We read with great interest the study reported by Davison et al1 in a recent issue of CHEST (July 2010). We completely agree with their hypothesis and part of their conclusion: Femoral venous oxygen saturation (Svo2) cannot always be used as a substitute for central mixed Svo2 in patients who need critical care. However, we think that femoral Svo2 can be used to guide resuscitation. It has been established that regional differences in oxygen consumption account for the regional differences in Svo22; hence, Svo2 measurements in the superior vena cava (SVC), inferior vena cava (IVC), and pulmonary artery (PA) can diverge significantly.1,2 Cerebral and cardiac consumption account largely for the Svo2 in the SVC and PA, whereas IVC Svo2 reflects mainly liver, kidney, gut, and skin consumption.3 Thus, the oxygen content in the IVC reflects only a part of the total oxygenation of a patient who is critically ill, but the clinical difference with the Svo2 in the SVC is lessened if the trend in femoral Svo2 is used, rather than the absolute value. Trends can be extremely helpful in the treatment of the patient who is critically ill. Moreover, in these patients, since the resting oxygen demands cannot be met, blood flow will be reduced through low-extraction tissues, such as the liver and gut, to be rerouted to essential tissues such as the brain; in this situation, the IVC Svo2 will reliably show an early decrease, indicating tissue hypoxia.3 The study by Davison et al1 demonstrates elegantly the discrepancy between Svo2 measurements in the upper and lower body in patients who are critically ill. However, considering the low margin between the mean Svo2 levels reported (73.1% ±11.6% vs 69.1% ±12.9%) and considering the utility of using trends rather than absolute values in the individual patient, IVC Svo2 is still clinically useful, though numerically different, and should not be discarded from the critical care armamentarium.