Information Domain Approach to the Investigation of Cardio-Vascular, Cardio-Pulmonary, and Vasculo-Pulmonary Causal Couplings

The physiological mechanisms related to cardio-vascular (CV), cardio-pulmonary (CP), and vasculo-pulmonary (VP) regulation may be probed through multivariate time series analysis tools. This study applied an information domain approach for the evaluation of non-linear causality to the beat-to-beat variability series of heart period (t), systolic arterial pressure (s), and respiration (r) measured during tilt testing and paced breathing (PB) protocols. The approach quantifies the causal coupling from the series i to the series j (C(ij)) as the amount of information flowing from i to j. A measure of directionality is also obtained as the difference between two reciprocal causal couplings (D(i,j) = C(ij) - C(ji)). Significant causal coupling and directionality were detected respectively when the median of C(ij) over subjects was positive (C(ij) > 0), and when D(i,j) was statistically different from zero (D(i,j) > 0 or D(i,j) < 0). The method was applied on t, s, and r series measured in 15 healthy subjects (22-32 years, 8 males) in the supine (su) and upright (up) positions, and in further 15 subjects (21-29 years, 7 males) during spontaneous (sp) and paced (pa) breathing. In the control condition (su, sp), a significant causal coupling was observed for C(rs), C(rt), C(st), and C(ts), and significant directionality was present only from r to t (D(r,t) > 0). During head-up tilt (up, sp), C(rs) was preserved, C(rt) decreased to zero median, and C(st) and C(ts) increased significantly; directionality vanished between r and t (D(r,t) = 0) and raised from s to t (D(s,t) > 0). During PB (su, pa), C(rs) increased significantly, C(rt) and C(ts) were preserved, and C(st) decreased to zero median; directionality was preserved from r to t (D(r,t) > 0), and raised from r to s (D(r,s) > 0). These results suggest that the approach may reflect modifications of CV, CP, and VP mechanisms consequent to altered physiological conditions, such as the baroreflex engagement and the dampening of respiratory sinus arrhythmia induced by tilt, or the respiratory driving on arterial pressure induced by PB. Thus, it could be suggested as a tool for the non-invasive monitoring of CV and cardiorespiratory control systems in normal and impaired conditions.