A 45-year-old hospitalized man is evaluated for shortness of breath and hypoxemia. The patient was admitted 3 days ago after a motor vehicle collision. At that time, evaluation revealed a femur fracture but no other injuries, and the patient underwent open reduction and internal fixation of the fracture. This morning, he suddenly began experiencing shortness of breath and sharp chest pain. The patient has no significant medical history and has been receiving morphine for pain control. He has smoked a pack of cigarettes daily for 15 years and drinks alcohol occasionally. Temperature is 36.1 C (97 F), blood pressure is 120/70 mm Hg, and pulse is 98/min. BMI is 42 kg/m². Chest x-ray is unremarkable, and arterial blood gas analysis on room air shows a pH of 7.51, PaCO₂ of 30 mm Hg, and PaO₂ of 65 mm Hg. Which of the following is the primary cause of hypoxemia in this patient?

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11ec160b_3ce2_f1b7_9249_558e7497b605_MD0001_00 Sudden-onset shortness of breath and chest pain in a hospitalized patient should raise suspicion for pulmonary embolism (PE). This patient has 2 important risk factors for PE: Immobilization (causes venous stasis) and recent surgery (inflammation induces a hypercoagulable state). The risk of PE is high after large orthopedic procedures; prophylactic anticoagulation reduces but does not eliminate this risk. Thrombi most commonly originate in the deep veins of the pelvis and lower extremities before embolizing to the lungs. Although the risk of fat embolism is also increased following a long-bone fracture, this patient lacks the typical skin rash and neurologic findings (eg, confusion). Thrombotic occlusion of the pulmonary circulation leads to redistribution of blood flow in the lungs, resulting in a ventilation/perfusion (V/Q) mismatch. With many well-ventilated alveoli now inaccessible to blood flow (dead-space ventilation), the remaining accessible alveoli are unable to fully oxygenate the volume of blood that continues to flow through the pulmonary circulation, and hypoxemia results. As with any V/Q mismatch that affects only part of the lungs, supplemental oxygen can help correct the hypoxemia by increasing the alveolar partial pressure of oxygen, allowing accessible alveoli to transfer additional oxygen to the blood. (Choice A) Alveolar hypoventilation refers to a global decrease in the alveolar partial pressure of oxygen caused by a decrease in tidal volume or respiratory rate. Patients also develop respiratory acidosis due to decreased CO₂ excretion. This patient's respiratory alkalosis is more typical of V/Q mismatch with subsequent hyperventilation, as V/Q mismatch does not hinder CO₂ removal as much as blood oxygenation. (Choice B) Decreased blood oxygen-carrying capacity (ie, decreased hemoglobin concentration) can occur due to postoperative bleeding. Although a decrease in hemoglobin can lead to tissue hypoxia, the measured PaO₂ (indicative of dissolved oxygen content rather than hemoglobin-bound oxygen content) will not be affected. (Choice C) Increased tissue oxygen extraction would cause venous oxygen levels to decrease but would not typically affect the arterial oxygen content. (Choice D) Hypoxemia due to a primary defect that impairs gas diffusion between the alveoli and pulmonary capillaries can occur in chronic lung diseases, including pulmonary fibrosis and emphysema. Educational objective: Pulmonary embolism is common in hospitalized and postoperative patients, and classically presents with sudden-onset shortness of breath and pleuritic chest pain. It causes hypoxemia due to ventilation/perfusion mismatch; arterial PCO₂ is usually normal or decreased.

A 45-Year-Old Hospitalized Man Is Evaluated for Shortness of Breath