Zvi Karni

Resistances and compliances of a compartmental model of the cerebrovascular system

A lumped parameter compartmental model for the nonsteady flow of the cerebrovascular fluid is constructed. The model assumes constant resistances that relate fluid flux to pressure gradients, and compliances between compartments that relate fluid accumulation to rate of pressure changes. Resistances are evaluated by using mean values of artery and cerebrospinal fluid (CSF) fluxes and mean compartmental pressures. Compliances are then evaluated from clinical data of simultaneous pulse wave recordings in the different compartments. Estimate of the average CSF compartmental deformation, based on the compliance between the CSF and brain tissue compartments, proves to be of the order of magnitude of actual experimental measurements.

A non-steady compartmental flow model of the cerebrovascular system.

A lumped parameter compartmental model for the cerebrovascular fluid system is constructed and solved for the general linear problem of a nonsteady flow with constant resistances and compliances. The model predicts the intracranial pressure waves in the various compartments of the brain in response to pressure changes in the vascular system.

Quasi-steady-state compartmental model of intracranial fluid dynamics

A lumped-parameter compartmental model for the cerebrovascular fluid system is constructed and solved for quasi-steady-state flow. The model predicts the pressure waves in the various compartments of the intracranial region in response to changes in the arterial pressure.

Isotropy and anisotropy of uterine muscle during labor contraction

Abstract A strain rosette analysis performed on 36 women during their 36–42 weeks of pregnancy at various points over the fundal region, shows that the functional performance of the uterine muscle is anisotropic in its character and a kinematic isotropy is a good approximation only during the early stages of labor. The various groups of fibers exhibit different viscoelastic properties, with the longitudinal group showing more resilience than the others. A case which proved an exception was that of an eighth month delivery showing the reverse behavior of strain hardening.

STRAIN UTEROGRAPHY IN LABOUR

Multi‐strain measurements of uterine contractions were made on 36 women at 36 to 42 weeks of pregnancy. The tracings obtained supported the existence of one dominant pacemaker area in the fundal region of the uterus and indicated that the contractile wave travelled in a downward direction with an average speed of 6 cm/second. The analysis of all normal labours showed that strain of the longitudinal muscle fibres changed from extension in the fundal region of the uterus to compression in its lower part, through a local zero strain in a narrow zone below the umbilicus. Intensity of the uterine contractions reached 6 per cent in extension and −2 per cent in compression. Information about the number of peak contractions, the frequency of contractions and the tonus of the myometrium during relaxation are of importance in the evaluation of uterine dysfunction.

Intracranial compartmental pulse-wave simulation

Abstract The general solution of the linear compartmental model for the cerebrovascular fluid system with constant resistances and compliances predicts the pressure waves in the compartments in response to an input pulse wave-arterial and/or jugular. Results are shown for a seven-compartment model and for a sinusoidal arterial pulse wave at a frequency of 1 Hz, with and without fluid drainage from the CSF compartment. z

An Outline of Continuum Modeling of Brain Tissue Mechanics

Modern, very sensitive, and noninvasive devices (strain-gauges, accelerometers, etc) detect deformations and motions in the order of a few microns. The observation that the rise in intracranial pressure produces measurable suture strain suggests that noninvasive determination of mechanical parameters in the intracranial cavity, such as density, velocity, pressure, energy, and their time rates, is clearly possible. Based on the above premise, and utilizing the concept of representative elementary volume (REV), this paper presents the conceptual part of a theory that defines the mechanical behavior of the brain and skull under various conditions. The systematic review of brain tissue mechanics leads to a continuum model of a mixed stress-displacement boundary value type, which defines the dynamics of the visco-elastic brain tissue in mathematical terms. (J Child Neurol 1986; 1:119-125).

A kinematic analysis of uterine deformation during labor

Abstract A thin shell model of the uterus is proposed and a kinematic solution of uterine deformation during labor derived. In vivo multi-point strain measurements performed during the various stages of labor up to delivery form boundary conditions for the numerical solution, which yields curvature and local obliqueness variations of the uterus during a contraction. The results are discussed in the light of their clinical applications.