System for determining characteristics of blood flow

Claims

What is claimed is:

1. A method for measuring a hemodynamic characteristic of a living body having elastic blood vessels comprising the steps of:

measuring a characteristic value across a section of said body, said characteristic value being at least representative of the variation in cross-sectional area of the blood vessels within that section;

measuring blood pressure in said body simultaneously with the measurement of said characteristic value; and

processing signals representative of the characteristic value and the blood pressure measurements with a processing means to obtain an electrical signal representing the hemodynamic characteristic; said processing step including the step of utilizing at least one hemodynamic equation which relates said characteristic value, said blood pressure and the modulus of elasticity of the blood vessels within said section as a function, and including the step of calculating said modulus of elasticity.

2. The method as set forth in claim 1 further characterized in that said step of measuring said characteristic value includes measuring the electrical impedance across said section.

3. The method as set forth in claim 2 characterized in that said step of processing includes the step of selecting said hemodynamic formula for the determination of cardiac output over a time period.

4. The method as set forth in claim 2 further comprising the step of selecting said hemodynamic formula for the determination of modulus of elasticity of the arterial walls in said body section.

5. The method as set forth in claim 2 wherein said step of utilizing a hemodynamic equation includes utilizing the equation ##STR1## where .rho.=resistivity of the blood

L=distance of the section over which the impedance is measured

Z=impedance variation due to blood flow variance

Zc=impedance due to the tissue comprising the section

Ao=unstretched cross sectional area of the blood vessels of the section.

P=pressure variation causing the blood flow variance

E'=(K.sup.2 -1/K.sup.2 +1)E

E=modulus of elasticity for the blood vessels of the section.

K=Ro/Ri the ratio of the outer to the inner diameters of the blood vessels of the section.

6. An apparatus for determining the value of a characteristic of blood flow in a section of a living body having elastic blood vessels comprising:

an inpedance plethysmograph for connection to said body having electrodes for attachment at spaced positions along said section of said body for selectively outputting a signal representative of the value of the electrical impedance of said body section,

means for simultaneously measuring blood pressure of said living body and for outputting a signal representative thereof; and

a processor having means for receiving said signals representing said electrical impedance and said blood pressure, means for storing corresponding values of the blood pressure and electrical impedance signals as a plurality of sets; and means for processing said simultaneously measured sets of said signals with corresponding sets of hemodynamic equations which relate said impedance, said blood pressure, and the modulus of elasticity of the blood vessels within said section as a function so as to calculate said modulus of elasticity, and so as to generate said blood flow characteristic.

7. The apparatus as in claim 6, further comprising:

means for outputting stored values, wherein said processor is additionally responsive to said stored values.

8. The apparatus as set forth in claim 6 including means for displaying the value of said blood flow characteristic.

9. The apparatus as set forth in claim 6 in which said blood flow characteristic is cardiac output.

10. The apparatus as set forth in claim 6 in which said blood flow characteristic is rate of blood flow.

11. The apparatus as set forth in claim 6 in which said blood flow characteristic is a function of the modulus of elasticity of the blood vessels within the body section.

12. The apparatus as set forth in claim 6 in which said blood flow characteristic is contractility of the left ventricle of the heart in said body.

13. The apparatus as in claim 6 further characterized in that said blood flow characteristic is cardiac index.

14. The apparatus as in claim 6 further characterized in that said blood flow characteristic is useful power output of the heart.

15. The apparatus as in claim 6 further characterized in that said blood flow characteristic is systemic vascular resistance.

16. The apparatus as in claim 6 further characterized in that said blood flow characteristic is stroke volume.

17. The apparatus as in claim 6 further characterized in that said blood flow characteristic is cross-sectional area of the blood vessels within the body section.

18. The apparatus as set forth in claim 6 wherein said processor processes simultaneous sets of the equations: ##STR2## where .rho.=resistivity of the blood

L=distance of the section over which the impedance is measured

Z=impedance variation due to blood flow variance

Zc=impedance due to the tissue comprising the section

Ac=unstretched cross sectional area of the blood vessels of the section.

