This application claims the benefit of a U.S. Provisional Application filed Aug. 28, 1997, entitled “Device and Method for In-Vivo Hematocrit Measurement Using Impedance and Pressure Plethysmography,” having application Ser. No. 60/057,166, now pending, and is also a continuation-in-part of U.S. patent application Ser. No. 09/121,000, filed Jul. 23, 1998, pending, which is a continuation-in-part of U.S. patent application Ser. No. 08/885,747, filed Jun. 30, 1997, abandoned, which is a continuation of U.S. patent application Ser. No. 08/602,700, filed Feb. 16, 1996, now U.S. Pat. No. 5,642,734, which is a continuation-in-part of a patent application filed Apr. 20, 1995, having Ser. No. 08/425,404, now U.S. Pat. No. 5,526,808, which is a continuation of U.S. patent application Ser. No. 08/298,795, filed Aug. 31, 1994, abandoned, which is a continuation of U.S. patent application Ser. No. 08/114,131, filed Aug. 30, 1993, abandoned, which is a continuation of U.S. patent application Ser. No. 07/592,851, filed Oct. 4, 1990, abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to devices and methods for noninvasive in vivo measurement of blood hematocrit and, more specifically, to devices and methods for such measurement that use impedance and pressure plethysmography.

2. Background of Related Art

The “hematocrit” of blood, which is defined as the percentage of whole blood volume occupied by erythrocytes (i.e., red blood cells), is an important measure of patient well being in cases of trauma, blood loss by disease, iron depletion in pregnancy, dietary iron deficiency, and a number of more specific medical conditions.

Hematocrit has traditionally been measured by centrifuging a column of blood, which has been extracted from the patient, in a glass tube, until the erythrocytes are compacted by centrifugal force to one end of the tube. The hematocrit is determined by measuring the length of the tube containing dark red material and dividing by the total length of the liquid column in the tube. These length observations are usually made visually, but are also made, in some cases, by automated optical means of various designs. Besides centrifugal hematocrit determinations, hematocrit is also derived and reported by various automated blood analyzers which count erythrocytes optically in unpacked blood. This erythrocyte count correlates with packed cell hematocrit and the derived hematocrit is reported.

It is noted that the above-described methods for obtaining hematocrit are invasive, that is they require that blood be removed from the patient in order to determine the hematocrit. A non-invasive method would be desirable because it would subject the patient to less pain and inconvenience and would preserve the patient's blood for its normal functions.

It has long been recognized by biomedical researchers that the electrical impedance of blood varies with hematocrit and that, as a result of this relationship, it should be possible to derive hematocrit from the measurement of blood impedance. Hematocrit has been successfully determined by measuring the impedance of blood that has been extracted from the patient and placed in an impedance measuring cell of controlled dimensions, where a fixed volume of the blood is contained, maintained at a known temperature, and agitated to maintain uniform cell distribution. Examples of such successful measurements are given by Okada and Schwan in “An Electrical Method to Determine Hematocrits,” IRE Transactions in Medical Electronics, ME-7:188-192 (1960) and by deVries et al. in “Implications of the Dielectrical Behavior of Human Blood for Continuous Online Measurement of Hematocrit,” Medical & Biological Engineering and Computing, pages 445-448 (1993) (hereinafter “deVries”). Like the centrifugal methods, these methods are invasive, however, and thus do not satisfy the need for a non-invasive hematocrit measurement. The impedance methods have, however, provided the inspiration for some ingenious inventions to measure hematocrit in-vivo and non-invasively.

The first in-vivo impedance measurement of hematocrit known to the inventors was reported by Yamakoshi et al. in “Noninvasive Measurement of Hematocrit by Electrical Admittance Plethysmography Technique,” IEEE Transactions, BMB-27, 3:156-161(1980). This measurement was made by immersing the finger of the test subject in a saline solution contained in a chamber fitted with impedance measuring electrodes. The electrolyte concentration of the saline solution was then varied by mixing in either water or more concentrated saline until the pulsatile variations of impedance caused by the increased volume of blood on each pulse were minimized. When this minimization of pulses occurred, the saline solution had the same resistivity as the blood in the pulsing arteries and this resistivity could be correlated against the known, previously determined relationship between resistivity and hematocrit.

