Oxygen saturation in intraosseous sternal blood measured by CO-oximetry and evaluated non-invasively during hypovolaemia and hypoxia – a porcine experimental study

Purpose: This study intended to determine, and non-invasively evaluate, sternal intraosseous oxygen saturation (SsO(2)) and study its variation during provoked hypoxia or hypovolaemia. Furthermore, the relation between SsO(2) and arterial (SaO(2)) or mixed venous oxygen saturation (SvO(2)) was investigated. Methods: Sixteen anaesthetised male pigs underwent exsanguination to a mean arterial pressure of 50 mmHg. After resuscitation and stabilisation, hypoxia was induced with hypoxic gas mixtures (air/N(2)). Repeated blood samples from sternal intraosseous cannulation were compared to arterial and pulmonary artery blood samples. Reflection spectrophotometry measurements by a non-invasive sternal probe were performed continuously. Results: At baseline SaO(2) was 97.0% (IQR 0.2), SsO(2) 73.2% (IQR 19.6) and SvO(2) 52.3% (IQR 12.4). During hypovolaemia, SsO(2) and SvO(2) decreased to 58.9% (IQR 16.9) and 38.1% (IQR 12.5), respectively, p < 0.05 for both, whereas SaO(2) remained unaltered (p = 0.44). During hypoxia all saturations decreased; SaO(2) 71.5% (IQR 5.2), SsO(2) 39.0% (IQR 6.9) and SvO(2) 22.6% (IQR 11.4) (p < 0.01), respectively. For hypovolaemia, the sternal probe red/infrared absorption ratio (SQV) increased significantly from baseline (indicating a reduction in oxygen saturation) + 5.1% (IQR 7.4), p < 0.001 and for hypoxia + 19.9% (IQR 14.8), p = 0.001, respectively. Conclusion: Sternal blood has an oxygen saturation suggesting a mixture of venous and arterial blood. Changes in SsO(2) relate well with changes in SvO(2) during hypovolaemia or hypoxia. Further studies on the feasibility of using non-invasive measurement of changes in SsO(2) to estimate changes in SvO(2) are warranted.