing. Her temperature is 37.1°C (98.8°F), blood pressure is 92/58 mm Hg, heart rate is 122/min, and respiratory rate is 26/min. Physical examination shows dry mucous membranes and diminished skin turgor.

Laboratory studies:
Serum Chemistries
Sodium140 mEq/L
Potassium3.2 mEq/L
Chloride86 mEq/L
Bicarbonate18 mEq/L
Blood urea nitrogen42 mg/dL
Creatinine1.6 mg/dL
Glucose480 mg/dL
Arterial Blood Gas (Room Air)
pH7.48
pCO2pCO_225 mm Hg
pO2pO_298 mm Hg

Which of the following best characterizes this patient's acid-base disturbances?

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

Answer

High anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis
This patient has a triple acid-base disorder. First, despite the alkalemic pH (7.487.48), the anion gap is massively elevated at 3636 mEq/L calculated as Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-) or 140−(86+18)140 - (86 + 18). This indicates a severe high anion gap metabolic acidosis (HAGMA), consistent with diabetic ketoacidosis. Second, the delta-delta gap must be assessed to uncover hidden metabolic disorders. The delta anion gap (ΔAG\Delta AG) is 36−12=2436 - 12 = 24. Subtracting this from a normal bicarbonate of 2424 mEq/L yields an expected bicarbonate of 00 mEq/L if HAGMA were the only metabolic disorder present. Because the measured bicarbonate (1818 mEq/L) is significantly higher than this expected value, a massive concurrent metabolic alkalosis is present, consistent with her history of intractable vomiting. Finally, Winter's formula (Expected pCO2=1.5×HCO3−+8±2Expected\ pCO_2 = 1.5 \times HCO_3^- + 8 \pm 2) is used to calculate the appropriate respiratory compensation for the measured bicarbonate. The expected pCO2pCO_2 is 1.5×18+8=35±21.5 \times 18 + 8 = 35 \pm 2 mm Hg. The patient's actual pCO2pCO_2 of 2525 mm Hg is substantially lower than this range, revealing an additional, tertiary primary respiratory alkalosis.

Step-by-Step Solution

1
Calculate the Anion Gap (AG) to screen for a primary metabolic acidosis regardless of the pH.
AG=Na+−(Cl−+HCO3−)=140−(86+18)=36AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (86 + 18) = 36 mEq/L.
An anion gap > 12 mEq/L confirms the presence of a High Anion Gap Metabolic Acidosis (HAGMA), which in this patient is driven by diabetic ketoacidosis.
2
Calculate the delta-delta gap to identify any hidden concurrent metabolic disorders.
ΔAG=36−12=24\Delta AG = 36 - 12 = 24. The Expected HCO3−HCO_3^- (if HAGMA were the only metabolic process) = 24−24=024 - 24 = 0 mEq/L. Since Measured HCO3−HCO_3^- (18 mEq/L) > Expected HCO3−HCO_3^- (0 mEq/L), a concurrent metabolic alkalosis is present.
The patient's severe vomiting has generated a profound metabolic alkalosis, which is independently raising the bicarbonate level above what would be expected from the HAGMA alone.
3
Apply Winter's formula to the measured bicarbonate to evaluate the expected respiratory compensation.
Expected pCO2=1.5×HCO3−+8±2=(1.5×18)+8=35±2pCO_2 = 1.5 \times HCO_3^- + 8 \pm 2 = (1.5 \times 18) + 8 = 35 \pm 2 mm Hg (Range: 33 to 37 mm Hg).
This establishes the appropriate physiological pCO2pCO_2 response to the patient's current bicarbonate level.
4
Compare the patient's actual pCO2pCO_2 to the expected pCO2pCO_2.
The measured pCO2pCO_2 (25 mm Hg) is significantly lower than the expected range (33 to 37 mm Hg).
Because the actual pCO2pCO_2 is lower than expected, a concurrent primary respiratory alkalosis is present, likely multifactorial due to pregnancy (progesterone effect), abdominal pain, and dehydration.

Key Concept

Diagnosis of triple acid-base disorders requires sequential calculation of the anion gap, delta-delta gap, and Winter's formula to uncover hidden metabolic and respiratory derangements that may be masked by a deceptively normal or alkaline pH.
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Nephrology

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