U.S. Pat. No. 5,526,808 (hereinafter “the '808 Patent”), issued to Kaminsky and assigned to Microcor, Inc., the assignee of the present invention, describes another impedance method for measuring hematocrit noninvasively and in vivo. This method draws upon the observation that hematocrit determines the frequency vs. impedance profile of blood. In addition, the method of the '808 Patent uses the pulsatile change of impedance in a finger or other limb of the body that occurs when each heartbeat pushes new blood into the organ where the measurement is made to separate the non-blood tissue impedance from the blood impedance.

The mathematical model upon which this method is based relies upon the assumption that, as blood pulses into a finger or other body part where the hematocrit measurement is being made, the admittance (i.e., the reciprocal of impedance) change that occurs is due to the increased volume of blood providing a new current path in parallel with the old current path present before the pulse occurs. Thus, the difference in admittance between baseline, when no new blood is in the limb, and during the pulse, when new arterial blood has entered the limb, is due to the new blood. The numerical value of this admittance difference is proportional to the volume of the new blood times the admittance of the new blood.

As shown in deVries, the admittance vs. frequency characteristics of blood have a characteristic shape that depends upon hematocrit. Comparing the shapes of either the magnitude or the phase versus the frequency of the admittance, derived for the pulsed blood, against known characteristic hematocrit-dependent shapes gives a measure of hematocrit. The known characteristic shapes can be derived from a database obtained from patients having hematocrits independently measured by the centrifugal method previously described.

U.S. Pat. No. 5,642,734 (hereinafter “the '734 Patent”), issued to Ruben et al. and assigned to Microcor, Inc., the assignee of the present invention, describes some additional methods to obtain in vivo hematocrit results. First, the '734 Patent describes using pressurized cuffs, in various ways, to change the amount of blood in the organ (e.g., the finger) at which hematocrit is noninvasively measured. Second, the '734 Patent describes a unique electronic system for driving electrodes attached to the body part under measurement and for deriving phase, as well as amplitude information from impedance measurements of the body part. Third, the '734 Patent teaches the use of a neural network computer algorithm to relate measured impedance and other data to hematocrit based upon matching a database obtained from a number of prior measurements of patients with separately-determined hematocrits.

In the field of blood oxygen saturation measurement, as opposed to the field of blood hematocrit measurement that has been under discussion thus far, U.S. Pat. No. 5,111,817 (hereinafter “the '817 Patent”), issued to Clark et al., observes that the accurate measurement of blood oxygen saturation levels in arteries (S

a

O

2

) in a body part under measurement, such as a finger, is typically hindered by different blood oxygen saturation levels in capillaries (S

c

O

2

) in the body part. The '817 Patent teaches a method for correcting measurements of S

a

O

2

for the effects of S

c

O

2

. In this method, a pressure cuff applies a pressure to the body part under measurement that is equal to the mean arterial blood pressure in the body part. As a result, measurements from the body part are dominated by the effects of the actual S

a

O

2

in the body part, so that the measured S

a

O

2

is closer to actual S

a

O

2

.

While the use of pressure cuffs are known in the art to assist in the noninvasive measurement of hematocrit and blood oxygen saturation, as is the use of electrode pairs to noninvasively measure hematocrit, the art does not teach specific configurations of apparatus that are used in the noninvasive measurement of hematocrit.

SUMMARY OF THE INVENTION

The present invention includes apparatus configured for use in the noninvasive measurement of the hematocrit of a patient. Apparatus incorporating teachings of the present invention include two or more pairs of electrodes. A pressurization component may also be associated with the apparatus of the present invention.

A first embodiment of electrodes incorporating teachings of the present invention includes four individual electrodes that are paired in inner and outer sets. The electrodes may be substantially L-shaped. A first member of each electrode is configured to contact and to be at least partially wrapped around a body part at which hematocrit is to be noninvasively measured. A second member of each electrode is configured to communicate with external electrical componentry that will either apply a voltage to the body part or measure impedance at the body part, as will be described hereinafter in greater detail. In a variation of the first embodiment, one or more of the electrodes may be substantially linear, with a first end thereof configured to be at least partially wrapped around a body part and a second end thereof configured to be connected to external electronic componentry.

