The long form of the alveolar gas equation involves several assumptions that make precise calculation of PAO2 virtually impossible. Despite this, past attempts to abbreviate the equation have sought an unwarranted degree of precision. In addition, well-defined normal values are not known for alveolar-arterial PO2 difference at all FIO2 levels and for all ages. Based on these considerations, any abbreviation of the alveolar gas equation should be simple, yet accurate enough for clinical purposes. The one equation that meets these criteria is PAO2 = PIO2 - 1.2(PaCO2). When FIO2 is above 0.60, the factor 1.2 can be eliminated for increased accuracy.
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