Central Venous-Arterial CO2 Difference in Cardiac Surgery Patients-A Parameter in Relationship to Cardiac Output and Altered Microcirculatory Blood Flow.

Abstract

To the Editor: With interest we read the recent article “High central venous-to-arterial CO2 difference is associated with poor outcomes inpatients after cardiac surgery: a propensity score analysis” (1). Chen et al. (1) study joins a series of previous investigations on central venous-to-arterial CO2 difference (P(v-a)CO2) in adult cardiac surgery patients. These studies demonstrated heterogeneous results regarding an association of P(v-a)CO2 with mortality. Whereas some (retrospective) studies observed associations of mortality in patients with elevated P(v-a)CO2 (cut-off values of 8 mm Hg and 8.6 mm Hg, respectively) (2, 3), an association with increased mortality was not noted in another study, in which a cutoff of 6 mm Hg for P(v-a)CO2 was used (4). In the current study by Chen et al. (1), P(v-a)Co2 >7.12 mm Hg was associated with poor outcome. mean P(v-a)CO2 in the poor outcome group compared with a control group was 11.21 ± 5.22 mm Hg versus 6.11 ± 2.94 mm Hg, P < 0.001 (1). Following analysis of earlier studies, Chen et al. hypothesized that P(v-a)CO2 could be used to evaluate cardiac output in postoperative cardiac surgery patients based on an negative correlation between P(v-a)CO2 and cardiac index. This association seems explained by the Fick equation which indicates that CO2 excretion (as equivalent to CO2 production) should be equivalent to the product of cardiac output and P(v-a)CO2. However, alterations in microcirculatory blood flow may further affect P(v-a)CO2. These alterations appear characterized by increased heterogeneity of perfused and non-perfused capillaries (5), which may result in increased P(v-a)CO2 even when cardiac output is sufficient (6). Hence, P(v-a)CO2 may be altered in cardiac surgery patients, especially after extracorporeal circulation (ECC) use due to impaired microcirculation (7) and/or a discrepancy of blood flow and/or altered metabolic demand (8). Thus, a cut-off value of 6 mm Hg, which is not exceeded under physiological conditions (9), may misinterpret pathophysiological alterations in ECC-treated patients. Based on underlying pathophysiology and the results of the multivariable logistic regression analysis in the excellent study by Chen, we are convinced that P(v-a)CO2 can likely be used in addition to other prognostical indices (e.g., lactate assessments) for prediction of poor outcome in adult ICU patients post-cardiac surgery.