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Difficulty: MediumNephrology

A 29-year-old woman is brought to the emergency department by her husband due to severe left-sided back pain, nausea, and shaking chills that began 4848 hours ago. Her temperature is 39.2∘C39.2^{\circ}C (102.6∘F102.6^{\circ}F), blood pressure is 86/5086/50 mmHg, pulse is 128128/min, and respiratory rate is 2828/min. Physical examination reveals flushed skin, dry mucous membranes, and marked left costovertebral angle tenderness. Laboratory testing reveals the following:

TestResult
Serum sodium140140 mEq/L
Serum potassium4.14.1 mEq/L
Serum chloride102102 mEq/L
Serum bicarbonate1212 mEq/L
Blood urea nitrogen2828 mg/dL
Serum creatinine1.41.4 mg/dL
Arterial blood gas analysis on room air shows:
TestResult
pH7.387.38
pCO2pCO_22020 mmHg
pO2pO_29696 mmHg

Based on the laboratory results, which of the following best characterizes this patient's acid-base status?

  1. Metabolic acidosis with an elevated anion gap and a concurrent primary respiratory alkalosisAnswer
  2. B
    Metabolic acidosis with an elevated anion gap and appropriate respiratory compensation
  3. C
    Metabolic acidosis with an elevated anion gap and a concurrent primary respiratory acidosis
  4. D
    Metabolic acidosis with a normal anion gap and a concurrent primary respiratory alkalosis
  5. E
    Primary respiratory alkalosis with appropriate renal metabolic compensation

Answer

The patient has a metabolic acidosis with an elevated anion gap (secondary to lactic acidosis) and a concurrent primary respiratory alkalosis (secondary to central hyperventilation).
The patient's arterial pH of 7.387.38 indicates an overall acidemic state (pH <7.40< 7.40). The low serum bicarbonate (1212 mEq/L) points to a primary metabolic acidosis. Calculating the anion gap (Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-)) yields 140−(102+12)=26140 - (102 + 12) = 26 mEq/L. This indicates a metabolic acidosis with an elevated anion gap, which is highly suggestive of lactic acidosis in the context of this patient's septic shock (hypotension, tachycardia) resulting from pyelonephritis.

To evaluate for a mixed disorder, Winter's formula (Expected pCO2=1.5×HCO3−+8±2Expected\ pCO_2 = 1.5 \times HCO_3^- + 8 \pm 2) must be used. Based on her bicarbonate of 1212 mEq/L, the expected pCO2pCO_2 is 26±226 \pm 2 mmHg (range 24−2824-28 mmHg). The patient's actual pCO2pCO_2 is 2020 mmHg. Because the actual pCO2pCO_2 is lower than the calculated lower limit of compensation (2424 mmHg), she has an independent, concurrent primary respiratory alkalosis. This mixed disorder is a classic presentation in severe sepsis, where tissue hypoperfusion causes lactic acidosis, while early systemic inflammation and high fever directly stimulate the brainstem respiratory center, leading to tachypnea and excessive CO2CO_2 clearance beyond what is expected for simple compensation.

Step-by-Step Solution

1
Determine the primary acid-base disorder by evaluating the pH and serum bicarbonate.
The pH is 7.387.38, which is acidemic (less than 7.407.40). The serum bicarbonate is severely low (1212 mEq/L), indicating the primary disorder is a metabolic acidosis.
The primary disorder matches the direction of the pH deviation. An acidemic pH driven by low bicarbonate confirms primary metabolic acidosis.
2
Calculate the anion gap to characterize the metabolic acidosis.
Anion Gap = Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-) = 140−(102+12)=26140 - (102 + 12) = 26 mEq/L.
The normal anion gap is 8−128-12 mEq/L. A calculated gap of 2626 mEq/L indicates the presence of unmeasured anions, likely lactic acid secondary to septic shock in this patient.
3
Apply Winter's formula to determine the expected respiratory compensation for the metabolic acidosis.
Expected pCO2pCO_2 = (1.5×HCO3−)+8±2(1.5 \times HCO_3^-) + 8 \pm 2 = (1.5×12)+8±2(1.5 \times 12) + 8 \pm 2 = 18+8±218 + 8 \pm 2 = 26±226 \pm 2 (range 2424 to 2828 mmHg).
Winter's formula calculates the expected physiological decrease in pCO2pCO_2 via hyperventilation in response to the primary low bicarbonate.
4
Compare the actual pCO2pCO_2 to the expected pCO2pCO_2 range to identify any secondary independent respiratory disorders.
The actual measured pCO2pCO_2 is 2020 mmHg, which is lower than the expected compensatory range of 24−2824-28 mmHg.
An actual pCO2pCO_2 that is lower than the expected minimum indicates that the patient is "blowing off" more carbon dioxide than compensation alone dictates. This defines a concurrent primary respiratory alkalosis.

Key Concept

Identification of mixed acid-base disorders using the anion gap and Winter's formula.
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Topics

Nephrology

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