A 44-year-old man comes to the emergency department due to sudden-onset shortness of breath that started a few hours ago. Last week, he sustained a crush injury after a motorcycle drove over his left foot, and he has been immobilized since then due to pain. The patient drinks alcohol occasionally and does not use tobacco or illicit drugs. Temperature is 36.1 C (97 F) , blood pressure is 118/76 mm Hg, pulse is 102/min, and respirations are 28/min. BMI is 37 kg/m². On physical examination, heart rhythm is regular with normal S1 and S2. The lungs are clear to auscultation bilaterally. The dorsum of the left foot has resolving ecchymoses and swelling compared to when he was evaluated last week. The right calf is swollen and tender. Which of the following arterial blood gas values are most likely to be present in this patient?

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11ec160b_3ce3_8df9_9249_894912e8a9ce_MD0001_00 Sudden-onset dyspnea in the setting of recent calf swelling is strongly suggestive of pulmonary embolism (PE). Dyspnea is the most common symptom of PE and risk factors include obesity, smoking, and prolonged immobilization. The embolus typically arises from a deep venous thrombus in the lower extremities (causes calf swelling) and lodges itself within the arterial blood supply of the lungs. PE occludes or significantly reduces blood flow to a portion of the pulmonary parenchyma, resulting in increased dead space ventilation and redistribution of pulmonary blood flow away from the affected segments. This leads to increased ventilation/perfusion (V/Q) mismatch in the remainder of the lungs with consequent hypoxemia. Stimulation of central respiratory drive occurs in response to dyspnea and inflammatory mediators released by ischemic pulmonary tissue. Hyperventilation does not significantly improve arterial oxygenation because hemoglobin is nearly fully saturated with O₂ in areas of normal V/Q ratio, allowing little capacity for high V/Q regions to increase blood O₂ content much further. In contrast, CO₂ removal is more directly dependent on ventilation, and high V/Q regions have large capacity to exhale additional CO₂. Therefore, the hyperventilation leads to hypocapnia (low PaCO₂) and respiratory alkalosis. The serum HCO₃^- is near normal in the acute setting, but metabolic compensation with renal bicarbonate loss takes place over the next 72 hours. (Choice A) Low blood pH of 7.30 with low serum HCO₃^- indicates metabolic acidosis, which is compensated by hyperventilation leading to low PaCO₂. (Choice B) A low pH of 7.32 with high PaCO₂ reflects respiratory acidosis. The high HCO₃^- indicates renal compensation by bicarbonate retention. Although PE may eventually lead to respiratory collapse with an acute increase in PaCO₂, a renal compensatory response would not be expected so abruptly. (Choice C) These arterial blood gas values are considered normal, reflecting an absence of acid-base disturbance. (Choice E) A high pH of 7.47 with low PaCO₂ reflects respiratory alkalosis. The low HCO₃^- indicates renal compensation, which is expected with chronic respiratory alkalosis but should not be present with acute PE. Normal PaO₂ can occasionally be present in acute PE but is less likely. (Choice F) A high pH of 7.48 with high serum HCO₃^- reflects metabolic alkalosis, which is compensated by hypoventilation leading to high PaCO₂. Educational objective: The combination of calf swelling, history of prolonged immobility, and sudden-onset dyspnea is strongly suggestive of acute pulmonary embolism (PE). PE typically presents with hypoxemia due to ventilation/perfusion mismatch and acute respiratory alkalosis (hypocapnia) due to hyperventilation. __________ References: Hypoxemia in acute pulmonary embolism. (http://www.ncbi.nlm.nih.gov/pubmed/4064770) Mechanisms of hypoxemia and hypocapnia in pulmonary embolism. (http://www.ncbi.nlm.nih.gov/pubmed/7599843)

A 44-Year-Old Man Comes to the Emergency Department Due to Sudden-Onset