Determination of Aortic Characteristic Impedance and Total Arterial Compliance From Regional Pulse Wave Velocities Using Machine Learning: An in-silico Study

In-vivo assessment of aortic characteristic impedance (Zao) and total arterial compliance (CT) has been hampered by the need for either invasive or inconvenient and expensive methods to access simultaneous recordings of aortic pressure and flow, wall thickness, and cross-sectional area. In contrast, regional pulse wave velocity (PWV) measurements are non-invasive and clinically available. In this study, we present a non-invasive method for estimating Zao and CT using cuff pressure, carotid-femoral PWV (cfPWV), and carotid-radial PWV (crPWV). Regression analysis is employed for both Zao and CT. The regressors are trained and tested using a pool of virtual subjects (n = 3,818) generated from a previously validated in-silico model. Predictions achieved an accuracy of 7.40%, r = 0.90, and 6.26%, r = 0.95, for Zao, and CT, respectively. The proposed approach constitutes a step forward to non-invasive screening of elastic vascular properties in humans by exploiting easily obtained measurements. This study could introduce a valuable tool for assessing arterial stiffness reducing the cost and the complexity of the required measuring techniques. Further clinical studies are required to validate the method in-vivo.

Changes in hemodynamics associated with metabolic syndrome are more pronounced in women than in men

The increase in cardiovascular risk associated with metabolic syndrome (MS) seems higher in women than in men. We examined hemodynamics during head-up tilt in 252 men and 250 women without atherosclerosis, diabetes, or antihypertensive medication, mean age 48 years, using whole-body impedance cardiography and radial pulse wave analysis. MS was defined according to Alberti et al. 2009. Men and women with MS presented with corresponding elevations of systolic and diastolic blood pressure (10-14%, p ≤ 0.001) versus controls. Supine pulse wave velocity (16–17%, p < 0.001) and systemic vascular resistance (7–9%, p ≤ 0.026), and upright cardiac output (6–11%, p ≤ 0.008) were higher in both MS groups than controls. Elevation of supine aortic characteristic impedance was higher in women than in men with MS (16% vs. 8%, p = 0.026), and in contrast to men, no upright impedance reduction was observed in women. When upright, women but not men with MS showed faster return of reflected pressure wave (p = 0.036), and smaller decrease in left cardiac work (p = 0.035) versus controls. The faster upright return of reflected pressure, lower upright decrease in left cardiac work, and higher elevation of aortic characteristic impedance may contribute to the greater increase in MS-related cardiovascular risk in women than in men.

Thoracic aortic pressure-flow relationships and vascular impedance in fetal sheep

Upper descending thoracic aortic pressures recorded from near-term fetal sheep resembled those recorded in mature animals, but diastolic thoracic aortic flows were much greater than those reported for adults. Furthermore, the pressure wave was not amplified during transmission along the aorta, as it is in adults. Descending thoracic impedance fell with increasing frequency to approximately 25% of resistance to steady flow and then showed oscillations typical of systems in which pulse wave reflections are important. Angiotensin II and norepinephrine infusions enhanced impedance oscillations and caused changes in apparent phase velocity in the aorta that were consistent with increased wave reflections. Angiotensin also increased aortic characteristic impedance from 0.0152 +/- 0.0013 to 0.0230 +/- 0.0024 mmHg.min.ml-1. The effects of nitroprusside infusions indicated reduced wave reflection effects. Embolization of the placenta and hindlimbs with 50-microns microspheres was used to elevate vascular resistance in these beds. Embolization caused aortic flow to fall to zero by late diastole, and changes in impedance and apparent phase velocity suggested increased wave reflections from the periphery. Characteristic impedance was increased to 0.0209 +/- 0.0031 mmHg.min.ml-1.