The A-a gradient is used to assess which physiologic parameter?

The A-a gradient is used to assess which physiologic parameter?

• A. Alveolar-arterial gradient
• B. Arterial-arterial gradient
• C. Alveolar-venous gradient
• D. Pulmonary venous gradient

Answer: (D)

The A-a gradient measures how effectively oxygen moves from the alveoli into the arterial blood. It is the difference between the alveolar oxygen tension (PAO2) and the arterial oxygen tension (PaO2). This gradient is calculated using the alveolar gas equation (PAO2 = FiO2 × (PB − PH2O) − (PaCO2/R)) and then subtracting PaO2 to get PAO2 − PaO2. A normal or small A-a gradient with hypoxemia usually points to hypoventilation or low FiO2, while an elevated gradient indicates impaired transfer from alveoli to blood—due to diffusion limitation, V/Q mismatch, or intrapulmonary shunt. In clinical questions like this, the A-a gradient specifically reflects the alveolar-arterial gradient, not a pulmonary venous gradient. Pulmonary venous gradient isn’t the parameter assessed by the A-a gradient.

The A-a gradient measures how effectively oxygen moves from the alveoli into the arterial blood. It is the difference between the alveolar oxygen tension (PAO2) and the arterial oxygen tension (PaO2). This gradient is calculated using the alveolar gas equation (PAO2 = FiO2 × (PB − PH2O) − (PaCO2/R)) and then subtracting PaO2 to get PAO2 − PaO2.

A normal or small A-a gradient with hypoxemia usually points to hypoventilation or low FiO2, while an elevated gradient indicates impaired transfer from alveoli to blood—due to diffusion limitation, V/Q mismatch, or intrapulmonary shunt. In clinical questions like this, the A-a gradient specifically reflects the alveolar-arterial gradient, not a pulmonary venous gradient.

Pulmonary venous gradient isn’t the parameter assessed by the A-a gradient.