Noninvasive - QT Dispersion Measurement

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QT Dispersion Measurement

QT interval dispersion calculated from the 12-lead ECG has emerged as a noninvasive measurement for quantifying the degree of myocardial repolarization inhomogeneity.^1-3QT Dispersion has been linked as a risk indicator for Arrhythmic Cardiac death in patient populations having Coronary Artery Disease⁴ myocardial infarction,⁵ acute myocardial infarction⁶ sustained ventricular arrhythmias,⁷ ventricular fibrillation,^8-9unstable angina pectoris and ischemia,¹⁰ long QT syndrome,^11-13 chronic heart failure,¹⁴ peripheral vascular disease,¹⁵ reprofusion therapy,¹⁶ drug arrhythmogenicity^17-20 and hypertrophic cardiomyopathy.²¹

Currently the most routinely used noninvasive method to assess ventricular recovery time is the QT interval measurement made from the 12-lead electrocardiogram. The QT interval measurement is usually made from a single lead, with very little standardization in dealing with measurement complications such as low amplitude T waves, U waves, P waves and small biphasic T waves.^22-24 This lack of a standard measurement technique has lead to poor sensitivity and specificity in using the isolated QT interval measurement for predicting susceptibility to life threatening ventricular arrhythmias.

More recently investigations have been looking at the homogeneity of ventricular recovery time instead of total ventricular recovery time. Homogeneity of recovery time has been postulated to protect against arrhythmias whereas dispersion of recovery time is arrhythmogenic. A single 12 lead electrocardiogram derived QT interval measurement gives no information on the inhomogeneity of ventricular recovery time.1 However, studies have shown that interlead variability of ventricular repolarization duration or QT dispersion, defined as the difference between the lead with the maximum QT interval and the lead with the minimum QT interval as measured from the 12 lead electrocardiogram has been suggested to give information about the spatial differences in myocardial recovery time.² Additionally studies have shown the benefit of using the peak of the T wave as one of the measurement points.13 In this case, the dispersion is measured as the difference between the lead with the maximum QTpeak interval and the lead with the minimum QTpeak interval.¹³

Although there is evidence to the benefit of looking at QT interval dispersion from the 12 lead electrocardiogram little has been done to standardize and automate the making of these measurements. Manual measurements of QT dispersion can be significantly affected by the speed and gain used to plot the electrocardiogram, and also suffer from both intra and inter observer variability.^6,25

Click to enlarge this report Research has indicated that accurate and consistent QT dispersion measurements are essential when attempting to identify small yet perhaps clinically significant changes in the 12-lead electrocardiogram.^29,30This is of particular importance when attempting to identify QT dispersion measurements as an independent and predictive value for defining and assessing risk. Automated measurement algorithms should offer consistency and reproducibility of these measurements. Additionally, automated analysis algorithms should allow the physician to inspect, modify and possibly eliminate measurements at their discretion, and be able to provide trending of measurements made on large numbers of electrocardiogram.^27-28Studies have shown the benefit of QT associated measurements made on all 12 leads versus only the precordial leads.²⁰ Thus, automated algorithms need to provide measurements based on all twelve leads termed "global" dispersion measurements, along with measurements based on the leads V1 through V6 often referred to as "precordial" dispersion measurements. It is also beneficial to indicate which leads were used to determine the minimum and maximum QT measurements and to graphically portray the range of dispersion on the 12 lead ECG.²²

Click to enlarge this report The benefit of automated analysis and trending of these QT measurements may be magnified when comparing serial changes of these values on repeat electrocardiograms during assessment of disease progression, regression, or when evaluating the effects of drug treatment.^22,26 Validation of the automated measurement algorithms is critical in establishing credibility and faith in the measurements themselves and should help to promote the use of the measurements in more varied clinical studies.

The exclusion of intra- and inter-observer variability on QT dispersion measurements made from the standard 12-lead electrocardiogram will help assure accuracy and repeatability that is required to permit their use to assess changes in ventricular recovery times that may possibly be due to therapeutic interventions or changes in the underlying disease process.^25,29,30

1) Day CP, McComb JM, Campbell RWF. QT dispersion: an indication of arrhythmia risk in patients with long QT intervals. Br Heart J 1990;63:342-344.

2) Mirvis DM. Spatial variation of QT intervals in normal persons and patients with acute myocardial infarction. J Am Coll Cardiol 1985;5:625-631.

3) Zabel M, Portnoy S, Franz MR. Electrocardiographic indexes of dispersion of ventricular repolarization: an isolated heart validation study. J Am Coll Cardiol 1995;25:746-752.

4) Zareba W, Moss AJ, le Cessie S. Dispersion of ventricular repolarization and arrhythmic cardiac death in coronary artery disease. Am J Cardiol 1994;74:550-553.

