Background: The objective of cardiopulmonary bypass (CPB) is to maintain an adequate balance between oxygen delivery (ḊO 2 ) and consumption. The critical ḊO 2 is that at which consumption becomes supply dependent. This study aimed to identify the critical ḊO 2 in neonates, who have higher metabolic rates than adults. Methods: In a retrospective cohort of neonates, ḊO 2 was calculated from CPB parameters recorded during aortic crossclamping. High lactate concentration measured after aortic unclamping (lactOFF) was used to identify anaerobic metabolism. Data were analysed using mixed linear and proportional odds regression models. The relationship between ḊO 2 and temperature was analysed in a subgroup of patients with lactOFF <2.5 mM, thought to have had balanced oxygen delivery and consumption. The estimated regression coefficient was further used to adjust hypothetical ḊO 2 thresholds, and ḊO 2 excursions below the threshold were quantified as magnitudeedurations. The lowest threshold that provided magnitudeedurations and linked with an increase in lactOFF was used as the lowest suitable (critical) ḊO 2 at 37 C. Results: Overall, 22 896 time points were analysed in 180 neonates. In 40 patients with lactOFF <2.5 mM, ḊO 2 varied by 22.87 (0.70) ml min À1 m À2 C À1 . When varying the ḊO 2 threshold between 340 and 380 ml min À1 m À2 , excursions below the threshold were linked with incremental lactOFF. A 100 ml m À2 excursion below the 340 ml min À1 m À2 ḊO 2 threshold increased the risk of a 1 mM increment in lactOFF by 22% (odds ratio: 1.22; 95% confidence interval: 1.02e1.45). Conclusions: It was found that 340 ml min À1 m À2 is likely to represent the lowest suitable ḊO 2 required in neonates to maintain aerobic metabolism during normothermic CPB.

Measurement of lactate concentrations during cardiac surgery with cardiopulmonary bypass (CPB) is a reliable monitoring tool for the assessment of the adequacy of perfusion, and a predictor of poor outcome. However, increased lactate production, which is multifactorial (anaerobic metabolism, hyperglycemia), increased lactate load by packed red blood cell (PRBC) transfusions, and decreased lactate clearance may all result in hyperlactatemia. The aim of this study was to estimate the clearance of lactate in infants undergoing surgery with CPB, using the lactate load from the PRBCs transfusions received during CPB. Retrospective cohort of infants <1 year of age with repeated lactate measurements during CPB, and a known lactate concentration in the PRBCs used during CPB were evaluated. All patients received PRBCs in the prime and during CPB to maintain hematocrit >35% and venous saturation >70%. Lactate kinetics were estimated across several time intervals between two lactate measurements, using a single compartment model. The lactate load was calculated as the product: PRBC‐lactate concentration * volume. The rate of endogenous lactate production was assumed to be unchanged (maintenance of high oxygen deliveries and normoglycemia throughout CPB). Overall, 87 calculations were performed in 27 patients, then averaged per patient. The mean lactate half‐life was 12.36 min [10.67–14.06], the mean clearance was 0.09 L/min [0.06–0.11], the indexed lactate clearance was 0.36 L/min/m2 [0.28–0.44]. Lactate clearance increased significantly with age. The half‐life of lactate in infants is comparable with that reported in adults with CPB, and lactate clearance is higher. Knowing the high lactate content of PRBCs, lactate clearance rather than absolute concentration is potentially a better indicator of the adequacy of perfusion during CPB in infants.

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