The Irregularity Factor: Mechanistic Insights into Beat-to-beat Irregularities in Atrial Fibrillation Cycle Length

BACKGROUND: Measures derived from atrial fibrillation cycle length (AFCL) are used to identify regions crucial for AF maintenance or predict (ablation) treatment outcomes. However, mechanisms underlying beat-to-beat irregularities in AFCL is unknown. OBJECTIVES: We aimed to 1) investigate beat-to-beat changes in AFCL (Δ-AFCL) by relating temporal variation in AFCL with quantified beat-to-beat changes in activation patterns and conduction times (Δ-CT) 2) test whether the Irregularity Factor (IF) (SD of the Δ-AFCL histogram), is indicative of activation patterns complexity. METHODS: ) of fibrillation waves from electrode array border towards mapping area center. RESULTS: The IF was larger during LSPAF (52±17 vs 25±10;P<0.001) compared to acute AF. Δ-CTs were also larger (25±8ms vs 19±7ms;P=0.005) and caused by changes in conduction velocity (Δ-CVs:36±6cm/s versus 29±6cm/s;P=0.001) and -direction (Δ-direction:94±13° versus 64±22°;P<0.001);Δ-pathlenghts (Δ-PL:13±4mm versus 14±3mm;P=0.333) were comparable. IF correlated with conduction block (ρ=0.887), lines of conduction block lengths (ρ=0.900), number of fibrillation waves (ρ=0.889) and focal waves (ρ=0.609), (all P<0.001). CONCLUSION: The IF indicates the underlying complexity of activation patterns, is higher during LSPAF compared to acute AF and is comparable throughout the atria during LSPAF. It is an easy-to-calculate, objective electrical biomarker which independently from the AF mechanisms can be used for e.g. guiding patient tailored treatment.