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 acidosisCevap
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

Cevap

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.

Adım Adım Çözüm

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.

Anahtar Kavram

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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