The ULTRA randomized controlled trial evaluated whether high-exchange subzero-balance ultrafiltration (H-SBUF) during pediatric cardiopulmonary bypass improves recovery compared with low-exchange SBUF. In 104 children under 15 kg, high-exchange ultrafiltration did not reduce peak postoperative vasoactive-ventilation-renal (VVR) scores or improve clinical outcomes. Inflammatory mediator profiles were largely similar, suggesting limited immunomodulatory benefit.