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1 The Heart and Vascular Research Center and 2 The Department of Biomedical Engineering, MetroHealth Campus, Case Western Reserve University, Cleveland 44109, and 3 The Veterans Affairs Medical Center, Cleveland, Ohio 44106
We determined the temporal stability of
T wave alternans (TWA) during constant rate stimulation and the
dependence of alternans on heart rate (HR) and
-adrenergic
stimulation. Although it is established that exercise can provoke
microvolt-level TWA in patients at risk for reentrant ventricular
arrhythmias, the mechanisms underlying TWA in humans are not well
understood. Specifically, the temporal stability of alternans at any
given HR and the influence of HR vs. sympathetic activation on
alternans remain unclear. TWA was measured during prolonged fixed-rate
atrial pacing at multiple cycle lengths (CLs) in 10 subjects referred
for electrophysiological testing and in 14 additional subjects in whom
atrial pacing was performed at identical pacing CLs with and without
isoproterenol. During constant CL stimulation, TWA amplitude oscillated
significantly over time (typically by 10 µV) in a quasiperiodic
fashion with periodicity of ~2-3 min. Alternans amplitude was
strongly dependent on HR but not on adrenergic stimulation. There was a
patient-specific threshold HR over which alternans appeared. At higher
HR, alternans amplitude increased and oscillations were less prominent.
Adrenergic stimulation was required to produce TWA that was not already
elicited by moderate elevation of HR in only 2 of 14 (14%) patients.
In conclusion, TWA 1) fluctuates spontaneously over 2-3
min and 2) increases monotonically with increased HR
(without a major adrenergic contribution in most patients). These data
suggest that increased HR rather than sympathetic activation is
responsible for arrhythmogenic microvolt-level TWA measured during exercise.
electrical alternans; repolarization; Q-T interval; adrenergic stimulation; ventricular tachycardia
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