APPARATUS AND METHOD FOR MEASURING NATIVE MAMMALIAN BLOOD VISCOSITY

Abstract: A clinical whole blood viscometer and a method for obtaining instantaneous whole blood viscosity measurements over a wide range of shear rates with a single blood sample. A conventionally acquired sample of fresh blood (inside 11) is drawn from a patient and directed under pressure in viscous flow so as to deflect a compliant diaphragm (20, 26), then allowing the diaphragm to relax and concurrently measuring the relaxation of said compliant diaphragm continuously so as to determine a profile of viscosity versus shear rate of said sample.

Description: APPARATUS AND METHOD FOR MEASURING NATIVE MAMMALIAN BLOOD VISCOSITY Λ . Field of The Invention This invention relates to the measurement of the full range of shear rate dependent viscosities of fresh mam¬ malian blood and of other pseudoplastic fluids, i.e., it generates a profile of viscosity vs. shear rate for non- Newtonian fluids. Description Of The Prior Art U.S. Patent No., 3,720,097 discloses a diagnostic method and apparatus by which a sample of fresh mammalian blood is taken by puncturing a blood vessel and introduced into a system including (1) a capillary generating a linear relationship between pressure and flow of New¬ tonian fluids, and which is calibrated to deliver 20 ml. of a normal saline control solution in one minute at -50 mm. Hg. at 37°C, (2) a programmed vacuum pump which draws the blood sample through the capillary at a variety of pressures in the range of 0 to 200 mm Hg. , (3) an elec¬ tromagnetic flowmeter which senses the flow rate of the blood through the capillary, and (4) X-Y recording means to instantaneously plot the resultant of the blood flow with respect to pressure variations from which infor¬ mation on the blood viscosity at various shear rates may be determined. U.S. Patent Nos. 3,999,538 and 4,083,363 disclose methods and apparatus by which a sample of fresh mammalian blood is taken by puncturing a blood vessel while the sample is maintained under a constant pre¬ determined pressure, and the sample is withdrawn through vφe a volume measuring device connected directly to a pressure measuring device. By correlating the time required to withdraw a specified amount of blood with constant pres¬ sure maintained during withdrawal, an indication of blood viscosity is obtained. While these methods and apparatus may function as described, their use has been heretofore limited to research projects, and the application of the apparatus has necessarily involved the introduction of fresh blood into costly, non-disposable laboratory equip¬ ment requiring elaborate cleaning procedures to decon¬ taminate the instrument between experiments. These com¬ plexities mitigate against the general, convenient and practical use of this viscometric approach in clinical practice. Nevertheless, further development work has resulted in a refinement of the apparatus described in U.S. Patent No. 3,720,097, resulting in a completely fluid-filled system, in which pressure taps filled with saline solution are used to access the pressure at each end of a capillary containing the blood sample to be measured. The pressure taps are connected to opposite sides of a fluid-filled high compliance differential pressure tran¬ sducer. After setting up an initial flow in the capillary with a syringe or other external pressure source, the external flow is stopped after the compliant transducer diaphragm has been deflected by an initial amount of differential pressure between the pressure taps. The stored elastic energy of the compliant- transducer dia¬ phragm then causes relaxation of the diaphragm and accom¬ panying liquid flow from the high pressure to low pressure side of the transducer with the flow path including the capillary containing the blood as the major resistance to flow. By properly choosing the dimensions and config¬ urations of the elements of the system, the system func¬ tions as a first order dynamic system exhibiting an exponential decay and time constant proportional to vis¬ cosity for a Newtonian fluid. Viscosity is determined from relationship of shear rate to shear stress. The shear stress is proportional to the pressure in the capillary, while shear rate is calculated from the time rate of pressure change across the capillary, which is propor¬ tional to the flow. Moreover, a non- Newtonian fluid will have instantaneous viscosities which vary as a function of the instantaneous shear rates. Therefore, the multiple shear rate dependent viscosities (i.e., the viscosity profile) of fresh blood, a non-Newtonian fluid, can be measured in a single experiment over a 50 - 100 X shear rate range; also by appropriate choice of capillary and diaphragm the experiment may be designed to last only a short period (typically 20 - 60 seconds). The shear rate range and experimental duration are selected to provide information of clinical interest (e.g., red cell aggre¬ gation effects, particularly at low shear rate) while the overall experimental time is short enough so that a fresh blood sample remains well mixed, without the necessity for anticoagulants. On the other hand, the time for the test is long enough so that very short term relaxation pheno¬ mena (less than a few seconds) in the blood do not influence the results. The technique and some of the studies carried out on and results obtained with it are described in the following publications: A. Downs, A., Litt, M, and Kron, R. E.