Aims: Higher values of heart rate variability (HRV) are commonly linked to better organization of feedback reflexes driving an organism's response to actual body needs. However, there are observations suggesting that abnormal levels of short-term HRV indices can be related to erratic rhythms resulting from remodeling of the cardiac tissue due to disease or aging. We hypothesize that decay with age of short-term measures of dynamical patterns of heart rhythm [a way in which accelerations (a) and decelerations (d) appear] provides a measure of healthy autonomic regulation in elderly people.
Methods: (1) Metrics of fragmentation, measuring probability of particular patterns of changes in RR-intervals: PIP=p(ad)+p(da), PAS=p(ada)+(pdad), PSS=1-p(paaa)-p(ddd) ; and (2) entropic measures, evaluating diversity in dynamical patterns: Shannon Entropy ShE, self-Transfer Entropy sTE and Entropy Rate S_T, were applied to signals with nocturnal RR-intervals (20000 points, binned with 8ms) extracted from Holter recordings. The 195 healthy participants considered were grouped according to their age decade: 20's(36 subjects), 30's(26 subjects), 40's(36 subjects), 50's(32 subjects), 60's(24 subjects), 70's(22 subjects), 80's( 18 subjects). Assuming that changes in a short-term index are approximated by a square function, by nonlinear regression and two-way ANOVA, we could estimate values which then allows to extract elderly people not affected by erratic rhythms.
Results: The statistical analysis provided the minimum of sTE decay at 60’s, min(sTE)=0.27 as the best separator. Accordingly, we divided subjects of 70’s and 80’s into two groups: sTE < min(sTE) and sTE > min(sTE). All entropic measures found these groups different (p<0.001, in t-test or Man-Whitney in case normality test failed). The fragmentation metrics PPP and PAS also provided these groups distinct. However,However, PSS, p(a) and p(dd) yielded that the groups are similar.
Conclusions: Frequent changes in heart rate acceleration sign are a dynamical signature for anomalous levels of short-term HRV in elderly people.