P=pressure variation causing the blood flow variance

E'=(K.sup.2 -1/K.sup.2 +1)E

E=modulus of elasticity for the blood vessels of the section.

19. A method of measuring a hemodynamic characteristic across a section of a living body having elastic blood vessels comprising the steps of:

determining the viscosity and resistivity of the blood of the body;

attaching a plethysmograph for measuring an electrical impedance across said section of the body said impedance being at least representative of the variance in the cross sectional area of the blood vessels within said section;

attaching means for measuring blood pressure;

measuring personal characteristics of said body;

processing signals representative of the characteristic value and the blood pressure measurements with a processing means to obtain an electrical signal representing the hemodynamic characteristic; said processing step including the step of utilizing at least one hemodynamic equation which relates said characteristic value, said blood pressure and the modulus of elasticity of the blood vessels within said section as a function, and including the step of calculating said modulus of elasticity; and

further processing said personal characteristics with a hemodynamic equation which relates said hemodynamic characteristic as a function of the calculated modulus of elasticity of said blood vessels, said viscosity, said resistivity, and said personal characteristics to determine the hemodynamic characteristic.

20. The method as in claim 19 wherein said personal characteristics are from the class of age, sex, height, and weight.

21. The method as in claim 19 further comprising the step of:

calculating the average cross-sectional area of vessels in the portion of the body as a function of the measured electrical impedance and blood pressure values.

22. The method as in claim 19 wherein said personal characteristic measuring step include the step of taking a blood sample from said body and performing a hematocrit on said sample to determine the viscosity and electrical resistivity of the blood sample.

23. A method of determining the kinetic energy of blood flow through a blood vessel comprising the steps of:

measuring an electrical property across a section of a body overlying the section of the blood vessel,

measuring blood pressure in said body simultaneously with the measurement of the electrical property;

processing values representative of multiple sets of the simultaneous measurements of the blood pressure and the electrical property to obtain a first value representing the mean area of the section of the blood vessel, a second value representing the rate of blood flow, and a third value representing the mean blood velocity; and

processing values representative of the rate of blood flow and mean blood velocity to obtain a value representing the kinetic energy of blood flow.

24. A method for measuring the total heart power comprising the steps of:

measuring an electrical property across a section of a body overlying the section of the blood vessel,

measuring blood pressure in said body simultaneously with the measurement of the electrical property;

processing values representative of multiple sets of the simultaneous measurements of the blood pressure and the electrical property to obtain a first value representing potential energy of blood flow, and a second value representing kinetic energy of blood flow; and

processing the values representing the potential and the kinetic energies of the blood flow to determine the value of the total heart power.

25. The method of claim 24 further comprising the steps of:

determining a value representing body surface area,

processing the values representing the total heart power and the body surface area to obtain a cardiac power index value representing the total heart power per body surface area.

26. A system for measuring a hemodynamic characteristic comprising:

means for non-invasively measuring a characteristic value across a section of a living body,

means for non-invasively measuring blood pressure in said body simultaneously with the measurement of said characteristic value; and

means for processing signals representative of the characteristic value and the blood pressure measurements through an electronic device to obtain an electrical signal representing the hemodynamic characteristic.

27. The system as set forth in claim 24 further comprising:

means for displaying the value representative of the hemodynamic characteristic.

28. The system as set forth in claim 27 wherein said measurements are taken over a time period and said display is in the face of a curve.

29. The system as in claim 26 further characterized in that said characteristic value is impedance.

30. The system as set forth in claim 29 wherein said processing is performed by passing signals representing said impedance and blood pressure signals through an electronic device for outputting said electrical signal representing a hemodynamic characteristic in accordance with selected formulae.

31. The system as set forth in claim 30 in which said formulae are selected for the determination of cardiac output over a time period.