-: Low Shear Rate Viscosity of Fresh Blood, Bio- rheology. Vol. 17, 25-35, Pergamon Press Ltd., 1980. B. Yepsen, G. , Boutin, D. , Litt, M. and Kron, R. E. : Rheological Modelling of Fresh Blood from Transient Pressure Measurements, Biorheology, Vol. 18, 475-484, Pergamon Press Ltd. , 1981. C. Seybert, J. , Kron, S. , Litt, M. and Kron, R. E. : A Portable Computerized Clinical Whole Blood Viscometer. Proceedings of the VI International Conference of the Cardio¬ vascular System Dynamics Society, 332-335, November 1984. The refinement of the viscometric system described in Patent No. 3,720,097 has several deficiencies which complicate its application in the general clinical set¬ ting, including complexity, cost and safety in blood handling, an important factor nowwith the increased risks from blood borne diseases such as AIDS and hepatitis. Moreover, undiluted fresh blood samples cannot easily be recovered from the instrumentation for additional labora¬ tory testing, the system must be disassembled to be cleaned if blood is inadvertently permitted to clot in the fluid-carrying elements and care must be taken to avoid contamination from spilling of blood or blood contact with the operator. If anticoagulated (rather than fresh) blood is used, the same deficiencies apply except for the clotting problem, but an additional complication of in- homogeneity due to RBC settling is added. With the introduction of FDA-approved drugs to re¬ duce blood viscosity in human beings, the need has arisen for a practical clinical method and apparatus for moni¬ toring hemorheological drug therapy and to determine fresh whole blood viscosity. The method must be in¬ expensive, safe, easy to use and accurate. Heretofore, "blood viscosity determinants" such as blood hematocrit and fibrinogen content have been used to estimate high shear rate blood viscosity but they suffer from wide individual variability. Also, these indirect determin¬ ants are time consuming and expensive to obtain and do not account for many factors such as red blood cell inter¬ actions and red blood cell flexibility that contribute heavily to low shear rate blood viscosity. Therefore, accurate, direct, immediate and clinically useful blood viscosity measures cannot be determined in an individual patient from knowledge of hematocrit and fibrinogen para¬ meters alone. Summary Of The Invention We have now invented a method and apparatus that provides from a small single sample of fresh whole blood a complete profile of viscosity versus shear rate over the full range of clinical interest in a safe and less complex manner than prior instrumentation and methods. In sum¬ mary, the method involves conventionally acquiring a sample of fresh whole blood from a patient and (1) estab¬ lishing a viscous flow (varying with time) of the blood sample within a constricted space (such as a capillary), said flow in one direction communicating with a body of liquid, and said flow in the opposite direction commun¬ icating with an enclosed body of liquid retained by a first compliant diaphragm, (2) establishing an enclosed body of air (or other compressible or non-compressible fluid) between said first compliant diaphragm and a second com¬ pliant diaphragm, the unenclosed side of said second compliant diaphragm being exposed to atmospheric pres¬ sure, and said second compliant diaphragm being asso¬ ciated with means to continuously measure the amount of deflection of said second compliant diaphragm in response to change of pressure of said body of air (or other compressible or non-compressible fluid), (3) maintaining said viscous flow of blood under external pressure suf¬ ficient to deflect the first compliant diaphragm and raise the pressure of the air (or other compressible or non- compressible fluid) whereby said second compliant dia¬ phragm is deflected and stressed a predetermined amount sensible to said measuring means; (4 " ) stopping the appli¬ cation of external pressure and continuing said viscous flow of blood in the capillary under only the pressure effected by the relaxation of said compliant diaphragms (and intervening air or other fluid) and (5) processing the output of said measuring means to determine the changing viscosity of said sample as a function of shear rates during the relaxation of said compliant diaphragms and intervening air or fluid, so as to obtain a profile of viscosity versus shear rate of said blood sample. Brief Description Of The Drawing Fig. 1 is a schematic diagram of the apparatus of an embodiment of this invention; Fig. 2 is a schematic diagram of the recording and data analysis instrumentation of an embodiment of this invention; Fig. 3 is a reproduction of a semi-log record of the viscosity measurements obtained using a sample of normal saline solution in accordance with the process and using the apparatus of an embodiment of this invention; and Fig. 4 is a reproduction of a semi-log record of the viscosity measurements obtained using one sample of fresh whole blood in accordance with the process and using the apparatus of an embodiment of this invention. Description Of The Preferred Embodiments Referring now to Fig. 1, a conventional hypodermic syringe 11 is shown containing a blood sample. The syringe 11 is adapted for connection to and is shown connected to a three-way valve 12 positioned medially of a conduit 13. One end of conduit 13 is connected through a connector 14 to a capillary 15, which is in turn connected into an open reservoir 16, shown containing a body of liquid L. The opposite end of conduit 13 is connected into an assembly 17 consisting of a recessed shell 18, a mating recessed shell 19, a first compliant diaphragm 20 and a venting system to air 21 containing valve VI. As constructed, the first compliant diaph¬ ragm 20 is sealed between the recessed shefl 18 and the mating recessed shell 19. Moreover, conduit 13 is con¬ nected into assembly 17 through recessed shell 18, and a conduit 22 is connected into assembly 17 through mating recessed shell 19 at one end thereof, while at the opposite end it is connected into the measuring assembly 23 con¬ sisting of a recessed shell 24, a mating recessed shell 25, a second compliant diaphragm 26, embedded coils 27, coil output means 28, and venting system to air 29 containing valve V2. Mating recessed shell 25 has an opening 30 formed therein open to the air, and the second compliant diaphragm 26 is sealed between recessed shell 24 and mating recessed shell 25. Referring now to Figs. 1 and 2, electrical leads 31, 32 connect the coil output means 28 to a transducing modulatordemodulator means 33. The output from the modu¬ lator-demodulator 33 is a voltage proportional to the deflection of the compliant diaphragm 26 sensed by coils 27 and is proportional to the pressure of the air or fluid in space A and is fed to a recorder 34 adapted to produce logarithmic pressure versus time curves in semi-log strip chart form, and also to differentiator and filter 35 adapted to produce a voltage proportional to the instan¬ taneous sensed pressure in space A, and a voltage propor¬ tional to the instantaneous time rate of pressure change (pressure-time derivative) in space A. These voltages are fed to the inputs of an analog to digital converter 36, which feeds digitized data to a programmed computer 37, which is served by a disk drive 38, a monitor 39, and a printer 40. In order to carry out the method of this invention, the apparatus is first calibrated with normal saline solution at 37°C. With reference to Figure 1A, the syringe 11 and three-way valve 12 are used to fill the space B which is the portion of assembly 17 bounded by the first compliant diaphragm 20 and the recessed shell 18, through conduit 13. The venting system 21 is used to eliminate air from space B. Space A consists of the remaining space in assembly 17 bounded by first compliant diaphragm 20 and mating recessed shell 19 and is connected through conduit 22 with the space in pressure measuring assembly 23 bounded by recessed shell 24 and second compliant diaphragm 26. Referring to Figure IB, the syringe 11 and three-way valve 12 are now used to fill the remainder of conduit 13 and capillary 15 with normal saline solution. Referring to Figure 1C, the three-way valve 12 is now set so that there is no flow and no net pressure difference between the portions of the apparatus filled with liquid L because diaphragms 20 and 26 are not deflected and the pressure in space A is atmospheric. Space A is adjusted to atmospheric pressure by means of the venting system 29. At this point, the output voltage of the modulator-demodulator 33 is set at zero to indicate atmospheric pressure throughout the system. The computer is activated with the required programs and test settings. The calibration run is begun by introducing more saline solution under pressure from syringe 11 through three-way valve 12 in the position shown in Figure 1A. This increased pressure causes the first compliant diaphr¬ agm 20 to deflect, thereby increasing the air pressure in the space A. In response to this increased air pressure, the second compliant diaphragm deflects against the at¬ mospheric pressure in space C which is between dia¬ phragm 26 and recessed shell 25, and that amount of deflection is sensed by the embedded coils 27, generating a signal which is fed through the electrical leads 31, 32 as input to the modulator-demodulator 33. Additional syringe pressure is applied until the voltage output from modulator-demodulator 33 is greater than 10 volts, at which point the recorder 34 and computer 37 are activated. Now the three-way valve 12 is set to close off syringe 11, as shown in Figure 1C, to permit fluid flow under the force due to the elastic relaxation of both compliant diaphragms and intervening fluid in space A, through con¬ duit 13, capillary 15 and into reservoir 16. As this flow continues, the pressure against first compliant dia¬ phragm 20 decreases with time, as does the pressure in the space A contained by the compliant diaphragms 20, 26, which in turn causes a decreasing voltage signal from modulator-demodulator 33. When this pressure-related signal drops below 10 volts, the computer program is automatically activated so as to acquire data represen¬ ting the relationship of fluid pressure in space A versus time. At the same time, the recorder 34 continuously prints out a semi-logarithmic plot of pressure versus time. Fig.3 is a reproduction of a print of the resulting visco etric relationships generated by recorder 34 during the calibration of the apparatus with normal saline solu¬ tion. With reference to Figure 3, Curve A represents the equilibrium base line pressure before pressure is applied with a syringe. Curve B represents the increase in pressure in space A resulting from compressing the sy¬ ringe. Curve C is the 10-volt maximum output of modulator¬ demodulator 33 when pressure is applied with the syringe. While the modulator-demodulator 33 output is above 10 volts, the valve 12 is turned to configuration shown in Figure 1C and the voltage output decays until it falls below 10 volts as shown at point D. Curve E shows the relaxation and incorporates the pressure-time data re¬ quired to calculate viscosity. Curve F represents the return of system to baseline atmospheric pressure. The fact that the plot generated for normal saline solution in Figure 3 (curve E) is a straight line shows that the system senses saline solution as a Newtonian fluid, whose first order decay in such a system is expected to be negatively exponential, with a single time constant which gives a falling straight line when plotted as a logarithm against time. The fact that the curve is completely linear also shows that no non-linear second order effects are present. The plotted curve shown in Fig. 4 was generated during a test run utilizing fresh whole blood. After filling the apparatus with normal saline solution as in the aforementioned calibration run, syringe 11 was filled with a sample of fresh whole blood, and by utilizing three- way valve 12, the syringe 11 was operated according to the configuration shown in Fig. IB so as to displace saline from the portion of conduit 13 between connector 14 and three-way valve 12, through capillary 15 and into the body of liquid L in reservoir 16. At this point, three-way valve 12 was set as in Fig. 1C so as to permit flow in each direction through conduit 13 in order to achieve pressure and flow equalization therein. The valve was then turned to configuration shown in Fig. 1A to deflect the dia¬ phragms 22 and 26 so that output of modulator-demodula¬ tor 33 exceeds 10 volts. Valve 12 is then turned to configuration shown in Fig. 1C, allowing pressure in space B to relax as in the aforementioned calibration test run. It is noted from comparing Figs. 3 and 4 that the time represented by Curve C between pressurization and drop of the pressure generated voltage below 10 volts is- sig¬ nificantly longer for blood than for the saline solution because of the greater viscosity of blood. Also, it is apparent that curve E, showing decay of the pressure in Fig. 4, is significantly prolonged for blood compared to saline and is curved concave upward. Such non-linear behavior is characteristic of a pseudoplastic (non-New¬ tonian) fluid such as blood, and indicates that the viscosity of the blood sample is increasing as the rate of flow (shear rate) -decreases. The computer data from the run generating the curve of Fig. 4 (Table I) showed a shear rate range from 1 sec~l to 46 sec~l, which covers a significant part of the shear rate range of clinical interest in analyzing blood rheology. Within said range, the viscosity of the blood sample varied from 2.5 cen- tipoise at 46 sec -1 to 8.6 σentipoise at 1 sec" 1 . The computer also fits a Power Law or Casson rheological equation of state to the viscosity data thereby providing parameters which have clinical utility in characterizing the rheology of blood. For the sample shown in Fig. 4, the value of the Power Law exponent n is 0.68 and the value of the parameter K is 0.085. These values are characteristic of fresh blood having the viscosity profile shown in Table I. In order to maintain physiologically meaningful test conditions, the equipment shown in Fig. 1 was en¬ closed in an air bath at 37°C. The temperature of the air bath may be set at other values if it is desired to study variation of blood rheology with relation to temperature, as in patients with cryoglobulinemia. TABLE I Computer Output of Viscosities of Blood Sample Shown in Figure 4 at Selected Shear Rates Shear Rate (sec" 1 ) Viscosity (centipoise) 1 8.56 10 4.11 19 3.34 28 2.96 37 2.70 46 2.52 It will be readily apparent that the method and apparatus of this invention provide an inexpensive, safe, easy to use and accurate determination of fresh whole blood viscosity. Importantly, by this means, blood samples may be tested without introduction into expensive and complex laboratory equipment and the necessity of elaborate cleaning procedures between test experiments. It is apparent that the portions of apparatus of this invention and which are enclosed in the dotted lines in Fig. 1 are either readily available as disposable items or may be conventionally engineered from disposable materials. It is likewise apparent that the establishment and maintenance of a viscous flow of fresh whole blood in accordance with the method of and through the use of the apparatus of this invention may be equivalently accomplished by utilizing conventional vacuum blood collection systems.

