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Difficulty: MediumNephrology

A 24-year-old woman is brought to the emergency department due to severe nausea, diffuse abdominal pain, and confusion. She has a history of type 1 diabetes mellitus but ran out of her insulin prescription three days ago. For the past 48 hours, she has had intractable vomiting, rendering her unable to tolerate oral intake. Her temperature is 37.1°C (98.8°F), blood pressure is 98/60 mm Hg, pulse is 118/min, and respirations are 24/min and deep. Physical examination reveals dry mucous membranes, decreased skin turgor, and diffuse abdominal tenderness without rebound or guarding.

Laboratory studies reveal:

Serum chemistryValue
Sodium140 mEq/L
Potassium3.2 mEq/L
Chloride94 mEq/L
Bicarbonate16 mEq/L
Blood urea nitrogen32 mg/dL
Creatinine1.4 mg/dL
Glucose480 mg/dL

Arterial blood gas (room air):

ParameterValue
pH7.33
pCO232 mm Hg
pO292 mm Hg

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

  1. Anion gap metabolic acidosis and concurrent metabolic alkalosisAnswer
  2. B
    Isolated anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    Anion gap metabolic acidosis and concurrent respiratory acidosis
  4. D
    Anion gap metabolic acidosis and concurrent respiratory alkalosis
  5. E
    Non-anion gap metabolic acidosis and concurrent metabolic alkalosis

Answer

Anion gap metabolic acidosis and concurrent metabolic alkalosis
This patient has a mixed acid-base disorder comprising an anion gap metabolic acidosis (AGMA) and a metabolic alkalosis, with appropriate respiratory compensation. First, the pH of 7.33 and low bicarbonate (16 mEq/L) indicate a primary metabolic acidosis. Next, calculating the anion gap (AG=Na+−(Cl−+HCO3−)\text{AG} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-)) yields 140−(94+16)=30140 - (94 + 16) = 30 mEq/L, indicating a severe AGMA, consistent with diabetic ketoacidosis. To determine if a secondary metabolic disorder is present, the delta gap must be analyzed. The anion gap increased by 18 (30−1230 - 12), so the bicarbonate should drop by exactly 18 to 6 mEq/L (24−18=624 - 18 = 6). However, the patient's actual bicarbonate is 16 mEq/L. Because the actual bicarbonate is significantly higher than the predicted bicarbonate, there is a concurrent metabolic alkalosis, likely secondary to her intractable vomiting (loss of gastric HCl). Finally, Winter's formula (pCO2=1.5×HCO3−+8±2\text{pCO}_2 = 1.5 \times \text{HCO}_3^- + 8 \pm 2) yields an expected pCO2\text{pCO}_2 of 1.5×16+8=321.5 \times 16 + 8 = 32 mm Hg. Because the measured pCO2\text{pCO}_2 is exactly 32 mm Hg, respiratory compensation is appropriate and there is no concurrent respiratory disorder.

Step-by-Step Solution

1
Determine the primary disturbance from the pH and bicarbonate.
The patient has acidemia (pH 7.33) with a low bicarbonate (16 mEq/L), indicating a primary metabolic acidosis.
Identifying the primary disorder provides the baseline for further acid-base calculations.
2
Calculate the anion gap (AG).
AG=Na+−(Cl−+HCO3−)=140−(94+16)=30\text{AG} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (94 + 16) = 30 mEq/L.
A significantly elevated AG (> 12 mEq/L) confirms an anion gap metabolic acidosis, typical of diabetic ketoacidosis.
3
Check respiratory compensation using Winter's formula.
Expected pCO2=1.5×16+8=32\text{pCO}_2 = 1.5 \times 16 + 8 = 32 mm Hg. The actual pCO2\text{pCO}_2 is 32 mm Hg.
Because the actual pCO2\text{pCO}_2 falls exactly within the expected range, there is appropriate respiratory compensation without a concurrent primary respiratory disorder.
4
Calculate the delta gap to identify hidden metabolic disorders.
ΔAG=30−12=18\Delta\text{AG} = 30 - 12 = 18. Expected HCO3−=24−18=6\text{HCO}_3^- = 24 - 18 = 6 mEq/L. Actual HCO3−\text{HCO}_3^- is 16 mEq/L.
Because the actual bicarbonate (16) is greater than the expected bicarbonate (6), a concurrent metabolic alkalosis is present, consistent with her history of intractable vomiting.

Key Concept

In complex patients (e.g., DKA with vomiting), evaluating the delta gap (or delta ratio) is required after calculating the anion gap to unmask hidden concurrent metabolic disorders.

Alternative Method

Instead of subtracting the delta gap from a normal bicarbonate, you can calculate the delta ratio (ΔAGΔHCO3−\frac{\Delta\text{AG}}{\Delta\text{HCO}_3^-}). A ratio between 1.0 and 2.0 indicates an isolated anion gap metabolic acidosis, while a ratio > 2.0 indicates a concurrent metabolic alkalosis. Here, the ratio is 188=2.25\frac{18}{8} = 2.25, confirming the concurrent metabolic alkalosis.
Estimated Time:2m 0s
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Nephrology

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