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Heart 1998;80:459-466 ( November )
Department of Cardiological Sciences, St George's
Hospital Medical School, Cranmer Terrace, London SW17 0RE, UK
Correspondence to: Dr Gang.
Accepted for publication 31 December 1997
Objective
To assess the stability and
reproducibility of computerised QT dispersion (QTd) measurement in
healthy subjects, as this is presently being incorporated into
commercial electrocardiographic systems.
Methods70 healthy volunteers (mean (SD) age
38 (10) years, 35 men, 35 women) with a normal 12 lead
electrocardiogram (ECG) were studied. From each subject, 70 ECG
recordings were taken using the MAC VU ECG recorder (Marquette). In
study A, 50 ECGs were recorded in each subject: 10 supine, 10 sitting,
10 standing, 10 holding breath in maximum inspiration, and 10 holding
breath in maximum expiration. After a mean interval of 8 (3) days
(range 7 to 23), 10 recordings in supine and 10 in the standing
position were repeated in each subject (study B). On measurements made
using a research version of the commercial software without manual
modification, the reproducibility of QTd was assessed by coefficient of
variance (CV) and relative error, and comparisons made with other ECG indices.
Results(1) QTd measurements were stable and
not influenced by changes in posture and respiratory cycle; (2) there
was no difference in QTd measurements between men and women, or between
age groups dichotomised at 35 years; (3) no correlation was found
between QTd and heart rate or QT interval; (4) short term
reproducibility of all QTd measurements (CV 15.6% to 43.8%) was worse
than that of conventional ECG indices (CV 1.4% to 5.3%); (5) long
term reproducibility of QTd measurements (relative error 27.4% to
31.0%) was also worse than that of conventional ECG indices (relative
error 1.8% to 7.9%) (p < 0.0001); (6) the reproducibility of QTd
measurements tended to increase when several serial recordings were averaged.
ConclusionsComputerised measurements of global
QTd and global QT-SD from 12 lead ECG by the MAC VU/QT Guard system are
not significantly altered by changes in posture and respiration. The
reproducibility of all QTd measurements is inferior to that of
conventional ECG indices in healthy subjects.
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