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J Am Coll Cardiol, 2008; 51:969-970, doi:10.1016/j.jacc.2007.08.069 © 2008 by the American College of Cardiology Foundation |
* Division of Cardiology Elmhurst Hospital Center, 79-01 Broadway, Elmhurst, New York 11373 (Email: madiasj{at}nychhc.org).
30% who had been referred for invasive electrophysiological testing for evaluation of syncope and/or nonsustained ventricular tachycardia. It is not clear to this reader whether this report’s database derives from the one employed in another study, emanating from the same laboratory, and published in another journal on the same month as the present study (2). However, even if this is the case, the present study provides an opportunity to evaluate the influence of the QRS duration on the magnitude of the MTWA. The authors in both studies (1,2) employed the traditional threshold of 
1.9 µV, derived from the spectral analysis of the signal, to characterize an MTWA value as positive. Although this analysis operates on this dichotomy to generate the "positive" and the "negative" response of a patient to MTWA testing, thus creating a qualitative context, there is a quantitative issue as its underpinning (i.e., a patient has to reach the threshold to qualify as "positive"). Moreover, current work indicates that the magnitude in µV of MTWA may be of importance, and higher values of calculated MTWA may imply worse prognosis (3,4), or therapy with various drugs, particularly beta-blockers, may attenuate the magnitude of MTWA (5). However, more work is needed to substantiate the claim that the quantitative assessment of MTWA may have advantages over the qualitative evaluation. In the meantime, it is important to deal with some possible confounders of the quantitative employment of the MTWA, 1 of which may be its T-wave amplitude dependence (6). Accordingly, in the presence of ventricular conduction delays, which often are associated with T waves larger in amplitude than the ones encountered in association with normal intraventricular conduction, a greater magnitude of MTWA in the former than in the latter may not mean a higher risk for sudden cardiac death or malignant ventricular arrhythmia, but merely an enhancing effect of the taller T waves on the magnitude of MTWA in patients with wide QRS complexes. Probably this was the reason for the increased rate of false-positive MTWA tests in patients with wide QRS complexes (120 ms) (specificity 22% in the patients with a wide QRS and 40% in the patients with a narrow QRS) in the authors’ other report (2). In that report they categorized their patients to those with QRS duration of <120 ms and 120 ms (2). In their present contribution (1), by employing 3 categories of patients in terms of QRS duration (Table 1) (QRS <110 ms, QRS 110 ms to 120 ms, and QRS >120 ms), they provide an opportunity to evaluate the effect of the amplitude of the T waves on the magnitude of MTWA in a "dose response" fashion. What is needed for this is a comparison of the values of MTWA in µV, and the T-wave amplitudes in mV in the above 3 categories of patients, according to their QRS duration. Could the authors provide us with these data? Evaluation of possible determinants of the MTWA and potential adjustment via a MTWA index (6) may upgrade both the currently used "qualitative" assessment and a future adoption of quantitative assessment of MTWA testing.  |
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