CAMH
I doubt this is news, but this article on T wave alternans is a good summary and reasonable viewpoint, IMO. Bold emphasis is mine.
Value of TWA test for primary-prevention-ICD screening varies by patient risk status
Jul 3, 2007 Steve Stiles
New York, NY - Microvolt T-wave alternans (TWA) testing (Cambridge Heart, Bedford, MA) can help identify patients who qualify for a primary-prevention implantable cardioverter-defibrillator (ICD) who are at low risk of ventricular arrhythmias, but its predictive value is better in some such patients than others, a prospective study suggests [1].
In particular, those who are device-eligible based on low-LVEF criteria but who also have nonsustained VT (NSVT) or syncope can be more reliably risk-stratified by the combination of TWA and electrophysiologic (EP) testing, conclude the study's authors, led by Dr Daniel J Cantillon (Cornell University Medical Center, New York, NY). Their report is scheduled for the July 10, 2007 issue of the Journal of the American College of Cardiology.
For such patients in the study, who had cardiomyopathy of either ischemic or nonischemic origin, the two tests together were more predictive than either alone, according to the authors. But, as coauthor Dr Sei Iwai (Cornell University Medical Center) told heartwire, the event rate associated with a negative TWA test by itself was high enough to question the safety of withholding primary-prevention device therapy.
Apples and oranges?
In the analysis of 286 adult patients with an LVEF <35% referred for EP testing due to NSVT or syncope, TWA testing was performed during atrial pacing in the course of a standard EP study. The technique differs from the more traditional approach of noninvasive TWA testing during exercise stress, complicating comparisons of the current TWA study with most others.
Iwai observed that there is support in the literature for both forms of TWA testing in risk stratification, although they tend to yield different predictive values. Exercise, unlike atrial pacing performed in an EP lab, greatly alters autonomic tone and catecholamine levels, "so you can't say the ways are comparable, and that's a bit of a limitation in the study."
The more important message, however, is that TWA testing probably has to be combined with another risk-stratification strategy, like EP testing, to identify patients like those in the study as at low enough risk to safely avoid receiving ICDs, Iwai said. In the current analysis, a negative result on both the TWA test and the EP study was associated with an 85% two-year rate of survival free from sustained ventricular arrhythmia.
Other findings
The results of TWA testing and EP testing were discordant in 49% of patients, "highlighting the fact that, although TWA and EP studies can both be used to potentially risk-stratify patients for sudden cardiac death, they identify different patient population sets," the authors write.
Number of subjects with each combination of positive and negative TWA and EP test results (and two-year arrhythmic event rate)
Finding
Negative TWA test (n=90), n (%)
NonnegativeTWA test* (n=196), n (%)
Positive EP study (n=115)
29 (28)
86 (33)
Negative EP study (n=171)
61 (15)
110 (35)
*Nonnegative tests=all positive plus all indeterminate tests, according to convention. TWA=T-wave alternans; EP=electrophysiologic
To download table as a slide, click on slide logo below
Without adjustment for potential confounders, a negative TWA result significantly predicted arrhythmia-free survival, with a hazard ratio (HR) of 2.33 (p<0.01) compared with only 1.27 (p=0.21) for a negative EP study. The HR for TWA testing remained significant in an analysis that controlled for age, sex, QRS duration, LVEF, NYHA class, ischemic vs nonischemic etiology, and subsequent ICD implantation.
In an accompanying editorial, Dr Thomas Klingenheben (JW Goethe University, Frankfurt, Germany) points out that EP testing has different predictive values in ischemic and nonischemic cardiomyopathy [2]. So findings based on the current study's mix of etiologies—in 75% and 25% proportions, respectively—don't necessarily apply to the majority of ICD candidates, who have ischemic disease.
For that group, Klingenheben notes, there are data from the Alternans Before Cardioverter-Defibrillator (ABCD) trial, which compared noninvasive TWA with EP studies in patients with ischemic disease, NSVT, no prior ventricular tachyarrhythmias, and an LVEF <40%. ABCD was presented at the American Heart Association 2006 Scientific Sessions and reported by heartwire at the time.
"In that study, both tests were equivalent in predicting appropriate ICD shocks or arrhythmic death, and their negative predictive values were 95%, rising to 98% if both tests were combined," Klingenheben notes. Those numbers are much higher than the 79% negative predictive value observed for TWA testing in the current study's ischemic patients, who had additional risk factors, he observes. "Putting the currently available studies of microvolt TWA in LV dysfunction in perspective, it can be concluded that the predictive efficacy of the test is largely dependent on the patient population studied."
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