Ecological assessment of heart rate complexity: Differences between high- and low-anxious adolescents

• 120 min long ECG recordings were taken while students performed regular activities.
• Anxious students show diminished HR time series' fractal dimension and entropy.
• Temperamental vulnerability is negatively correlated with HR fractal dimension.

Nonlinear measures can capture the complex structure of the heart beating, and recordings taken while the individual performs daily activities may help to understand the cardiac system's output in natural conditions. As healthy systems are characterized by having highly complex outputs, we hypothesized that the cardiac output from high anxious adolescents should be less complex than the output from their low anxious counterparts. In this study ECG was recorded for two hours in 50 adolescents while they performed regular school activities. Fractal dimension (FD), scaling exponents and multiscale entropy were calculated on the interbeat intervals time series. Both FD and entropy were significantly lower in the high-anxious group than the low-anxious group. These results suggest different heart-related regulation in adolescents who suffered from high anxious symptomatology.