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D I T O R ’ S P A G E

s Mechanical Dispersion a Raven of Ventricularrrhythmias?

illiam A. Zoghbi, MD,* Jagat Narula, MD, PHD†

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he ancients believed a bird’s flight could foretell theuture (1) and Domitian looked to the raven tooretell his immediate future. Ironically, Suetonius, a

oman historian and senator, records, “A few monthsefore he (Domitian) was killed, a raven perched onhe Capitalium and cried, ‘All will be well,’ an omen

hich some interpreted as follows: . . . a raven . . .ould not say, ‘It is well,’ only declared ‘It will be

ell’ ” (2). Emperor Domitian died soon after and allas well (3)!

erious ventricular arrhythmias account forthe majority of sudden cardiac death afteracute myocardial infarction (MI), andidentification and treatment of patients at

isk of sudden cardiac death remains a major fo-us of clinical practice (4). Implantable cardio-erter defibrillators (ICD) have been the mainstayf therapy for this condition and are currently in-icated in patients with reduced cardiac systolicunction, based on a left ventricular ejection frac-ion (LVEF) �35%. Although ICD is effective ineducing the occurrence of sudden cardiac deathhe device never delivers therapy in a large pro-ortion of recipients of an ICD based on reducedVEF. Also, the majority of patients who die

uddenly would not have qualified for ICD underurrent LVEF-based guidelines (5).

While a depressed LVEF portends a poor out-ome, the accuracy of prediction of sudden car-iac death with LVEF alone is moderate to low.n the post-infarct setting, several studies havedditionally focused on the electrical disturbancehat occurs—including electrical dispersion evalu-

rom the *Cardiovascular Imaging Institute, Department of Cardiology,he Methodist DeBakey Heart and Vascular Center, Houston, Texas;

wnd the †Division of Cardiology, University of California-Irvine, Irvine,alifornia.

ted with signal-averaged electrocardiograms, QTispersion, and heart rate variability—with vari-ble and somewhat disappointing results. Practi-ally none of these methodologies are currentlysed clinically to further subclassify individuals atisk of sudden cardiac death and help refine ICDherapy. Thus, the quest to improve upon the ac-uracy of markers to predict serious arrhythmias istill an ongoing effort.

Echocardiographic imaging has evolved enor-ously, from the traditional measurement of left

entricular volumes and ejection fraction to, moreecently, quantitation of regional function withlaborate parameters such as speckle tracking.hese have included regional deformation param-

ters of strain and strain rate. Regional deforma-ion can be accurately timed and comparedhroughout the ventricle. Thus, dyssynchrony ofontraction can be measured, expressed as maxi-al time difference among segments or standard

eviation of the time to maximal contraction.hese parameters have been investigated to quan-

itate cardiac dyssynchrony and evaluate selectionf patients for resynchronization therapy. In this

ssue of iJACC, Haugaa et al. (6) propose to use thisarameter as an index of “mechanical dispersion”—annnovative suggestion for identifying patients atisk of ventricular arrhythmias.

The use of mechanical dispersion to predictentricular arrhythmias is promising at firstlance. The authors studied individuals after MIho had a clinical indication for an ICD, or pa-

ients who had an ICD for secondary prevention.he higher the index of mechanical dispersion,

he more prevalent was a composite end point ofCD shocks for ventricular fibrillation or ventricu-ar arrhythmias needing pace termination. This

as seen in the primary and secondary prevention

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Zoghbi and Narula

Editor’s Page

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ubjects. Mechanical dispersion was a strong andndependent predictor of arrhythmias requiringCD therapy. A mechanical dispersion of 70 mshowed a sensitivity of 65% and a specificity of0% for identifying arrhythmic events. Possibleimitations of such an index are in segments thatisplay no deformation, where timing of contrac-ion may be difficult. In addition, patients with aide QRS complex on the electrocardiogram may

lready have a large mechanical dispersion. Fi-ally, though a test with high specificity is wel-ome, less than perfect sensitivity may not allowhe proposed test to become a part of routinelinical practice algorithm for management of aisease associated with high mortality.The present dispersion index may represent theechanical correlate of electrical or tissue hetero-

eneity seen after MI. Cardiac magnetic resonancemaging has demonstrated that tissue heterogeneityost-MI is related to increased susceptibility to ven-ricular arrhythmias induced by programmed ven-ricular stimulation (7). And, 2 months ago, iJACClso brought you data extracted from clinical studiesased on the molecular imaging of neuronal dys-

folklore.html. Accessed February 7, 2010. dial infarction for risk p

ent (8). The sympathetic nervous system is an im-ortant trigger for major arrhythmic events, and aismatch of myocardial perfusion and innervation

oses substantially greater risk. Whether the aboveechanical, electrical, and histological indexes are

orrelated, additive, or merely provide redundantrognostic information needs to be evaluated in fu-ure prospective investigations. Nonetheless, com-ining imaging and electrophysiologic modalitieshould offer improved accuracy in future selection ofatients with heart failure for ICD placement.

hile such novel imaging applications are of para-ount interest, one cannot be too cautious when

ealing with identification of new risk factors forredicting sudden cardiac death, as this condition isomplex with numerous predisposing risks, fre-uently variable, dynamic, and even genetic!

ddress for correspondence:

agat Narula, MD, PhDditor-in-Chief, JACC: Cardiovascular Imaging

655 Nobel Drive, Suite 630an Diego, California 92112

unction to predict appropriateness of ICD place- E-mail: narula@uci.edu

7

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E F E R E N C E S

. Kanitz L. Domitian the Man Revealedby His Coins. Journal for the Study ofAncient Numismatics 1973–1974;5:45–7.

. Rolfe, J. (trans.) Suetonius, The Livesof the Caesars, Domitian. Loeb Classi-cal Library. Cambridge, MA: HarvardUniversity, 1992:385.

. Folklore and Superstition. Available at:http://www.shades-of-night.com/aviary/

4. Turakhia M, Tseng ZH. Sudden car-diac death: epidemiology, mechanismsand therapy. Curr Probl Cardiol 2007;32:501–46.

5. Buxton A, Lee KL, Hafley GE, et al.Limitations of ejection fraction for pre-diction of sudden death risk in patientswith coronary artery disease: lessons fromthe MUSTT study. J Am Coll Cardiol2007;50:1150–7.

6. Haugaa KH, Smedsrud MK, Steen T,et al. Mechanical dispersion assessed bymyocardial strain in patients after myocar-

rediction of ventric-

ular arrhythmia. J Am Coll Cardiol Img2010;3:247–56.

. Schmidt A, Azevedo CF, Cheng A,et al. Infarct tissue heterogeneity bymagnetic resonance imaging identi-fies enhanced arrhythmia susceptibil-ity in patients with left ventriculardysfunction. Circulation 2007;115:2006 –14.

. Gerson MC, Abdallah M, Muth JN,Costea AI. Will imaging assist in theselection of patients with heart failurefor an ICD? J Am Coll Cardiol Img

2010;3:101–10.