A 28-year-old primigravida at 32 weeks of gestation presents with dyspnea on exertion. Spirometry shows FEV1 85% predicted, FVC 82% predicted, and FEV1/FVC ratio 0.88. The diagram shows pulmonary function changes in pregnancy. The pattern marked **B** (reduced FRC and ERV with maintained FEV1) explains her rapid desaturation during a simulated apnea test. Which of the following best describes the PRIMARY mechanism underlying the reduction in FRC and ERV seen in this patient?

Question

Accepted Answer

C. Mechanical elevation of the diaphragm by the gravid uterus reducing expiratory reserve volume. ## Why "Mechanical elevation of the diaphragm by the gravid uterus reducing expiratory reserve volume" is right

The pattern marked **B** (reduced FRC and ERV with maintained FEV1) is the hallmark physiologic adaptation in pregnancy. Williams Obstetrics and Harrison's both emphasize that the gravid uterus elevates the diaphragm by approximately 4 cm at term, which mechanically restricts the volume available for expiration. This reduces both ERV (by ~20%) and RV (by ~20%), thereby lowering FRC by 20–30% from the 2nd trimester onward. Critically, FEV1 and FVC remain normal because large airway function is preserved—only the expiratory reserve is lost. This reduced FRC is the reason pregnant women desaturate rapidly during apnea, making pre-oxygenation essential before anesthetic induction.

## Why each distractor is wrong

- **Progesterone-mediated bronchoconstriction decreasing small airway patency**: Progesterone stimulates central respiratory drive and increases minute ventilation; it does not cause bronchoconstriction. The FEV1/FVC ratio remains normal in physiologic pregnancy, ruling out obstructive disease.
- **Increased airway resistance from pulmonary edema secondary to gestational hypertension**: Pulmonary edema would produce a restrictive pattern (reduced FVC and TLC) and is pathologic, not physiologic. The patient's FVC is preserved, and there is no mention of hypertension or edema.
- **Decreased inspiratory muscle strength from myasthenia gravis-like syndrome of pregnancy**: No such syndrome exists in pregnancy. Inspiratory reserve volume actually increases to compensate for the loss of expiratory reserve, maintaining vital capacity and normal FEV1.

**High-Yield:** FEV1 is preserved in pregnancy—any obstruction or restriction is pathologic. The reduced FRC and ERV are mechanical, not functional airway disease.

[cite: Williams Obstetrics 26e; Harrison's Principles of Internal Medicine 21e]

Answer

A. Progesterone-mediated bronchoconstriction decreasing small airway patency

Answer

B. Decreased inspiratory muscle strength from myasthenia gravis-like syndrome of pregnancy

Answer

D. Increased airway resistance from pulmonary edema secondary to gestational hypertension

Explanation

Why "Mechanical elevation of the diaphragm by the gravid uterus reducing expiratory reserve volume" is right

Why each distractor is wrong

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