A second embodiment of electrodes useful in apparatus of the present invention has two elements, each including a pliable substrate and two electrodes, an electrode of an outer set and an electrode of an inner set. The pliable substrate preferably conforms to the shape of the body part at which hematocrit is to be noninvasively measured and may include a substantially planar member or be configured to at least partially receive the body part (e.g., an open- or close-ended tube configured to at least partially receive a finger). Again, the electrodes may be substantially L-shaped and include a first member and a second member. At least a portion of the first member of each electrode is secured to and carried by the pliable substrate. Thus, as the first members of each of the L-shaped electrodes are brought into contact with a body part at which hematocrit is to be noninvasively measured and the pliable substrate of each of the two elements is at least partially wrapped around the body part, the first member of each electrode is also at least partially wrapped around the body part. In a variation of the second embodiment, one or more of the electrodes may be substantially linear, with a first end thereof configured to be at least partially wrapped around a body part and a second end thereof configured to be connected to external electronic componentry.

A third embodiment of electrodes includes a single, pliable substrate that at least partially carries four electrodes arranged relative to the substrate in inner and outer sets. The pliable substrate preferably conforms to the shape of the body part at which hematocrit is to be noninvasively measured and may include a substantially planar member or be configured to at least partially receive the body part (e.g., as an open-ended or close-ended tube configured to at least partially receive a finger). The electrodes may be L-shaped, as described previously herein with respect to the first and second electrode embodiments, or substantially linear, and are configured to communicate with external electronic componentry.

Apparatus incorporating teachings of the present invention also include a pressurization component configured to apply a predetermined amount of pressure to the body part at which hematocrit is to be noninvasively measured.

A first embodiment of the pressurization component includes a pliable bladder configured to be at least partially wrapped around the body part, over the inner and outer pairs of electrodes, so as to apply increased pressure to the body part as pressure within the bladder is increased (e.g., with air or another fluid).

In a second embodiment, a pliable substrate upon which portions of the electrodes are carried, as in the second and third electrode embodiments described previously herein, comprises a pliable bladder. Accordingly, at least a portion of at least one electrode of each of the inner and outer electrode pairs may be secured to or otherwise carried by the pliable bladder. Prior to introducing pressure into the pliable bladder, the bladder may be substantially planar or configured to at least partially receive the body part at which hematocrit is to be noninvasively measured (e.g., as an open-ended or close-ended tube configured to at least partially receive a finger).

These pliable bladder embodiments of the pressurization component are configured to be connected to a source of pressure. As the pliable bladder is pressurized (e.g., by air pressure or pressure of another fluid), pressure is applied to at least a portion of the body part.

In addition, these pliable bladders may line a receptacle formed in a rigid member and configured to at least partially receive the body part.

In a third embodiment of pressurization component incorporating teachings of the present invention, inner and outer pairs of electrodes, such as those of the first, second, and third electrode embodiments described previously herein, are placed in contact with and at least partially wrapped around a body part at which hematocrit will be measured. The body part is at least partially inserted into a pressure chamber, which is in fluid communication with a source of positive pressure. Portions of one or both electrodes of the inner and outer pairs of electrodes in contact with the body part may also be inserted into the pressure chamber. As the body part is positioned within the pressure chamber, an at least partial seal is formed around the body part. Accordingly, as a positive pressure forms within the pressure chamber, pressure will be applied to the body part.

A system for noninvasively measuring the hematocrit of blood perfusing a living body part (e.g., a finger) in accordance with teachings of the present invention includes a non-invasive hematocrit measurement apparatus and external electronic componentry associated therewith. The electronic componentry of the system includes circuitry that drives first and second alternating currents of different frequencies (e.g., 100 KHz and 10 MHz) between separate points on the body part. The alternating currents may be applied to the body part through input electrodes attached to the body part at the separate points. Also, additional circuitry monitors first and second signals (e.g., voltage waveforms) induced in the body part by the first and second currents (e.g., by monitoring output electrodes attached to the body part), and other circuitry generates first and second pulsatile signals and first and second baseline signals from the first and second induced signals. Determining circuitry then calculates the hematocrit of the blood from the first and second pulsatile signals and the first and second baseline signals. This calculation may be performed, for example, by determining the hematocrit (H) from the following equation:

[(1+(

f

−1)

H

)/(1

−H

)]{1+[((

af

(

e

−bx

−c

))

x

/(1

−x

))−1

]H

}/{1+[((

af

(

e

−bx

−c

))/(1

−x

))−1

]H}=C

·(&Dgr;Volt

H

/V

H

2

)/(&Dgr;Volt

L

/Volt

L

2

),

where (f, a, b, x, c, and C) are various constants, as will be described below, &Dgr;Volt

H

and &Dgr;Volt

L

are the first and second pulsatile signals, and V

H

and V

L

are the first and second baseline signals.

