Prospective, observational study of carbon dioxide gaps and free energy change and their association with fluid therapy following cardiac surgery

Background Venoarterial carbon dioxide pressure (p_v-aCO₂) and content (C_v-aCO₂) differences, including the ratio to arteriovenous oxygen content difference (C_a-vO₂), and free energy changes (−∆∆G_a-v) may reflect tissue hypoperfusion. The associations with changes in cardiac output (CO) or oxygen consumption (VO₂) following fluid bolus administration were investigated. Methods Single-centre, observational study of 89 adult post-operative cardiac surgical patients admitted to ICU. The p_v-aCO₂, C_v-aCO₂ and their ratios to C_a-vO₂ as well as the −∆∆G_a-v were determined before and after a 250-500 mL fluid bolus using arterial, central venous and mixed venous blood gas analyses. Responses associated with changes ≥ or <15% in CO or oxygen consumption (VO₂) were compared. Results In 234 boluses, the mixed venous to arterial p_v-aCO₂ and its ratio to C_a-vO₂ were independently associated with an increase in CO; odds ratio 1.3 (95% CI 1.1-1.5) and 1.7 (95% CI 1.5-1.9) respectively, P <.001) and VO₂; odds ratio 2.1 (95% CI 1.3-3.1), P <.001 for C_a-vO₂. No measures of p_v-aCO₂, C_v-aCO₂ or related ratios to the C_a-vO₂ were associated with an increase in CO ≥15% following a single volume bolus. The mixed venous and central venous C_v-aCO₂ to C_a-vO₂ ratios were different for the first bolus episode only; mean differences 0.81 (95% CI 0.13-1.5), P =.02 and 0.44 (95% CI 0.06-0.82), P =.02, respectively, for increased VO₂ ≥ 15%. The −∆∆G_a-v did not change. Conclusion The venoarterial carbon dioxide gradients and related calculations to assess the adequacy of tissue perfusion before a fluid bolus were not associated with subsequent increases in CO of oxygen consumption.