32. The system as set forth in claim 30 in which said formulae are selected for the determination of modulus of elasticity of the arterial walls in said body section.

33. An apparatus for measuring hemodynamic characteristics comprising:

means for measuring across a section of a living body and determining a characteristic value;

means for measuring blood pressure in said body simultaneously with the measurement of the characteristic value;

means for determining a hemodynamic characteristics from the simultaneous measurements; and

means for displaying the determined hemodynamic characteristics.

34. The apparatus as in claim 33 further comprising:

means for repeating the simultaneous measurements a plurality of times; and

means for determining hemodynamic characteristics responsive to the plurality of simultaneous measurements.

35. The apparatus as in claim 33 or 34 further comprising:

means for measuring an electrical characteristic value across the section of living body, and

means for determining the characteristic value from the measured electrical characteristic value.

36. The apparatus of claim 35 further characterized in that said electrical characteristic value is a function of electrical impedance Z.

37. The apparatus as in claim 36 further comprising:

means for determining the electrical impedance measurements corresponding to systolic P.sub.S and diastolic P.sub.D blood pressure measurements;

means for determining an elasticity value E from the simultaneous blood pressure and impedance measurements corresponding to systolic and diastolic pressure, and a third pair of simultaneous measurements of pressure and impedance;

means for determining the respective cross sectional area of the vessel for each set of measurements from the respective pressure and impedance measurements and E.

38. The apparatus as in claim 37 further comprising:

means for determining the rate of blood flow Q as a function of the determined elasticity value E and the measured impedances and pressures.

39. The apparatus as in claim 38 further comprising:

means for determining the stroke volume V as a function of the rate of blood flow Q integrated over the time of a cardiac cycle.

40. The apparatus as in claim 38 further comprising:

means for taking a plurality of time-sequential impedance and pressure measurements and averaging said plurality to determine a surface impedance, Z base, a mean pressure, P mean, and a mean elasticity, E mean,

means for deriving a mean area, A mean, responsive to Z base, P mean and E mean; and

means for determining an average velocity of blood responsive to the rate of blood flow Q and the mean area, A mean.

41. The apparatus of claim 35 further characterized in that said electrical characteristic value is the first time derivative dz/dt of the electrical impedance Z.

42. The apparatus as in claim 34 wherein the means for measuring blood pressure is further comprised of:

means for attaching a blood pressure measuring device to the patient's body; and

means for generating signals from the device which vary with time during the cycle of blood flow.

43. The apparatus as in claim 34 or 42 wherein said means for measuring said characteristic value is further comprised of:

means for attaching first and second electrode sets in spaced relationship to one another across the section of the body;

means for injecting an electrical signal into said first electrode set;

means for detecting an electrical signal as received by said second electrode set; and

means for outputting said signal representative of said characteristic value.

44. A system for measuring and determining a hemodynamic characteristic comprising:

means for determining viscosity and resistivity of blood of a patient;

means for attaching a plethysmograph to measure an electrical property across a portion of the patient's body, means for simultaneously measuring blood pressure and the electrical property across the section of the patient;

means for inputting personal characteristics of the patient; and

means for processing the personal characteristics and the simultaneously measured values of the blood pressure and the electrical property, to determining the hemodynamic characteristic.

45. The system as in claim 44 wherein said personal characteristics are from the class of the patient's age, sex, height, and weight.

46. The system as in claim 44 further comprising:

means for calculating the average cross-sectional area of vessels in the portion of the body as a function of the measured electrical property and blood pressure values.

47. The system as in claim 44 wherein said electrical property is impedance.

48. The system as in claim 44 further comprising means for taking a blood sample and determining a hemotocrit therefrom.

49. A system for measuring the cross-sectional area of a section of a blood vessel comprising:

means for measuring an electrical property across a section of a body overlying the section of the blood vessel,

means for measuring blood pressure in said body simultaneously with the measurement of the electrical property,

means for processing values representative of the simultaneous measurements of the blood pressure and the electrical property to obtain a value representing the cross-sectional area of the section of the blood vessel.