In accordance with another embodiment of the system of the present invention, a system for measuring the hematocrit of blood perfusing a living body part includes electrodes positioned on the surface of the body part. A measuring device measures the electrical impedance at one or more frequencies between the electrodes. Also, a chamber is positioned to surround the body part between the electrodes, and a measuring apparatus measures pulsatile blood volume by the pulsatile-related change in internal pressure within the chamber. Further, a calculating device (e.g., a programmed microprocessor) determines the blood hematocrit from the measurements of impedance and pulsatile blood volume. The device may determine the hematocrit H in accordance with the following equations:

H

=(&rgr;−58)/(0.01 &rgr;+0.435), and

&rgr;=&Dgr;

VZ

0

2

/L

2

&Dgr;Z,

where &Dgr;V is the change in pulsatile blood volume at any point in time, &Dgr;Z is the change of impedance at the same point in time, L is a constant which will be described below, and Z

0

is the baseline impedance at the beginning of each pulse.

In a further embodiment, the system for determining blood hematocrit includes circuitry that produces a current signal including a first, relatively low frequency portion and a second, relatively high frequency portion, and the hematocrit measuring apparatus, which stimulates a living body part containing blood with the current signal. Also, additional circuitry of the hematocrit measuring apparatus is used to sense voltages at the first and second frequencies induced in the body part by the stimulation thereof, and further circuitry detects signal envelopes of the sensed voltages, with each signal envelope having a pulsatile component and a baseline component. Isolation circuitry isolates the pulsatile components and baseline components of the detected signal envelopes, and extraction circuitry extracts one or more sets of time-matched segments of the isolated pulsatile components and one or more sets of time-matched segments of the isolated baseline components. Further, other circuitry effectively correlates the blood hematocrit to the product of the ratio of the time-matched segments of the pulsatile components and the inverse ratio of the squares of the time-matched segments of the baseline components.

Another embodiment of the system includes an apparatus for determining the hematocrit of blood perfusing a living body part from relatively low frequency pulsatile and baseline signals induced in the body part, and from relatively high frequency pulsatile and baseline signals also induced in the body part, includes circuitry that effectively determines the ratio of the product of the relatively high frequency pulsatile signal and the square of the relatively low frequency baseline signal to the product of the relatively low frequency pulsatile signal and the square of the relatively high frequency baseline signal. The apparatus also includes circuitry that correlates the blood hematocrit to the effectively determined ratio.

Other embodiments of the invention include methods of measuring the hematocrit of blood perfusing a living body part, and a method of determining blood hematocrit, that generally correspond to the systems and apparatus described above.

Other features and advantages of the present invention will become apparent to those of skill in the art through a consideration of the ensuing description, the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

is a top view of a substantially linear electrode useful in apparatus according to the present invention;

FIG. 1A

is a variation of the substantially linear electrode shown in

FIG. 1

;

FIG. 2

is a top view of a variation of the electrode shown in

FIG. 1

, wherein the electrode is configured with an L-shape;

FIG. 3

is a perspective view depicting four of the electrodes shown in

FIG. 2

partially wrapped around a finger of a patient;

FIG. 4

is a top view of an embodiment of an element of an apparatus incorporating teachings of the present invention that includes two of the electrodes shown in

FIG. 2

being carried by a substantially planar substrate;

FIG. 5

is a perspective view depicting two of the elements depicted in

FIG. 4

secured to a finger of a patient;

FIG. 6

is a top view of another embodiment of apparatus according to the invention, including four of the electrodes shown in

FIG. 1

secured to a substantially planar, pliable substrate;

FIG. 7

is a perspective view illustrating the apparatus of

FIG. 6

secured to a finger of a patient and electrically connected to external electronic componentry by way of an exemplary connector;

FIG. 8

is a perspective view illustrating four of the electrodes shown in

FIG. 2

secured to a finger of a patient with a bladder embodiment of a separate pressurization component secured around the electrodes to the finger;

FIG. 9

is a top view of an embodiment of an apparatus of the present invention that includes four of the electrodes shown in

FIG. 1

carried on a surface of a substantially planar pliable bladder;

FIG. 10

is a perspective view of the assembly of the apparatus shown in FIG.

