Correspondence
The Long-QT Syndrome
N Engl J Med 1993; 328:287January 28, 1993
- Article
To the Editor:
Vincent and colleagues (Sept. 17 issue)1 ably describe the spectrum of symptoms and QT intervals related to the long-QT syndrome. It is not surprising that the QT interval corrected for heart rate (QTc) was not always adequate for an accurate diagnosis. Indeed, it is amazing that the QTc works as well as it often does. The authors' formula for QTc is the original formula and one of many2. Based on the study of 39 patients, it was published in 1920 by Bazett3. The probable reason it is helpful in a variety of situations is that it does reduce the variance in the QT interval due to changes in cycle length expressed as heart rate4. A number of the other formulas are not based on the square root of the RR interval, as is Bazett's, and a new linear formula appears to yield optimal results5. Moreover, two additional considerations arise: at very rapid and very slow heart rates, the QT interval has special relations to the cycle length. For example, at elevated heart rates, most of the cycle length is the QT interval itself, so a high correlation during tachycardias is largely self-correlation2. Finally, the variance might also be reduced by measuring the JT interval, the more appropriate measure of ventricular cell repolarization, which is the point of attack for most influences altering QT intervals4. Would the authors consider recalculating their results on the basis of the recent linear formula5 and investigating the use of the JT interval4 as a more appropriate (and perhaps more useful) physiologic expression of repolarization?
5 ReferencesDavid H. Spodick, M.D., D.Sc.
Saint Vincent Hospital, Worcester, MA 016041
Vincent GM ,Timothy KW ,Leppert M ,Keating M . The spectrum of symptoms and QT intervals in carriers of the gene for the long-QT syndrome. N Engl J Med 1992;327:846-852
Full Text | Web of Science | Medline2
Spodick DH ,Rifkin RD ,Rajasingh MC . Effect of self-correlation of the relation between QT interval and cardiac cycle length. Am Heart J 1990;120:157-160
CrossRef | Web of Science | Medline3
Bazett HC . An analysis of the time-relations of electrocardiograms. Heart 1920;7:353-3704
Spodick DH . Reduction of QT-interval imprecision and variance by measuring the JT interval. Am J Cardiol 1992;70:103-103
CrossRef | Web of Science | Medline5
Sagie A ,Larson MG ,Goldberg RJ ,Bengtson JR ,Levy D . An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study). Am J Cardiol 1992;70:797-801
CrossRef | Web of Science | Medline
Author/Editor Response
Dr. Vincent replies:
To the Editor: Spodick raises a common and important question -- namely, how to correct the QT interval for variation in heart rate. We1 used Bazett's formula because it is the most commonly used correction formula and has been used in almost all publications about the long-QT syndrome. There are a number of formulas that have been proposed for QT-interval correction, and we too were interested in the new method proposed by Sagie et al2. None of these formulas have been carefully studied in a well-defined population of subjects with the long-QT syndrome. We recently decided, therefore, to evaluate a number of the correction formulas with our genetically determined data base. We shall be pleased to include the recent linear formula,2 and we shall also include measurement of the JT interval3 in that evaluation. An additional important question, not addressed by the correction formulas or JT-interval measurement, is where the T wave ends. We use the initial change in slope at the end of the rapid downstroke of the T wave1,4. Sagie et al.2 used the point at which the terminal limb of the T wave joins the base line. In patients with a rather slow and long terminal approach to the base line, these two measurements can vary by as much as 0.04 second. This, of course, makes comparing studies quite difficult and indicates that this problem requires as much interest and study as the problem of correction for heart rate.
4 ReferencesG. Michael Vincent, M.D.
LDS Hospital, Salt Lake City, UT 841431
Vincent GM ,Timothy KW ,Leppert M ,Keating M . The spectrum of symptoms and QT intervals in carriers of the gene for the long-QT syndrome. N Engl J Med 1992;327:846-852
Full Text | Web of Science | Medline2
Sagie A ,Larson MG ,Goldberg RJ ,Bengston JR ,Levy D . An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study). Am J Cardiol 1992;70:797-801
CrossRef | Web of Science | Medline3
Spodick DH . Reduction of QT-interval imprecision and variance by measuring the JT interval. Am J Cardiol 1992;70:103-103
CrossRef | Web of Science | Medline4
Vincent GM ,Jaiswal D ,Timothy KW . Effects of exercise on heart rate, QT, QTc and QT/QS2 in the Romano-Ward inherited long QT syndrome. Am J Cardiol 1991;68:498-503
CrossRef | Web of Science | Medline
