A clinically oriented nomogram for the derivation and interpretation of alveolar-arterial oxygen gradients

This paper is designed for those familiar with arterial blood gases, alveolar-arterial (A-a) oxygen gradients, and their application in the clinical setting. The concept of a gradient is reviewed, and several examples of clinical applications are given. A nomogram of A-a gradients is presented for two representative altitudes. Using only the PaO2 and PaCO2 available from blood gases, the corresponding A-a gradient can be found quickly. Further, the nomogram is presented in such a way as to facilitate interpretation of results. Finally, we present a computer program that will allow the production of a nomogram of A-a gradients appropriate for a specific altitude.