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Heart rate and T-wave
alternans (TWA) both increase prior to the onset of spontaneous ventricular
tachyarrhythmias (VTA). However, because TWA is usually increased at higher
heart rate (HR), it is unclear whether the pre-arrhythmic upsurge in TWA is
driven solely by an increase in HR or by a combination of several factors.
To answer this question, we examined the relationship between HR and TWA in
Holter ECGs with spontaneous sustained (>30 sec) VTA.
Methods. Ambulatory ECGs from 38 patients (age: 65±10y.o, all
had structural heart disease, 79% ischemic, LVEF:33±18%) were examined at
effective resolution of 2.5μV. The lead
with the highest magnitude of the T-wave was selected for analysis. TWA was
examined using the Spectral, Modified Moving Average, and Intra-Beat
Average analyses. In each subject, 15-min arrhythmia-free (Control) periods
were identified using 2 criteria: 1) time interval >1hr from the onset
of VTA, and 2) sinus rate ≥ than that during 15 min before the onset
of VTA. The levels of TWA were compared using the Wilcoxon matched pairs
test.
Results. TWA increased 10% during 15min before the onset of VTA
compared to 60-120min before VTA and reached 22±11μV (p=0.01). HR was also significantly
higher 15min before the onset of VTA than during 60-120min before the event
(85±18 and 78±18bpm, p<10-3). Although during the Control
periods (7±5hr from the onset of VTA), HR was 5% higher than 15min before
the onset of VTA (p<10-3), TWA was significantly lower during
these periods than before the onset of the arrhythmia (p=0.006, Figure).
Conclusions. An increase in the magnitude of TWA before the onset of
VTA was associated with an increase in heart rate. However, the levels of
TWA were often lower at similar or faster heart rates during
arrhythmia-free periods. This suggests that heart rate is not the only
driving force behind the increase in TWA that precedes the onset of the
arrhythmia. 
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