Background:After cardiac surgery,central venous oxygen saturation (ScvO2) and serum lactate concentration are often used to guide resuscitation;however,neither are completely reliable indicators of global tissue hypoxia.This observational study aimed to establish whether the ratio between the veno-arterial carbon dioxide and the arterial-venous oxygen differences (P(v-a)CO2/C(a-v)O2) could predict whether patients would respond to resuscitation by increasing oxygen delivery (DO2).Methods:We selected 72 patients from a cohort of 290 who had undergone cardiac surgery in our institution between January 2012 and August 2014.The selected patients were managed postoperatively on the Intensive Care Unit,had a normal ScvO2,elevated serum lactate concentration,and responded to resuscitation by increasing DO2 by ＞10％.As a consequence,48 patients responded with an increase in oxygen consumption (VO2) while VO2 was static or fell in 24.Results:At baseline and before resuscitative intervention in postoperative cardiac surgery patients,a P(v-a)CO2/C(a-v)O2 ratio ≥1.6 mmHg/ml predicted a positive VO2 response to an increase in DO2 of＞1 0％ with a sensitivity of 68.8％ and a specificity of 87.5％.Conclusions:P(v-a)CO2/C(a-v)O2 ratio appears to be a reliable marker of global anaerobic metabolism and predicts response to DO2 challenge.Thus,patients likely to benefit from resuscitation can be identified promptly,the P(v-a)CO2/C(a-v)O2 ratio may,therefore,be a useful resuscitation target.