Atrial Flutter Cycle Length Oscillations and Role of the Autonomic Nervous System

To the Editor: In a recent issue of Circulation , Stambler et al1 provide an intriguing but possibly misleading set of results regarding the variability of atrial flutter cycle length (AFCL) and the role of the autonomic nervous system in AFCL modulation. Previous studies2 3 4 5 identified in the AFCL variability the existence of 2 rhythmic oscillations, 1 controlled by the ventricular rate and 1 by respiratory activity, suggesting a modulatory effect of atrial stretch. Using spectral analysis, Stambler et al claimed the existence of an additional low-frequency oscillation (band 1) mediated by sympathetic tone. Their conclusions are not supported by their study design and results.In fact, the newly proposed low-frequency oscillation band was concentrated in the extremely low-frequency region of the spectrum and on average was centered around 0.006±0.002 Hz (Table 2 of the article by Stambler et al). This should correspond to an AFCL oscillation with a period of 166 seconds, clearly longer than the 2 minutes of analyzed recording time. This means that we are dealing with a baseline trend rather than a rhythmic oscillation. In other words, band 1 is simply a DC component of the spectrum reflecting the nonstationary nature of the analyzed time series and thus should be avoided in the interpretation of the power spectra.6 In their study, Stambler et al also examined the role of the autonomic nervous system in the modulation of AFCL by studying the effect of pharmacological interventions on the power-band distribution of AFCL spectra. Stimulation and inhibition of sympathetic tone obtained by intravenous infusion of isoproterenol and long-term therapy with …