Lower limit of adequate oxygen delivery for the maintenance of aerobic metabolism during cardiopulmonary bypass in neonates.

Background: The objective of cardiopulmonary bypass (CPB) is to maintain an adequate balance between oxygen delivery (Ḋo) and consumption. The critical Ḋo is that at which consumption becomes supply dependent. This study aimed to identify the critical Ḋo in neonates, who have higher metabolic rates than adults.

Methods: In a retrospective cohort of neonates, Ḋo was calculated from CPB parameters recorded during aortic cross-clamping. 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 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 thresholds, and Ḋo excursions below the threshold were quantified as magnitude-durations. The lowest threshold that provided magnitude-durations and linked with an increase in lactOFF was used as the lowest suitable (critical) Ḋo at 37°C.

Results: Overall, 22 896 time points were analysed in 180 neonates. In 40 patients with lactOFF <2.5 mM, Ḋo varied by 22.87 (0.70) ml min m °C. When varying the Ḋo threshold between 340 and 380 ml min m, excursions below the threshold were linked with incremental lactOFF. A 100 ml m excursion below the 340 ml min mḊo threshold increased the risk of a 1 mM increment in lactOFF by 22% (odds ratio: 1.22; 95% confidence interval: 1.02-1.45).

Conclusions: It was found that 340 ml min m is likely to represent the lowest suitable Ḋo required in neonates to maintain aerobic metabolism during normothermic CPB.