QT INTERVAL AND QT DISPERSION MEASURED WITH THE THRESHOLD METHOD DEPEND ON THRESHOLD LEVEL

Various computerized methods with multiple parameter options for measu rements of the QT interval now are available. The optimum parameter se tting for most algorithms is not known. This study evaluated the influ ence of the threshold level applied on the T wave differential on the QT interval and its dispersion measured in normal and abnormal electro cardiograms (ECGs). Seven hundred sixty ECGs recorded in 76 normal sub jects and 630 in 63 patients with hypertrophic cardiomyopathy (HCM) (1 0 consecutive recordings in each individual) were analyzed. in each le ad of each EGG, the QT interval was measured by the threshold method a pplied to the first differential of the T wave. The threshold level wa s varied between 5% and 30% of the T wave maximum in 1% steps, resulti ng in 26 different choices of QT measurements. With each choice the ma ximum QTc and the QT dispersion (QTd, standard deviation of the QT in all 12 leads) were obtained for each recording. The maximum QTc was si gnificantly longer in HCM patients than in normal subjects (P < 0.001) at all threshold levels except between 5% and 7%. The QTd was signifi cantly greater in HCM patients at all threshold levels. The QTc and QT d changed significantly with the threshold level. The maximum QTc vari ed up to 60 ms in normal subjects and up to 70 ms in HCM patients, dep ending on the threshold level. Thus, the QT interval and its dispersio n measured with the threshold method applied to the first T wave diffe rential depended significantly on the threshold level in both normal a nd diseased hearts. All programmable options of available automatic in struments should be examined carefully before any study, and all algor ithmic details should be systematically presented.