Relation Between Oxygen Consumption and Oxygen Delivery After Cardiac Surgery

In Response: In our article [1], we warned against the risk of mathematical coupling of data when oxygen uptake (VO2) and oxygen delivery (DO2) are calculated from the same values of hemoglobin, arterial oxygen saturation (SaO2), and especially cardiac index. There is generally a good agreement between direct and indirect determinations of VO2[2], but both methods have their limitations. In this study [1] as in others [3,4], we preferred to assess the relationship between cardiac index and oxygen extraction, because this relationship is not subject to mathematical coupling of data [5]. We strongly disagree with Dr. Myles and Dr. McRae when they state that a strong relationship between VO2 and DO2 implies VO2/DO2 dependency. On the contrary, we wrote that the close relationship between VO2 and DO2 should not be interpreted as VO2/DO2 dependency. The demonstration of VO2/DO2 dependency should be based on repeated assessments of VO2 during an acute change in DO2[6]. We claimed that the progressive increased in VO2 reflected the progressive increase in oxygen demand after cardiac surgery, and that changes in cardiac index were adaptive. When the cardiac index response was insufficient, mixed venous oxygen saturation decreased, reflecting an increase in oxygen extraction. Jean-Louis Vincent, MD, PhD Christina Routsi, MD Department of Intensive Care, Cliniques Universitaires de Bruxelles, Hospital Erasme, Brussels, Belgium