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Difficulty: MediumNephrology

A 24-year-old woman presents to the emergency department with a 2-day history of severe right-sided flank pain, nausea, chills, and fever. She has no significant past medical history and takes no medications. On presentation, her temperature is 39.4°C (102.9°F), blood pressure is 98/55 mm Hg, pulse is 128/min, and respirations are 32/min. Physical examination reveals exquisite right costovertebral angle tenderness and a flushed appearance.

Laboratory results are as follows:

ParameterValue
Sodium138 mEq/L
Potassium4.2 mEq/L
Chloride102 mEq/L
Bicarbonate14 mEq/L
Blood urea nitrogen22 mg/dL
Serum creatinine1.3 mg/dL
Serum lactate4.2 mmol/L

Arterial blood gas analysis on room air reveals:

ParameterValue
pH7.32
pCO2pCO_222 mm Hg
pO2pO_296 mm Hg

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

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

Answer

The patient has a mixed acid-base disorder consisting of a high anion gap metabolic acidosis and a concomitant respiratory alkalosis.
The correct characterization is a high anion gap metabolic acidosis (HAGMA) with a concomitant respiratory alkalosis. The patient is acidemic (pH < 7.40) with a low bicarbonate, indicating primary metabolic acidosis. The anion gap is elevated at 22 mEq/L (due to lactic acid production from early sepsis/pyelonephritis). Using Winter's formula, the expected pCO2pCO_2 is (1.5×14)+8=29±2(1.5 \times 14) + 8 = 29 \pm 2 (range: 27-31 mm Hg). The measured pCO2pCO_2 is 22 mm Hg. Because the actual pCO2pCO_2 is significantly lower than the expected compensatory range, an independent primary respiratory alkalosis is also present, driven by central stimulation of respiration from systemic inflammatory response syndrome (SIRS), fever, and pain.

Step-by-Step Solution

1
Determine the primary acid-base disorder based on the pH.
The pH is 7.32, which is less than 7.40, indicating acidemia. The low bicarbonate (14 mEq/L) aligns with the low pH, confirming a primary metabolic acidosis.
Establishing the primary disturbance directs which compensation formula must be applied.
2
Calculate the serum anion gap.
AG=Na+−(Cl−+HCO3−)=138−(102+14)=22AG = Na^+ - (Cl^- + HCO_3^-) = 138 - (102 + 14) = 22. Normal is roughly 10-14 mEq/L, indicating a High Anion Gap Metabolic Acidosis (HAGMA).
Differentiating between normal and high anion gap narrows down the underlying etiology (in this case, lactic acidosis due to sepsis).
3
Apply Winter's formula to calculate the expected respiratory compensation.
Expected pCO2=(1.5×HCO3−)+8±2pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2.
Expected pCO2=(1.5×14)+8=21+8=29±2pCO_2 = (1.5 \times 14) + 8 = 21 + 8 = 29 \pm 2.
The expected pCO2pCO_2 range is 2727 to 3131 mm Hg.
This establishes the target pCO2pCO_2 baseline if the lungs were purely compensating for the metabolic derangement.
4
Compare the actual pCO2pCO_2 to the expected pCO2pCO_2.
The patient's actual pCO2pCO_2 is 22 mm Hg. Since 22<2722 < 27, the patient is hyperventilating more than expected.
A pCO2pCO_2 lower than the expected compensatory range indicates a separate, primary respiratory alkalosis (often driven by pain, fever, or early sepsis stimulating the respiratory center).

Key Concept

In patients with metabolic acidosis, Winter's formula (pCO2=1.5×HCO3−+8±2pCO_2 = 1.5 \times HCO_3^- + 8 \pm 2) must be used to calculate expected respiratory compensation. An actual pCO2pCO_2 lower than the expected range indicates a concomitant primary respiratory alkalosis.

Alternative Method

To ensure absolute precision, you can also calculate the Delta Ratio (ΔAG/ΔHCO3−\Delta AG / \Delta HCO_3^-). ΔAG=22−12=10\Delta AG = 22 - 12 = 10. ΔHCO3−=24−14=10\Delta HCO_3^- = 24 - 14 = 10. The ratio is 1.0. A ratio between 1.0 and 2.0 confirms a pure high anion gap metabolic acidosis without a hidden normal anion gap acidosis or metabolic alkalosis.
Estimated Time:1m 30s
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Nephrology

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