50. A system for measuring the elasticity of a section of a blood vessel comprising:

means for measuring an electrical property across a section of a body overlying the section of the blood vessel,

means for measuring blood pressure in said body simultaneously with the measurement of the electrical property, and

means for processing values representative of multiple sets of the simultaneous measurements of the blood pressure and the electrical property to obtain a value representing the elasticity of the section of the blood vessel.

51. A system for measuring the rate of blood flow in a section of a blood vessel comprising:

means for measuring an electrical property across a section of a body overlying the section of the blood vessel,

means for measuring blood pressure in said body simultaneously with with the measurement of the electrical property, and

means for processing values representative of multiple sets of simultaneous measurements of the blood pressure and the electrical property to obtain a value representing the rate of blood flow in the section of the blood vessel.

52. The system as in claim 51 further comprising:

means for integrating the rate of blood flow over the time of one cardiac cycle to determine pulse stroke volume.

53. The system as in claim 51 further comprising:

means for integrating the rate of blood flow over one minute to determine cardiac output.

54. The system as in claim 53 further comprising:

means for determining a value for the body surface area, and

means for determining a cardiac index as a function of the cardiac output and the body surface area.

55. The system as in claim 53 further comprising:

means to determining heart rate;

means for determining stroke volume as a function of the cardiac output divided by the heart rate.

56. A system for measuring the mean velocity of blood across a section of a blood vessel comprising:

means for noninvasively measuring an electrical property across a section of a body overlying the section of the blood vessel,

means for noninvasively measuring blood pressure in said body simultaneously with the measurement of the electrical property;

means for processing values representative of multiple sets of the simultaneous measurements of the blood pressure and the electrical property to obtain a value representing the mean area of the section of the blood vessel;

means for processing values representative of multiple sets of the simultaneous measurements to obtain a value representating the rate of blood flow in the section of the blood vessel, and

means for deriving the mean velocity of blood responsive to the rate of blood and the mean area.

57. The system as in claim 56 further comprising means for displaying the mean velocity of blood, rate of blood flow, and mean area.

58. A system for measuring vascular resistance comprising:

means for measuring blood pressure in said body at two points on said body, overlying said section;

means for processing values representative of the two-point measurements of the blood pressure to obtain a value representing differential blood pressure of the section of the blood vessel,

means for determining cardiac output; and

means for determining vascular resistance as a function of the cardiac output and the differential blood pressure.

59. The method as in claim 58 wherein the vascular resistance is systemic; and

the two points measured are mean right artial pressure and mean arterial pressure.

60. The system as in claim 58 further comprising:

means for detemining the rate of blood flow, and

means for determining a value for the potential energy of blood flow as a function of the differential blood pressure and the rate of blood flow.

61. A system for determining the kinetic energy of blood flow through a blood vessel comprising:

means for measuring an electrical property across a section of a body overlying the section of the blood vessel,

means for measuring blood pressure in said body simultaneously with the measurement of the electrical property;

means for processing values representative of multiple sets of the simultaneous measurements of the blood pressure and the electrical property to obtain a first value representing the rate of blood flow, and a second value representing the mean blood velocity; and

means for processing values representative of the rate of blood flow and mean blood velocity to obtain a value representing the kinetic energy of blood flow.

62. A system for measuring the total heart power comprising:

means for measuring an electrical property across a section of a body overlying the section of the blood vessel,

means for measuring blood pressure in said body simultaneously with the measurement of the electrical property;

means for processing values representative of multiple sets of the simultaneous measurements of the blood pressure and the electrical property to obtain a first value representing potential energy of blood flow, and a second value representing kinetic energy of blood flow; and

means for processing the values representing the potential and the kinetic energies of blood flow to determine the value of the total heart power.

63. The system of claim 62 further comprising:

means for determining a value representing body surface area, and

means for processing the values representing the total heart power and the body surface area to obtain a value representing the total heart power per body surface area.

