abstract
presented
at the 1997 International Congress of Non Invasive Cardiovascular Dynamics
T Wave Alternans.
A Marker of Vulnerability to Ventricular Tachyarrhythmias
Journal of Cardiovascular Diagnosis and Procedures
1997; Vol. 14/Number 2:pg. 82
Nabil El-Sherif, MD, State University of New York
Health Science Center and Veterans Affairs Medical Center, Brooklyn,
New York, USA
Alternation of the configuration and/or duration
of the repolarization wave of the electrocardiogram (ECG); usually
referred to as T wave alternans, is seen under diverse experimental
and clinical conditions. Interest in repolarization alternans
is attributed to the hypothesis that it may reflect underlying
dispersion of repolarization in the ventricle, a well-recognized
electrophysiologic substrate for reentrant ventricular arrhythmias
(RVA). Recent experimental observations in the canine anthropleurin-A
model of long QT syndrome and Torsade de Pointes, a surrogate
for the clinical LQT-3 syndrome, have shed more light on the relationship
between T wave alternans and RVA. During T wave alternans there
is marked dispersion of the duration of local repolarization,
measured as activation-recovery intervals (ARI) calculated from
unipolar electrograms. The dispersion is greatest across the
ventricular wall between mid myocardial regions (M) with longer
repolarization and both epicardial (EP) and endocardial (EN) regions
with shorter repolarization. The M region also exhibits marked
disparity of ARIs in alternate cycles compared to EP and EN regions.
This has the effect of decreasing the degree of dispersion across
the ventricular wall during the cycle with "short" QT
or even reversing the gradient of dispersion across the wall.
A premature focal impulse arising following the cycle with marked
dispersion of repolarization can initiate functional arcs of conduction
block and circulating wavefronts. Although overt T wave alternans
in the ECG are not common, in recent years digital signal-processing
techniques capable of detecting subtle degrees of T wave alternans
have suggested that the phenomenon may be more prevalent than
previously recognized and could represent an important marker
of vulnerability to RVA. Recent technical improvements allow
detection of microvolt T wave alternans during sinus rhythm with
the heart rate moderately elevated using bicycle exercise. In
preliminary studies the predictive value of T wave alternans was
high suggesting its clinical utility as a non-invasive risk stratifier.
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