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Difficulty: MediumNephrology

A 65-year-old woman is evaluated in the emergency department for three days of worsening right-sided flank pain, subjective fever, and profound weakness. Her past medical history is significant for hypertension and type 2 diabetes mellitus. On physical examination, her temperature is 38.9∘C38.9^\circ C (102.0∘F102.0^\circ F), blood pressure is 88/5588/55 mmHg, pulse is 118118/min, and respiratory rate is 2626/min. The patient appears lethargic but is arousable. Mucous membranes are dry, and there is marked tenderness to percussion over the right costovertebral angle.

Laboratory studies reveal the following:

TestResult
Sodium135135 mEq/L
Potassium5.25.2 mEq/L
Chloride9898 mEq/L
Bicarbonate1212 mEq/L
Blood urea nitrogen4545 mg/dL
Creatinine2.12.1 mg/dL
Glucose180180 mg/dL

Arterial blood gas analysis on room air shows:

TestResult
pH7.157.15
PaCO2PaCO_23636 mmHg
PaO2PaO_27070 mmHg

Based on the provided laboratory data, which of the following combinations best characterizes this patient's current acid-base status?

  1. Elevated anion gap acidosis and a concurrent respiratory acidosisAnswer
  2. B
    Elevated anion gap acidosis with appropriate ventilatory compensation
  3. C
    Normal anion gap acidosis and a concurrent respiratory acidosis
  4. D
    Primary respiratory acidosis with compensatory renal bicarbonate retention
  5. E
    Elevated anion gap acidosis and a concurrent metabolic alkalosis

Answer

Elevated anion gap acidosis and a concurrent respiratory acidosis
This patient presents with signs of severe pyelonephritis progressing to urosepsis, leading to hypotension, renal failure, and poor tissue perfusion. The arterial blood gas demonstrates an acidemia (pH 7.157.15). The low bicarbonate (1212 mEq/L) identifies this as a metabolic acidosis. The anion gap is calculated as 135−(98+12)=25135 - (98 + 12) = 25 mEq/L, revealing a superimposed unmeasured anion burden (likely lactic acid from shock and uremic acids from acute kidney injury), which categorizes it as an elevated anion gap metabolic acidosis. To assess respiratory compensation, Winter's formula is applied: Expected PaCO2=1.5×[HCO3−]+8±2=(1.5×12)+8=26±2PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = (1.5 \times 12) + 8 = 26 \pm 2 mmHg. The patient's measured PaCO2PaCO_2 is 3636 mmHg, which is considerably higher than the 24−2824-28 mmHg range expected for adequate compensation. Although 3636 mmHg is below the normal physiologic baseline of 4040 mmHg, it represents a relative failure of ventilation in the context of severe metabolic acidosis (likely due to lethargy and fatigue). This excess CO2CO_2 defines a superimposed primary respiratory acidosis.

Step-by-Step Solution

1
Determine the primary acid-base disorder by evaluating the pH and bicarbonate.
The pH is acidemic (7.157.15) and the bicarbonate is significantly reduced (1212 mEq/L), indicating a primary metabolic acidosis.
pH below 7.357.35 confirms acidosis, and a low HCO3−HCO_3^- confirms its metabolic origin.
2
Calculate the serum anion gap to classify the metabolic acidosis.
Anion Gap =Na+−(Cl−+HCO3−)=135−(98+12)=25= Na^+ - (Cl^- + HCO_3^-) = 135 - (98 + 12) = 25 mEq/L.
An anion gap above 1212 mEq/L indicates the presence of unmeasured acid anions, confirming an elevated anion gap metabolic acidosis.
3
Calculate the expected respiratory compensation using Winter's formula.
Expected PaCO2=(1.5×12)+8±2=18+8±2=26±2PaCO_2 = (1.5 \times 12) + 8 \pm 2 = 18 + 8 \pm 2 = 26 \pm 2 mmHg (Range: 24−2824 - 28 mmHg).
Winter's formula determines if the lungs are adequately blowing off CO2CO_2 to compensate for the metabolic acidosis.
4
Compare the expected PaCO2PaCO_2 to the measured PaCO2PaCO_2 to identify any secondary disorders.
The measured PaCO2PaCO_2 is 3636 mmHg, which is significantly higher than the expected range of 24−2824-28 mmHg.
Because the actual CO2CO_2 retention is higher than what physiological compensation dictates, there is an additional primary respiratory acidosis superimposed on the metabolic acidosis.

Key Concept

In patients with metabolic acidosis, Winter's formula (1.5×[HCO3−]+8±21.5 \times [HCO_3^-] + 8 \pm 2) must be used to calculate the expected PaCO2PaCO_2. If the measured PaCO2PaCO_2 is higher than the calculated range, a concurrent respiratory acidosis is present.
Estimated Time:1m 30s
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Nephrology

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