Following 4 days of profuse, watery diarrhea, a 70-year-old man with a long-standing histo… — USMLE Step 2 CK

Following 4 days of profuse, watery diarrhea, a 70-year-old man with a long-standing history of severe chronic obstructive pulmonary disease presents to the emergency department for evaluation of extreme weakness. He reports having 8 to 10 bowel movements daily since attending a family picnic. His home medications include a long-acting muscarinic antagonist and an inhaled corticosteroid.

Vital signs show a temperature of $37.4^\circ\text{C}$ ( $99.3^\circ\text{F}$ ), blood pressure of 102/64 mm Hg, pulse of 115/min, and respiratory rate of 14/min. Physical examination reveals dry mucous membranes, decreased skin turgor, and diffuse expiratory wheezes bilaterally.

Laboratory studies are obtained:

Parameter	Value
Sodium	$140 \text{ mEq/L}$
Potassium	$2.9 \text{ mEq/L}$
Chloride	$114 \text{ mEq/L}$
Bicarbonate	$16 \text{ mEq/L}$
Blood urea nitrogen	$35 \text{ mg/dL}$
Creatinine	$1.4 \text{ mg/dL}$

Arterial blood gas on room air:

Parameter	Value
pH	$7.15$
$PaCO_2$	$48 \text{ mm Hg}$
$PaO_2$	$62 \text{ mm Hg}$

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

Normal anion gap metabolic acidosis and respiratory acidosisAnswer
B
Normal anion gap metabolic acidosis with adequate respiratory compensation
C
High anion gap metabolic acidosis and respiratory acidosis
D
Normal anion gap metabolic acidosis and respiratory alkalosis
E
Chronic respiratory acidosis with fully compensated metabolic alkalosis

Answer

Normal anion gap metabolic acidosis and respiratory acidosis

The patient's severe diarrhea leads to direct gastrointestinal loss of bicarbonate, causing a primary normal anion gap metabolic acidosis (NAGMA). In a healthy individual, the expected physiologic response is hyperventilation to blow off

CO_2

. Using Winter's formula (

PaCO_2 = 1.5 \times [\text{HCO}_3^-] + 8 \pm 2

), the expected

PaCO_2

for this patient should be between

30

and

34 \text{ mm Hg}

. However, his actual

PaCO_2

48 \text{ mm Hg}

. Because his actual

PaCO_2

is much higher than the expected compensatory range, he has a superimposed respiratory acidosis. This occurs because his severe underlying COPD prevents him from adequately increasing his minute ventilation to compensate for the metabolic acidemia.

Step-by-Step Solution

Determine the primary disturbance by looking at pH and bicarbonate.

The pH is

7.15

(acidemia) and bicarbonate is

16 \text{ mEq/L}

(low). This establishes a primary metabolic acidosis.

Acidemia with a corresponding decrease in serum bicarbonate indicates that a metabolic acidosis is the primary driving process.

Calculate the anion gap (AG) to classify the metabolic acidosis.

\text{AG} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (114 + 16) = 10 \text{ mEq/L}

An AG of

10 \text{ mEq/L}

falls within the normal range (

8-12 \text{ mEq/L}

), identifying a normal anion gap metabolic acidosis (NAGMA).

Calculate the expected respiratory compensation using Winter's formula.

Expected

PaCO_2 = (1.5 \times 16) + 8 \pm 2 = 32 \pm 2 \text{ mm Hg}

(Range:

30-34 \text{ mm Hg}

Winter's formula determines if the lungs are appropriately blowing off

CO_2

to compensate for the primary metabolic acidosis.

Compare the actual

PaCO_2

to the expected

PaCO_2

range.

The actual

PaCO_2

48 \text{ mm Hg}

, which is significantly higher than the expected

30-34 \text{ mm Hg}

Because the actual

PaCO_2

is higher than expected, there is a concomitant, uncompensated primary respiratory acidosis.

Key Concept

Patients with underlying chronic lung disease (like COPD) may fail to mount an appropriate respiratory compensation (hyperventilation) to a primary metabolic acidosis, resulting in a mixed metabolic and respiratory acidosis.

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