A 52-year-old man is brought to the emergency department after being found unresponsive on the street. The patient's medical history is unknown. Temperature is 36.2 C (97.2 F), blood pressure is 108/62 mm Hg, and pulse is 72/min. On physical examination, he is unresponsive to verbal and tactile stimuli but moans to deep sternal rub. An arterial blood gas analysis on room air shows a partial pressure of oxygen of 60 mm Hg. The partial pressure of oxygen in his alveoli is calculated to be 68 mm Hg. Which of the following is the most likely cause of this patient's symptoms?

Question

A 52-year-old man is brought to the emergency department after being found unresponsive on the street.  The patient's medical history is unknown.  Temperature is 36.2 C (97.2 F), blood pressure is 108/62 mm Hg, and pulse is 72/min.  On physical examination, he is unresponsive to verbal and tactile stimuli but moans to deep sternal rub.  An arterial blood gas analysis on room air shows a partial pressure of oxygen of 60 mm Hg.  The partial pressure of oxygen in his alveoli is calculated to be 68 mm Hg.  Which of the following is the most likely cause of this patient's symptoms?

Quizplus · Accepted Answer

11ec160b_3cda_661a_9249_879b780e1c75_MD0001_00 The partial pressure of oxygen in the alveoli (PAO₂) is normally 104 mm Hg, and due to the high rate of O₂ diffusion across the alveolar-capillary membrane, the O₂ level in the alveolar capillary blood rapidly equilibrates with the PAO₂. However, the blood O₂ level then drops slightly due to the addition of deoxygenated blood from the bronchial circulation; therefore, the partial pressure of oxygen in arterial blood (PaO₂) is normally around 100 mm Hg. This discrepancy between alveolar and arterial O₂ concentration is termed the alveolar-arterial (A-a) gradient. In healthy individuals, a normal A-a gradient is typically between 4 and 15 mm Hg, with older individuals having higher normal values due to an age-related decline in O₂-diffusing capacity. This patient has low PaO₂ and PAO₂ with a normal A-a gradient (68 − 60 = 8), indicating that his low PaO₂ is directly due to low PAO₂. Possible causes of hypoxemia in the setting of a normal A-a gradient include alveolar hypoventilation and low partial pressure of inspired oxygen (PiO₂) (ie, high altitude). Common causes of alveolar hypoventilation include suppressed central respiratory drive (eg, sedative overdose) and diseases that decrease inspiratory capacity (eg, myasthenia gravis, obesity). (Choices B and D) Dead space ventilation is one extreme of ventilation-perfusion mismatch that occurs when the alveoli are adequately ventilated, but there is no alveolar perfusion (eg, pulmonary embolism). An intrapulmonary shunt is the other extreme of ventilation-perfusion mismatch that occurs when blood perfuses alveoli that are not ventilated (eg, pneumonia, pulmonary edema). The A-a gradient is elevated in ventilation-perfusion mismatch. (Choice C) Gas diffusion is impaired in diseases that disrupt the alveolar-capillary membrane, such as alveolar hyaline membrane disease (acutely) or emphysema (chronically). Diffusion impairment causes an elevated A-a gradient because O₂ cannot be effectively transported into the blood. (Choice E) A left-to-right shunt occurs when oxygenated blood from the left side of the heart is shunted into the right side of the heart (eg, atrial or ventricular septal defect). Left-to-right shunts do not cause hypoxemia; however, if left untreated, they may progress into a right-to-left shunt (ie, Eisenmenger syndrome) with hypoxemia, cyanosis, and an elevated A-a gradient. Educational objective: There are 5 major causes of hypoxemia (low arterial partial pressure of oxygen [PaO₂]): alveolar hypoventilation, low partial pressure of inspired oxygen, ventilation-perfusion mismatch, diffusion impairment, and right-to-left shunting. The A-a gradient is normal with alveolar hypoventilation and low partial pressure of inspired oxygen, which helps distinguish these causes from other causes of hypoxemia.

A 52-Year-Old Man Is Brought to the Emergency Department After