A comparison between measured and calculated central venous oxygen saturation in critically ill patients

BACKGROUND: Central venous oxygen saturation (ScvO(2)) is often used to help to guide resuscitation of critically ill patients. The standard gold technique for ScvO(2) measurement is the co-oximetry (Co-oximetry_ScvO(2)), which is usually incorporated in most recent blood gas analyzers. However, in some hospitals, those machines are not available and only calculated ScvO(2) (Calc_ScvO(2)) is provided. Therefore, we aimed to investigate the agreement between Co-oximetry_ScvO(2) and Calc_ScvO(2) in a general population of critically ill patients and septic shock patients. METHODS: A total of 100 patients with a central venous catheter were included in the study. One hundred central venous blood samples were collected and analyzed using the same point-of-care blood gas analyzer, which provides both the calculated and measured ScvO(2) values. Bland and Altman plot, intra-class correlation coefficient (ICC), and Cohen’s Kappa coefficient were used to assess the agreement between Co-oximetry_ScvO(2) and Calc_ScvO(2). Multiple linear regression analysis was performed to investigate the independent explanatory variables of the difference between Co-oximetry_ScvO(2) and Calc_ScvO(2). RESULTS: In all population, Bland and Altman’s analysis showed poor agreement (+4.5 [-7.1, +16.1]%) between the two techniques. The ICC was 0.754 [(95% CI: 0.393–0.880), P< 0.001], and the Cohen’s Kappa coefficient, after categorizing the two variables into two groups using a cutoff value of 70%, was 0.470 (P <0.001). In septic shock patients (49%), Bland and Altman’s analysis also showed poor agreement (+5.6 [–6.7 to 17.8]%). The ICC was 0.720 [95% CI: 0.222–0.881], and the Cohen’s Kappa coefficient was 0.501 (P <0.001). Four independent variables (PcvO(2), Co-oximetry_ScvO(2), venous pH, and Hb) were found to be associated with the difference between the measured and calculated ScvO(2) (adjusted R(2) = 0.8, P<0.001), with PcvO(2) being the main independent explanatory variable because of its highest absolute standardized coefficient. The area under the receiver operator characteristic curves (AUC) of PcvO(2) to predict Co-oximetry_ScvO(2) ≥ 70% was 0.911 [95% CI: 0.837–0.959], in all patients, and 0.903 [95% CI: 0.784–0.969], in septic shock patients. The best cutoff value was ≥ 36 mmHg (sensitivity, 88%; specificity, 83%), in all patients, and ≥ 35 mmHg (sensitivity, 94%; specificity, 71%) in septic shock patients. CONCLUSIONS: The discrepancy between the measured and calculated ScvO(2) is clinically not acceptable. We do not recommend the use of calculated ScvO(2) to guide resuscitation in critically ill patients. In situations where the Co-oximetry technique is not available, relying on PcvO(2) to predict the measured ScvO(2) value above or below 70% could be an option.