A 44-year-old woman is admitted to the hospital for a total left hip arthroplasty. On post-operative day 1, she is found to be hypoxemic with an oxygen saturation of 85%. Her hypoxemia developed over the last 12 hours, during which her pain medication, hydromorphone, was increased. Physical examination reveals a lethargic woman with clear lung sounds throughout. An arterial blood gas done on room air reveals a pH of 7.28, PaO2 that is decreased at 55 mmHg, and PaCO2 that is elevated at 60 mmHg consistent with an acute respiratory acidosis. The calculated Alveolar-arterial (A-a) gradient is 15 (normal for her age).
Which of the following most likely explains her hypoxemia?
The correct answer is:
C) Hypoventilation
Hypoventilation can result from processes such as decreased tidal volume or decreased respiratory rate, both of which make up an individual’s minute ventilation (MV). Patients with hypoxemia secondary to these processes typically have a normal A-a gradient and elevated PaCO2. This patient’s hypoventilation was mostly likely caused by her recent increase in opioid medication, which can cause respiratory depression through activation of the Mu receptor in the central nervous system.
Answer choice A: Decreased fraction of inspired oxygen (FiO2), is incorrect. The FiO2 on room air is 21%, and the only way to increase this value is by giving supplemental oxygen. This patient is on room air, so it is impossible to have a lower FiO2. High altitude only affects the atmospheric pressure by decreasing it; however, the FiO2 stays the same at high altitude.
Answer choice B: Diffusion limitation, is incorrect. Diffusion limitation results from processes that thicken the air-blood interface. Such processes include interstitial lung disease and pulmonary hypertension. They typically occur over longer periods of time and are unlikely to have caused this patient’s acute hypoxemia. Additionally, hypoxemia secondary to diffusion limitation usually presents with an elevated A-a gradient.
Answer choice D: Physiologic shunt, is incorrect. Physiologic shunt is a type of V/Q mismatch where alveolar ventilation is limited but alveolar perfusion is preserved. A classic clinical example of this is atelectasis or acute respiratory distress syndrome (ARDS).
Answer choice E: Ventilation/perfusion (V/Q) mismatch, is incorrect. V/Q mismatch encompasses physiologic shunt and alveolar dead space. V/Q mismatch is the most common cause of hypoxia. Alveolar dead space are areas in the lung with adequate ventilation but no corresponding perfusion such as a pulmonary embolism. Patients with hypoxemia secondary to V/Q mismatch typically have a widened A-a gradient and respond to supplemental oxygen.
Key Learning Point
There are many etiologies of hypoxemia. In patients with an elevated A-a gradient, treatment with supplemental oxygen can help distinguish V/Q mismatch (does correct) from anatomical shunt such as an intracardiac shunt (does not correct). In patients with a normal A-a gradient that respond to supplemental oxygen, think about alveolar hypoventilation.
