Response to Letter Regarding Articles “Electrocardiographic Abnormalities That Predict Coronary Heart Disease Events and Mortality in Postmenopausal Women: The Women’s Health Initiative” and “Electrocardiographic Predictors of Incident Congestive Heart Failure and All-Cause Mortality in Postmenopausal Women: the Women’s Health Initiative”
We appreciate Dr Lehmann’s supportive comments on our articles1,2 in Circulation. Dr Lehmann also brings up a relevant question: Why did we not include QRS duration as a covariate in our QT adjustment models?
This question was examined in some detail in previous publications.3,4 Adding the QRS interval as a covariate with the R-R interval did not increase the R2 value in men, and it increased by less than 1% in women.
What is the reason for this seemingly counterintuitive paradox? The QT interval is, without exception, the sum of the excitation time and the action potential duration of the myocardial cells in the region repolarizing last. This region is excited in normal conduction during the initial half of the QRS interval because repolarization sequence is semireverse with respect to depolarization; therefore, its excitation time is perhaps smaller than expected. Excitation times in the so called “incomplete bundle-branch blocks”(QRS <120 ms) vary.5 The significance of the QRS contribution to QT adjustment, if included as a covariate, is again a different question as noted.
In major ventricular conduction defects, repolarization becomes more concordant with respect to the depolarization sequence. QRS duration contributes more heavily to the QT interval, and it must be included as a covariate with the R-R interval in QT adjustment formulas. As an alternative, a univariate formula for JT without the QRS interval can be used. Properly established upper percentile limits for adjusted QT or JT intervals for normal conduction and for bundle-branch blocks must be used in evaluation of prolonged repolarization.3,4,6
Rautahajru PM, Kooperberg C, Larson JC, LaCroix A. Electrocardiographic abnormalities that predict coronary heart disease events and mortality in postmenopausal women: the Women’s Health Initiative. Circulation. 2006; 113: 473–480.
Rautaharju PM, Kooperberg C, Larson JC, LaCroix A. Electrocardiographic predictors of incident congestive heart failure and all-cause mortality in postmenopausal women: the Women’s Health Initiative. Circulation. 2006; 113: 481–489.