A 26-year-old woman, gravida 1 para 0, at 33 weeks gestation comes to the office due to dyspnea. The patient has had anxiety attacks during the pregnancy and reports a recent mild upper respiratory illness. She smoked 2-3 cigarettes daily prior to conception. Family history is significant for scleroderma in the patient's mother. BMI is 38 kg/m². Pulmonary evaluation shows the following: Which of the following is the most likely explanation for this patient's findings?

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11ec25c8_d4b1_7829_bc81_d9948ccd581a_MD0003_00 This patient's pulmonary function tests are typical of pregnancy, making her shortness of breath most likely due to dyspnea of pregnancy. Most women experience dyspnea during pregnancy due to the normal physiologic changes that occur. Progesterone likely triggers a sensation of dyspnea and stimulates the respiratory drive in the medulla, resulting in increased minute ventilation, mostly via increased tidal volume and a slight increase in respiratory rate. The hyperventilation increases PaO₂ to facilitate oxygen delivery to the fetus and decreases PaCO₂ (respiratory alkalosis) to encourage unloading of acidic wastes from the fetus. Because of the gravid uterus, the resting position of the diaphragm elevates in pregnancy, compressing the lower portion of the lungs. The resulting basilar atelectasis causes decreased residual volume and functional residual capacity (FRC), as well as a slight decrease in total lung capacity. However, diaphragmatic excursion is not impaired and actually increases, which, combined with increased chest wall expansion, allows for larger tidal volume. Although breathing takes place at a lower baseline lung volume (ie, lower FRC), vital capacity is unchanged. larger tidal volume (Choice A) Decreased FEV1 is expected with asthma. In addition, FRC is unchanged or may be increased due to air trapping. FEV1 is unaffected by the physiologic changes of pregnancy. (Choice B) Hyperventilation syndrome involves frequent increases in minute ventilation that are often associated with anxiety. FRC remains normal. (Choice D) Obesity-related restrictive ventilation (ie, obesity hypoventilation syndrome) is characterized by the restriction of chest wall expansion by an excessive mass of thoracic tissue. Vital capacity and tidal volume are decreased, and, therefore, minute ventilation is decreased. (Choice E) Pulmonary thromboembolism causes ventilation-perfusion mismatch and typically increases minute ventilation, mostly via an increase in respiratory rate. Lung volumes are generally unaffected. Educational objective: Normal physiologic changes in pregnancy include a progesterone-induced sensation of dyspnea and an increase in minute ventilation that is driven mostly by increased tidal volume. Basilar lung compression by the gravid uterus decreases residual volume and functional residual capacity. Vital capacity and FEV1 are unchanged from normal levels. References: Dyspnea in pregnancy. (http://www.ncbi.nlm.nih.gov/pubmed/28805596)

A 26-Year-Old Woman, Gravida 1 Para 0, at 33 Weeks