Central Venous to Arterial CO: Many Unknowns2: Many Unknowns Difference After Cardiac Surgery in Children: Many Unknowns

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We read with great interest the recent issue of Pediatric Critical Care Medicine by Rhodes et al (1), who demonstrated that elevated central venous to arterial CO2 difference (AVCO2) at ICU admission was associated with poor clinical outcomes in infants and neonates after cardiac surgery with cardiopulmonary bypass (CPB). AVCO2 has emerged as a promising marker for identifying patients with impaired tissue perfusion in septic shock and other surgical conditions (2, 3). In adult patients who undergo cardiac surgery with CPB, reports concerning the use of AVCO2 in predicting poor clinical outcomes are conflicting (4, 5). In the pediatric population, this is the first study to report the association of elevated AVCO2 with “poor outcome,” and “normal values” of AVCO2 up to 24 hours after cardiac surgery with CPB. However, our own experience (article in preparation) has not confirmed this work. Thus, we encourage the readers to interpret this article with caution.
There are several issues in the study by Rhodes et al (1). First, in their study, poor outcome was defined as a composite of mortality and several morbidities, including an Inotrope Score (IS) greater than 15. However, we would argue that IS greater than 15 per se does not necessarily represent a poor outcome. Among 34 patients who had poor outcome, seven patients had an IS greater than 15. Thus, we are curious to know whether elevated AVCO2 would have been significantly associated with poor outcome had they not included IS. Second, as they rightly point out, AVCO2 may not be useful in different clinical settings such as ours with heterogeneous patients, since their study population was limited to neonates and young infants. Third, we speculate it would be more difficult to find the association between elevated AVCO2 and poor clinical outcomes in hospitals with a lower mortality rate than theirs (10.1%). Fourth, there was considerable overlap of AVCO2 values between patients with poor outcomes and those without. Although the authors demonstrated that high AVCO2 was significantly correlated with poor outcomes, there likely is no clear cutoff for “elevated” AVCO2 to predict such poor outcomes. Finally, AVCO2 values at 6, 12, and 24 hours after admission were derived from only a fraction of patients, who were considered critically ill to warrant central venous blood analysis. Therefore, these are probably not normal values and caution should be exercised while interpreting the cardiac output surrogate variables over time in Figure 1 (1).

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