Role of respiration in setting causality among cardiovascular variability series

We checked whether the observed shift of the dominant causality from heart period (HP) to systolic arterial pressure (SAP) in supine position to the reverse causal direction in upright position could be the result of the exogenous action of respiration on both variables. A model-based approach exploiting a multivariate dynamic adjustment class was utilized to decompose HP and SAP dynamics into partial processes and cancel respiratory-related influences from HP and SAP series. Causality was assessed in the information domain through a bivariate approach based on cross-conditional entropy. After canceling respiratory-related influences we observed the same trend on causality from supine to upright position as detected from the original series, thus suggesting that respiratory influences are not responsible per se for HP-SAP causal relations.