Influence of age on cardio-respiratory interaction assessed by joint symbolic dynamics

Cardio-respiratory dynamics exhibit complex behaviour and underlying mechanisms responsible for their interaction have not been fully understood. The aim of this paper was to validate the approach for detection of cardio-respiratory interaction based on joint symbolic dynamics using two simulated non-linearly coupled systems, and to investigate the influence of age on the interaction between cardiac and respiratory cycles in healthy subjects. For symbolic analysis, the time series were transformed into ternary symbol vectors based on the changes between two successive time points of each series. Subsequently, words of length `3' were formed and the correspondence between words of the two series was determined. The process was repeated using surrogate data to determine the effectiveness of the joint symbolic dynamics approach. Subsequently, the electrocardiograms (ECG) and respiratory signals of 20 young (age: 21-34 yrs) and 20 elderly (age: 68-85 yrs) healthy subjects were collected from Physionet, from which the R-R intervals and respiratory phases were extracted, respectively. Using the joint symbolic dynamics approach and an additional adaptive delay in the cardiac oscillator based on the maximization of the cross-correlation between the phases of respiration and the delayed R-R intervals, we found a significantly higher percentage of similarity in the symbolic dynamics of R-R intervals and respiratory phases in young subjects compared to elderly subjects (22.8±6.7 vs. 16.5±6.6 %, p<;0.01). In conclusion, joint symbolic dynamics provides an efficient technique for the analysis of cardio-respiratory interaction.