Arterial wall diameter and viscoelasticity variability

The aim of this study was to evaluate and characterize arterial diameter and arterial wall viscoelastic variability. An animal study was performed on seven sheep instrumented in the brachycephalic artery. ECG, arterial diameter and pressure waveforms were simultaneously measured. Four different hemodynamic conditions were considered: 1) under anesthesia, 2) conscious steady state, 3) vascular smooth muscle (VSM) activation and 4) VSM relaxation. A system modeling identification approach was applied in order to estimate viscoelastic indexes. The linear autoregressive with exogenous input model (ARX) was applied to the single beat pressure-diameter data to assess the arterial system dynamics. The elastic and viscous indexes were derived from the identified ARX model. Arterial pressure and diameter, heart rate and viscoelastic indexes variability analysis were performed in the time and frequency domains. While systolic pressure and arterial diameter oscillation show a similar pattern to heart rate at all frequency bands in control condition, wall viscosity variability does not match with these parameters (p<0.05). Compared with control condition, arterial diameter variability was lower during anesthesia and higher during VSM relaxation (p<0.05). The elastic index variability was lower during anesthesia and VSM relaxation and higher during VSM activation. The different behavior of arterial wall viscoelasticity suggests that intrinsic mechanisms related with vascular tone and vasomotion might be involved in the oscillatory pattern of the arterial wall.