9

and an electrical connector therefor;

FIG. 11

is a perspective view illustrating, wrapped around a finger of a patient, an element of yet another embodiment of an apparatus that includes a substantially planar bladder member with two of the electrodes shown in

FIG. 1

secured to the surface thereof, as well as the element of

FIG. 4

wrapped around an adjacent portion of the finger;

FIG. 12

is a perspective view of yet another embodiment of apparatus including electrodes shown in

FIG. 1

carried on an inner surface of a tubular bladder configured to at least partially receive a finger;

FIG. 13

is a perspective view of a rigid member including a receptacle at least partially lined by the apparatus shown in

FIG. 9

;

FIG. 14

is a perspective view of another embodiment of a pressurization component of the present invention, including a close-ended pressure chamber configured to receive a finger of a patient;

FIG. 15

is a perspective view of still another embodiment of a pressurization component incorporating teachings of the present invention, including a pressure chamber with two open ends, through which a portion of a finger of a patient extends, and depicting a finger and the portions of four of the electrodes shown in

FIG. 2

in contact therewith inside the pressure chamber;

FIG. 16

is a perspective view illustrating the pressure chamber of

FIG. 15

being disposed around a finger of a patient and around portions of two electrodes of the type shown in

FIG. 1

that contact the finger, as well as the element of

FIG. 4

being wrapped around an adjacent portion of the finger;

FIG. 17

is a block diagram of a hematocrit measurement system in accordance with this invention;

FIG. 18

is a block diagram of another hematocrit measurement system in accordance with this invention;

FIG. 19

is a diagram of a calibration system of the hematocrit measurement system of FIG.

18

.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The disclosures of U.S. Pat. No. 5,111,817, issued to Clark et al.; U.S. Pat. No. 5,526,808, issued to Kaminsky; and U.S. Pat. No. 5,642,734, issued to Ruben et al., are hereby incorporated in their entireties by this reference.

Apparatus Securable to a Body Part to Effect a Noninvasive Measurement of Hematocrit

FIG. 1

illustrates a first embodiment of an electrode

10

that is useful in effecting the method of the present invention. Electrode

10

is formed from a conductive material, such as a thin metal or metal-lined sheet, and includes a first end

12

and an opposite, second end

14

. First end

12

is configured to contact and, preferably, to be at least partially wrapped around a body part, such as a finger, at which hematocrit is to be measured in accordance with teachings of the present invention. Second end

14

is configured to be connected to external electronic componentry (not shown) that effects the hematocrit measurement. First end

12

may include a retention component

17

thereon, depicted as being quantity of pressure sensitive adhesive, so as to hold electrode

10

in contact with or around a body part. The pressure sensitive adhesive of retention component

17

is preferably an electrically conductive adhesive that secures electrode

10

directly to the body part. Exemplary conductive adhesives include that sold under the trade name HYDROGEL® from a number of suppliers, such as Avery Dennison, Specialty Tape Division, of Painesville, Ohio, and that sold as AQUATRIX™ II by Hydromer, Inc. of Somerville, N.J. Although adhesive retention component

17

is illustrated as covering substantially all of electrode

10

, an adhesive retention component

17

may alternatively cover a much smaller region of electrode

10

that will adequately secure electrode

10

into contact with the body part. For example, the adhesive of retention component

17

may be disposed between electrode

10

and the body part, or retention component may be configured and located to adhere to another portion of electrode

10

and to secure the same at least partially around the body part.

A variation of a substantially linear electrode

10

′ is shown in FIG.

1

A. Electrode

10

′ includes a retention component

17

′, in this case a sleeve, located between a first end

12

′ and a second end

14

′ of electrode

10

′. As depicted, retention component

17

′ is configured to receive and retain first end

12

′ of electrode

10

′ as first end

12

′ is looped around a body part at which hematocrit is to be measured. By pulling second end

14

′ taut through retention component

17

′, electrode

10

′ may be snugly secured to the body part.

Another embodiment of an electrode

20

that may be used in the present invention is illustrated in FIG.

2

. Electrode

20

is substantially L-shaped and includes a first member

22

and a second member

24

. Electrode

20

is formed from a conductive material, such as a thin metal sheet or a metal-lined sheet. First member

22

of electrode

20

is configured to contact and, preferably, to be at least partially wrapped around a body part, such as a finger, at which hematocrit is to be measured in accordance with the method of the present invention.