Claims: CLAIMS Having thus described our invention, we claim: 1. The method of measuring the viscosity profile (shear rate dependent viscosity) of a fresh (or anti- coagulated) whole blood sample or other non-Newtonian liquid over a predetermined range of shear rates which comprises: (1) establishing a viscous flow of the sample within a constricted space, said flow in one direction communicating with a body of liquid, and said flow in the opposite direction communicating with an enclosed body of liquid retained by a first compliant diaphragm; (2) establishing an enclosed body of air (or other com¬ pressible or non-compressible fluid) between said first compliant diaphragm and a second compliant diaphragm, the unenclosed side of said second compliant diaphragm being exposed to atmospheric pressure, and said second compliant diaphragm being associated with means adapted to continuously measure the amount of change in de¬ flection of said second compliant diaphragm in response to change of pressure of said air or other compressible or non-compressible fluid; (3) varying said viscous flow of blood or other liquid sample within the constricted space under external pressure sufficient to change the pressure of the air (or other fluid) by a predetermined amount sensible to said measuring means, whereby said first and second compliant diaphragms are stressed; (4) stopping the external pressure and continuing said viscous flow of blood or other liquid sample under only the pressure effected by the stored elastic energy of relaxation of said first and second compliant diaphragms and enclosed air or other fluid; (5) processing the output of said measuring means to determine the changing viscosity of said sample as a function of shear rates during the relaxation of said first compliant diaphragm so as to obtain a profile of viscosity versus shear rate of said sample. 2. An apparatus for measuring the viscosity of a fresh or anticoagulated whole blood sample or other non- Newtonian liquid comprising in combination: (1) capil¬ lary or other constricted space means; (2) first conduit means in fluid flow communication with said capillary or constricted space means; (3) three-way valve means posi¬ tioned medially of said conduit means and adapted to receive the contents of a hypodermic syringe or other source of liquid under pressure; (4) an enclosed assembly comprising a pair of mating recessed shell portions and a first compliant diaphragm sealed therebetween, the distal end of said conduit means communicating into the interior of said assembly through one shell portion; (5) second conduit means communicating at one end thereof into the assembly (4) through the mating shell portion thereof; (6) an enclosed measuring assembly comprising a second pair of mating recessed shell portions having a second compliant diaphragm sealed therebetween, the dis¬ tal end of said second conduit means communicating into the interior of said measuring assembly through one shell portion of said second pair of shell portions, the mating shell portion of said second pair of shell portions having an opening formed therein; (7) coil means embedded within each of said second pair of shell portions adapted to sense the deflection of the second compliant dia¬ phragm; and (8) output means adapted to transmit gen¬ erated voltage from said coils. 3. The apparatus of Claim 2, further including (1) a modulator-demodulator adapted to generate an output vol¬ tage utilizing the voltage from said coils as input; (2) differentiating and filtering means adapted to generate one output voltage proportional to the output voltage of said modulator-demodulator, and another output voltage proportional to the time derivative of the output voltage of said modulator-demodulator; (3) an analog to digital converter adapted to convert the output voltages of said differentiating and filtering means to digital outputs; (5) a computer adapted to receive the digital output of said analog to digital converter; and (6) a program for converting the digital inputs to the computer to values of viscosity versus shear rate, and for computing para¬ meters of rheological equations of state. 4. The apparatus of Claim 3, including in com¬ bination a recorder adapted to print continuous log¬ arithm of pressure versus time curves in strip chart form utilizing the output voltage of said modulator-demod¬ ulator as input.

Inventors: KRON REUBEN E, LITT MITCHELL, KRON, REUBEN, E., LITT, MITCHELL
Applicants: KDL TECHNOLOGIES INC, KDL TECHNOLOGIES, INC.
Application year: 1987
Main classification: A61B5
IPC codes: G01N11