A 25-year-old primigravid woman at 32 weeks gestation is evaluated in the outpatient clini… — USMLE Step 2 CK

A $25$ -year-old primigravid woman at $32$ weeks gestation is evaluated in the outpatient clinic. She reports a two-week history of feeling "out of breath" and frequently needing to take deep sighs, particularly when sitting or resting. She has no chest pain, cough, orthopnea, or paroxysmal nocturnal dyspnea. Her pregnancy has been uncomplicated, and she takes a daily prenatal vitamin. Her temperature is $36.8^\circ$ C ( $98.2^\circ$ F), blood pressure is $112/68$ mm Hg, pulse is $86$ /min, and respirations are $18$ /min. Pulse oximetry is $99\%$ on room air. Physical examination reveals clear breath sounds bilaterally. Cardiac auscultation reveals a grade $2/6$ systolic ejection murmur at the left upper sternal border. Trace bilateral lower extremity edema is present. An arterial blood gas analysis on room air reveals the following:

pH: $7.45$
PaCO $_2$ : $29$ mm Hg
PaO $_2$ : $102$ mm Hg
HCO $_3^-$ : $18$ mEq/L

Which of the following is the most appropriate next step in the management of this patient?

Reassurance and continuation of routine prenatal careAnswer
B
Computed tomography angiography of the chest
C
Measurement of plasma D-dimer levels
D
Transthoracic echocardiography
E
Intravenous sodium bicarbonate therapy

Answer

Reassurance and continuation of routine prenatal care

The correct approach is reassurance and continuation of routine prenatal care. Dyspnea is a common physiologic complaint during pregnancy, affecting a large majority of pregnant women. It is primarily driven by progesterone, which directly stimulates the central respiratory center. This leads to an increase in tidal volume and minute ventilation, resulting in a primary chronic respiratory alkalosis. The kidneys compensate by increasing the excretion of bicarbonate, which lowers the serum bicarbonate level. The patient's arterial blood gas reflects this perfectly normal physiologic state (high-normal pH, decreased PaCO

_2

, and appropriately decreased HCO

_3^-

). Furthermore, her mild systolic ejection murmur (due to increased blood volume and cardiac output) and trace peripheral edema are standard benign findings in the third trimester. Because she has no hypoxia, tachycardia, or other focal symptoms, no further intervention is required.

Step-by-Step Solution

Analyze the patient's symptoms and vital signs in the context of her pregnancy.

The patient has dyspnea and a need to sigh, but vital signs are stable, pulse oximetry is normal (

99\%

), and there are no signs of acute pathology like wheezing, crackles, or asymmetric edema.

Differentiating between physiologic dyspnea of pregnancy and pathological conditions (like pulmonary embolism or heart failure) requires careful clinical assessment.

Interpret the arterial blood gas (ABG) results.

The ABG shows a mild alkalemia (pH

7.45

), hypocapnia (PaCO

_2

29

mm Hg), and decreased bicarbonate (HCO

_3^-

18

mEq/L) with normal oxygenation.

ABG values change predictably during pregnancy. The increased minute ventilation driven by progesterone causes a chronic respiratory alkalosis, which is met by increased renal excretion of bicarbonate.

Synthesize the clinical and laboratory findings to determine the next step.

The combination of the systolic ejection murmur, trace edema, and compensated respiratory alkalosis perfectly aligns with normal third-trimester physiologic adaptations. No further diagnostic testing or treatment is needed.

Recognizing normal physiology prevents unnecessary, costly, and potentially harmful medical interventions.

Key Concept

Normal physiologic changes of pregnancy include progesterone-mediated hyperventilation, which leads to a primary chronic respiratory alkalosis and a compensatory decrease in serum bicarbonate.

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