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Significance of Short QT Intervals
Abstract & Commentary
By John P. DiMarco, MD, PhD, Professor of Medicine, Division of Cardiology, University of Virginia, Charlottesville. Dr. DiMarco is a consultant for Novartis, and does research for Medtronic and Guidant.
Source: Anttonen O, et al. Prevalence and prognostic significance of short QT intervals in a middle-aged Finnish population. Circulation 2007;116;714-720.
Anttonen and his colleagues analyzed the prevalence and long-term prognostic significance of a short QT interval in a sample of the Finnish population. The study group included 10,957 men and women between 30 and 59 years of age. The initial screening exam for entry into this longitudinal study was between 1966 and 1972 and included 12 lead ECG was obtained. A recording on which the QT interval could be reliably measured was available in 10,822 subjects. QT intervals were measured by trained technicians and all QT intervals measuring less than 360 ms were confirmed by one of the authors. Raw QT intervals were corrected for heart rate by the Bazett's (QTc) and Fridericia's (QTfc) formulae and by a population based nomogram (QTnc). Subjects were followed long-term as an epidemiologic cohort with the primary end points for this analysis being all cause mortality, cardiovascular mortality, and death due specifically to coronary artery disease. The significance of a short QT interval was adjusted for other variables known to effect cardiovascular mortality.
There were 11 of 10,822 (0.1%) subjects who had a QTc < 320 ms using the Bazett's correction. This number was reduced to 9 and 7 if the Fridericia or nomogram based corrections were used. If a cut-off of <340 ms was used, 43 (0.4%), 33 and 34 subjects met criteria using the 3 correction methods. The increased prevalence of a short QT interval using the Bazett's formula was due to overcorrection of the QT interval at low heart rates — a phenomenon well described previously.
As had also been noted in other studies, QT intervals were longer in women with 4.4% of males and only 1.3% of woman having a QTc < 360 ms. Short QT intervals also were associated with younger age, lower heart rates, and lower systolic blood pressure. During long-term follow-up, a short QT interval was not associated with any significant change in total, cardiovascular, or sudden death mortality. This was true when cut-off values of <320, <340, and <360 ms were used. For example, in the patients with a QTfc <340 ms there were 15 deaths and only 3 cardiovascular deaths during the over 30 year follow-up period. Additional data indicated that none of the individuals with a short QTc interval had been hospitalized for syncope or ventricular arrhythmias or had been prescribed anti-arrhythmic medications. The authors concluded that the incidental finding of a short QT interval in an asymptomatic individual without a familiar history of ventricular arrhythmias does not require intervention.
Recently several groups have described families with the "short QT syndrome." Effected individuals had short QT intervals defined by using rare cut-offs ranging from 310 to 340 ms. Four genetic patterns have been described with the syndrome due to "gain-of-function" mutations in K+ channel genes(HERG, KvLQT1, KCNJ2) or a "loss of function" mutation in a Ca++ channel gene (CACN2 ).
In the reported cases, the short QT interval was associated with a high risk for atrial and ventricular arrhythmias and cardiac arrest. In this paper, Anttonen and his colleagues used a population based approach rather than a symptomatic proband-based approach to assess the significance of a short QT interval. Their data suggest that although rare, short QT intervals are not associated with an adverse prognosis in the general population. Unless arrhythmias are present or a positive family history is noted, no action needs to be taken when a short QT is detected.