Role of Alveolar-Arterial Gradient in Partial Pressure of Oxygen and PaO2/Fraction of Inspired Oxygen Ratio Measurements in Assessment of Pulmonary Dysfunction.

Assessment of pulmonary dysfunction is vital to anesthetists. Measurements including the gradient between the alveolar partial pressure of oxygen (PAo2) and the arterial partial pressure of oxygen (Pao2), called the PAo2 - Pao2 , and the ratio of the Pao2 to the fraction of inspired oxygen (FIo2) (Pao2/FIo2 ratio) are useful in determining the extent of acute lung injury. A literature review via MEDLINE using the terms PAo2 - Pao2 , Pao2/FIo2 ratio, and pulmonary dysfunction was performed to identify articles on the use of these measures in the perioperative period. Both measures have been found to predict clinical outcomes in most settings. We also developed a mathematical model to calculate values of the PAo2 - Pao2 and the Pao2/FIo2 ratio. In model results, as in clinical findings, both respond appropriately to reflect worsening pulmonary dysfunction when shunt or diffusion barrier (alveolar Po2 - pulmonary capillary partial pressure of oxygen) is increased. However, both are also sensitive to the FIo2. The increase in the Pao2/FIo2 ratio as the FIo2 increases is particularly problematic because it could disguise a deterioration in the patient's pulmonary status. The PAo2 - Pao2 and the Pao2/FIo2 ratio should be used with an understanding of their limitations. Copyright© by the American Association of Nurse Anesthetists.