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Rechner für die alveolo-arterielle Sauerstoffdifferenz

Berechnen Sie die alveolo-arterielle Sauerstoffdifferenz aus einer arteriellen Blutgasanalyse, mit dem altersabhängigen Normwert, PaO2/FiO2, SpO2/FiO2 und ROX-Index.

Medizinisch geprüft von Dr. Ivan IbáñezNº Col. 17/0548731. Juli 2026
Diagram of an alveolus and its capillary showing alveolar PAO₂, arterial PaO₂ and the gap between them, which is the A-a gradient.
The gradient is the gap between the oxygen in the alveolus and the oxygen that reaches the artery. A normal gap points away from the lung; a widened one points at it.

How to use this calculator

  1. Take PaO₂ and PaCO₂ straight from the arterial blood gas, in mmHg.
  2. Enter the FiO₂ the patient was breathing: 21% on room air, or the delivered fraction on oxygen.
  3. Enter the age — the normal gradient widens with age, so an 80-year-old is not compared against a 20-year-old.
  4. Adjust the barometric pressure if you are above sea level. This matters: at 2 240 m the same PaO₂ that looks abnormal at sea level can be entirely expected.
  5. Read the mechanism first, the number second. The point of the gradient is to tell you whether the lung is the problem.

FAQ

What is the alveolar-arterial gradient?

It is the difference between the oxygen tension in the alveolus (PAO₂, from the alveolar gas equation) and the one measured in arterial blood (PaO₂). It quantifies how much oxygen is lost crossing from air into blood.

What is a normal gradient for age?

The most widely used estimate is (age + 10) divided by 4 — about 10 mmHg at 30 years and about 20 mmHg at 70. That is why the gradient is always read with the age in front of it.

Why does the gradient separate the causes of hypoxaemia?

If someone is breathing too little, or the inspired air is oxygen-poor at altitude, alveolus and artery fall together and the gap between them stays normal. If the lung itself is the problem, oxygen does not cross properly and the gap widens.

Why adjust the barometric pressure?

The alveolar gas equation is pressure-dependent. At altitude there is less pressure and PAO₂ is physiologically lower. Leaving 760 mmHg when the blood gas was drawn in Mexico City or Bogotá invents a gradient that is not there.

CalcVita. (2026). Rechner für die alveolo-arterielle Sauerstoffdifferenz. CalcVita. Retrieved 31. Juli 2026, from https://calcvita.com/de/rechner/alveolo-arterielle-differenz