64. A method for measuring a hemodynamic characteristic in a section of a living body having elastic blood vessels comprising:

measuring an electrical parameter across said body section to determine an impedance value for said section;

measuring a blood pressure value in said body simultaneously with the measurement of the impedance value;

determining impedance values corresponding to the systolic and the diastolic blood pressure of the body;

calculating an elasticity value E from the simultaneous blood pressure and impedance measurements corresponding to said systolic and diastolic pressures, and a third pair of measurements of mean pressure and impedance, and determining the respective cross sectional area of the blood vessels within said section for each set of measurements from the respective pressure and impedance measurements and E.

65. The method as set forth in claim 64 further comprising the step of:

determining the rate of blood flow Q as a function of the determined elasticity value E and the measured impedances and pressures.

66. The method as in claim 65 further comprising the step of:

determining the stroke volume V as a function of the rate of blood flow Q integrated over a fixed time t of a cardiac cycle.

67. The method as in claim 65 further comprising the step of:

determining cardiac output as a function of the rate of blood flow Q integrated over a set of predetermined limits.

68. The method as in claim 65 wherein the step of measuring blood pressure further comprises:

attaching a blood pressure measuring device to said body; and

generating signals from the device which vary with time during the cycle of blood flow.

69. The method as set forth in claim 68 wherein the step of measuring said characteristic value is further comprised of the steps of:

attaching a first electrode set and second electrode set to said body in spaced relationship to one another;

injecting an electrical signal into said first electrode set;

detecting an electrical signal as received by said second electrode set; and

outputting said signal representative of said representative value.

70. The method as set forth in claim 64 further comprising the steps of:

taking a plurality of time-sequential impedance and pressure measurements and averaging said plurality to determine a base impedance, Z base, a mean pressure, P mean, and a mean elasticity, E mean,

deriving a mean area, A mean, corresponding to Z base, P mean and E mean; and,

determining an average velocity of blood corresponding to Z base, P mean and E mean; and,

determining an average velocity of blood corresponding to Q and A mean.

71. A method for measuring the instantaneous rate of blood flow in a section of a living body having elastic blood vessels comprising the steps of:

measuring an electrical parameter across a section of said body having the blood vessels within, said electrical property being at least representative of the variance in cross sectional area of the blood vessels within that section;

measuring blood pressure in said body simultaneously with the measurement of said electrical parameter;

processing values of multiple sets of simultaneous measurements of the blood pressure and the electrical parameter;

solving a set of hemodynamic equations which relates the axial velocity of the blood in said vessels as a function of various unknowns, said equations having of the same number of unknown variables as the number of measurement sets, wherein one of the unknowns is the modulus of elasticity of the blood vessels within said section;

multiplying the axial velocity of the blood flow by the cross sectional area of the blood vessels of said section to yield an instantaneous rate of blood flow therethrough.

72. The method as set forth in claim 71 further comprising the step of:

integrating the rate of blood flow over a predetermined period to determine cardiac output.

73. The method as in claim 72 further comprising the step of:

determining the heart rate; and

determining stroke volume as a function of the cardiac output divided by the heart rate.

74. A method as set forth in claim 73 comprising:

measuring blood pressure in said body at two points, on said body;

processing values representative of the two-point measurement of the blood pressure to obtain a value representing differential blood pressure of the blood vessels of said section,

determining cardiac output; and

determining vascular resistance as a function of the cardiac output and the differential blood pressure.

75. The method as in claim 73 wherein the vascular resistance is sytemic; and the two points measured are mean right atrial pressure and mean arterial pressure.

76. The method as in claim 74 further comprising the steps of:

determining the instantaneous rate of blood flow, and

determining a value for the potential energy of blood flow due to the power output of the heart as a function of the differential blood pressure and the rate of blood flow.

77. The method as in claim 74 further comprising the step of:

measuring blood pressure in said body at said mean right atrial pressure and pressure at a point at any other artery.

