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Abstract
To determine whether the automatic measurement of the QT interval is consistent with
the manual measurement, this study evaluated the reproducibility and agreement of
both methods in 70 normal subjects and 54 patients with hypertrophic cardiomyopathy.
The mean, minimum, and maximum QT interval and QT dispersion were computed in a set
of 6 consecutive electrocardiograms (3 in the supine and 3 in the standing position)
obtained from each subject. The automatic method determined the T-wave end as the
intersect of the least-squares-fit line around the tangent to the T-wave downslope
with the isoelectric baseline. Manual measurements were obtained using a high-resolution
digitizing board. QT dispersion was defined as the difference between the maximum
and minimum QT interval and as standard deviations of the QT interval duration in
all and precordial leads. In patients with hypertrophic cardiomyopathy, the absolute
values of the QT interval and QT dispersion were significantly higher than those in
normal subjects (p < 0.0001). In both groups, the intrasubject variability of the
QT interval was significantly lower with automatic than with manual measurement (p
< 0.05). The agreement between automatic and manual QT interval measurements was surprisingly
poor, but it was better in patients with hypertrophic cardiomyopathy (r2 = 0.46 to
0.67) than in normal subjects (r2 = 0.10 to 0.25). In both groups, the reproducibility
and agreement of both methods for QT dispersion were significantly poorer than for
QT interval. Hence, the automatic QT interval measurements are more stable and reproducible
than manual measurement, but the lack of agreement between manual and automatic measurement
suggests that clinical experience gained with manual assessment cannot be applied
blindly to data obtained from the automatic systems.