A reduction in muscle oxygen was also found to be an earlier indicator of hypovolaemia than the standard clinical measures new post (heart rate and blood pressure) [35]. Nevertheless, previous studies have suggested that resting StO2 values are insensitive to the assessment of tissue hypoperfusion [36]. Our results, which indicate similar resting StO2 values in spite of an insufficient flow, support the findings of these previous studies.We also found a 50% increase in the StO2 recovery slope with fluid loading. This suggests that restoration of intravascular volume in preload-dependent patients improves muscle tissue oxygenation and increases SV and CO. This finding may have important clinical implications, because the ultimate goal of resuscitation should be improvement of tissue oxygenation and perfusion.

We hypothesise that this is due to improved microvessel recruitment during the fluid challenge, together with an increase venular blood compartment volume, despite the absence of macrocirculatory changes. It is unlikely that changes in the StO2 recovery slope with VE were due to changes in rheologic factors, because we found no significant differences in haemoglobin levels before and after VE. In addition, Creteur et al. [37] recently performed VOT before and after red blood cell transfusion and reported no differences despite the different haemoglobin levels. It must be stressed that the StO2 recovery slope remained low, or even decreased, in some of our patients after VE (Figure (Figure2),2), even though CO improved with VE.

This is in agreement with the hypothesis that VE can cause apparent improvement in systemic parameters, even though microcirculation and tissue oxygenation remain uncorrected. In addition, a 500-mL fluid infusion might have been insufficient in some patients.Our study has several limitations that need to be addressed. First, we studied only surgical patients. Although major surgery is associated with significant impairment in both microvascular flow and tissue oxygenation [38], our data should not be extrapolated to other, more specific patient populations (with an increased intersubject variability) until further investigations are carried out. In addition, repeated measurements were performed in some patients. An advantage of analysing the response to repeated fluid loading is that it reproduces daily practice, when fluid responsiveness has to be evaluated in the same patient on different occasions.

Second, we did not evaluate patient outcomes. In other words, we did not determine whether higher StO2 recovery slopes were associated reduced organ failure. Third, we placed the NIRS probe on the thenar eminence, a region with very little Batimastat fat, and therefore there was little interference with the spectroscopic measurements.