The potential reasons for these findings and the underlying pathophysiologic aspects are numerous. First, at a length of 20 cm in a normal-sized adult, a femoral catheter remains in the iliac vein. In this position, the distal tip does not sample the venous return of any intraabdominal organs. Second, in times of physiologic stress, perfusion to kidneys, muscle, and splanchnic regions of the body may be decreased, while flow to the myocardium and brain is relatively preserved.14 Other investigators have postulated that this redistribution of blood flow and oxygen delivery accounts for the numeric discrepancy between Svo2 and Scvo2.7,11,12,14,47,48 During states of circulatory collapse, Scvo2 values are higher when compared with Svo2, reflecting regional differences in perfusion and oxygen delivery.43,47 Ruokonen et al49 measured blood flow distribution and regional oxygen delivery in septic shock before and after vasopressor therapy. Oxygen delivery and oxygen consumption increased dramatically in splanchnic and leg blood flow during vasopressor therapy. This was out of proportion to the changes seen in systemic oxygen delivery, and thus the authors concluded that regional changes in oxygen delivery in septic shock cannot predict systemic changes. Sander et al50 compared Svo2 and Scvo2 in patients with cardiac conditions and found that Scvo2 overestimated Svo2 when Svo2 levels were low. Conversely, Scvo2 underestimated Svo2 when Svo2 levels were high. The regional oxygen extraction rate was the principal difference between Svo2 and Scvo2. Similarly, differences in regional extraction may account for the lack of correlation of Scvo2 and Sfvo2 in our study.