Comparison of non-invasive peripheral venous saturations with venous blood co-oximetry

The estimation of venous oxygen saturations using photoplethysmography (PPG) may be useful as a noninvasive continuous method of detecting changes in regional oxygen supply and demand (e.g. in the splanchnic circulation). The aim of this research was to compare PPG-derived peripheral venous oxygen saturations directly with venous saturation measured from co-oximetry blood samples, to assess the feasibility of non-invasive local venous oxygen saturation. This paper comprises two similar studies: one in healthy spontaneously-breathing volunteers and one in mechanically ventilated anaesthetised patients. In both studies, PPG-derived estimates of peripheral venous oxygen saturations (SxvO₂) were compared with co-oximetry samples (ScovO₂) of venous blood from the dorsum of the hand. The results were analysed and correlation between the PPG-derived results and co-oximetry was tested for. In the volunteer subjects,moderate correlation (r = 0.81) was seen between SxvO₂ values and co-oximetry derived venous saturations (ScovO₂), with a mean (±SD) difference of +5.65 ± 14.3% observed between the two methods. In the anaesthetised patients SxvO₂ values were only 3.81% lower than SpO₂ and tended to underestimate venous saturation (mean difference = -2.67 ± 5.89%) while correlating weakly with ScovO₂ (r = 0.10). The results suggest that significant refinement of the technique is needed to sufficiently improve accuracy to produce clinically meaningful measurement of peripheral venous oxygen saturation. In anaesthetised patients the use of the technique may be severely limited by cutaneous arteriovenous shunting.