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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