Impedance of a fibrin clot in a cylindrical tube: relation to clot permeability and viscoelasticity.

By use of an impedance to quantify the pressure-to-flow relation for a clot-filled tube, a simple model is developed that encompasses both viscoelastic and porous properties of the clot. Measurements over a range of frequencies are used to separate the role of clot permeability from clot matrix elasticity. The theoretical impedance model consists of a series resistance and capacitance (representing structural flow) in parallel with a resistance (representing permeating flow). The viscoelasticity of the matrix, permeability and effective pore size are related to these three impedance elements. The validity of the model has been verified for a range of vessel sizes approximating small arteries (1 to 3 mm in diameter). The presence of dextran T40 in clotted fibrinogen solutions changes the clot impedance by increasing clot permeability and decreasing clot viscoelasticity. Because the flow contains two components, the behavior of a clot in vivo under pulsatile pressure cannot be predicted from the viscoelastic properties obtained from non-tube flow instruments nor from steady flow permeation measurements; a combination of the two as provided by oscillatory tube-flow measurements is required.

A new method for the analysis of blood and plasma coagulation.

A new method of measuring and analyzing the clinically significant structural characteristics of blood clots is described, using an oscillatory flow instrument (Vilastic-3 Viscoelasticity Analyzer). The development of the viscous and elastic properties of the clot are monitored continuously at known levels of stress and strain in a cylindrical tube [1], and specific transitional events and their timing are obtained. In clinical practice, two types of clot analysis currently are in use. One measures the time required for the clot to reach some instrument-determined degree of development, which provides no information about the mechanical character of the clot. The second yields a continuous depiction of the way the forming clot affects the instrument [2] and gives numerical values that are inseparable from instrument characteristics. For greater universality, it is preferable to obtain the clot properties per se, as opposed to integrated clot-instrument parameters, and to document the timing of well-defined viscoelastic events. Measurements on clotting blood and plasma show that the dependence of clotting characteristics on measurements conditions and hence the need for precise control of stress or strain during the process.