A 19-year-old university student is evaluated in the emergency department for severe gener… — USMLE Step 2 CK

A $19$ -year-old university student is evaluated in the emergency department for severe generalized weakness, abdominal cramping, and frequent vomiting over the past $24$ hours. She has a known history of type 1 diabetes mellitus and reports that her insulin pump malfunctioned yesterday. Vital signs show a temperature of $37.1^\circ$ C ( $98.8^\circ$ F), blood pressure of $105/65$ mm Hg, pulse of $115$ /min, and respiratory rate of $28$ /min with deep, sighing breaths.

Laboratory results are as follows:

Test	Result
Sodium	$138$ mEq/L
Potassium	$5.1$ mEq/L
Chloride	$103$ mEq/L
Bicarbonate	$10$ mEq/L

An arterial blood gas analysis on room air reveals:

Parameter	Result
pH	$7.22$
PaCO $_2$	$23$ mm Hg
PaO $_2$	$98$ mm Hg

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

Primary metabolic acidosis with appropriate respiratory compensationAnswer
B
Primary metabolic acidosis with a superimposed primary respiratory alkalosis
C
Primary metabolic acidosis with a superimposed primary respiratory acidosis
D
Primary respiratory alkalosis with appropriate metabolic compensation
E
Primary respiratory acidosis with appropriate metabolic compensation

Answer

The patient has a primary metabolic acidosis with appropriate respiratory compensation.

The correct answer identifies that the patient has a primary metabolic acidosis (due to a pH of 7.22 and a low bicarbonate of 10 mEq/L) and that the respiratory compensation is fully appropriate. According to Winter's formula (1.5 × 10 + 8 = 23), the expected PaCO2 is exactly 23 mm Hg. Because the measured PaCO2 matches the expected value, no additional acid-base disorder is present.

Step-by-Step Solution

Determine the primary acid-base disturbance based on the pH.

The patient's pH is

7.22

, which is less than the normal range (

7.35 - 7.45

), indicating an acidemia. The primary disorder driving this must be an acidosis.

The direction of the pH deviation firmly identifies whether the primary process is an acidosis or alkalosis.

Identify the primary driver of the acidemia using the PaCO2 and bicarbonate levels.

The bicarbonate level is low (

10

mEq/L), which would cause an acidemia. The PaCO

_2

is also low (

23

mm Hg), which would typically cause an alkalemia. Therefore, the low bicarbonate is the primary pathological driver.

Matching the direction of the pH change to the corresponding metabolic or respiratory parameter identifies the primary disorder as a metabolic acidosis.

Calculate the expected respiratory compensation using Winter's formula.

Expected PaCO

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

. Plugging in the values:

(1.5 \times 10) + 8 = 15 + 8 = 23 \pm 2

mm Hg. The expected range is

21

25

mm Hg.

Winter's formula determines if the respiratory response (hyperventilation/Kussmaul respirations) to a metabolic acidosis is fully appropriate, or if there is a concurrent respiratory disorder.

Compare the expected PaCO2 to the measured PaCO2.

The measured PaCO

_2

23

mm Hg, which falls perfectly within the calculated expected range of

21 - 25

mm Hg.

Because the measured PaCO

_2

matches the expected compensation perfectly, the patient does not have a superimposed primary respiratory acidosis or alkalosis.

Key Concept

In patients presenting with a primary metabolic acidosis, the appropriate physiological respiratory compensation can be calculated using Winter's formula: Expected PaCO2 = (1.5 × [HCO3-]) + 8 ± 2. If the measured PaCO2 matches this expected value, the respiratory compensation is appropriate and there is no concurrent primary respiratory disorder.

Estimated Time:1m 0s

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