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Difficulty: MediumNephrology

A medical rapid response team evaluates a 72-year-old female resident of a skilled nursing facility who developed profound lethargy after 3 days of intractable emesis and poor oral intake. Upon transfer to the emergency department, her blood pressure is 82/50 mm Hg, pulse is 124/min, and respirations are 24/min. Physical examination reveals dry mucous membranes, flat neck veins, and delayed capillary refill. Laboratory testing reveals the following:

TestResult
Serum sodium140140 mEq/L
Serum potassium3.13.1 mEq/L
Serum chloride9090 mEq/L
Serum bicarbonate1515 mEq/L
Blood urea nitrogen5252 mg/dL
Serum creatinine2.42.4 mg/dL
Arterial pH7.327.32
Arterial pCO2pCO_23030 mm Hg
Arterial pO2pO_28888 mm Hg

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

  1. Primary high anion gap metabolic acidosis and concurrent metabolic alkalosisAnswer
  2. B
    Primary high anion gap metabolic acidosis and concurrent respiratory alkalosis
  3. C
    Primary high anion gap metabolic acidosis and concurrent non-anion gap metabolic acidosis
  4. D
    Pure high anion gap metabolic acidosis with appropriate respiratory compensation
  5. E
    Primary non-anion gap metabolic acidosis and concurrent metabolic alkalosis

Answer

Primary high anion gap metabolic acidosis and concurrent metabolic alkalosis
The correct answer accurately reflects all three steps of acid-base interpretation in this patient. First, the patient is acidemic (pH=7.32pH = 7.32) with a low bicarbonate (1515 mEq/L), defining a primary metabolic acidosis. Second, the anion gap is very high at 3535 (140−105140 - 105), indicating a high anion gap metabolic acidosis (HAGMA), likely from lactic acidosis secondary to hypoperfusion. Third, Winter's formula (1.5×15+8=30.5±21.5 \times 15 + 8 = 30.5 \pm 2) confirms the measured pCO2pCO_2 of 3030 represents appropriate respiratory compensation, ruling out a primary respiratory disturbance. Finally, the delta gap (excess anion gap) is 35−12=2335 - 12 = 23. Adding this to the measured bicarbonate yields a corrected bicarbonate of 3838 mEq/L (15+2315 + 23). Because 3838 is much greater than the normal 2424, a concurrent metabolic alkalosis is present, which is consistent with the patient's history of intractable emesis.

Step-by-Step Solution

1
Evaluate the primary disturbance using pH and serum bicarbonate.
The arterial pHpH is 7.327.32 (acidemia) and serum HCO3−HCO_3^- is low (1515 mEq/L), indicating a primary metabolic acidosis.
Establishing the primary acid-base disorder is the first required step in interpreting arterial blood gases.
2
Calculate the anion gap (AG=Na+−(Cl−+HCO3−)AG = Na^+ - (Cl^- + HCO_3^-)).
AG=140−(90+15)=35AG = 140 - (90 + 15) = 35 mEq/L. This is significantly elevated (normal 8−128-12 mEq/L), meaning there is a primary high anion gap metabolic acidosis (likely due to lactic acidosis from hypovolemic shock).
Determining the anion gap separates metabolic acidosis into gap and non-gap etiologies and dictates the need for a delta gap calculation.
3
Apply Winter's formula (pCO2=1.5×HCO3−+8±2pCO_2 = 1.5 \times HCO_3^- + 8 \pm 2) to check for respiratory compensation.
Expected pCO2=1.5×15+8=22.5+8=30.5±2pCO_2 = 1.5 \times 15 + 8 = 22.5 + 8 = 30.5 \pm 2 mm Hg. The measured pCO2pCO_2 is 3030 mm Hg, which falls exactly in the expected range. There is no concurrent respiratory disorder.
Checking respiratory compensation identifies superimposed respiratory alkalosis or acidosis.
4
Calculate the delta gap to identify additional hidden metabolic disturbances.
Delta gap = Measured AG−NormalAG(12)=35−12=23AG - Normal AG (12) = 35 - 12 = 23. Expected normal HCO3−HCO_3^- = Measured HCO3−+DeltaGap=15+23=38HCO_3^- + Delta Gap = 15 + 23 = 38 mEq/L. Because 3838 is much greater than 2424, there is a concurrent metabolic alkalosis.
The delta gap reveals if the drop in bicarbonate is proportional to the rise in unmeasured anions. A disproportionate result indicates a third concurrent disorder.

Key Concept

Patients with mixed acid-base disorders require a step-wise approach: identifying the primary disorder, calculating the anion gap, applying Winter's formula, and calculating the delta-delta to reveal hidden metabolic derangements (such as metabolic alkalosis from vomiting masked by a severe high-anion gap metabolic acidosis).
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Topics

Nephrology

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