An Improved Method for Adjusting the QT Interval for Heart Rate (the Framingham Heart Study) Alex Sagie, MD, Martin G. Larson, ScD, Robert J. Goldberg, PhD, James R. Bengtson, MD, and Daniel Levy, MD Several form&s have been proposed to adjust the QT Interval for heart rate, the most commonly used bslng the QT correctlon formula (QT, = QT/m) propwed In 1920 by Raxett. The QT, formula was derived from observations In only 39 young subjects. Recently, ths adequacy of Ba&t’s formula has been questtoned. To evaluate ths heart rate QT assocMon, the QT Interval was measured on the hdtlal baseline electrocardiogram of S,OlS subjects (2,229 men and 2,779 women) from the Framlngham Heart Study with a mean age of 44 years (range 26 to 62). Persons wlth coronary artery disease were excluded. A linear regmdon modei was developed for correcting QT according to RR cycle length. The large sample allowed for subdlvlslon of the populatlon into sexspeclflc deciles of RR Intervals and for comparison of QT, Raxett’s QT, and linear corrected QT (QT&. The mean RR interval was 0.81 second (range 0.5 to 1.47) heart rate 74 bsats/mln (range 41 to 120), and mean QT was 0.35 second (range 0.24 to 0.49) In men and 0.36 second (range 0.26 to 0.46) In women. The linear regression model yleldedacorrectlonformula(fora&erenceRR Interval of 1 second): QTu: = QT + 0.154 (1 - RR) that applies for men and women. This equatlon corrects QT more reliably than the Razett’s formula, which overcorrects the QT Interval at fast heart rates and undercorrects lt at low heart rates. Lower and upper limits of normal QT values In relation to RR were generated. A simple linear equation was developed that Is mere accurate than Baxett’s correction at dlfferent cycle lengths and more convenient for cllnkal practke. This formula allevlates the need to apply sscondary corrections to 9axett’s fomuda. Addltlonal Investlgatlon Is warranted to determined whether QTLC Improves the ldentlftcation of subjects at high risk for mallgnant arrhythmias or sudden death. (Am J Cardlol1992;70:797-601) From the Framingham Heart Study, Framingham, the Divisions of Epidemiology and Preventive Medicine of Boston University School of Medicine, Boston, the Department of Medicine, University of Massachusetts Medical School, Worcester, and the Divisions of Cardiology and Clinical Epidemiology, Beth Israel Hospital, Boston, Massachusetts; and the National Heart, Lung, and Blood Institute, Bethesda, Maryland. Dr. Sagie was a visiting Research Fellow from the Beiin Medical Center, Tel Aviv, Israel. Manuscript received May 1, 1992; revised manuscript received and accepted May 21,1992. Address for reprints: Daniel Levy, MD, Framingham Heart Study, 5 Thurber Street, Framingham, Massachusetts 01701.
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rolongation of the QT interval is associated with increased risk for malignant ventricular arrhythmias and sudden cardiac death in congenital long QT syndromes, electrolyte disturbances, after use of several antiarrhythmic drugs and after myocardial infarction.‘-‘* The duration of the QT interval has been shown to depend on the length of the precedmg cardiac cycle. It has been over 70 years since a formula for correcting QT for cycle length (QTJ was introduced by Bazett.13 However, the adequacy of thii approach (dividing the measured QT by the square root of the measured RR) to adjust correctly for differences in heart rate has been questioned. i4-*0 Although many formulas have been proposed to adjust the QT interval for heart rate, only a few have been derived or validated in a large population-based cohort.14-22 In the present study we examined the initial baseline electrocardiogram in over 5,000 persons originally enrolled in the Framingham Heart Study. This report examines the relation of QT (and QT,) to heart rate and presents results of a linear regression equation (QTLc) that better corrects QT for heart rate. METHODS The original cohort of the Framingham Heart Study consisted of 5,209 subjects who were between the ages of 28 and 62 years on entering the study >40 years ago. The initial electrocardiograms of these subjects were used to measure the duration of the QT interval. From 1985 to 1986 all initial electrocardiograms were measured manually by 2 trained readers with the aid of calipers and magnifying lens. Measurements were obtained from a 1Zlead resting electrocardiogram, with a paper speed of 25 mm/s. The average of 2 to 3 QT intervals from the electrocardiographic lead with the longest QT interval was analyzed. The QT interval was measured from the beginning of the QRS complex to the end of the T wave where its terminal limb joined the baseline. Each week a blinded duplicate review was performed on a random sample of all electrocardiograms read during that week. There was a high degree of inter- and intraobserver agreement. Any disagreements found were subsequently adjudicated and corrected. Additionally, a sample of 480 baseline electrocardiograms were blindly and independently reread 5 times. A high degree of correlation was obtained between the multiply read samples and the singly read baseline measurements with regard to the mean RR, and average QT interval (p
P
rolongation of the QT interval is associated with increased risk for malignant ventricular arrhythmias and sudden cardiac death in congenital long QT syndromes, electrolyte disturbances, after use of several antiarrhythmic drugs and after myocardial infarction.‘-‘* The duration of the QT interval has been shown to depend on the length of the precedmg cardiac cycle. It has been over 70 years since a formula for correcting QT for cycle length (QTJ was introduced by Bazett.13 However, the adequacy of thii approach (dividing the measured QT by the square root of the measured RR) to adjust correctly for differences in heart rate has been questioned. i4-*0 Although many formulas have been proposed to adjust the QT interval for heart rate, only a few have been derived or validated in a large population-based cohort.14-22 In the present study we examined the initial baseline electrocardiogram in over 5,000 persons originally enrolled in the Framingham Heart Study. This report examines the relation of QT (and QT,) to heart rate and presents results of a linear regression equation (QTLc) that better corrects QT for heart rate. METHODS The original cohort of the Framingham Heart Study consisted of 5,209 subjects who were between the ages of 28 and 62 years on entering the study >40 years ago. The initial electrocardiograms of these subjects were used to measure the duration of the QT interval. From 1985 to 1986 all initial electrocardiograms were measured manually by 2 trained readers with the aid of calipers and magnifying lens. Measurements were obtained from a 1Zlead resting electrocardiogram, with a paper speed of 25 mm/s. The average of 2 to 3 QT intervals from the electrocardiographic lead with the longest QT interval was analyzed. The QT interval was measured from the beginning of the QRS complex to the end of the T wave where its terminal limb joined the baseline. Each week a blinded duplicate review was performed on a random sample of all electrocardiograms read during that week. There was a high degree of inter- and intraobserver agreement. Any disagreements found were subsequently adjudicated and corrected. Additionally, a sample of 480 baseline electrocardiograms were blindly and independently reread 5 times. A high degree of correlation was obtained between the multiply read samples and the singly read baseline measurements with regard to the mean RR, and average QT interval (p
