Arterial blood gases Arterial oxygen tension

The arterial oxygen tension (Pao2) in a normal subject breathing air at atmospheric pressure ranges from 12.5 kPa (94 mmHg) at age 20 years to 10.8 kPa (81 mmHg) at age 60 to 69 years.The alveolar partial pressure of oxygen PAo2 varies with the inspired oxygen concentration as shown by the simplified alveolar air equation

where Fio2 is the inspired oxygen fraction, PB is the atmospheric pressure (100 kPa), Ph2o is the water vapor pressure, Paco2 is the alveolar partial pressure of carbon dioxide, and RQ is the respiratory quotient (RQ = 0.8).

Thus a healthy patient with a normal Paco2 breathing 60 per cent oxygen should have a calculated Pao2 of about 50 kPa (375 mmHg). A Pao2 of less than a third of the predicted value represents a very significant impairment of respiratory function, and mechanical ventilation should be considered. Use of the Pao2/Fio2 ratio relieves the clinician from calculations of this type. At the American-European Consensus Conference (Bernard et al. 1992), acute lung injury was defined as a Pao^/Fio2 < 300 mmHg (40 kPa) and acute respiratory distress syndrome as Pao^Fio2 < 200 mmHg (26.7 kPa).

Falling oxygen saturation can also be a useful indicator of respiratory failure. However, this can be a pitfall for the inexperienced. A saturation of 100 per cent corresponds to any Pao2 over 12 kPa (90 mmHg). Thus a patient with severe acute respiratory distress syndrome on 60 per cent oxygen may have a Pao2 of only 12 kPa (90 mmHg) and yet a saturation of 100 per cent. On this inspired mixture the patient should have a Pao2 of 50 kPa (375 mmHg) and is clearly in respiratory failure. Misinterpretation of measurements in this way may slow the referral of critically ill patients to the intensive care unit.

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