This retrospective study examined paediatric patients undergoing cardiac surgery with crystalloid priming to evaluate outcomes related to postoperative hyperlactataemia. Among 186 patients, 53% experienced hyperlactataemia, which was associated with longer cardiopulmonary bypass and ICU stays but did not increase ventilation duration. Findings suggest transient hyperlactataemia may not indicate tissue hypoxaemia, but longer inotropic support and ICU stays necessitate careful management post-surgery.