Low and “supranormal” central venous oxygen saturation and markers of tissue hypoxia in cardiac surgery patients: a prospective observational study

To characterize incidence of low, normal and “supranormal” central venous oxygen saturation (ScvO₂) and the relation to markers of tissue hypoxia, course and outcome in cardiac surgery patients.

Prospective, observational study in a university multidisciplinary 50-bed intensive care unit including 205 consecutive patients undergoing elective cardiac surgery. Data were split into training and test data sets and subjected to 50 replications of fivefold cross-validation to estimate lower and upper bounds of ScvO₂ indicative of impaired tissue oxygenation.

Both low (≤60.8%) and supranormal (≥77.4%) ScvO₂ were associated with an unfavorable course, while the logistic EuroSCORE for risk adjustment was comparable between groups. Incidences of abnormal ScvO₂ were 13.2% low and 30.7% supranormal. Patients with low ScvO₂ and an uneventful course initially presented with normal lactate levels, whereas patients with supranormal ScvO₂ displayed consistently higher serum lactate levels. High ScvO₂ values were associated with the use of β-mimetics and signs of systemic inflammation. Mortality rates were comparable for patient populations presenting either low (14.8%) or supranormal ScvO₂ (7.9%) and higher than normals (0%, p < 0.001). Lactate was comparably increased in patients that ultimately died, irrespective whether they had low or supranormal ScvO₂ values. In contrast, neither low nor supranormal ScvO₂ was associated with altered gastric pCO_2.

High ScvO₂ is an under-recognized warning sign for impaired tissue oxygenation in the peri-operative period. Including values ≥77.4% as ‘normal’ impaired performance of ScvO₂ monitoring to predict a complicated perioperative course.