Hidden information in three-axial ECG data of normal subjects: Fractal dimensions of corresponding points from successive QRS loops as a potential sport and age dependent marker.

This study is aimed to estimate spatial variability of normal QRS loops trajectories in successive cardiac cycles analyzed in XYZ phase space among 27 young and 27 older subjects, both male and female. Among young subjects, 10 individuals were professional football players, while the remaining 17 were students without regular physical activities. Modified three-axial leads (Frank's lead system) were used continually to record high-resolution ECG (1 kHz sampling rate) at rest during 200 seconds by Biopac Student Lab System. Variability of all the three spherical coordinates of the ventricular electrical vector in five characteristic positions regarding the R peak was analyzed. It was found that although fractal dimensions were not gender-dependent (p > 0.05), the calculated values in all tested five positions were greater in young than in older individuals (p < 0.05). Besides that, among young subjects, several fractal dimensions were greater in athletes than in physically inactive students (p < 0.05). These results suggest that the availability of possible phase space trajectories for QRS vector motion is related to individual physical activity and ageing, leaving less variability to physically inactive or older individuals. Recognizing non-invasive features of heart activity might help early detection of heart problems caused by aging and sedentary life style.