This retrospective study evaluated the impact of lactate levels and lactate clearance on mortality in patients under 18 who required venoarterial ECMO support after surgery for congenital heart defects. Analyzing 55 patients from 1,844 congenital heart surgeries, it found that lower lactate levels at the 6th, 12th, and 24th hours and higher lactate clearance at the 24th hour were significantly associated with successful weaning from ECMO and lower mortality. Lactate levels ≥2.9 and lactate clearance ≤69.44% were predictive of mortality, highlighting the importance of these factors in managing ECMO patients.