title: Stability of the arterial/alveolar oxygen partial pressure ratio. Effects of low ventilation/perfusion regions.
abstract: The alveolar-arterial oxygen partial pressure difference (AaDO2) and the arterial/alveolar oxygen partial pressure ratio (a/APO2) were compared for stability when inspired oxygen concentration (FIO2) changed. The analysis was based on a three-compartment lung model and experimental results in 10 patients with respiratory failure receiving assisted ventilation. It was found that a/APO2 was more stable than AaDO2 and more useful for: (1) comparing gas exchange in patients receiving different levels of FIO2, (2) following gas exchange in the same patient as FIO2 is changed, and (3) estimating the PaO2 expected at a given level of FIO2 if blood gas data are available at another level. However, areas with low ventilation/perfusion (V/Q) ratios may cause sudden changes in a/PO2 at certain critical values of PAO2. Most stable is a/APO2 and, therefore, most useful at FIO2 levels greater than 0.3, and PaO2 levels less than 100 torr.
mesh_terms: D000328:Adult; D000368:Aged; D001784:Blood Gas Analysis; D006801:Humans; D008168:Lung / Q000502:physiology; D008875:Middle Aged; D008954:Models, Biological; D010100:Oxygen / Q000032:analysis* / Q000097:blood; D010313:Partial Pressure; D011650:Pulmonary Alveoli / Q000378:metabolism*; D012121:Respiration, Artificial; D012131:Respiratory Insufficiency / Q000378:metabolism / Q000628:therapy; D014692:Ventilation-Perfusion Ratio
chemical_list: D010100:Oxygen