Diurnal variations of the dominant cycle length of chronic atrial fibrillation

High-resolution digital Holter recording was carried out in 21 patients (15 men, 64 +/- 12 yr) with chronic atrial fibrillation. Dominating atrial cyc le length (DACL) was derived by frequency domain analysis of QRST-reduced e lectrocardiograms. Daytime mean DACL was 150 +/- 17 ms, and nighttime mean was 157 +/- 22 ms (P = 0.0002). Diurnal fluctuation in DACL differed among patients: it tended to be virtually absent in those with a short mean DACL, but in those with longer DACL the night-day difference was as much as 23 m s (R = 0.72, P< 0.001, correlation of mean DACL to night-day difference). M ean DACL also correlated with ventricular cycle length (R = 0.40, P< 0.001) , particularly at night (r = 0.49). The shorter cycle lengths found in this study during the day are consistent with sympathetic and/or other physiolo gical modulation, but since increased vagal tone shortens atrial refractori ness in most models, parasympathetic influences are not likely to play a ma jor role. Alternatively, atrial effective refractory period may not be the sole determinant of atrial cycle length during atrial fibrillation.