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Sustained Anxiety Scores are Associated with the Fractal Dynamics of the Heartbeat in Early Adolescents.

The risk of suffering anxiety disorders is associated with sustained subthreshold symptoms of anxiety. This study evaluated the stability of anxiety scores (high, moderate or low) across a six-month period in early adolescents (N = 95). The associations between sustained anxiety, vagally-mediated heart rate variability (vmHRV), sympathetic activity, and heart rate fractal dynamics in everyday life conditions were analyzed. The anxiety scores from 71.50% of participants remained at the same level. The linear correlations between anxiety and cardiac measures were weak but a group-based approach revealed that the fractal dimension (FD) from stable-low anxiety participants was higher than the FD from participants with stable-moderate anxiety scores but not higher than the FD from the stable-high anxiety group. The short-term correlations' exponent a1 from the stable-high anxiety group was higher than the a1 from the stable-moderate anxiety group but not higher than the exponent from the stable-low anxiety group. No differences were found in the vmHRV nor sympathetic activity. The lack of a direct association between the complexity of the heart rate and the level of sustained anxiety suggests a nonlinear pattern of associations that would be in accordance with the optimum variability principle.