Limb lead interchange in thorough QT/QTc studies.

The investigators analyzed 85,133 electrocardiograms (ECGs) recorded in 484 subjects from 5 thorough QT/QTc studies (3 using Holter devices, 2 using 12-lead ECGs) for inadvertent limb lead interchanges using a dedicated quality control process in a central ECG laboratory. Limb lead interchanges were present in 2919 (3.4%) ECGs in 17.9% of subjects and were more frequent with Holter devices (7.5% vs 0.8%, P < .0001), where leads remain connected for prolonged periods, affecting data from several time points. Left arm-left leg interchange was seen in 54% of 12-lead ECGs and right arm-left arm interchange in 38%. The ECG device itself could identify 21.7% of interchanges, whereas experienced readers blinded to subject and visit identified 79% of interchanges; 21% of interchanges were identified only during the quality control process. If correctly identified, QT measurement could be performed in a precordial lead. If undiagnosed, incorrect QT interval measurements and morphological diagnosis may confound results.