Significance of Qt Dispersion to Improve the Diagnostic Accuracy of Exercise Stress Tests for Myocardial Ischaemia

Abstract

Background: This study consisted of 60 randomly selected patients (49 men and 11
women) who had undergone both symptom-limited treadmill exercise tests and
coronary angiography, within one month, for clinical indications. This was a prospective
study done during the period of December 1998 to July 1999 in NICVD, Dhaka
Bangladesh. The ECG is generally considered to provide a summation of the electrical
activity of the heart and to be insensitive to local electrophysiological events. Recent
studies suggest that important local electrophysiological differences can be detected
from a standard 12 -lead ECG, allowing new insights into the electrical behavior of
patient’s hearts. The QT dispersion recorded as the difference between maximum
and minimum QT intervals on a 12-lead ECG, is sensitive to myocardial ischaemia
and may improve the accuracy of exercise testing.
Methods and Results: Exercise ECGs were analyzed in 60 patients who had undergone
exercise ECG and coronary angiography for clinical indications:
1) The true-positive (TP) group consisted of 22 patients with mean ±SD age of 52 ± 9
years. These patients had a positive treadmill test result with ³1 mm horizontal STsegment
depression and an abnormal coronary angiogram, defined as ³ 50% diameter
stenosis of a major epicardial coronary artery. Two patients were female in this group.
2) The true negative (TN) group consisted of 20 patients who had a negative exercise
test and non significant CAD based on coronary angiography. The mean ± SD age of
the group was 44 ± 8 years. Five patients were female in this group.

3) Eight patients with positive treadmill test result with abnormal exercise ECGs and
non significant coronary artery disease (False positive [FP] group). This group had a
mean age of 43± 10 years. Three patients were female in this group.
4) The false negative (FN) group consisted of 10 patients with mean ±SD age of 48± 7
years. These patients had negative exercise ECG and an abnormal coronary angiogram.
This group had one female patient.
The exercise QTD Was 41±14ms in TN, 78±22ms in TP (P<0.001 versus TP) and
47±16ms in FP (P=NS versus TN and P< 0.001 versus TP) groups. The exercise QTD
was 84±17ms in FN (P=NS versus TP) group.
A stress QTD of >60ms had a sensitivity of 88% and specificity of 93% for the diagnosis
of significant CAD compared with 68% (P<0.05) and 71% (P<0.01) respectively, for
³ 1mm ST-segment depression during stress. When QTD of >60ms was added to STsegment
depression as a condition for positive test, the specificity increased to 100%.
Conclusion: Exercise QTD is an easily measurable ECG variable that significantly
increases the accuracy of exercise testing.
Key Words: QT dispersion . coronary disease . electrocardiography . exercise.