Purpose: To investigate the interchangeability of mixed and central venous-arterial carbon dioxide differences and the relation between the central difference (pCO₂ gap) and cardiac index (CI). We also investigated the value of the pCO₂ gap in outcome prediction.
Methods: We performed a post hoc analysis of a well-defined population of 53 patients with severe sepsis or septic shock. Mixed and central venous pCO₂ were determined earlier at a 6 h interval (T = 0 to T = 4) during the first 24 h after intensive care unit (ICU) admittance. The population was divided into two groups based on pCO₂ gap (cut off value 0.8 kPa).
Results: The mixed pCO₂ difference underestimated the central pCO₂ difference by a mean bias of 0.03 ± 0.32 kPa (95 % limits of agreement: -0.62-0.58 kPa). We observed a weak relation between pCO₂ gap and CI. The in hospital mortality rate was 21 % (6/29) for the low gap group and 29 % (7/24) for the high gap group; the odds ratio was 1.6 (95 % CI 0.5-5.5), p = 0.53. At T = 4 the odds ratio was 5.3 (95 % CI 0.9-30.7); p = 0.08.
Conclusions: From a practical perspective, the clinical utility of central venous pCO₂ values is of potential interest in determining the venous-arterial pCO₂ difference. The likelihood of a bad outcome seems to be enhanced when a high pCO₂ gap persists after 24 h of therapy.