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A 47-year-old man comes to the emergency department due to a 30-minute history of sudden-onset severe chest pain and shortness of breath. He is on an RV road trip with his family and has had pain in and swelling of the left leg for the past 2 days. The patient has no significant medical history and takes no medications. Temperature is 37.2 C (99 F), blood pressure is 90/56 mm Hg, pulse is 124/min, and respirations are 32/min. Oxygen saturation is 82% on room air. BMI is 36 kg/m2. ECG shows sinus tachycardia. Chest x-ray shows no abnormalities.
Item 1 of 2
Which of the following is most likely increased in this patient?
Show Explanatory Sources
This patient has developed a deep venous thrombosis in his left leg following a lengthy car trip and now has clinical features highly consistent with acute pulmonary embolism (PE). Obstruction of the pulmonary circulation by an embolus causes increased dead space ventilation (ie, alveoli are ventilated but not perfused); the term "dead space" refers to the volume of inspired air that does not participate in gas exchange. As dead space ventilation increases, blood that continues to flow through the pulmonary circulation cannot be fully oxygenated by the decreased number of accessible alveoli (ventilation/perfusion (V/Q) mismatch), leading to hypoxemia.
(Choice A) Functional residual capacity refers to the volume of air remaining in the lungs after a normal expiration. It is increased in chronic obstructive pulmonary disease (COPD) due to air trapping; however, it is unchanged in PE.
(Choice B) Diffusion capacity is reduced by conditions that disrupt the alveolar-capillary interface (eg, pulmonary fibrosis), but it is not altered in PE.
(Choice D) Pulmonary compliance (ability to expand) is reduced in restrictive lung disease (eg, pulmonary fibrosis) and is increased in COPD (due to emphysematous destruction of the distal airways). However, it is not significantly altered in the setting of PE.
(Choice E) Total airway resistance is increased by bronchoconstriction, which occurs in patients with asthma. PE does not affect airway resistance.
Educational objective:
Pulmonary embolism causes dead space ventilation, which leads to hypoxemia due to the consequent ventilation/perfusion mismatch.