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Division of Cardiothoracic and Vascular Anesthesia and Intensive Care; University Hospital; Vienna, Austria; martin.dworschak{at}meduniwien.ac.at
To the Editor:
Murkin et al. (1) showed that goal-directed intraoperative interventions (i.e., preventing hyperventilation, raising mean arterial pressure >50 mm Hg, decreasing central venous pressure <10 mm Hg, increasing pump flow >2 L · m–2 · min–1, giving transfusions when hematocrit decreases <20%, increasing Fio2, deepening anesthesia, and pulsatile flow) driven by a decrease in cerebral regional oxygenation (rSO2) to <75% of baseline were associated with a marginally shorter ICU stay and slightly lower postoperative major organ morbidity and mortality after cardiac surgery when compared with patients who were treated in the same way but without rSO2 monitoring.
It remains unclear, however, if relative rSO2 changes from the right or the left hemisphere were used as trigger for an intervention. Interestingly, only absolute values correlated with LOS >10 days. Without hemispheric related differences (4), using one expensive sensor would also have been sufficient. Furthermore, the exact algorithm is not specified, as well as the priorities of the selected interventions, and the projected goal of the optimization (desired rSO2 versus parameter in the optimal range). Moreover, interventions in the treatment group, unlike those in controls, were actually preemptive measures. They already started 15 s after rSO2 decreased to less than the predetermined threshold value of 75% of baseline and therefore minimized the duration of cerebral desaturations defined as rSO2 <70% baseline for 
1 min. It is therefore surprising that a relatively arbitrary parameter, i.e., rSO2 AUC <70% baseline >150 min but not rSO2 AUC <40% baseline or rSO2 minimum, was statistically different between groups. This procedure nevertheless could have introduced a methodological flaw.
Unlike SvO2, which, like rSO2, does not require pulsatile flow, rSO2 has not yet been validated for its adequacy to guide a goal-directed therapy aiming to optimize oxygen transport and tissue oxygenation (2,3). It does neither reflect global brain oxygenation nor the oxygen balance of the whole body. Heterogenous tissue oxygenation may therefore either give a false sense of security or could lead to incorrect conclusions. Although particular interventions to improve oxygen delivery carried out as part of a goal-directed therapy may also increase rSO2 one should be cautious before recommending rSO2 as a surrogate indicator of the global well-being. Its usefulness as a guide to institute and evaluate nonspecific therapeutic measures other than those directed to optimize brain oxygenation still requires further investigations.
REFERENCES
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