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      Evaluation of a 12-Lead Digital Holter System for 24-Hour QT Interval Assessment


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          Background: Drug induced QT prolongation may precipitate life threatening cardiac arrhythmias. Evaluation of the QT prolonging effect of new pharmaceutical agents in a ‘thorough QT/QTc study’ is being mandated by FDA. The purpose of this study was to evaluate an automated 12-lead digital Holter system for a thorough QT/QTc study. Methods: Five healthy volunteers underwent 24-hour digital Holter monitoring. Each recording underwent a fully automated QT analysis (AQA) followed by an onscreen complete manual over read (MOR). Each recording was analyzed twice at least 2 weeks apart. The effect of data sampling (5-min segment/hour), the system sensitivity to detect 5-ms increase in QT, and the ability to assess circadian variation were evaluated. Results: The AQA resulted in identical QT for the first and second analyses, but with obvious errors in QT measurements. Compared to the complete onscreen MOR, the mean QT was longer with AQA (416 ± 41 vs. 387 ± 30 ms, p < 0.001), correlation; r = 0.3. The reproducibility of AQA with complete MOR was very good (QT: 387 ± 30 vs. 387 ± 30 ms, coefficient of variation: 0.2%, r = 0.986. The 5-min mean QT intervals correlated well with the hourly mean QT intervals (r = 0.994, p < 0.001, coefficient of variation = 1 ms) and both showed a similar circadian variation. The system was sensitive to detect a 5-ms change in QT intervals (5 ± 2 ms, coefficient of variation = 0.6%, r = 0.998, p < 0.001). Conclusions: The AQA is not an acceptable method, while the automatic analysis with complete MOR is a highly sensitive and reproducible method. Data sampling by analyzing 5-min segments per hour is sensitive and reproducible.

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          Most cited references 21

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          An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study)

          Several formulas have been proposed to adjust the QT interval for heart rate, the most commonly used being the QT correction formula (QTc = QT/square root of RR) proposed in 1920 by Bazett. The QTc formula was derived from observations in only 39 young subjects. Recently, the adequacy of Bazett's formula has been questioned. To evaluate the heart rate QT association, the QT interval was measured on the initial baseline electrocardiogram of 5,018 subjects (2,239 men and 2,779 women) from the Framingham Heart Study with a mean age of 44 years (range 28 to 62). Persons with coronary artery disease were excluded. A linear regression model was developed for correcting QT according to RR cycle length. The large sample allowed for subdivision of the population into sex-specific deciles of RR intervals and for comparison of QT, Bazett's QTc and linear corrected QT (QTLC). The mean RR interval was 0.81 second (range 0.5 to 1.47) heart rate 74 beats/min (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 0.48) in women. The linear regression model yielded a correction formula (for a reference RR interval of 1 second): QTLC = QT + 0.154 (1-RR) that applies for men and women. This equation corrects QT more reliably than the Bazett's formula, which overcorrects the QT interval at fast heart rates and undercorrects it at low heart rates. Lower and upper limits of normal QT values in relation to RR were generated.(ABSTRACT TRUNCATED AT 250 WORDS)
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            Rate-corrected QT interval: techniques and limitations.

            The duration of the QT interval on the surface electrocardiogram represents the time required for all ventricular depolarization and repolarization processes to occur. Among the many physiologic and pathologic factors that contribute to the QT interval, heart rate plays a major role. Several approaches have been used to correct the QT interval, all of which take into account the heart rate at which the interval is measured. The simplest and most common approach to correcting the QT interval is to divide its value by the square root of the preceding RR interval expressed in seconds, i.e., by using Bazett's formula. This calculation provides a corrected QT (QTc) interval that represents the QT interval normalized for a heart rate of 60 beats/min. However, several studies have shown that Bazett's correction formula is not optimal. Fridericia's cube-root formula has been shown to perform better in correcting the QT interval for heart rate. Other formulas require the measurement of several QT-RR pairs at various heart rates to obtain a reliable QTc interval and are therefore not easily usable. Any correction formula is likely to introduce an error in assessing the QTc interval. Although the importance of this error should not be minimized, the corrected QT interval remains useful in assessing the effects of drugs on the duration of repolarization. For this purpose, Fridericia's cube-root formula is preferable to Bazett's square-root formula.(ABSTRACT TRUNCATED AT 250 WORDS)
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              Diurnal pattern of QTc interval: how long is prolonged? Possible relation to circadian triggers of cardiovascular events.

              This study sought to evaluate the range and variability of the QT and corrected QT (QTc) intervals over 24 h and to assess their pattern and relation to heart rate variability. Recent Holter monitoring data have revealed a high degree of daily variability in the QTc interval. The pattern of this variability and its relation to heart rate variability remain poorly characterized. We developed and validated a new method for continuous measurement of QT intervals from three-channel, 24-h Holter recordings. Average RR, QT, QTc and heart rate variability were measured from 5-min segments of data from 21 healthy subjects. Measurement of 6,048 segments showed mean (+/- SD) RR, QT and QTc intervals of 830 +/- 100, 407 +/- 23 and 445 +/- 16 ms, respectively (mean QTc interval for men 434 +/- 12 ms, 457 +/- 10 ms for women, p or = 500 ms in 6 subjects and > or = 490 ms in 13. The 95% upper confidence limit for the mean 24-h QTc interval was 452 ms (men 439 ms, women 461 ms). The RR, QT and QTc intervals and the high frequency component of heart rate variability were greater during sleep. Both the QTc interval and the variability between hourly minimal and maximal QTc intervals reached their circadian peak shortly after awakening, before declining to daytime levels. The maximal QTc interval over 24 h in normal subjects is longer than heretofore thought. Both QT and QTc intervals are longer during sleep. The QTc interval and QTc variability reach a peak shortly after awakening, which may reflect increased autonomic instability during early waking hours, and the time of the peak value corresponds in time to the period of reported increased vulnerability to ventricular tachycardia and sudden cardiac death. These findings have implications regarding the definition of QT prolongation and its use in predicting arrhythmias and sudden death.

                Author and article information

                S. Karger AG
                November 2006
                15 November 2006
                : 106
                : 4
                : 224-232
                aRFUMS, The Chicago Medical School, bRush University, and cUniversity of Illinois at Chicago, Chicago, Ill., USA
                93190 Cardiology 2006;106:224–232
                © 2006 S. Karger AG, Basel

                Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.

                Page count
                Figures: 5, Tables: 4, References: 28, Pages: 9
                Original Research


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