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Difficulty: HardGeneral Internal Medicine

A 65-year-old man is brought to the emergency department due to profound weakness and lethargy. He has experienced large-volume, watery diarrhea for the past 5 days following a recent trip abroad. His medical history is significant for severe chronic obstructive pulmonary disease (COPD) requiring 2 L/min of home oxygen, and hypertension. Current medications include inhaled tiotropium, inhaled fluticasone-salmeterol, and lisinopril.

Temperature is 37.2°C (99.0°F), blood pressure is 98/60 mm Hg, pulse is 114/min, and respirations are 26/min. Physical examination shows dry mucous membranes, delayed capillary refill, and diffusely decreased breath sounds with a prolonged expiratory phase.

Laboratory studies show:
Serum sodium: 136 mEq/L
Serum potassium: 3.1 mEq/L
Serum chloride: 114 mEq/L
Serum bicarbonate: 12 mEq/L
Blood urea nitrogen: 48 mg/dL
Serum creatinine: 1.9 mg/dL

Arterial blood gas (on room air):
pH: 7.24
PaCO2PaCO_2: 36 mm Hg
PaO2PaO_2: 62 mm Hg

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

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

Answer

The patient has a normal anion gap metabolic acidosis and a concurrent respiratory acidosis.
The patient presents with an acidemic pH (7.24) and a low serum bicarbonate (HCO3−HCO_3^-) of 12 mEq/L, indicating a primary metabolic acidosis. The anion gap is calculated as Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-), which equals 136−(114+12)=10136 - (114 + 12) = 10 mEq/L. Because the normal anion gap is 8-12 mEq/L, this represents a normal anion gap metabolic acidosis, consistent with severe gastrointestinal bicarbonate loss from diarrhea. Next, Winter's formula (PaCO2=1.5×[HCO3−]+8±2PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2) must be used to assess the adequacy of respiratory compensation. For this patient, the expected PaCO2PaCO_2 is 1.5×12+8=26±21.5 \times 12 + 8 = 26 \pm 2 mm Hg (range 24-28 mm Hg). The actual measured PaCO2PaCO_2 is 36 mm Hg. Because the actual PaCO2PaCO_2 is significantly higher than the expected range, a concurrent, superimposed respiratory acidosis is present. The patient's severe underlying chronic obstructive pulmonary disease (COPD) prevents him from mounting the necessary hyperventilatory response to adequately compensate for the metabolic acidosis.

Step-by-Step Solution

1
Assess the pH and primary disturbance.
The pH is 7.24 (acidemia) and the serum HCO3−HCO_3^- is 12 mEq/L (low).
An acidemic pH driven by a low bicarbonate level indicates a primary metabolic acidosis.
2
Calculate the Anion Gap (AG).
AG=Na+−(Cl−+HCO3−)=136−(114+12)=10AG = Na^+ - (Cl^- + HCO_3^-) = 136 - (114 + 12) = 10 mEq/L.
Determines whether the acidosis is driven by unmeasured anions (high AG) or bicarbonate loss (normal AG). A value of 10 mEq/L indicates a normal anion gap metabolic acidosis (NAGMA).
3
Apply Winter's formula to assess respiratory compensation.
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 mm Hg (range 24-28 mm Hg).
Provides the physiological target for PaCO2PaCO_2 if the lungs are adequately compensating for the metabolic acidosis.
4
Compare the actual PaCO2PaCO_2 to the expected PaCO2PaCO_2.
The actual PaCO2PaCO_2 is 36 mm Hg, which is significantly higher than the expected range of 24-28 mm Hg.
A higher than expected PaCO2PaCO_2 signifies an inability to adequately ventilate, revealing a superimposed, concurrent respiratory acidosis.

Key Concept

Mixed acid-base disorders and the application of Winter's formula
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Topics

General Internal Medicine

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