Extracorporeal membrane oxygenation (ECMO) represents an established modality of intraoperative circulatory and respiratory support during lung transplantation (LTx). Systemic anticoagulation with unfractionated heparin (UFH) remains essential to prevent circuit thrombosis; however, the optimal monitoring strategy during this procedure remains uncertain. Conventional assays, including antifactor Xa activity (anti-Xa), activated partial thromboplastin time (aPTT), aPTT ratio (aPTTr), and activated clotting time (ACT), demonstrate variable sensitivity and reliability, particularly at low UFH concentrations where ACT is often inadequate. This study aimed to evaluate the interrelationship between standard coagulation monitoring methods and to assess the feasibility of using viscoelastic testing, specifically the ROTEM INTEM/HEPTEM clotting time ratio (I/Hr), as a bedside alternative to ACT. A total of 79 patients undergoing LTx with intraoperative ECMO support were analyzed. Unfractionated heparin was administered in all cases, and coagulation parameters were assessed preoperatively, before ECMO cannulation, and during ECMO support. A strong correlation was observed between I/Hr and anti-Xa, with satisfactory agreement with aPTT and aPTTr. Bland–Altman analysis confirmed narrower limits of agreement for I/Hr-derived versus ACT-derived anti-Xa predictions. These findings support I/Hr as a reliable and practical bedside surrogate for UFH monitoring during ECMO-assisted LTx.