FIG. 2

illustrates electrode

20

as including an exemplary retention component

27

, which, as illustrated, includes a receptacle

28

at the junction between first member

22

and second member

24

. Receptacle

28

is configured to receive an end portion

29

of first member

22

as first member

22

is looped around a body part at which hematocrit is to be measured. As shown in

FIG. 2

, end portion

29

is configured with a series of retaining regions

29

a

, adjacent regions having therebetween constricted regions

29

b

of lesser widths. Retaining regions

29

a

are wider than the width of receptacle

28

, while the widths of constricted regions

29

b

are about the same or less than the width of receptacle

28

. Thus, as end portion

29

is inserted into receptacle

28

, a retaining region

29

a

that has been pulled through receptacle

28

secures end portion

29

within receptacle

28

to interconnect receptacle

28

and end portion

29

. In this manner, receptacle

28

and end portion

29

interlock to securing first member

22

in place around a body part. Second member

24

of electrode

20

is configured to be connected to external electronic equipment that effects the method of the present invention.

Alternatively, electrodes

10

or

20

may be formed from a deformable material that substantially sets upon deformation so as to retain a portion of electrode

10

,

20

in contact with and secure that portion of electrode

10

,

20

to the body part at which hematocrit is to be measured.

An example of the use of electrodes

20

is illustrated in

FIG. 3

, which shows first members

22

of four electrodes

20

in contact with and being wrapped around a body part, in this case a finger

1

, at which hematocrit is to be measured in accordance with teachings of the present invention. As depicted, electrodes

20

are arranged in outer and inner sets

21

O and

21

I, respectively, with electrodes

20

of inner set

21

I spaced apart from one another and electrodes

20

of outer set

21

O positioned adjacent and outside of electrodes

20

of inner set

21

I. The adjacent electrodes

20

of inner set

21

I and outer set

21

O are spaced more closely to one another than electrodes

20

of inner set

21

I.

FIG. 4

depicts an assembly

30

including two electrodes

20

that are partially carried upon a pliable substrate

32

. As illustrated, pliable substrate

32

supports first members

22

of electrodes

20

. Exemplary materials from which pliable substrate

32

may be fabricated include, without limitation, fabrics, polymers, and other flexible materials. As shown in

FIG. 4

, pliable substrate

32

also includes a retention component

27

, in this case a quantity of adhesive material, between first members

22

. Adhesive material

27

is preferably a pressure sensitive adhesive that will maintain contact between first members

22

and the body part at which hematocrit is to be measured, as well as to maintain the positions of first members

22

relative to the body part.

An example of the use of assembly

30

for effecting the method of the present invention is illustrated in FIG.

5

. As shown, two assemblies

30

are positioned upon a body part, in this case a finger

1

, with first members

22

of electrodes

20

contacting the body part. As depicted, assemblies

30

are spaced a greater distance apart from one another than are electrodes

20

of each assembly

30

.

Another embodiment of an electrode assembly

40

that is useful in carrying out the method of the present invention is shown in FIG.

6

. Assembly

40

includes a pliable substrate

42

, similar to that depicted in FIG.

4

and described with reference thereto, and four electrodes

10

carried thereby. As illustrated, electrodes

10

are oriented upon pliable substrate

42

in substantially mutually parallel relation. Assemblies with electrodes

10

that are not oriented parallel to one another are, however, also within the scope of the present invention. Electrodes

10

are arranged in an inner set

11

I and an outer set

11

O. Electrodes

10

of inner set

11

I are positioned adjacent one another and electrodes

10

of outer set

11

O are positioned outside of electrodes

10

of inner set

11

I, with electrodes

10

of inner set

11

I being spaced apart a greater distance than each electrode

10

of inner set

11

I and the adjacent electrode

10

of outer set

11

O. Second ends

14

of electrodes

10

are preferably configured to couple with an electrical connector of a known type, such as connector

44

depicted in FIG.

7

.

FIG. 7

illustrates the manner in which assembly

40

is used in determining the hematocrit of a patient. Assembly

40

is positioned relative to a body part of a patient, such as finger

1

, with first ends

12

of electrodes

10

(see

FIG. 6

) in contact with the body part. As shown, first ends

12

of electrodes

10

are at least partially wrapped around finger

1

. Second ends

14

of electrodes

10

extend beyond an outer periphery of pliable substrate

42

and are coupled with corresponding terminals of an electrical connector

44

, which communicates with external electronic components (not shown) that are used to effect the method of the present invention.

An apparatus according to the present invention may also include a pressurization device, such as an inflatable bladder

50

illustrated in FIG.

