Letter Regarding Article by Schmidt et al, “Infarct Tissue Heterogeneity by Magnetic Resonance Imaging Identifies Enhanced Cardiac Arrhythmia Susceptibility in Patients With Left Ventricular Dysfunction”
To the Editor:
I read with interest the article by Schmidt et al.1 The authors have made an important observation relating differences in signal intensity on contrast-enhanced magnetic resonance imaging, used as an index of tissue heterogeneity, to inducibility of monomorphic ventricular tachycardia (MVT). The electrophysiological testing, as described by the authors, has important features that should be highlighted because they may affect the interpretability of the results. The authors report that electrophysiological testing was performed in standard fashion (3 extrastimuli at 2 different cycle lengths delivered from the right ventricular apex and outflow tract) in 12 patients (26%) and with noninvasive programmed stimulation only from the right ventricular apex via the implantable cardioverter defibrillator in 35 patients (74%). Of note, 9 (75%) of 12 of those undergoing electrophysiological testing had inducible monomorphic ventricular tachycardia (MVT), and 11 (31%) of 35 undergoing noninvasive programmed stimulation had inducible MVT. This is a vast difference in the rate of inducibility (P<0.02). This finding is consistent with prior data that show that the use of a second site for programmed ventricular stimulation increases the sensitivity of electrophysiological testing.2 Although the difference in inducibility rates noted by Schmidt et al are quite dramatic, only 12 patients underwent full electrophysiological testing. In any case, a distinct possibility exists that a significant number of patients who underwent noninvasive programmed stimulation might have had inducible MVT had they undergone full electrophysiological testing. Reclassifying these patients may affect the magnetic resonance imaging findings.
The authors also classified the patients into 2 groups on the basis of inducible MVT or no inducible MVT. In a similarly designed study evaluating infarct-size characteristics (infarct size, infarct surface area) derived from contrast-enhanced magnetic resonance imaging, we3 classified 48 patients who underwent standard electrophysiological testing into 3 groups: 18 with inducible MVT, 21 with no inducible ventricular tachycardia, and 9 with inducible polymorphic ventricular tachycardia, ventricular fibrillation, or ventricular flutter. Because these latter arrhythmias are considered nonspecific, they were not included in our primary analysis comparing patients with inducible MVT with those that were not inducible. In fact, the magnetic resonance imaging findings of this nonspecific group were intermediate between the group with inducible MVT and those without any inducible ventricular tachyarrhythmia. On the basis of the definition provided, Schmidt et al probably classified these results as “noninducible for MVT.” Although it is unclear how to analyze these patients, at a minimum, the number of patients with these inducible arrhythmias should be provided. Addressing these issues will further enhance the important contribution made by Schmidt and colleagues relating anatomy to physiology.
Schmidt A, Azevedo CF, Cheng A, Gupta SN, Bluemke DA, Foo TK, Gerstenblith G, Weiss RG, Marban E, Tomaselli GF, Lima JA, Wu KC. Infarct tissue heterogeneity by magnetic resonance imaging identifies enhanced cardiac arrhythmia susceptibility in patients with left ventricular dysfunction. Circulation. 2007; 115: 2006–2014.
Doherty JU, Kienzle MG, Waxman HL, Buxton AE, Marchlinski FE, Josephson ME. Programmed ventricular stimulation at a second right ventricular site: an analysis of 100 patients, with special reference to sensitivity, specificity and characteristics of patients with induced ventricular tachycardia. Am J Cardiol. 1983; 52: 1184–1189.