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 is described. A conventionally acquired sample of fresh blood is drawn from a patient and introduced into the viscometer so as to establish viscous flow, the viscosity measurements being taken by remote sensing devices (45) activated by a compressible fluid in contact with the sample and causing the sample to pass through a constriced space (41) as the fluid, which has been pressurised, is allowed to relax.

Description: This invention relates to the measurement of the full range of shear rate dependent viscosities of mammalian 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 Newtonian fluids, and which is calibrated to deliver 20 ml. of 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 electromagnetic 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 information 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by puncturing a blood vessel while the sample is maintained under a constant predetermined pressure, and the sample is withdrawn through a volume measuring device connected directly to a pressure measuring device.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 pressure 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 in certain cases the application of the apparatus has necessarily involved the introduction of fresh blood into equipment requiring elaborate cleaning procedures to decontaminate the instrument between experiments. These complexities mitigate against the general, convenient and practical use of this viscometric approach in clinical practice. Later developments to overcome these problems related to U.S. Patent No. 3,720,097 led to a completely liquid-filled pressure measuring 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 transducer. 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 diaphragm then causes relaxation of the diaphragm and accompanying 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 configurations of the elements of the system, the system functions as a first order dynamic system exhibiting an exponential decay and time constant proportional to viscosity 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 proportional 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 aggregation effects, particularly at low shear rate) while the overall experimental time is short enough so that any blood sample remains well mixed, and fresh blood samples can be tested without the necessity for anticoagulants. On the other hand, the time for the test is long enough so that very short term relaxation phenomena (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, Biorheology, 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 Conferece of the Cardiovascular System Dynamics Society, 332-335, November 1984. Despite the above described improvements of U.S. Patent No. 3,720,097 there still remain deficiencies which complicate their application in the general clinical setting, including complexity, cost and safety in blood handling, an important factor with 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 laboratory 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. Also, because it must be completely liquid filled, the system is sensitive to errors due to the presence of air within any of the fluid enclosures. It anticoagulated (rather than fresh) blood is used, the same deficiencies apply except for the clotting problem, but an additional complication of inhomogeneity due to RBC settling is added. Most importantly, these prior developments are not amenable to the design of a cost effective, disposable device. With the introduction of FDA-approved drugs to reduce blood viscosity in human beings, the need has arisen for a practical clinical method and apparatus for monitoring hemorheological drug therapy and to determine whole blood viscosity. The method must be inexpensive, 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 determinants are time consuming and expensive to obtain and do not account for many factors such as red blood cell interactions 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 parameters alone. GB-2198542 discloses a method and apparatus that provides from a single small sample of blood (or other non-Newtonian liquid) 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. The method involves acquiring a sample of blood and establishing a body of the sample within a constricted space (such as a capillary), said body in one direction communicating with a body of liquid and said body in the other direction communicating with an enclosed body of blood (or any other liquid) retained by a compliant diaphragm, establishing a body of air of known mass between the first compliant diaphragm and a second compliant diaphragm, the opposite side of the second compliant diaphragm being exposed to atmospheric pressure and being associated with means to measure continuously the amount of deflection of the second compliant diaphragm in response to change of pressure of the body of air, maintaining the viscous flow of blood under external pressure sufficient to deflect the first compliant diaphragm and raise the pressure of the air, whereby the second compliant diaphragm is deflected and stressed a predetermined amount sensible to the measuring means, stopping the application of external pressure and continuing the viscous flow of blood in the capillary under only the pressure effected by the relaxation of the compliant diaphragms, and processing the output of the measuring means to determine the changing viscosity of the sample as a function of shear rates during the relaxation of tie compliant diaphragms and intervening air, so as to obtain a profile of viscosity versus shear rate of the blood sample. This method is very effective, but involves the use of apparatus which is too expensive to be disposed of after an analysis is carried out and which involves a large number of parts which require sterilization. US-A-3435665 discloses a capillary viscometer which also poses similar problems. The apparatus comprises a manometer whose manometric liquid directly contacts the fluid to be tested. Thus, the manometer must be sterilised after every operation of the apparatus. The present invention provides a method and apparatus therefor in which the apparatus may be cheaply manufactured, so as to be disposable after an analysis has been carried out. The apparatus may be in one piece, so that few connections need be made, reducing the risk of contact by the analyst with blood. The number of parts which require sterilization is substantially reduced. The present invention provides a method of measuring a continuous profile of viscosity versus shear rate of a non-Newtonian liquid, such as blood, comprising providing a compliant means comprising a body of compressible fluid held in a chamber, establishing a sample of the non-Newtonian liquid within a constricted space between a fluid at known pressure in a fluid pressure chamber and the fluid of the compliant means and thereby establishing in the chamber of the compliant means direct contact of the compressible fluid with a free surface of said sample, and also establishing in the fluid pressure chamber a fluid sample interface, applying pressure by pressurising means to the fluid pressure chamber and thereby establishing an initial viscous flow of the sample to pressurise the fluid of the compliant means, removing the application of pressure and so allowing the fluid of the compliant means to relax and the sample to flow in the constricted space, sensing with a pressure sensor the changing pressure with time of the fluid of the compliant means during such relaxing flow, and processing the information with processing means to provide a continuous profile of viscosity versus shear rate of the sample. The present invention also provides apparatus for measuring a continuous profile of viscosity versus shear rate of a non-Newtonian liquid, such as blood, comprising constricting means defining a constricted space, compliant means comprising a chamber holding a body of compressible fluid, the chamber being in direct communication with one side of the constricting means, a fluid pressure chamber in fluid communication with the other side of the constricting means, holding means for holding a sample of the liquid and connectible with the constricting means via the fluid pressure chamber to admit the sample to the constricting means and the chamber, thereby providing direct contact of the fluid with a free surface of the sample within the chamber and to establish in the fluid pressure chamber a fluid sample interface, means for pressurising the fluid pressure chamber and thereby the liquid and the fluid, a pressure sensor for sensing the changing pressure with time of the fluid of the compliant means as the fluid of the compliant means is allowed to relax and cause the sample to flow through the constricting means, and a processing means for processing information from the sensor to provide a continuous profile of viscosity versus shear rate of the 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; 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; and Fig. 5 is a schematic diagram of the apparatus of another embodiment of this invention. Referring now to Fig. 1, a capillary 41 is connected to two chambers 42 and 43, in which interfaces are provided between the body of liquid and the air in spaces A1 and A2. The chamber 42 is connected through tubing means 44 to a three-way valve V3, which can be vented to the atmosphere or connected to pressure measuring means 45, which communicates to the system of Fig 2 through wires 48 and 49. Chamber 43 is similarly connected through tubing means 46 to three-way valve V4, which can be vented to the atmosphere or connected to syringe 47. Referring now to Fig. 2, electrical leads 31, 32 connect pressure measuring means 45 to a transducing modulator-demodulator means 33. The output from the modulator-demodulator 33 is a voltage proportional to the pressure of the air or fluid in space A1 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 instantaneous sensed pressure in space A1, and a voltage proportional to the instantaneous time rate of pressure change (pressure-time derivative) in space A1. 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 system is initially empty of all liquid. Valve V3 is turned to vent the space A1, and valve V4 is turned to connect syringe 47, which contains a test or calibration liquid with tubing 46 and chamber 43. The test liquid is introduced to the system using syringe 47, filling the capillary 41 and partially filling chambers 42 and 43 so as to establish air-liquid interfaces in these chambers. Valve V3 is then turned to connect space A1 with the pressure measuring means 45. At this point the output voltage of a modulator-demodulator 33 (Fig. 2) is set to zero to indicate atmospheric pressure in space A1. The sample syringe 47 is then replaced with an air filled syringe or other pressurization means and pressure applied to chamber 43, the liquid, and the air in space A1. When the pressure output is greater than 10 volts, the valve V4 is turned to vent space A2 to the atmosphere. The higher pressure in space A1 then causes the liquid to flow through capillary 41 from chamber 42 to 43, with reduction in pressure in space A1. Suitable design allows correction for the small changes in pressure head in chambers 42 and 43 during this process. The result is a pressure-time curve as shown in Fig. 4, which may be processed to determine the viscosity of the test fluid as a function of shear rate. As the flow through capillary 41 continues, the pressure in space A1 decreases with time, 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 representing the relationship of fluid pressure in space A1 versus time. At the same time, the recorder 34 continuously prints out a semilogarithmic plot of pressure versus time. Fig. 3 is a reproduction of a print of the resulting viscometric relationships generated by recorder 34 during the calibration of the apparatus with normal saline solution. With reference to Fig. 