Letter by Selvaraj and Chauhan Regarding Article, “Upsurge in T-Wave Alternans and Nonalternating Repolarization Instability Precedes Spontaneous Initiation of Ventricular Tachyarrhythmias in Humans”
To the Editor:
We read with interest the article by Shusterman et al1 concerning repolarization dynamics preceding spontaneous initiation of ventricular tachyarrhythmias in humans. Two related time-domain methods measured a larger magnitude of “alternans” at 30 minutes compared with 60 to 120 minutes preceding arrhythmia onset. Spectral analysis of T-wave amplitude showed a nonuniform increase in power in all frequency ranges. The authors interpret these findings as a surge in alternans and nonalternating repolarization complexity preceding the arrhythmia.
An increase in sympathetic activity preceding the onset of ventricular arrhythmia was reported by the authors in the same population.2 This can lead to increased noise in the ECG recordings from perspiration and an increase in motion and respiratory artifact.3
The modified moving average and, to a larger extent, the intrabeat average measurements are affected by noise that cannot be completely removed by preprocessing.3 Using these time-domain methods, the authors have previously emphasized the need to measure surrogate alternans in the isoelectric T end to P onset segment to show that an increase in T-wave alternans magnitude is not artifactually attributable to noise.3 It is unclear why the authors did not use a similar analysis of surrogate alternans in the present study1 to ensure that noise was not a confounder.
Using the spectral method, white noise can spuriously increase the spectral power in the alternans frequency, and subtraction of the spectral power in a “noise band” is required to measure true alternans.4 Therefore, the increase in spectral power seen in all frequency ranges could be explained by an increase in noise rather than an increase in “nonalternating repolarization complexity.” The possibility of subharmonics influencing the measured power in the alternans range is another important confounder, particularly respiratory subharmonics (0.15 to 0.25 cpb), which increase in spectral power before the tachyarrhythmia. The authors feel the contribution of respiratory subharmonics is improbable because the increase in T-wave alternans magnitude has been confirmed by 3 independent methods. However, the concordant findings do not exclude the confounding effects of subharmonics, because these methods are all inherently influenced by subharmonics.3 Although the increase in alternans may not be accounted for entirely by respiratory subharmonics, this is likely an important contribution and is not unexpected, because respiratory excursions that influence T-wave amplitude can increase during sympathetic surges preceding tachyarrhythmia. Thus, we feel that white noise and respiratory oscillations may significantly contribute to the apparent increase in alternating and nonalternating repolarization instability.
Shusterman V, Goldberg A, London B. Upsurge in T-wave alternans and nonalternating repolarization instability precedes spontaneous initiation of ventricular tachyarrhythmias in humans. Circulation. 2006; 113: 2880–2887.
Shusterman V, Aysin B, Gottipaty V, Weiss R, Brode S, Schwartzman D, Anderson KP. Autonomic nervous system activity and the spontaneous initiation of ventricular tachycardia. ESVEM Investigators. Electrophysiologic Study Versus Electrocardiographic Monitoring Trial. J Am Coll Cardiol. 1998; 32: 1891–1899.