USE OF LEAD ADJUSTMENT FORMULAS FOR QT DISPERSION AFTER MYOCARDIAL-INFARCTION

Objective-To determine whether lead adjustment formulas for correcting QT dispersion measurements are appropriate in patients after myocardi al infarction. Design-Retrospective analysis of QTc dispersion measure ments in 461 electrocardiograms (ECGs). Data are presented as uncorrec ted QTc dispersion ''adjusted'' for a number of measurable leads and c oefficient of variation of QTc intervals for ECGs in which between six and 12 leads had a QT interval that could be measured accurately. Pat ients-Patients were drawn from the placebo arm of the second Leicester Intravenous Magnesium Intervention Trial. Some 163 patients who subse quently died and an equal number of known survivors had ECGs recorded on day 2 or 3 of acute myocardial infarction. ECGs were also available in 135 of these patients from at least 1 month postinfarct. Results-T he most common lead in which a QT interval measurement was omitted was aVR (n = 176), the least common lead was V3 (n = 13). The longest QTc interval measured was most usually in lead V4 (n = 72) and the shorte st in lead V1 (n = 67). As the number of measurable leads decreased th ere was a small, nonsignificant increase in QTc dispersion from 12 lea d to eight lead ECGs (mean (so) 100 (35.5) v 109.5 (47.9) ms). Lead ad justed QTc dispersion (QTc dispersion/square root of the number of mea surable leads) showed a large, significant increase when the number of measurable leads decreased from 12 to eight (28.9 (10.3) v 38.7 (16.1 ) ms, P < 0.001). A similar trend was seen for coefficient of variatio n of QTc intervals (standard deviation of QTc intervals/mean QTc inter val 64.3 (2.19) v 8.45 (3.94)%, P < 0.001). Conclusions-Lead adjustmen t formulas for QT dispersion are not appropriate in patients with myoc ardial infarction. Large differences in lead adjusted QTc dispersion a re produced, dependent on the number of measurable leads, for very sma ll differences in QTc dispersion. It is recommended that QT dispersion is presented as unadjusted QT and QTc dispersion, stating the mean (S D) of the number of leads in which a QT interval was measured.