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 syringe. 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 V4 is turned to vent space A2 to the atmosphere and the voltage output decays until it falls below 10 volts as shown at point D. Curve E shown the relaxation and incorporates the pressure-time data required 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 Fig. 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 tame. 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. 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 significantly 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-Newtonian) 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 (not shown) from the run generating the curve of Fig. 4 showed a shear rate range from 1 sec⁻¹ to 46⁻¹, 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 centipoise to 46 sec⁻¹ to 8.6 centipoise 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. It is not essential to the operation of this invention that the test fluid be introduced using a syringe. Fig. 5 shows an alternative embodiment of this invention in which the sample of blood or other test fluid is supplied in a standard vacuum-type blood collecting tube. Referring now to Fig. 5, vacuum collecting tube 51 contains the blood sample and is closed by rubber stopper RS. Needle 52, a long hypodermic type needle, is connected using fitting 61 to capillary 50, which in turn is connected to chamber 54. Chamber 54 is connected by tubing means 55 to three-way valve V6, which can either vent air space A3 to atmosphere or connect it to the pressure measuring means 56. Means 56 is connected by wires 57 and 58 to the system shown in Fig. 2. Also, short needle 53 is connected by fitting 62 to tubing means 60, which in turn is connected to three-way valve V5, which can either vent the air space A4 to atmosphere or connect it to air-filled syringe 59. In order to carry out the method of this embodiment, the apparatus is initially empty of liquid. Stopper RS is pierced by needle 53 whose end extends into air space A4, and needle 52, which extends into the liquid sample to the bottom of tube 51. Value V6 is turned to vent air space A3 to atmosphere. Valve V5 is turned to connect syringe 59 to needle 53 and air space A4. The pressure in air space A4 is then raised using the syringe, pushing liquid from the bottom of tube 51 through needle 52 to fill capillary 50 and partially fill chamber 54 to establish an air-liquid interface in chamber 54. Valve V6 is then turned to close off the vent and connect the pressure measuring means 56 to air space A3. The modulator-demodulator in Fig. 2 is then set to zero to indicate atmospheric pressure in air space A3. Additional pressure on the syringe pressurizes the gas in air space A3 until 10 volts output is obtained for the pressure in air space A3. Valve V5 is then turned to vent the pressure in air space A4 to atmosphere. The higher pressure in air space A3 then causes the liquid to flow from chamber 54 through the capillary means 50 and needle 52 back into the tube 51, reducing the pressure in air space A3 as a function of time. Suitable design allows for the correction in pressure due to the small height changes in chamber 54 and tube 51. The result is a pressure -time curve comparable to that obtained in the previously described embodiment which can be processed by the system of Fig. 2 to determine viscosity as a function of shear rate and yield results comparable to those found with the previously described embodiments. In alternative embodiments of this invention, the capillary means described in the embodiments of Figs. 1 and 5 may be replaced by any fluid resistive device, linear or non-linear, whose flow characteristics are known or may be determined by suitable calibration. In another embodiment of this invention, the analysis and recording system shown in Fig. 2 may be replaced by any suitable means in which the pressure-time signal resulting from the flow is differentiated by analog or digital means to obtain values proportional to the instantaneous shear stress and shear rate, from which values of viscosity versus shear rate and rheological parameters may be calculated. It should be evident to anyone familiar with the art of constructing transient flow systems that other embodiments of this invention are possible, in which a time-varying flow of a liquid is used to produce a time-varying pressure in an enclosed air (or other compressible fluid), space such that the viscosity-shear rate characteristics of the liquid can be determined from the dynamic behavior and time constant of the system. It will also 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 inventions which are enclosed in the dotted lines in Figs. 1 and 5 are either readily available as disposable items or may be conventionally engineered from disposable materials. It is also evident that this system is especially applicable to measuring the viscosity of blood because it requires small sample sizes, provides rapid measurement with rapid data reduction, and maximum safety in handling and cleanup since all components that contact the blood may be disposed of. The embodiments of this invention can also be used without adding anticoagulants to the blood sample if desired, since the determination is so rapid. This invention is also applicable to measuring the viscosity-shear rate behavior of non-Newtonian fluids other than blood, but has its greatest advantages when used with biological fluids. It will also be readily apparent that this invention is applicable to the analysis, diagnosis and treatment of various hematologic disorders such as hyperviscosity syndromes and blood clotting. This invention also has application to measurements necessary during the production and utilization of polymeric fluid systems in which the time and shear dependent flow properties of the sample are important, such as monitoring the course of polymerization reactions or in quality control of industrial products, such as paints, plastics, pharmaceuticals and foods.