78. The method as set forth in claim 74 further comprising the step of integrating the rate of blood flow over one cardiac cycle to determine stroke volume.

79. The method as in claim 71 further comprising the steps of:

determining a value for the body surface area, and

determining a cardiac index as a function of the cardiac output and the body surface area.

80. The system as in claim 26 or 33 or 34 or 44 or 40 or 50 or 51 or 56 or 58 or 61 or 62 further comprising:

means for displaying the hemodynamic characteristic in a format from the class of formats of absolute values, relative values, deviation from mean, range of values, time averaged values, minimum values, average minimum values, mean values, maximum values, average maximum values, instantaneous rate of change of values, maximum rate of change of values, minimum rate of change of values, mean rate of change of values, and average rate of change of values.

This invention relates to a process and apparatus for determining hemodynamic characteristics from the monitoring of the flow of blood in a section of the living body. More particularly, this invention relates to a process and apparatus in which an electrical characteristic value representative of at least the variance in cross sectional area of a blood vessel is measured across the section of living body, simultaneously with the measurement of blood pressure, wherein the measured values are utilized in combination with a set of hemodynamic equations to produce an output representation of the hemodynamic characteristics of the living body.

Heretofore, a number of methods and means have been devised and utilized for determining blood flow characteristics. These include indicator dilution methods, magnetic flow meters, ultrasonic blood flow meters, impedance cardiography, blood flow determination by radiographic methods, blood flow determination by catherization known as the Swan Ganz method, and blood flow determination by simultaneous blood sampling from a vein and an artery coincident with measurement of oxygen consumption known as the Fick method. The two most widely used methods are the Swan Ganz and Fick methods. These methods are supported by long-established empirical data, and are held in high esteem, being more reliable and repeatable than the other mentioned methods and means are.

The above methods and means all have problems and inadequacies in meeting the needs of surgeons, patients, and others in medical practice. A major objection to most of the existing methods is the lack of accuracy, repeatability, and supporting empirical data. Swan Ganz and Fick methods are the most widely accepted and used, but are invasive techniques requiring insertion of foreign objects into the body and blood stream. While these methods give an approximate repeatability variation of plus and minus 6%, better than other preexisting methods, these methods are relatively dangerous to the patient. With both the Swan Ganz and Fick invasive methods, there is potential for infection and local tissue damage as a result of the measurement process. Because of this, patient acceptance of these methods is limited, and the use of these methods is restricted to very serious cases. Additionally, the Swan Ganz method can result in potential cardiac irregularities, and pulmonary injuries, as well as potential damage to the lungs and veins (puncture). Since the Fick and Swan Ganz methods are dangerous to the patient's well being, the amount of time the invasive means may be utilized is limited. Continuous measurements and sequential measurements are not practicably possible, as prolonged or repeated insertion of the invasive means into the patient's body and blood stream is potentially harmful, traumatic, discomforting and relatively dangerous to the patient. Thus, a small number of discrete samples are taken, which can be quite unsuitable in unstable patients, as this can lead to large errors.

Special skills are required for utilizing invasive blood flow measurement, such as the Swan Ganz or Fick methods, and therefore utilization of these methods is generally restricted to the catherization lab, operating room, and to a limited extent to the cardiac critical unit. These techniques are not utilized, as a rule, in the patient's room, on out-patients, or in a doctor's office, due to the hazardous and traumatic nature of these techniques of measurement. Additionally, these methods create discomfort and pain in the patient being tested. Thus, these methods are typically restricted to patients who are in acute state, and are not generally utilized for patients undergoing preventive check-up, rehabilitation, or in a chronic disease states.

In U.S. Pat. No. 3,996,925, by one of the present inventors, a system is disclosed for determining blood flow as a function of the electrical impedance measured across a section of the living blood in accordance with an analogue electronic processor. This system provided for the determination of stroke volume and cardiac output, utilizing the measured impedance value over a given time and utilizing a number of constant values (not measured) representing physiological and personal characteristics of the patient. However, there is a long standing need for a system which is more accurate, and which is effective for determining other hemodynamic characteristics and which is responsive to actual personal characteristics of the patient and the section of the body across which measurements are made. For example, it would be desirable to determine a value individually for each patient based on measured values, instead of assuming a constant value for all patients.

