The successful use of central venous oxygen saturation (ScvO₂) as a haemodynamic goal in the management of early sepsis has led to interest in the use of this parameter in surgical patients 1. ScvO₂ measurement requires placement of a central venous catheter so that the tip lies in the superior vena cava. Readings may be taken intermittently by blood sampling and co-oximetry, or continuously with a spectrophotometric catheter. Experimental studies have shown that changes in ScvO₂ closely reflect circulatory disturbances during periods of hypoxia, haemorrhage and subsequent resuscitation 23. Fluctuations correlate well with those of mixed venous saturation (SvO₂), although absolute values differ 23. Observational studies have described changes in ScvO₂ in various groups 4. In particular, the prognostic significance of ScvO₂ reductions to below 65% has been demonstrated in trauma 5, severe sepsis 6, myocardial infarction 7 and cardiac failure 8. However, the only interventional trial of ScvO₂ conducted so far used a goal of 70% 1.

Although the association between cardiac index (CI), oxygen delivery index (DO₂I) and related parameters and outcome after major surgery has been well described 91011121314, only limited data are available describing ScvO₂ values in the peri-operative period 15. The physiology of ScvO₂ disturbances is complex. The value of ScvO₂ is determined by changes in oxygen delivery and consumption, both of which are subject to considerable variation during the peri-operative period 4. It is not appropriate to assume that either the normal value or fluctuations in ScvO₂ will be similar to those of other patient groups. If ScvO₂ is to be used in the haemodynamic assessment of surgical patients, more detailed information is required describing fluctuations during the peri-operative period. The aim of this study was to describe changes in ScvO₂ after major general surgery and their relationship to outcome.

Data was collected from 117 patients between November 2002 and August 2004. Five patients were excluded from the analysis because ScvO₂ data were collected with a spectrophotometric catheter. Sixty-four patients developed 123 complications in all. There were 12 deaths (10.2%). The mean ± SD age was 66.8 ± 11.4 years. Twenty patients (17%) underwent emergency surgery and 77 patients (66%) were male. The APACHE II score was 9.5 ± 4.1, with a predicted mortality of 10.3 ± 9.0%. The P-POSSUM score was 38.6 ± 7.7, with a predicted mortality of 16.7 ± 17.6%. Fifty-seven (49%) patients were extubated within 1 hour of surgery and a further 29 (25%) were extubated before the end of the 8-hour study period.

The major finding of this study is the occurrence of considerable fluctuations in ScvO₂ after major general surgery that have prognostic significance. Multivariate analysis identified the lowest ScvO₂ value, lowest CI value and P-POSSUM score as independent predictors of complications. This observation supports the hypothesis that the association between reductions in ScvO₂ and outcome is similar to that observed previously for CI and DO₂I 910111213. It is interesting to note that P-POSSUM score was an independent predictor of complications, but APACHE II score was not. This may be because P-POSSUM score was designed for use in surgical patients using data from the UK, whereas APACHE II was designed for use in mixed groups of critically ill patients using data from North America 1718. As might be expected, the use of GDT was associated with fewer post-operative complications. However, this association was not independent of other predictors of outcome. The observation of collinearity between CI, DO₂I and the use of GDT suggests that the level of DO₂I achieved by individual patients is more important than the approach to haemodynamic management.

The optimal cutoff value of ScvO₂ for prediction of complications was 64.4%. This is very similar to the value (65%) identified in other patient groups 567. Large fluctuations in ScvO₂ occur during the peri-operative period. Values of ScvO₂ decreased significantly during the first hour after surgery, while CI and DO₂I remained unchanged. A significant increase in oxygen consumption therefore occurred during this period despite the fact that fewer than half of the patients were extubated within 1 hour of surgery. This finding is consistent with previous findings in cardiac surgical patients 14, as well as earlier work by Shoemaker 13. Post-operative oxygen consumption is determined by various factors including pain, emergence from anaesthesia, body temperature and shivering. Peri-operative disturbances of ScvO₂ cannot therefore be assumed to relate solely to DO₂I.

The 8-hour mean of ScvO₂ was 75.0% in patients who did not develop post-operative complications. This value was comparable to previous measurements in healthy conscious patients 1920, but higher than those taken immediately before induction of anaesthesia 15 and in patients with good outcome after trauma, severe sepsis, cardiac failure or myocardial infarction 5678. It is notable that derangements in CI, DO₂I and ScvO₂ were observed in the absence of similar disturbances in other commonly measured biochemical and physiological variables. This was despite the high rates of morbidity and mortality in the study population. It is possible that disturbances in ScvO₂, CI and DO₂I might indicate the presence of occult tissue hypoperfusion before disturbances in other parameters.

The use of observational data from an interventional trial has both advantages and disadvantages. In this study, goals for arterial oxygen saturation, haemoglobin, heart rate, mean arterial pressure, serum lactate and urine output were the same in all patients. All clinical management and data collection were closely supervised by a member of the research team in accordance with a carefully defined treatment protocol. The benefit of such rigorous study design must be offset against the fact that, in some patients, intravenous fluid administration was guided by central venous pressure, whereas in others fluid management was guided by stroke volume and supplemented with low-dose dopexamine. It is an inherent problem with studies of this type that the predictive nature of certain variables may relate both to the initial cardiovascular disturbance and subsequent attempts to correct it. The large number of statistical comparisons performed in the univariate analyses may seem speculative. This is not the case; comparisons made were of variables in which an association with outcome had previously been suggested 91011121314171821. We were therefore obliged to identify all such associations in the available data.

Reductions in ScvO₂ are common after major surgery and are associated with an increased rate of post-operative complications. Peri-operative changes in ScvO₂ relate to both oxygen consumption and delivery. Further evaluation of peri-operative trends in ScvO₂ should be performed before this variable is used as a haemodynamic goal in surgical patients.

• The successful use of central venous saturation in the management of severe sepsis has led to interest in the use of this variable in surgical patients.

• This analysis suggests that central venous saturation may have prognostic significance following major surgery.

• Further evaluation of peri-operative trends in central venous saturation is required.

APACHE = Acute Physiology and Chronic Health Evaluation; CI = cardiac index; DO₂I = oxygen delivery index; GDT = goal-directed therapy; ICU = intensive care unit; OR = odds ratio; P-POSSUM = Portsmouth Physiologic and Operative Severity Score for the enUmeration of Mortality and morbidity; ScvO₂ = central venous saturation.

RP received a travel grant from LiDCO Ltd. to present data at an international meeting. JF has previously performed consultancy work for LiDCO Ltd. DB currently performs consultancy work for LiDCO Ltd. and has previously performed consultancy work for Deltex Ltd. No other competing interests are declared.

RP, AR and DB were responsible for study design. RP, DD and JF were responsible for data collection. All authors were involved in data analysis and drafting the manuscript and approved the final version. All authors had full access to data and take responsibility for the integrity of the data and the accuracy of the analysis.