Patent ID: 6796168
Filing Date: 2004-09-28
Classification: A61B,G01N

Abstract:
A method for determining the viscosity of blood flowing through the cardiovascular system of a living being at any point in the system, said method comprising the steps of:(a) determining a characteristic relationship for the blood between viscosity and shear rate, said step of determining a characteristic relationship comprising: (1) subjecting a portion of the blood to a plurality of shear rates as the blood flows through a device, said device comprising a scanning viscometer having a capillary tube and at least one riser tube, said device having some known parameters including the diameter and length of said capillary tube and the diameter of said at least one riser tube; (2) collecting a plurality of data points corresponding to movement of the blood through said plurality of shear rates, said collected data points corresponding to the changing height of at least one column of blood that moves through said capillary tube and said at least one riser tube, said at least one column of blood comprising a rising column of blood and a falling column of blood; and (3) using said collected data points and some known parameters of said device to generate said characteristic relationship, said characteristic relationship comprising an equation that describes the viscosity of the blood in terms of the shear rate of the blood, said shear rates of the blood being variable from approximately zero shear rate to infinite shear rate, said use of said collected data points comprising: (i) computing estimated values of the mean flow velocity of the blood as it flows through said device; (ii) selecting a first model for a constitutive equation that relates shear rate with shear stress, said first model comprising a Casson model based on: â€ƒÏ„&it;â€ƒ={â€ƒ&it;Ï„y+a3&it;Î³.for&it;â€ƒ&it;Ï„&GreaterEqual;Ï„y0&it;â€ƒ&it;and&it;â€ƒ&it;Î³.=0for&it;â€ƒ&it;0&leq;Ï„<Ï„y&it;â€ƒâ€ƒ&it;â€ƒwhere&tgr; represents the shear stress of the blood; &tgr;r represents the yield stress of the blood and is constant; {dot over (&ggr;)} is the shear rate of the blood; and a3 is a constant and is one of said constitutive equation parameters that minimizes the sum of squares of the deviations of said theoretical values of said mean flow velocity and said estimated values of said mean flow velocity; (iii) applying said constitutive equation and principles of fluid dynamics to determine a second model that provides the mean flow velocity of the blood as a function of time along with physical and constitutive equation parameters and which generates theoretical values of the mean flow velocity; (iv) substituting said collected data points into said second model to generate an over-determined system of equations for said constitutive equation parameters; (v) applying a non-linear curve-fitting algorithm to determine the values of said constitutive equation parameters that minimize the sum of squares of the deviations of said theoretical values of the mean flow velocity and said estimated values of said mean flow velocity; and (vi) using said determined values of said constitutive equation parameters to generate said characteristic relationship for the blood between viscosity and shear rate, and wherein said characteristic relationship comprises a characteristic blood viscosity-shear rate relationship as follows: V=f1+f2S+f3SwhereV is said viscosity of the blood; S is the shear rate of the blood; and f1, f2 and f3 are constant coefficients that are defined as: Æ’1=a3f2=&it;a2&it;Ï &it;â€ƒ&it;g&it;â€ƒ&it;Rc2&it;Lf3=&it;2&it;Ï &it;â€ƒ&it;g&it;â€ƒ&it;Rc&it;a2&it;a3LwhereRc is the radius of said capillary tube L is the length of said capillary tube; &rgr; is the density of the blood; g is the gravitational constant; and a2 is another one of said constitutive equation parameters that minimizes the sum of squares of the deviations of said theoretical values of said mean flow velocity and said estimated values of said mean flow velocity; and (b) obtaining a shear rate of the blood as it moves through at least one position in the system; and (c) determining the viscosity of the blood at said at least one position by applying said shear rate to said characteristic relationship.