A-a Gradient Calculator

The Alveolar-Arterial (A-a) gradient is the difference between the oxygen concentration in the alveoli (PAO₂) and the oxygen measured in arterial blood (PaO₂). It reflects how efficiently oxygen is transferred from the lungs into the bloodstream. A widened gradient suggests impaired gas exchange.

The alveolar gas equation is used first: PAO₂ = (FiO₂ × (760 − 47)) − (PaCO₂ / 0.8). This assumes 100% humidity at sea level and a respiratory quotient of 0.8. The A-a gradient is then calculated as PAO₂ − PaO₂.

A normal A-a gradient is generally less than 10 mmHg on room air. However, normal values increase with age. A common age-adjusted estimate of the upper normal limit is: (Age / 4) + 4 mmHg. This calculator uses that formula to flag elevated gradients.

A high A-a gradient most commonly results from ventilation-perfusion (V/Q) mismatch or intrapulmonary shunting. Common causes include pulmonary embolism, pneumonia, pulmonary edema, ARDS, and atelectasis. Impaired diffusion (e.g., pulmonary fibrosis) can also widen the gradient.

A normal A-a gradient in a hypoxemic patient suggests the lungs are functioning properly. The hypoxemia is likely due to hypoventilation (e.g., opioid overdose, neuromuscular disease) or low ambient oxygen (high altitude), rather than a pulmonary pathology.

Room air has an FiO₂ of 0.21 (21%). If the patient is receiving supplemental oxygen, use the appropriate FiO₂ for their delivery method — for example, 0.44 for 4 L/min nasal cannula or 1.0 for a non-rebreather mask at high flow.

The standard atmospheric pressure at sea level is 760 mmHg. The formula subtracts 47 mmHg for water vapor pressure at body temperature. At higher altitudes, the lower atmospheric pressure reduces PAO₂, which affects the A-a gradient calculation — adjustments are needed for high-altitude settings.

The A-a gradient can support the clinical suspicion for PE — most PE patients have a widened gradient — but it is not diagnostic on its own. A normal gradient does not rule out PE, and a widened gradient has many other causes. It is best used alongside clinical assessment and other diagnostic tools.