The mathematical analysis of circulation is formulated in hemodynamics. It is a combination of fluid mechanics and the theory of elasticity applied to the pulsating flow of blood through blood vessels and the corresponding periodic displacements of vascular walls. The main problem in the application of hemodynamics to clinical measurements is how to obtain reliable values of important hemodynamic parameters, such as diameters of arteries, modulus of elasticity of arterial walls, length of blood vessels, etc.

Electrical impedance plethysmography can be combined with hemodynamics. Electrical impedance plethysmography is based on the measurement of transthoracic impedance Z and its first time derivative DZ/DT, which in accordance with hemodynamic theory provide valuable information about intravascular and extravascular fluid volume, heart function, and vascular response to the heart function.

Variation of electrical impedance of a section of the human body as a function of time is the basis of impedance plethysmography. The resultant form of the impedance signal is related to a timing parameter and events in the cardiac cycle to deduce stroke volume by applying a single semi-empirical formula such as that derived by Kubicek, as set forth in the annals of N.Y. Academy of Science, 1970, page 729, to the geometrical parameters of the wave-form. One problem with this technique is that very limited cardiovascular data is obtainable. Another, more serious, limitation and problem with this method is that of accuracy.

Utilization of hemodynamics for modeling requires specific numerical values of various coefficients which are a part of the model, but which depend on individual characteristics of the patient and of the selected part of the body, such as diameters and lengths of arteries, elastic moduli of their walls, thickness of the walls, etc. All these characteristics vary from the patient and from one location in the body to another.

The elastic properties of the vascular system are represented by the modulus of elasticity, which is defined as the ratio of stress and strain. The elastic properties of arteries depend on several factors such as the degree of arteriosclerosis, blood pressure, and vasoconstriction and vasodilation. Sclerotic changes of arterial walls cause intrinsic changes of mechanical properties and relative thickness of the walls. They increase the modulus of elasticity. Modulus of elasticity is a nonlinear monotonically increasing function of blood pressure in the physiological range. Any agent (neurogenic, hormonal, drug) which causes vasoconstriction will effectively increase the apparent modulus of elasticity. A vasodilator will cause an effective decrease of the modulus. Thus, the measured apparent modulus depends on the vascular tone of the patient under test. These factors, and others, are the main reasons why a fixed value cannot be assumed for all patients for modulus of elasticity. Rather, the modulus of elasticity should be calculated individually for each patient based upon measured values. Only in this way can the mathematical model be properly utilized in computing hemodynamic characteristics from measured values.

Accordingly, it is an object of the present invention to provide methods and means for determining and displaying hemodynamic characteristics of a patient based on measured values from the body of the patient under test.

It is a further object to provide and display hemodynamic characteristic values, alternatively or simultaneously as absolute values, a range of values, percent deviation of values, and/or other statistical forms.

It is a further object to provide a means and method for continuous and/or sequential measurement of data to provide for the determination and display of hemodynamic characteristics related to continuous or sequential measurement.

It is still a further object to provide for the determination and display of multiple hemodynamic characteristic values utilizing noninvasive measurements.

It is a further object to utilize time correlated measured indications obtained from the body under test, and empirically established models, and a computer, to eliminate the use of most general physical constants, replacing the general physical constants with a value determined from measured values, in the method and means for determining and displaying selective hemodynamic characteristics.

It is yet another object to provide a means and method of measuring data and determining hemodynamic characteristics which has high repeatability, (exhibiting a small variation in the repeatability of results).

It is still a further object to provide a hemodynamic measurement system which achieves high patient acceptance level, which has virtually no potential for infection, local tissue damage, cardica irregularities, pulmonary injury, or lung injury, and which permits preventive medical therapy as a result of utilization of cardiovascular function data.

