abstract
presented
at the 1995 ACC
New Orleans, Louisiana
March 20-22, 1995
How to Detect ECG T-Wave Alternans in Patients
at Risk for Sudden Cardiac Death
J Am Coll Cardio Feb. 1995; Special Issue:1027-40
David S. Rosenbaum, Xiaohua Fang, Judith A. Mackall,
Case Western Reserve University, Cleveland, OH
We have shown previously that subtle and visually
inapparent T wave alternans (TWA) is a marker of susceptibility
to sudden cardiac death (SCD). Since the characteristics of TWA
in patients with SCD are poorly understood, the optimal technique
for detecting TWA remains controversial. To resolve this problem,
high-fidelity orthogonal ECGs were recorded during atrial pacing
(100 bpm) from 35 patients undergoing EP testing. TWA measured
by fast Fourier transform (FFT) and complex demodulation (CD)
techniques were compared in two groups of patients: 1. High
Risk Group (n=15): Inducible sustained monomorphic VT at
EP testing or SCD during 20 month follow up, and 2. Low Risk
Group (n=20): Negative EP test and no SCD during 20 month
follow up. With FFT, beat-to-beat T wave amplitude oscillations
occurring at the alternans frequency are measured relative to
spectral noise. With CD, beat-to-beat T wave amplitude oscillations
were modeled as a sinusoid whose amplitude is assumed to be a
measure of TWA without regard to noise levels. The magnitude
of TWA measured by FFT and CD correlated well (r2 =
0.97) when TWA levels were relatively high (>5µV). However,
the detection of very low level (1µV - 5µV), but prognostically
important TWA differed considerably between the two techniques.
Using a cutoff TWA level of 2 µV to define TWA positivity,
FFT predicted high risk for SCD with a sensitivity of 70% and
specificity of 80% whereas CD had a sensitivity of 90% and specificity
of only 5%. Computer simulations performed with an idealized
alternans wave form demonstrated that the poor specificity of
CD was due to an inability of CD to selectively discriminate TWA
from noise. When the phase of TWA was reset on random beats (to
simulate ectopic beats), TWA was measured more accurately by CD
than FFT. However, phase resetting was found in only 2.7% of
ECG complexes analyzed from patients and its presence did not
substantially obscure the detection of TWA by FFT compared to
CD. Conclusions: Very low level TWA is a prognostically
significant marker for SCD and is more accurately measured using
the FFT technique. The enhanced specificity of FFT compared to
CD results from the capability of FFT to discriminate low level
TWA from noise.
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