Elucidating the Mechanisms of Atrial Flutter Cycle Length Variability Using Power Spectral Analysis Techniques

BACKGROUND: The mechanism of the small beat-to-beat variations in cycle length of atrial flutter in humans has not been fully explained. We investigated the beat-to-beat control of atrial flutter cycle length using time and frequency analysis techniques. METHODS AND RESULTS: Mean, SD, and power spectra of atrial cycle lengths were calculated from atrial recordings in 28 patients with type I atrial flutter. In control patients, mean and SD values of atrial cycle length were 265 +/- 37 and 4.9 +/- 1.7 ms. Power spectra contained two or three major peaks with 10.6 +/- 9.2% in band 1 (0.0 to 0.18 Hz), 26.7 +/- 15.9% in band 2 (0.18 to 0.6 Hz), and 63.1 +/- 17.7% in band 3 (0.6 to 2.2 Hz). Isoproterenol infusion (n = 8) increased percentage of total power in band 1 (7.1 +/- 5.6% to 25.7 +/- 18.9%, P < .001). Percentage of total power in band 1 was less in patients receiving (n = 5) versus not receiving (n = 18) oral beta-blockers (2.2 +/- 1.9% versus 10.6 +/- 9.2%, P = .003). Standard deviation (2.5 +/- 1.3 versus 4.9 +/- 1.7 ms, P = .009) and total power (2025 +/- 1350 versus 9768 +/- 8874 ms2, P = .005) were less in heart transplant recipients (n = 5) than control patients. Increases in respiratory rate (n = 6) shifted band 2 frequency peak to higher frequencies (0.26 +/- 0.13 to 0.38 +/- 0.18 Hz, P < .05). Atrial cycle length was longer and monophasic action potential duration was shorter during inspiration than during expiration. Band 3 frequency peak was correlated with heart rate (r = .797, P < .0001). CONCLUSIONS: Atrial flutter cycle length variability has an underlying periodic pattern that is detected by spectral analysis. Atrial flutter is modulated on a beat-to-beat basis by an interplay between the autonomic nervous and respiratory systems and the ventricular rate.