ARDS was first described in 1967, but the definition of the condition remained vague, leading to conflict over just what patients had the syndrome and what the mortality rate was. It was finally defined as a combination of a ratio between arterial blood gases and the percentage of oxygen in the gases being breathed by the patient in excess of 300mm of mercury, combined with bilateral infiltrates on a lung x-ray. For example, in a normal patient, arterial blood oxygen should be close to 100% in an normal atmosphere of 21% oxygen. If the amount of oxygen in the breathing air goes up while the arterial blood oxygen remains low, ARDS should be suspected and then confirmed with an x-ray.