In accordance with the illustrated embodiments of the present invention, a method and a means for implementing the method are disclosed for measuring and displaying hemodynamic characteristics. A characteristic value representative of at least the variation in cross sectional area of a blood vessel is measured across a section of a living body, blood pressure is measured in the body simultaneously with the measurement of the characteristic value, and signals representative of the characteristic value and the blood pressure measurements are utilized (processed) to obtain an electrical signal representing the hemodynamic characteristic. In a preferred embodiment, any or all of selected hemodynamic characteristics are displayed.

Preferably the measurement of the characteristic value is done noninvasively. Alternatively, both the blood pressure and characteristic value are measured noninvasively. In the illustrated embodiment, the characteristic value measured is the electrical impedance across a section of the body. The hemodynamic characteristic is determined from measurements of the electrical characteristic and the pressure measurements in accordance with selected hemodynamic formulae. Various formulae are utilized, enabling the determination of numerous important hemodynamic characteristics such as cardiac output, cardiac index, stroke volume, cardiac power index, work of heart, elasticity, contractility, base impedance, and so forth. Furthermore, the hemodynamic characteristics may be determined as absolute values, a range of values, in terms of percent deviation from mean value, as a value over a single cycle or averages (and other statistical functions) of multiple cycles or cardiac system measurement.

In the illustrated embodiment, personal characteristics of the patient being measured are input for use by the processor in determining hemodynamic characteristics. The personal information can include age, weight, height, sex, race, the spacing distance of the plethysmograph electrodes, and/or the viscosity and resistivity of the blood of the patient under test. The viscosity and resistivity of the blood as determined by appropriate means is coupled to the means for processing (computer or logic circuitry) for use in determining and displaying hemodynamic characteristics.

Other objects and advantages of the present invention will become apparent upon reading the following detailed description, while referring to the attached drawing, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph plotting blood pressure, cross-sectional area of arterial lumen, and electrical conductance against a common time axis;

FIG. 2 is a graph showing the relationship of (dz/dt)min to Zo;

FIG. 3 is a partial block diagram illustrating one embodiment of an electrode configuration for use with the present invention;

FIG. 4 is a graph plotting impedance signal (as measured in FIG. 3) against time;

FIG. 5 is a block diagram of a system for determining hemodynamic characteristics in accordance with the present invention;

FIG. 6 is a graphical illustration of simultaneously measured values of impedance Z and measured values of pressure P against time;

FIG. 7 is a side view of a blood vessel illustrating vessel deformation (expansion and contraction) caused by blood flow;

FIG. 8 is a pictorial representation of the cross-section of the blood vessel illustrated in FIGS. 7A-B, showing the deformation of the blood vessel for different times (and pressures) during a cardiac cycle;

FIG. 9 is a sectional view illustrating the variation in the cross-sectional area of the blood vessel corresponding to the vessel deformation of FIG. 4;

FIG. 10 is a system block diagram of an embodiment of an electrohemodynamic system in accordance with the present invention;

FIGS. 11,23,24A,24B, and 25-27 are graphs illustrating the relationship between the measured electrical system parameters and the corresponding mnemonics representative of the electrical signal values as derived from the measured values and utilized in the selected formula to determine the hemodynamic characteristics; and

FIG. 28 is a pictorial representation of a report printout illustrating one form which the display can take.

A Fortran listing at pages 63a et seq. provided as an illustrative embodiment of the reduction to computer program form of the techniques and teachings of the present invention.

In accordance with the present invention, a method and means is provided for measuring a hemodynamic characteristic comprising the measurement of a characteristic value across a section of a living body (such as non-invasive measurement of electrical impedance or conductance), the measurement of blood pressure in the body simultaneously with the measurement of the characteristic value, and the processing of signals representative of the characteristic value and the blood pressure measurements through an electronic device to obtain an electrical signal representing the hemo