The alveolar-arterial (A-a) oxygen gradient is a core tool for working up hypoxemia — it compares the oxygen level the lungs should theoretically be able to deliver (calculated from the alveolar gas equation) against the oxygen level actually measured in arterial blood. The gap between the two narrows down whether low oxygen is from simple hypoventilation or from a diffusion, shunt, or ventilation-perfusion problem. This guide explains the alveolar gas equation, how to interpret the gradient, and why age changes what counts as normal.

The alveolar gas equation and where PAO2 comes from

The alveolar gas equation, PAO2 = FiO2 × (Pb − PH2O) − PaCO2 ÷ RQ, estimates the partial pressure of oxygen in the alveoli. FiO2 is the fraction of inspired oxygen (0.21 on room air, up to 1.0 on 100% supplemental oxygen). Pb is barometric pressure, 760 mmHg at sea level and lower at altitude. PH2O, the water vapor pressure of fully humidified air at body temperature, is a fixed constant of 47 mmHg. PaCO2 is measured directly from an arterial blood gas, and RQ (the respiratory quotient — the ratio of CO2 produced to O2 consumed) defaults to 0.8 for a typical mixed diet.

PAO2 is a calculated value — it cannot be measured directly at the bedside. PaO2, by contrast, is measured directly from the same arterial blood gas sample. The A-a gradient is simply PAO2 − PaO2.

Interpreting a normal versus an elevated gradient

A normal A-a gradient with hypoxemia points toward hypoventilation as the cause — the lungs are working normally, but the patient simply isn't moving enough air (for example, from opioid overdose, neuromuscular weakness, or severe obesity hypoventilation). An elevated A-a gradient with hypoxemia points toward a problem within the lung itself: a diffusion defect (e.g. pulmonary fibrosis), a shunt (blood bypassing ventilated alveoli entirely, as in some congenital heart defects or severe pneumonia), or ventilation-perfusion (V/Q) mismatch (blood flowing to poorly ventilated regions, as in pulmonary embolism, COPD, or asthma).

The gradient itself doesn't diagnose which of these is happening — it narrows the differential and directs further workup, such as imaging or a response to supplemental oxygen (shunt physiology characteristically responds poorly to added oxygen, while V/Q mismatch responds better).

Why age-adjustment matters, and the calculator's limits

The A-a gradient normally widens with age, independent of any disease — a reflection of the gradual, normal increase in ventilation-perfusion mismatch that comes with aging lungs. A flat threshold (often quoted as roughly 5–10 mmHg for a young adult on room air) will over-call mild elevations in older patients as abnormal. The age-adjusted normal upper limit used here, approximately (age ÷ 4) + 4 mmHg, corrects for that so an older patient's gradient is judged against a realistic expectation for their age.

This calculator assumes steady-state arterial blood gas values and a typical respiratory quotient; it is a decision-support and education tool, not a diagnosis. The A-a gradient is one data point in a hypoxemia workup — it should always be interpreted alongside the full clinical picture by a qualified clinician, not used on its own to rule a cause in or out.