Bivariate point process modeling and joint non-stationary analysis of pulse transit time and heart period

Pulse transit time (PTT) is strictly related to pulse wave velocity and may be used for non-invasive monitoring of arterial stiffness and pressure, whose assessment is fundamental to detect cardiovascular dysfunctions. We propose a new model to characterize instantaneous PTT dynamics, and the interactions between PTT and R-R interval (RRI). In this model, PTT is described as a point process whose probability function depends on previous PTT and RRI values. From the model coefficients, instantaneous powers, coherence and directed coherence of each spectral component are estimated. We used this framework to study the changes that tilt table test provoked in PTT and RRI dynamics in 17 healthy subjects. Time-varying spectral and coherence analysis revealed that, although PTT and RRI were locally correlated, direct contribution of RRI on PTT was low during the entire test in high frequency band, and just after postural changes in low frequency band. We conclude that PTT may add valuable information for a more accurate characterization of cardiovascular regulation.