Arterial Oxygen Saturation vs. Alveolar Oxygen Tension in Anatomical Venous-Arterial Shunting

¹ From the Departments of Physiology, Surgery and Mathematical Biology, University of Chicago, Chicago, Illinois

The ‘saturation-tension’ curve relating O₂ saturation to alveolar Po₂, previously described by us for evaluation of venous admixture and diffusion difficulty in the lung, was quantitatively studied in dogs with right-left anatomical shunts prepared by anastomosing pulmonary artery and vein following pneumonectomy. This curve, obtained by oximeter, end-tidal sampler and O₂ analyzer, rises in essentially linear fashion at high O₂ tensions, both for arterial and for mixed venous blood. When saturation of arterial blood is replotted against alveolar minus arterial O₂ tension, the resulting curve is truly linear at high O₂ tensions. This was indicated by a theoretical expression derived from the so-called mixing equation for calculating fraction of shunting. The latter may alternatively be calculated using an extrapolation procedure based on the above equation, which was also used to construct theoretical saturation-tension curves for various fractions of shunting. These curves indicate the quantity of O₂ carried by hemoglobin, with anatomical shunting and also without. Saturation-tension curves may therefore be used for diagnosing right-left anatomical shunts in patients, as proven in several cases, without recourse to blood samples.