Temporal organization of atrial activity and irregular ventricular rhythm during spontaneous atrial fibrillation: An in vivo study in the horse

Introduction: Atrial fibrillation (AF) is common in healthy horses. We stud ied the temporal organization of AF to test the hypothesis that the arrhyth mia is governed by a high degree of periodicity and therefore is not random in the horse. Further, we surmised that concealed conduction of AF impulse s in the AV node results in an inverse relationship between AF frequency an d ventricular frequency. Methods and Results: Fast Fourier transform (FFT) analysis of atrial activi ty was done on signal-averaged ECGs (n = 11) and atrial electrograms (n = 3 ) of horses with AF at control (C), after quinidine sulfate (22 mg/kg by mo uth every 2 hours) at 50% time to conversion (T50), and immediately before conversion (T90) to sinus rhythm. FFT always revealed a single dominant fre quency peak. The mean dominant frequency decreased until conversion (C = 6. 84 +/- 0.85 Hz, T50 = 4.87 +/- 1.5 Hz, T90 = 3.41 +/- 1.18 Hz; P < 0.001). Mean AA intervals (n = 500) gradually increased after quinidine, Mean RR in tervals (n = 500), standard deviation of the mean (SDM), Poincare plots, an d serial autocorrelograms (SACs) of 500 RR intervals were measured at C and T90 to determine the ventricular response to AF and quinidine-induced chan ges in the variability of the ventricular response. Mean RR interval and SD M were reduced after quinidine (C = 1431 +/- 266 msec and 695 +/- 23 msec; T90 = 974 +/- 116 msec and 273 +/- 158 msec, respectively; P < 0.01). Poinc are plots and SAC at C and at T90 revealed a significant correlation of con secutive RR intervals typical of a system with a deterministic behavior, At T90, the variability of RR intervals was reduced and the overall periodici ty of RR intervals was increased after quinidine administration. Conclusion: In the horse, AF is a complex arrhythmia characterized by a hig h degree of underlying periodicity. The inverse AA-to-RR interval relations hip and reduced variability of RR intervals after quinidine suggest that th e ventricular response during AF results from rate-dependent concealment of AF wavelets bombarding the AV node, which nevertheless results in a signif icant degree of short-term predictability of beat-to-beat changes in RR int ervals.