C22 - Relationship Between Nonlinear R-R Interval Dynamics and Mean Heart Rate in Dogs with Atrial Fibrillation

Abstract: Background In humans, non linear indices of heart rate (HR) irregularity provide complementary information to 24-hour heart rate (HR) but has not been reported in dogs with atrial fibrillation (AF). Objective To evaluate the relationships between nonlinear R–R interval indices from 24-hour ambulatory ECG (Holter) recordings and 24-hour HR in dogs with AF. Study Design Retrospective, exploratory, observational study. Animals Fifty dogs with electrocardiographic diagnosis of AF. Methods From each dog’s extracted R–R interval series, we calculated HR summary statistics and nonlinear indices (CVRR, Poincaré SD1, SD2, SD1/SD2, detrended fluctuation analysis exponent [DFA α], and sample entropy) using open-source software. Associations between 24-hour mean HR and nonlinear metrics were assessed using Spearman rank correlation. Rhythm-pattern visualization used Poincaré plots (RRn vs RRn+1). Results Mean HR was 133.3 ± 33.3 bpm. Mean HR had predictable inverse correlation with standard deviation-based measures of R-R interval irregularity (SD1 (rho=−0.908, p=9.44×10⁻²⁰), SD2 (rho=−0.868, p=3.16×10⁻¹⁶) and CVRR (rho=−0.730, p=1.75×10⁻⁹)), and DFA α (ρ=−0.653, p=2.72×10⁻⁷), while sample entropy increased with HR (rho=+0.579, p=1.06×10⁻⁵). SD1/SD2 was not correlated with mean HR (rho=+0.086, p=0.554). In subgroup analyses, mean HR did not differ detectably by Holter timing/context (Pre vs Post; Routine vs Uncontrolled), but SD1/SD2 differed for Pre vs Post (p=0.020) and DFA α differed for Pre vs Post (p=0.047); sample entropy did not differ. Conclusions Mean HR alone may incompletely characterize AF ventricular response patterns. Nonlinear indices, particularly SD1/SD2 and DFA α, may capture timing or therapy-related differences, supporting their complementary value in AF Holter assessment.