Claims: A method of measuring a continuous profile of viscosity versus shear rate of a non-Newtonian liquid, such as blood, comprising providing a compliant means comprising a body of compressible fluid held in a chamber (42,54), establishing a sample of the non-Newtonian liquid within a constricted space (41,50) between a fluid at known pressure in a fluid pressure chamber (43,51) and the fluid of the compliant means and thereby establishing in the chamber (42,54) of the compliant means direct contact of the compressible fluid with a free surface of said sample, and also establishing in the fluid pressure chamber (43,51) a fluid sample interface, applying pressure by pressurising means (47,59) to the fluid pressure chamber (43,51) and thereby establishing an initial viscous flow of the sample to pressurise the fluid of the compliant means, removing the application of pressure and so allowing the fluid of the compliant means to relax and the sample to flow in the constricted space (41,50), sensing with a pressure sensor (45,56) the changing pressure with time of the fluid of the compliant means during such relaxing flow, and processing the information with processing means (35,37) to provide a continuous profile of viscosity versus shear rate of the sample. Apparatus for measuring a continuous profile of viscosity versus shear rate of a non-Newtonian liquid, such as blood, comprising constricting means (41,50) defining a constricted space, compliant means comprising a chamber (42, 54) holding a body of compressible fluid, the chamber (42, 54) being in direct communication with one side of the constricting means, a fluid pressure chamber (43,51) in fluid communication with the other side of the constricting means, holding means (47, 59) for holding a sample of the liquid and connectible with the constricting means via the fluid pressure chamber (43, 51) to admit the sample to the constricting means (41, 50) and the chamber (42, 54), thereby providing direct contact of the fluid (A1, A3) with a free surface of the sample within the chamber (42, 54), and to establish in the fluid pressure chamber (43, 51) a fluid sample interface, means for pressurising the fluid pressure chamber and thereby the liquid and the fluid, a pressure sensor (45, 56) for sensing the changing pressure with time of the fluid of the compliant means as the fluid of the compliant means is allowed to relax and cause the sample to flow through the constricting means, and a processing means (35, 37) for processing information from the sensor to provide a continuous profile of viscosity versus shear rate of the sample. Apparatus according to Claim 2 including a first three-way valve (V3, V6) actuable to communicate the chamber (42, 54) alternately with the pressure sensor (45, 56) and with a body of fluid of known pressure, a second three-way valve (V4, V5) actuable to communicate the fluid pressure chamber (43, 51) alternately with the holding means (47, 59) or with the means for pressurising, and with a body of fluid at known pressure. Apparatus according to Claim 3, wherein said fluid pressure chamber is a vacuum type blood collecting tube (51) sealed by a stopper (RS), said apparatus comprising long needle means (52) having one end pierceably received in the stopper of said collecting tube (51) so as substantially to extend to the bottom of the tube, and the other end of said long needle means (52) being connected with said other side of said constricting means (50), short needle means (53) pierceably received at one end in the stopper of said collecting tube (51) so as to extend only above the surface of liquid contained therein, the other end of said short needle means being in communication with said second three-way valve (V5). Apparatus according to any one of Claims 2, 3 and 4 comprising wire means connecting said processing means (35, 37) with said pressure sensor (45, 56) and wherein said pressure sensor (45, 56) is operable to generate a first output voltage which is proportional to the sensed pressure of the compressible fluid and the processing means (35, 37) is operable to generate a second output voltage which is proportional to the first output voltage. The apparatus of Claim 5, wherein said processing means (35, 37) comprises a differentiating and filtering means (35) and a computer (37), said apparatus further including a modulator-demodulator means (33) connected between said pressure sensor (45, 56) and said differentiating and filtering means (35) and being responsive to said pressure sensor (45, 56) to produce an output, said differentiating and filtering means (35) being operable to receive the output of said modulator-demodulator means (33) to generate one output voltage proportional to the received voltage, and another output voltage proportional to the time derivative of the received voltage an analog to digital converter (36) connected between said differentiating and filtering means (35) and said computer for receiving and converting the output voltages of said differentiating and filtering means (35) to digital outputs, said computer (37) being programmed to receive the digital outputs of said analog to digital converter (36) and convert the digital inputs to the computer (37) to values of viscosity versus shear rate. The apparatus of Claim 5, wherein said processing means (35, 37) comprises a computer (37), said apparatus further including modulator-demodulator means (33) connected between said pressure sensor (45,56) and said computer (37), an analog to digital converter means (36) connected between said modulator-demodulator means (33) and said computer (37) for receiving and converting the output from said modulator/demodulator means to digital output, said computer (37) being programmed to receive the digital output of said analog to digital converter means (36), to differentiate the digital input to the computer and convert the original and differentiated digital inputs to values of viscosity versus shear rate. The apparatus of Claims 6 or 7 including a recorder (34) operable to print continuous logarithmic curves of pressure versus time in strip chart form utilizing the output of said modulator-demodulator means (33) as input.

Inventors: KRON REUBEN E, LITT MITCHELL, KRON, REUBEN E., LITT, MITCHELL
Applicants: KDL TECHNOLOGIES INC, KDL TECHNOLOGIES, INC.
Application year: 1989
Main classification: G01N11
IPC codes: G01N11