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Difficulty: EasyNephrology

An emergency medical services team brings a 6262-year-old man to the hospital following 44 days of profuse, nonbloody diarrhea. His past medical history is significant for severe chronic obstructive pulmonary disease (COPD) managed with a daily inhaled tiotropium. Upon arrival, he reports profound fatigue and lightheadedness.

Vital signs include a heart rate of 108/min108/\text{min}, blood pressure of 105/65 mm Hg105/65\text{ mm Hg}, respiratory rate of 24/min24/\text{min}, and temperature of 37.2∘C37.2^\circ\text{C} (99.0∘F99.0^\circ\text{F}). Physical examination reveals dry oral mucosa and diminished skin turgor. The lungs have diffuse expiratory wheezes bilaterally.

Initial laboratory investigations reveal the following:

Laboratory TestResult
Sodium138 mEq/L138\text{ mEq/L}
Potassium3.4 mEq/L3.4\text{ mEq/L}
Chloride111 mEq/L111\text{ mEq/L}
Bicarbonate14 mEq/L14\text{ mEq/L}
Blood urea nitrogen32 mg/dL32\text{ mg/dL}
Creatinine1.4 mg/dL1.4\text{ mg/dL}

Arterial blood gas (on room air):

Arterial Blood GasResult
pH7.227.22
pCO2\text{pCO}_236 mm Hg36\text{ mm Hg}
pO2\text{pO}_262 mm Hg62\text{ mm Hg}

Based on the laboratory results, which of the following is the most accurate characterization of this patient's acid-base status?

  1. Mixed metabolic acidosis and respiratory acidosisAnswer
  2. B
    Metabolic acidosis with appropriate respiratory compensation
  3. C
    Mixed metabolic acidosis and respiratory alkalosis
  4. D
    Primary respiratory acidosis with appropriate metabolic compensation
  5. E
    Uncompensated metabolic acidosis

Answer

Mixed metabolic acidosis and respiratory acidosis
This patient has a low pH (7.227.22) and a low serum bicarbonate (14 mEq/L14\text{ mEq/L}), which establishes the presence of a primary metabolic acidosis (non-anion gap, given the gap of 1313, which is consistent with severe diarrhea). To determine if the respiratory compensation is appropriate, Winter's formula must be applied: Expected pCO2=(1.5×HCO3−)+8±2\text{pCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2. For this patient, the expected pCO2\text{pCO}_2 is (1.5×14)+8=21+8=29±2(1.5 \times 14) + 8 = 21 + 8 = 29 \pm 2 (range: 27−31 mm Hg27-31\text{ mm Hg}). The patient's actual pCO2\text{pCO}_2 is 36 mm Hg36\text{ mm Hg}. Because the actual pCO2\text{pCO}_2 is greater than the expected range, there is a concomitant primary respiratory acidosis. This is a common finding in patients with advanced COPD who cannot adequately increase their minute ventilation in response to acidemia.

Step-by-Step Solution

1
Identify the primary acid-base disorder by evaluating the pH and serum bicarbonate.
The pH is 7.227.22 (acidemia) and the bicarbonate is 14 mEq/L14\text{ mEq/L} (low). This indicates a primary metabolic acidosis.
The pH determines the primary direction of the derangement, and the concordant change in bicarbonate confirms a metabolic etiology.
2
Calculate the expected pCO2\text{pCO}_2 using Winter's formula to assess for mixed disorders.
Expected pCO2=(1.5×14)+8=29±2 mm Hg\text{pCO}_2 = (1.5 \times 14) + 8 = 29 \pm 2\text{ mm Hg} (range 27−31 mm Hg27-31\text{ mm Hg}).
Winter's formula determines if the respiratory response (hyperventilation) to a metabolic acidosis is appropriate.
3
Compare the actual pCO2\text{pCO}_2 to the expected pCO2\text{pCO}_2.
The actual pCO2\text{pCO}_2 (36 mm Hg36\text{ mm Hg}) is higher than the expected range (27−31 mm Hg27-31\text{ mm Hg}).
A higher-than-expected pCO2\text{pCO}_2 indicates inadequate ventilation, meaning a superimposed respiratory acidosis is present, likely secondary to the patient's severe COPD.

Key Concept

Patients with a primary metabolic acidosis must be evaluated with Winter's formula to determine if respiratory compensation is adequate; a pCO2 higher than expected indicates a concomitant respiratory acidosis.
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Topics

Nephrology

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