5) Glancy JM, Garratt CJ, Woods KL, de Bono DP. QT dispersion and mortality after myocardial infarction. Lancet 1995; 345: 945-948.

6) Van de Loo A, Arendts W, Hohnloser SH. Variability of QT dispersion measurements in the surface electrocardiogram in patients with acute myocardial infarction and in normal subjects. Am J Cardiol 1994;74:1113-1118.

7) Pye M, Quinn AC, Cobbe SM. Qt interval dispersion: a non-invasive marker of susceptibility to arrhythmia in patients with sustained ventricular arrhythmias. Br Heart J 1994;71:511-514.

8) Higham PD, Furniss SS, Campbell RWF. Increased QT dispersion in patients with ventricular fibrillation following myocardial infarction. Abstract Circulation 1992;86:II-61.

9) Campbell R. Commentary: QTc dispersion may reflect vulnerability to ventricular fibrillation. Br Heart J 1996;312:878-879.

10) Cin VG, Celik M, Ulucan S. QT dispersion ratio in patients with unstable angina pectoris (a new risk factor?). Clin Cardiol 1997;20:533-535.

11) De Ambroggi L, Negroni MS, Monza E, Bertoni T, Schwartz PJ. Dispersion of ventricular repolarization in the long QT syndrome. Am J Cardiol 1991;68:614-620.

12) Priori SG, Napolitano C, Diehl L, Schwartz PJ. Dispersion of the QT interval a marker of therapeutic efficacy in the idiopathic long QT syndrome. Circulation 1994;89:1681-1689.

13) Alberti M, Merri M, Benhorin J, Locati E, Moss AJ. Electrocardiographic precordial interlead variability in normal individuals and patients with long QT syndrome. Computers in Cardiology 1991;475-478.

14) Davey PP, Bateman J, Mulligan IP, Forfar C, Barlow C, Hart G. QT interval dispersion in chronic heart failure and left ventricular hypertrophy: relation to autonomic nervous system and Holter tape abnormalities. Br. Heart J 1994;71:268-273.

15) Darbar D, Luck J, Davidson N, Pringle T, Main G, McNeil G, Struthers AD. Sensitivity and specificity of QTc dispersion for identification of risk of cardiac death in patients with peripheral vascular disease. Br. Heart J;1996:312:874-878.

16) Moreno F, Villanueva T, Karagounis LA, Anderson JL. Reduction in QT interval dispersion by successful thrombolytic therapy in acute myocardial infarction. Circulation 1994;90:94-100.

17) Day CP, McComb JM, Matthews J, Cambell RW. Reduction in QT dispersion by sotalol following myocardial infarction. Eur Heart J 1991;12:423-427.

18) Cui G, Sen L, Sager P, Uppal P, Singh BN. Effects of Amiodarone, sematilide, and Sotalol on QT dispersion. Am J Cardiol 1994;74:896-900.

19) Sedwick ML, Rasmussen HS, Cobbe SM. Effects of the class III antiarrhythmic drug dofetilide on ventricular monophasic action potential duration and QT interval dispersion in stable angina pectoris. Am J Cardol 1992;70:1432-1437.

20) Hii JTY, Wyse DG, Gillis AM, Duff HJ, Solylo MA, Mitchell LB. Precordial QT interval dispersion as a marker of torsade de pointes diparate effects of class Ia antiarrhythmic drugs and amiodarone. Circulation 1992;86:1376-1382.

21) Buja G, Miorelli M, Turrini P, Melacini P, Nave A. Comparison of QT dispersion in hypertrophic cardiomyopathy between patients with and without ventricular arrhythmias and sudden death. Am J Cardiol 1993;72:973-976.

22) Higham PD, Campbell RWF. QT dispersion. Br Heart J 1994;71:508-510.

23) Morganroth J. Relations of QTc prolongation on the electrocardiogram to torsades de pointes: definitions and mechanisms. Am J Cardiol 1993;72:10B-13B.

24) Garson A. How to measure the QT interval - what is normal? Am J Cardiology 1993;72:14B-16B.

25) Murray A, McLaughlin NB, Bourke JP, Doig CJ, Furniss SS, Campbell RWF. Errors in manual measurement of QT intervals. Br Heart J 1994;71:386-390.

26) Doroghazi RM, Childers R. Time-related changes in the Q-T interval in acute myocardial infarction: possible relation to local hypocalcemia. Am J Cardiol 1978;41:684-688.

27) Xue Q, Reddy S. New algorithms for QT dispersion analysis. Computers in Cardiology 1996;293-296.

28) Xue Q, Reddy S. Computerized QT analysis algorithms. Submitted to J of Electrocardiography 1997;30 supplement.

29) Woosley RL, Sale M. QT Interval: A measure of drug action. AM J Cardiol 1993;72:36B-43.

30) Murry A, McLaughlin NB, Campbell RWF. Measuring QT dispersion: man versus machine. Heart 1997;77:539-542.

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