Central venous-to-arterial carbon dioxide gradient as a marker of occult tissue hypoperfusion after major surgery

The central venous-arterial carbon dioxide tension gradient ('CO₂gap') has been shown to correlate with cardiac output and tissue perfusion in septic shock. Compared to central venous oxygen saturation (SCVO2), the CO₂gap is less susceptible to the effect of hyperoxia and may be particularly useful as an adjunctive haemodynamic target in the perioperative period. This study investigated whether a high CO₂gap was associated with an increased systemic oxygen extraction (O2ER >0.3) or occult tissue hypoperfusion in 201 patients in the immediate postoperative period. The median CO₂gap of all patients was 8 mmHg (IQR 6 to 9), and a large CO₂gap was very common (> 6mmHg in 139 patients [69%], 95% CI 63 to 75; >5 mmHg in 170 patients [85%], 95% CI 79 to 89). A CO₂ gap >5 mmHg had a higher sensitivity (93%) and negative predictive value (74%) than a CO₂gap >6 mmHg in excluding occult tissue hypoperfusion. Of the four variables that were predictive of an increased O₂ER in the multivariate analysis-CO₂gap, arterial pH, haemoglobin and arterial lactate concentrations-the CO₂gap (odds ratio 4.41 per mmHg increment, 95% CI 1.7 to 11.2, P=0.002) was most important and explained about 34% of the variability in the risk of occult tissue hypoperfusion. In conclusion, a normal CO₂ gap (<5 mmHg) had a high sensitivity and negative predictive value in excluding inadequate systemic oxygen delivery and may be useful as an adjunct to other haemodynamic targets in avoiding occult tissue hypoperfusion in the perioperative setting when high inspired oxygen concentrations are used.