A 58-year-old woman comes to the office due to gradually worsening shortness of breath. She has a history of emphysema secondary to alpha-1 antitrypsin deficiency and uses inhaled bronchodilators and glucocorticoids. Lately the patient has been feeling short of breath even with minimal exertion. She has no other medical conditions. The patient is a former smoker with a 5-pack-year history. Blood pressure is 130/80 mm Hg, pulse is 82/min, and respirations are 18/min. Physical examination shows increased anteroposterior chest diameter, prolonged expiratory phase, and decreased breath sounds bilaterally. The heart sounds are distant. Chest x-ray reveals hyperinflated lungs with no consolidation, pleural effusion, or pneumothorax. Which of the following sets of arterial blood gas findings on room air is most likely to be seen in this patient?

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11ec160b_3b2a_1ebe_9249_375a03e07c67_MD0001_00 This patient's increased anteroposterior chest diameter, prolonged expiratory phase, distant heart sounds, and hyperinflation of the lungs on chest x-ray are consistent with severe emphysema. Severe disease results in impaired gas exchange due to destruction of the alveolar-arterial membrane, which can lead to hypoxemia. In addition, increased alveolar compliance (reduced lung elastic recoil) and collapse of floppy airways (airway obstruction) leads to CO₂ retention. The kidneys make up for the chronic retention of acidic CO₂ by increasing reabsorption of bicarbonate (secondary metabolic alkalosis) to help normalize pH. Therefore, patients with severe emphysema are expected to have chronic respiratory acidosis with metabolic compensation (high PaCO₂, compensatory high bicarbonate, slightly acidic pH), often accompanied by hypoxemia (PaO₂ <75 mm Hg on room air). (Choice A) These values (low pH, high PaCO₂, normal bicarbonate) show acute respiratory acidosis with hypoxemia, such as occurs with hypoventilation due to acute opioid overdose. The kidneys require several days to compensate for respiratory acid-base disturbances; in acute settings, they do not have time to cause significant bicarbonate retention. (Choice C) Blood pH 7.4, PaCO₂ 40 mm Hg, and plasma bicarbonate 24 mEq/L are normal values. Deviations from these values represent an acid-base disturbance. (Choice D) These values (slightly high pH, elevated bicarbonate, elevated PaCO₂) show compensated metabolic alkalosis such as occurs with vomiting. The loss of hydrogen ions from the gastrointestinal tract is rapidly compensated for by CO₂ retention in the lungs (secondary respiratory acidosis). The blood pH corrects close to normal but remains slightly alkaline. (Choice E) These values (high pH, low PaCO₂, near normal bicarbonate) show acute respiratory alkalosis with hypoxemia, such as occurs with hyperventilation due to acute pulmonary embolism. The kidneys have not had time to excrete bicarbonate to compensate for the increase in CO₂ exhalation. Educational objective: Patients with severe emphysema typically have chronic CO₂ retention leading to chronic respiratory acidosis with metabolic compensation (high PaCO₂, compensatory high bicarbonate, slightly acidic pH), often accompanied by hypoxemia (PaO₂ <75 mm Hg on room air).

A 58-Year-Old Woman Comes to the Office Due to Gradually