Results
84 patients met the eligibility criteria. All patients experienced TdP and in 49 (58.3%) patients early PVCs were present. Mean (SD) age was 58.8 (8.8) years and 52 (61.9%) were female. Prior medical history included hypertension in 21 (25.0%), ischemic heart disease in 13 (15.5%), heart failure in 12 (14.3%), atrial fibrillation in 8 (9.5%) and renal impairment in 11 (13.1%) patients. Figure 1 shows the medications that were suspected to cause QT interval prolongation. The three most common offending drug classes were anti-arrhythmic drugs, psychiatric drugs and antibiotics. Amiodarone (n=10), Methadone (n=9) and Dofetilide (n=7) were the most frequent individual drugs. In 26 (31.0%) cases, patients took a combination of potentially QT interval prolonging drugs.
Mean (SD) QRS duration was 100 (26) ms and the mean (SD) heart rate was 66 (20) bpm. Figure 2 shows the Bland Altmann plot with a wide variation in the QTc differences between the limb and chest leads. Comparing the limb and chest leads, mean (SD) QT durations were 645 (129) and 661 (125) ms (p=0.03) and mean (SD) QTc durations were 655 (97) and 671 (102) ms (p=0.02), respectively. Using only the limb leads for QT interval interpretation, 18 (21.4%) ECGs were non-interpretable: 10 (11.9%) due to too flat T-waves, 7 (8.3%) due to frequent, early PVCs and 1 (1.2%) due to insufficient ECG recording quality. In the chest leads, it was not possible to interprete the QT interval in 9 (10.7%) patients: 6 (7.1%) due to frequent, early PVCs, 1 (1.2%) due to insufficient ECG quality, 2 (2.4%) due to missing chest leads but none due to too flat T-waves. Single limb and chest leads that were judged most often as the best for QT interval interpretation were II and V5, respectively.
Figure 3 shows detailed data on T-wave morphology for each individual lead and cumulative for limb and chest leads. The most common, cumulative T-wave morphologies in the limb leads were flat T-waves in 51.0%, broad T-waves in 14.3% and late peaking T-waves in 12.6%. The most common, cumulative morphologies in the chest leads were inverted T-waves in 35.5%, flat T-waves in 19.6% and biphasic T-waves in 15.2%. T-wave alternans was present in 6 (7.1%) patients. Beside a high variability in T-wave morphology between patients, there was also high variability over the individual leads within the same patient as showcased in Figure 4.