6 research outputs found
Fractional-order viscoelasticity applied to describe uniaxial stress relaxation of human arteries.
Viscoelastic models can be used to better understand arterial wall mechanics in physiological and pathological conditions. The arterial wall reveals very slow time-dependent decays in uniaxial stress-relaxation experiments, coherent with weak power-law functions. Quasi-linear viscoelastic (QLV) theory was successfully applied to modeling such responses, but an accurate estimation of the reduced relaxation function parameters can be very difficult. In this work, an alternative relaxation function based on fractional calculus theory is proposed to describe stress relaxation experiments in strips cut from healthy human aortas. Stress relaxation (1 h) was registered at three incremental stress levels. The novel relaxation function with three parameters was integrated into the QLV theory to fit experimental data. It was based in a modified Voigt model, including a fractional element of order α, called spring–pot. The stressrelaxation predictionwas accurate and fast. Sensitivity plots for each parameter presented a minimum near their optimal values. Least-squares errors remained below 2%. Values of order α = 0.1–0.3 confirmed a predominant elastic behavior. The other two parameters of the model can be associated to elastic and viscous constants that explain the time course of the observed relaxation function. The fractional-order model integrated into the QLV theory proved to capture the essential features of the arterial wall mechanical response
Simulation of the arterial elasticity influence on the Ambulatory Arterial Stiffness Index AASI
Recientemente se propuso un Ãndice de rigidez arterial denominado AASI (Ambulatory Arterial Stiffness
Index) derivado de mediciones ambulatorias de presión arterial durante 24 horas. Su asociación como Ãndice de rigidez y la
infl uencia estadÃstica de la dispersión en los valores presivos continúa bajo discusión. Proponemos estudiar estas controversias
en el contexto de un modelo estadÃstico. Se realizó una simulación con valores similares a los de pacientes de arterias normales,
rÃgidas y compliantes, utilizando 3 curvas exponenciales presión-diámetro. Se generaron diámetros pulsátiles aleatorios siguiendo
distribuciones normales y se obtuvieron presiones sistólicas y diastólicas en tiempos paramétricos equivalentes a 24 horas. Se
calculó el AASI como uno menos la pendiente de la regresión de presión arterial sistólica y diastólica. El AASI del grupo normal
resultó 0,42, aumentó a 0,50 en el rÃgido y disminuyó a 0,34 en el compliante (siempre con r2>0,9). Disminuir la dispersión del
rango de presiones provocó una disminución de r2 en la regresión de la nube de puntos de presión sistólica y diastólica, aumentando
artifi cialmente el AASI. Por primera vez la elasticidad no-lineal de la pared arterial ayuda a explicar la asociación del AASI como
Ãndice de rigidez arterial. La simulación corrobora que la dispersión de los valores presivos condicionan el cálculo del AASI debido
a su naturaleza estadÃstica.Recently, an arterial stiffness index called AASI (Ambulatory Arterial Stiffness Index) calculated from ambulatory
blood pressure measurements during 24 hours was proposed. The associations with arterial stiffness and the pressure dispersion
dependence remain under discussion. We propose to study these controversies in a statistical model framework. A simulation was
performed including values similar to the ones in patients with normal, rigid and compliant arteries. Three exponential curves of
pressure-diameter were simulated. Based on diameters randomly generated following normal distributions, systolic and diastolic
pressures were calculated in a 24h parametric time. AASI was calculated as one minus the slope of the regression of systolic to
diastolic pressure. The AASI for the normal group was 0,42, increased to 0,50 in the rigid group and decreased to 0,34 in the
compliant case (always r2>0,9). A dispersion decrease in the pressure values was followed by an r2 decrease in the diastolic vs
systolic pressure regression, artifi cially increasing AASI. For the fi rst time the non-linearity of the arterial wall helps to explain the
association of AASI with a stiffness index. The simulation corroborates that 24 h pressure variability conditions AASI values due
to its statistical nature