A 68-year-old man with a history of severe chronic obstructive pulmonary disease (COPD) is… — USMLE Step 2 CK

A 68-year-old man with a history of severe chronic obstructive pulmonary disease (COPD) is brought to the emergency department by his wife due to worsening shortness of breath, profound fatigue, and confusion. According to the wife, the patient has had severe, large-volume, watery diarrhea for the past four days. His temperature is $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), blood pressure is $100/60$ mm Hg, pulse is $112$ /min, and respiratory rate is $24$ /min. Physical examination reveals dry mucous membranes, decreased skin turgor, and diminished breath sounds bilaterally with a prolonged expiratory phase.

Laboratory results are as follows:

Test	Result
Sodium	$136$ mEq/L
Potassium	$3.1$ mEq/L
Chloride	$112$ mEq/L
Bicarbonate	$14$ mEq/L
Blood urea nitrogen	$42$ mg/dL
Creatinine	$1.6$ mg/dL
Albumin	$4.0$ g/dL

Arterial blood gas analysis on room air:

Test	Result
pH	$7.14$
pCO $_2$	$43$ mm Hg
pO $_2$	$62$ mm Hg

Which of the following best describes this patient's primary acid-base disturbance?

Mixed normal anion-gap metabolic acidosis and respiratory acidosisAnswer
B
Uncompensated normal anion-gap metabolic acidosis
C
Mixed high anion-gap metabolic acidosis and respiratory acidosis
D
Normal anion-gap metabolic acidosis with appropriate respiratory compensation
E
Mixed high anion-gap metabolic acidosis and respiratory alkalosis

Answer

Mixed normal anion-gap metabolic acidosis and respiratory acidosis

The correct interpretation involves a multi-step analysis. First, the patient has severe acidemia (pH 7.14). The low bicarbonate (14 mEq/L) identifies a primary metabolic acidosis, which is clinically consistent with his history of severe diarrhea (intestinal loss of bicarbonate). Calculating the anion gap [136 - (112 + 14) = 10 mEq/L] confirms a normal anion-gap metabolic acidosis (NAGMA). To determine if respiratory compensation is appropriate, Winter's formula must be applied: Expected pCO2 = (1.5 × HCO3-) + 8 ± 2. In this patient, Expected pCO2 = (1.5 × 14) + 8 ± 2 = 29 ± 2 mm Hg. The patient's actual pCO2 is 43 mm Hg, which is significantly higher than the expected target of 27-31 mm Hg. Because the patient is retaining more CO2 than expected (due to his severe underlying COPD), he has a superimposed, concomitant respiratory acidosis.

Step-by-Step Solution

Evaluate the pH to determine the primary acid-base direction.

The pH is 7.14, which indicates significant acidemia.

Establishing acidemia or alkalemia is the mandatory first step in acid-base interpretation.

Examine the serum bicarbonate (HCO3-) and pCO2 to identify the primary disorder.

The serum bicarbonate is markedly low at 14 mEq/L, confirming a primary metabolic acidosis. This is clinically consistent with massive bicarbonate loss from severe diarrhea.

A low pH driven by a low bicarbonate defines metabolic acidosis.

Calculate the serum anion gap (AG).

AG = Na+ - (Cl- + HCO3-) = 136 - (112 + 14) = 136 - 126 = 10 mEq/L.

Determining the anion gap (normal 8-12 mEq/L) is required to categorize the metabolic acidosis. An AG of 10 mEq/L indicates a normal anion-gap (hyperchloremic) metabolic acidosis.

Apply Winter's formula to determine the expected respiratory compensation (expected pCO2).

Expected pCO2 = (1.5 × 14) + 8 ± 2 = 21 + 8 ± 2 = 29 ± 2 mm Hg (Range: 27 to 31 mm Hg).

Winter's formula determines if the lungs are adequately blowing off CO2 to compensate for the metabolic acidosis.

Compare the expected pCO2 to the measured pCO2.

The measured pCO2 is 43 mm Hg. Because the measured pCO2 (43) is significantly higher than the expected pCO2 (27-31), there is an additional primary respiratory acidosis.

When actual pCO2 exceeds expected pCO2 in a metabolic acidosis, a superimposed respiratory acidosis is present. The patient's severe COPD prevents adequate hyperventilation.

Key Concept

Acid-base interpretation using Winter's formula to identify mixed acid-base disorders in patients with underlying lung disease.

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic