A 52-year-old man with a history of obesity hypoventilation syndrome and type 2 diabetes m… — USMLE Step 2 CK

A 52-year-old man with a history of obesity hypoventilation syndrome and type 2 diabetes mellitus is brought to the emergency department by his wife due to profound weakness and confusion. For the past 4 days, he has had severe, watery diarrhea with up to 10 bowel movements daily, accompanied by abdominal cramping. He has not been able to tolerate oral intake. His temperature is $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ ), blood pressure is $90/60\text{ mm Hg}$ , pulse is $112\text{/min}$ , and respirations are $14\text{/min}$ . Physical examination shows dry mucous membranes and decreased skin turgor. The patient appears somnolent but is arousable.

Laboratory studies show:
Serum sodium: $135\text{ mEq/L}$
Serum potassium: $3.2\text{ mEq/L}$
Serum chloride: $113\text{ mEq/L}$
Serum bicarbonate: $12\text{ mEq/L}$
Blood urea nitrogen: $42\text{ mg/dL}$
Serum creatinine: $1.6\text{ mg/dL}$
Serum albumin: $4.0\text{ g/dL}$

Arterial blood gas analysis on room air:
pH: $7.08$
$\text{PaCO}_2$ : $42\text{ mm Hg}$
$\text{PaO}_2$ : $65\text{ mm Hg}$

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

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

Answer

Mixed normal anion gap metabolic acidosis and respiratory acidosis

This patient has a mixed normal anion gap metabolic acidosis and respiratory acidosis. The primary metabolic acidosis is diagnosed by the low pH (

7.08

) and low bicarbonate (

12\text{ mEq/L}

), caused by profound bicarbonate loss from severe diarrhea. The anion gap is calculated as

\text{Sodium} - (\text{Chloride} + \text{Bicarbonate})

, which is

135 - (113 + 12) = 10\text{ mEq/L}

. Since this is within the normal range of

8-12\text{ mEq/L}

, the primary disorder is a normal anion gap (hyperchloremic) metabolic acidosis. Next, Winter's formula must be applied to determine if respiratory compensation is adequate: Expected

\text{PaCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2

. For this patient, the expected

\text{PaCO}_2

(1.5 \times 12) + 8 = 26 \pm 2\text{ mm Hg}

(range

24-28\text{ mm Hg}

). The patient's actual

\text{PaCO}_2

42\text{ mm Hg}

. Because the actual

\text{PaCO}_2

is significantly higher than the expected range, he has a superimposed primary respiratory acidosis. This indicates a failure to mount the necessary hyperventilatory response, perfectly aligning with his history of obesity hypoventilation syndrome and current somnolent state.

Step-by-Step Solution

Identify the primary acid-base disturbance using pH and serum bicarbonate.

The pH is

7.08

(severe acidemia) and the serum bicarbonate is

12\text{ mEq/L}

(low), indicating a primary metabolic acidosis.

Determining the primary disorder is the critical first step in any acid-base evaluation.

Calculate the serum anion gap.

Anion Gap =

\text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-)

135 - (113 + 12) = 10\text{ mEq/L}

. This is within the normal range (

8-12\text{ mEq/L}

Differentiating between normal and high anion gap metabolic acidosis narrows the differential diagnosis. This patient's profound diarrhea caused a normal anion gap metabolic acidosis due to gastrointestinal bicarbonate loss.

Calculate the expected respiratory compensation using Winter's formula.

Expected

\text{PaCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2 = (1.5 \times 12) + 8 \pm 2 = 26 \pm 2\text{ mm Hg}

. The actual

\text{PaCO}_2

42\text{ mm Hg}

, which is significantly higher than the expected range.

If the actual PaCO2 is higher than the expected PaCO2, there is a concurrent primary respiratory acidosis. This patient's obesity hypoventilation syndrome and somnolence prevent him from adequately hyperventilating to compensate for the acidemia.

Key Concept

Evaluating complex acid-base disorders requires a stepwise approach: identify the primary disturbance, calculate the anion gap, and apply Winter's formula to determine if respiratory compensation is appropriate.

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