Background: Protamine neutralises heparin after separation from cardiopulmonary bypass. This study aimed to evaluate the effects of lidocaine on protamine induced pulmonary vascular constriction in paediatric cardiothoracic surgery. Methods: This was a single-centre, prospective, double-blind and randomised study conducted among eighty pediatric patients with acyanotic congenital cardiac disease, scheduled for elective on-pump cardiac surgery under general anaesthesia. In the study, the participants were divided into four groups: Group NPHL- nonpulmonary hypertension with lidocaine preconditioning, group NPHS- nonpulmonary hypertension with normal saline (as placebo), group PHL- pulmonary hypertension with lidocaine preconditioning, and group PHS- pulmonary hypertension with normal saline (as placebo). Results: Pulmonary vasoconstriction occurred in 11.25% of cases after protamine administration. Both the NPHS and PHS groups exhibited an increase in mean airway pressure (Paw), Respiratory index (RI), alveolar-arterial oxygen difference (A-aDO2), pulmonary artery pressure (PAP) and decreased dynamic pulmonary compliance (Cydn) and oxygen index (OI) after protamine administration. However, these changes were not observed in the NPHL and PHL groups with lidocaine preconditioning. Plasma levels of TXB2 in the NPHS and PHS groups were higher than the NPHL and PHL groups, but 6-keto-PGF1 alpha levels were lower in the NPHS and PHS groups than in the NPHL and PHL groups. Conclusion: In congenital heart disease, repair without cardiopulmonary bypass is not possible in most cases. Prior to reversing heparin with protamine, preconditioning lidocaine reverses protamine-induced pulmonary vasoconstriction and improves lung function.