A 42-year-old woman presents to the emergency department complaining of sudden-onset shortness of breath and right-sided pleuritic chest pain. She recently returned from a 14-hour international flight. She has no significant past medical history and takes no medications other than oral contraceptives.
Vital signs:
Temperature: 37.1°C (98.8°F)
Blood pressure: 128/78 mm Hg
Heart rate: 115/min
Respiratory rate: 26/min
Oxygen saturation: 88% on room air
Arterial blood gas analysis on room air reveals:
pH: 7.48
: 28 mm Hg
: 55 mm Hg
: 20 mEq/L
Assume the inspired partial pressure of oxygen () is 150 mm Hg and the respiratory quotient () is 0.8.
What is this patient's alveolar-arterial () oxygen gradient in mm Hg?
Answer: 60 mm Hg
Answer
60
The alveolar-arterial (A-a) gradient evaluates the integrity of the alveolar-capillary membrane and helps differentiate causes of hypoxemia. It is calculated by first determining the alveolar oxygen tension () using the alveolar gas equation: . Given mm Hg, mm Hg, and , the is mm Hg. The A-a gradient is then , which is mm Hg. A normal A-a gradient is typically less than 10-15 mm Hg (or approximated by ). An elevated gradient in this patient is highly consistent with a pulmonary embolism, which creates a ventilation-perfusion (V/Q) mismatch.
Step-by-Step Solution
Key Concept
Calculation of the alveolar-arterial (A-a) oxygen gradient to evaluate hypoxemia and ventilation-perfusion mismatch.