Letter to the Editor
What Is the Organization of Waves in Ventricular Fibrillation?
To the Editor:
Analysis of the spatiotemporal organization of wave propagation during ventricular fibrillation (VF) can provide important clues about underlying mechanisms.1,2 Although VF may be the result of the breakup of multiple, randomly propagating wavelets,3 it may also result from rotor-generating wavefronts that break at fixed heterogeneities.4 These two apparently vastly different mechanisms are hypothesized to have different signatures in terms of VF dynamics, the latter showing, of course, more organization and stability in its patterns of wave propagation.5,6 In a recent article published in Circulation Research, Choi et al7 presented results of a study of the dynamics of VF in the Langendorff-perfused guinea pig heart. They used optical mapping and various methods of analysis, including spectral and velocity quantification, to characterize the nature of wave organization during VF. The study provides important and useful information, particularly in regard to the existence of action potential duration gradients during VF. However, some of the authors’ conclusions seem premature and potentially misleading and may introduce unnecessary confusion to our current understanding of VF.
For example, on the basis of their analysis of frequency components of optical data from a selected and sparse number of epicardial recording sites, Choi et al7 reach the conclusion that multiple peaks, with “no indication of a single dominant frequency” (page e53), characterize their spectra. This conclusion is not surprising given the fact that the authors used an incorrect definition of dominant frequency.5 The dominant frequency, as the frequency with the maximal power, is uniquely determined not only for single peak spectra, as Choi et al used, but also for more complex, multiple peak spectra. Although Choi et al do not show the spatial distribution of dominant frequencies, it seems reasonable to suspect that the failure of these authors “to observe large areas with a monolithic dominant frequency” (page e56) was also the result of insufficient spatial resolution. This in itself invalidates the comparison of their data with those of Zaitsev et al,5 who used thousands of recording sites.5,6
Similarly, potentially misleading is the vector analysis of the local propagation velocity on the epicardial surface from which unsubstantiated conclusions about spatiotemporal periodicity of activation during VF are made. Choi et al7 analyze the magnitude and direction of conduction velocity vectors from two groups of five adjacent sites, one on the apex and the other on the base “to detect organized repetitive wavefronts” (page e52) and “quantitatively assess the occurrence of Wenckebach-like conduction on the epicardium during VF” (page e57). Vector analysis can indeed be useful in analyzing propagation and spatiotemporal periodicities,6 but by no means is the analysis of only two distant groups of sites sufficient to draw conclusions regarding organization during VF. An important limitation of the approach used by Choi et al is that it overlooks breakthroughs that are common during VF6 and thus undermines the level of existing organization. In addition, the approach potentially underestimates preference in direction of wavefront propagation because it utilizes spatial averaging of the velocity data that necessarily reduces the likelihood of detection of periodicities when the recording resolution is insufficient. Although the authors recognize some of the pitfalls of their measurements, they should be more conservative in their conclusions.
In summary, the question of whether wave propagation during VF is organized or random has not yet been answered. Importantly, achieving such an answer should not depend on the tools of analysis; only critically evaluated and well thought out investigative methods and conditions should be used for constructive comparison of one’s results with those of other studies. The study by Choi et al7 would have had greater impact on the debate over the nature of VF if its authors would have adhered to such critical self-evaluation.
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