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Difficulty: HardNephrology

A 72-year-old woman is brought to the emergency department from a skilled nursing facility due to severe lethargy and altered mental status over the past 24 hours. Facility staff report she has had multiple episodes of voluminous, watery diarrhea for the past 3 days after completing a course of clindamycin for a cellulitis infection. Her medical history is significant for advanced chronic obstructive pulmonary disease (COPD) requiring home oxygen, hypertension, and heart failure with preserved ejection fraction.

Vital signs are:
Temperature: 37.8°C (100.0°F)
Heart rate: 112/min
Blood pressure: 90/60 mm Hg
Respiratory rate: 26/min
Oxygen saturation: 88% on her baseline 2 L/min nasal cannula.

Physical examination reveals dry mucous membranes and delayed capillary refill. Lung auscultation demonstrates diffusely decreased breath sounds with prolonged expiration and scattered end-expiratory wheezes. The patient is somnolent but arousable to physical stimuli.

Serum chemistry results are as follows:
Sodium: 135 mEq/L
Potassium: 3.1 mEq/L
Chloride: 110 mEq/L
Bicarbonate: 14 mEq/L
Blood urea nitrogen: 42 mg/dL
Creatinine: 1.6 mg/dL (baseline 0.9 mg/dL)

Arterial blood gas analysis (on 2 L/min oxygen) reveals:
pH: 7.19
PaCO2PaCO_2: 38 mm Hg
PaO2PaO_2: 58 mm Hg

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

  1. A
    Normal anion gap metabolic acidosis with appropriate respiratory compensation
  2. B
    High anion gap metabolic acidosis with appropriate respiratory compensation
  3. Normal anion gap metabolic acidosis and concurrent respiratory acidosisAnswer
  4. D
    High anion gap metabolic acidosis and concurrent respiratory acidosis
  5. E
    Primary respiratory acidosis with appropriate metabolic compensation

Answer

Normal anion gap metabolic acidosis and concurrent respiratory acidosis
The correct answer identifies two simultaneous acid-base disorders. First, the patient's severe diarrhea has resulted in a profound loss of bicarbonate, leading to a metabolic acidosis. The anion gap is calculated as Na+−(Cl−+HCO3−)=135−(110+14)=11Na^+ - (Cl^- + HCO_3^-) = 135 - (110 + 14) = 11 mEq/L, which falls squarely into the normal range (8-12 mEq/L), confirming a normal anion gap metabolic acidosis (NAGMA). Second, to determine if the respiratory compensation is appropriate, Winter's formula must be applied: Expected PaCO2=(1.5×HCO3−)+8±2PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. For this patient, Expected PaCO2=(1.5×14)+8=29±2PaCO_2 = (1.5 \times 14) + 8 = 29 \pm 2 mm Hg. The patient's actual measured PaCO2PaCO_2 is 38 mm Hg. Because the actual PaCO2PaCO_2 is significantly higher than the expected range of 27-31 mm Hg, the patient has a concurrent respiratory acidosis. This failure to adequately ventilate is highly consistent with impending respiratory failure in a patient with severe underlying COPD who is fatiguing under the stress of acute hypovolemic shock.

Step-by-Step Solution

1
Analyze the pH to determine the primary disturbance.
The pH is 7.19 (acidemia). The HCO3−HCO_3^- is 14 mEq/L (low) and the PaCO2PaCO_2 is 38 mm Hg (near normal). The primary disturbance is metabolic acidosis.
A low pH driven by a low bicarbonate indicates a primary metabolic acidosis.
2
Calculate the serum anion gap (AG).
AG = Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-) = 135−(110+14)=135−124=11135 - (110 + 14) = 135 - 124 = 11 mEq/L.
Determining the anion gap distinguishes between bicarbonate loss (normal AG, 8-12 mEq/L) and unmeasured acid accumulation (high AG, >12 mEq/L). The patient's diarrhea has caused a normal AG metabolic acidosis.
3
Calculate expected respiratory compensation using Winter's formula.
Expected PaCO2=(1.5×14)+8±2=21+8±2=29±2PaCO_2 = (1.5 \times 14) + 8 \pm 2 = 21 + 8 \pm 2 = 29 \pm 2 mm Hg (range 27-31 mm Hg).
This determines if the respiratory system is appropriately compensating for the metabolic acidosis.
4
Compare actual PaCO2PaCO_2 to expected PaCO2PaCO_2.
Actual PaCO2PaCO_2 (38 mm Hg) is greater than Expected PaCO2PaCO_2 (27-31 mm Hg).
Because the actual PaCO2PaCO_2 is higher than expected, there is inadequate ventilation, indicating a concurrent (superimposed) respiratory acidosis. This is likely due to diaphragmatic fatigue secondary to her underlying COPD and acute illness.

Key Concept

Acid-base analysis requires calculating the anion gap to categorize metabolic acidosis, followed by Winter's formula to uncover hidden secondary respiratory disorders; a 'normal' absolute PaCO2 value is highly abnormal in the setting of severe metabolic acidosis.
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Topics

Nephrology

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