Monitoring changes in mixed venous oxygen saturation using photoplethysmography on the sternum - a proof of concept study.

Mixed venous oxygen saturation (SvO2) is a valuable indicator of the balance between oxygen delivery and consumption in the body. However, the invasiveness and risk of complications limit its use. This study aimed to test the feasibility of continuously monitoring variations in SvO2 from photoplethysmography (PPG) measurements of sternal intramedullary oxygen saturation. In an animal model, including anesthetized pigs (n = 15, mean 33.1 (SD 2.0) kg), alterations in SvO2 were instituted by separate sessions: incremental decrease in arterial oxygen saturation using hypoxic gas mixtures (N2/air), and hemorrhage of 20% plus 20% of estimated blood volume. Estimations of non-invasive sternal oxygen saturation (nSsO2) were derived from calibrating PPG measurements against SvO2 sampled from the pulmonary artery. Individual linear correlations were found between SvO2 and PPG measurements. nSsO2 and SvO2 co-varied, except in severe hypovolemia. A concordance rate between nSsO2 and SvO2 was determined to 87% (95% CI 79.1-95.0). In this study, we demonstrated linear correlations between SvO2 and PPG-measurements on the sternum, enabling estimation of nSsO2, which trends with changes in SvO2. The results support further technical development of non-invasive monitoring of SvO2 from sternal measurements.