Stability of the arterial/alveolar oxygen partial pressure ratio. Effects of low ventilation/perfusion regions

Crit Care Med. 1979 Jun;7(6):267-72. doi: 10.1097/00003246-197906000-00003.


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

  • Adult
  • Aged
  • Blood Gas Analysis
  • Humans
  • Lung / physiology
  • Middle Aged
  • Models, Biological
  • Oxygen / analysis*
  • Oxygen / blood
  • Partial Pressure
  • Pulmonary Alveoli / metabolism*
  • Respiration, Artificial
  • Respiratory Insufficiency / metabolism
  • Respiratory Insufficiency / therapy
  • Ventilation-Perfusion Ratio


  • Oxygen