A 27-year-old woman is brought to the emergency department by her roommate due to persiste… — USMLE Step 2 CK

A 27-year-old woman is brought to the emergency department by her roommate due to persistent nausea, vomiting, and diffuse abdominal pain for the past 24 hours. She has a history of type 1 diabetes mellitus and reports that she ran out of her insulin prescription two days ago. On physical examination, she appears lethargic but is easily arousable. Her respiratory rate is $26/\text{min}$ and breaths are deep and rapid.

Laboratory testing reveals the following:

Laboratory Test	Result
Serum sodium	$136 \text{ mEq/L}$
Serum potassium	$5.1 \text{ mEq/L}$
Serum chloride	$98 \text{ mEq/L}$
Serum bicarbonate	$12 \text{ mEq/L}$
Arterial blood pH	$7.22$
Arterial $\text{PaCO}_2$	$26 \text{ mm Hg}$

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

Elevated anion gap metabolic acidosis with adequate respiratory compensationAnswer
B
Elevated anion gap metabolic acidosis with superimposed respiratory alkalosis
C
Elevated anion gap metabolic acidosis with superimposed respiratory acidosis
D
Non-anion gap metabolic acidosis with adequate respiratory compensation
E
Non-anion gap metabolic acidosis with superimposed respiratory alkalosis

Answer

Elevated anion gap metabolic acidosis with adequate respiratory compensation

The patient's low pH (

7.22

) and low bicarbonate (

12 \text{ mEq/L}

) demonstrate a metabolic acidosis. The anion gap is calculated as

136 - (98 + 12) = 26

, which signifies an elevated anion gap metabolic acidosis (consistent with diabetic ketoacidosis). Using Winter's formula, the expected compensatory

\text{PaCO}_2

is calculated as

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

. Since the patient's actual

\text{PaCO}_2

26 \text{ mm Hg}

, the respiratory compensation is entirely adequate, and there are no superimposed respiratory disturbances.

Step-by-Step Solution

Determine the primary disturbance by analyzing the pH and serum bicarbonate.

The pH is

7.22

(acidemia) and serum bicarbonate is

12 \text{ mEq/L}

(low), indicating a primary metabolic acidosis.

Identifying the initial direction of pH and primary buffering molecule changes establishes the core acid-base status.

Calculate the serum anion gap.

\text{Anion Gap} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 136 - (98 + 12) = 136 - 110 = 26

. A value of

26

is greater than the normal range (

8-12

), confirming an elevated anion gap metabolic acidosis.

An elevated anion gap is characteristic of conditions producing unmeasured organic acids, such as diabetic ketoacidosis.

Apply Winter's formula to determine if respiratory compensation is appropriate.

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

. Calculation:

(1.5 \times 12) + 8 = 18 + 8 = 26

. The expected range is

24

28 \text{ mm Hg}

Comparing the expected carbon dioxide level to the patient's measured

\text{PaCO}_2

(

26 \text{ mm Hg}

) reveals if there is a secondary, superimposed respiratory disorder.

Interpret the final results.

The measured

\text{PaCO}_2

matches the expected value perfectly.

Because the measured value falls precisely within the calculated range, the patient has an elevated anion gap metabolic acidosis with adequate respiratory compensation, and no secondary acid-base disorder.

Key Concept

Identifying complex acid-base disorders requires calculating the anion gap to classify metabolic acidosis and applying Winter's formula to assess for appropriate respiratory compensation.

Estimated Time:1m 0s

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic