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.