8

. Inflatable bladder

50

may be of any type known in the art, such as an infant blood pressure cuff, which may be secured around a body part, such as a finger, at which hematocrit is to be measured by known means, such as complementary hook and loop type fasteners. As depicted, bladder

50

is separate from electrodes

20

, which are wrapped around a finger

1

in substantially the same manner as that depicted in and described with reference to FIG.

3

. Bladder

50

is at least partially wrapped around a body part at which hematocrit is to be measured, such as finger

1

, so as to apply pressure thereto in order to facilitate the hematocrit measurement. Bladder

50

, as well as any other embodiment of pressurization component used in the present invention, may be positioned relative to the body part so as to apply pressure between the innermost pair

21

I of electrodes

20

. Preferably, bladder

50

or another pressurization component abuts or at least partially overlaps each electrode of innermost pair

21

I so as to apply pressure to an entire portion of the body part located between innermost pair

21

I.

Bladder

50

communicates with an external pressure source

54

as known in the art, such as by use of a conduit

52

, such as a tube. As an example of the manner in which the internal pressure of bladder

50

may be increased, air or another fluid may be supplied to bladder

50

from external pressure source

54

by way of conduit

52

. As the internal pressure of bladder

50

increases, pressure is applied to the body part at which hematocrit is to be measured. Although

FIG. 8

depicts bladder

50

as being wrapped around finger

1

and all four electrodes

20

thereon, bladder

50

may be positioned elsewhere on finger

1

or another body part, such as between electrodes

20

of inner set

21

I or over two adjacent electrodes

20

.

FIG. 9

illustrates another embodiment of an assembly

60

that may be used to measure hematocrit in accordance with teachings of the present invention. Apparatus

60

includes a pliable substrate

42

, which comprises a bladder

50

′ that may be pressurized. Bladder

50

′ includes a port

51

to facilitate connection to an external pressure source, such as external pressure source

54

depicted in FIG.

8

. For example, an end of a conduit, or tube, may be inserted into port

51

and retained therein by way of an externally protruding, circumferentially extending rib formed around the end of the conduit. Apparatus

60

also includes four electrodes

10

carried upon a surface of bladder

50

′. As depicted, apparatus

60

includes four electrodes

10

that are arranged in mutually parallel relation to one another. Electrodes

10

are arranged in two sets, including an inner set

11

I and an outer set

11

O. Electrodes

10

of inner set

11

I are spaced apart from one another, while electrodes

10

of outer set

11

O are positioned outside of electrodes

10

of inner set

11

I, adjacent thereto, and spaced apart therefrom a lesser distance than electrodes

10

of inner set

11

I are spaced apart from one another. Preferably, second end

14

of each electrode

10

includes a terminal end

15

that extends beyond an outer periphery of bladder

50

′ so as to facilitate electrical connection of electrodes

10

and assembly

60

to external electronic componentry (not shown) that effects the method of the present invention. For example, terminal ends

15

may at least partially wrap around a peripheral edge of bladder

50

′.

FIG. 10

illustrates connection of assembly

60

to an electrical connector

70

of a type known in the art. Electrical connector

70

includes terminals

72

that correspond to terminal ends

15

of electrodes

10

to connect electrodes

10

with the appropriate external electronic componentry (not shown).

FIG. 11

illustrates an assembly

60

′, which is a variation of assembly

60

, that includes two electrodes

10

, as well as an electrical connector

70

′ configured to connect assembly

60

′ to external electronic componentry (not shown) that facilitates the determination of a patient's hematocrit in accordance with teachings of the present invention. When assembly

60

′ is employed, two additional electrodes, such as electrodes

20

of assembly

30

must also be brought into contact with a body part, such as finger

1

, at which hematocrit is to be measured. The outermost electrode

20

of assembly

30

and the outermost electrode

10

of assembly

60

′ comprise an outer set

61

O′, while the innermost electrode

20

of assembly

30

and the adjacent electrode

10

of assembly

60

′ comprise an inner electrode set

61

I′ to facilitate the measurement of hematocrit in accordance with teachings of the present invention. The pliable substrate of assembly

60

′ is an inflatable bladder

50

′ that includes a port

51

′ through which bladder

50

′ may be pressurized and depressurized.

FIG. 12

illustrates yet another embodiment of an assembly

60

″ for measuring hematocrit in accordance with the method of the present invention. Assembly

60

″ includes an inflatable bladder

50

″ having a tubular shape. Bladder

50

″ includes a receptacle

56

configured to at least partially receive a body part, such as a finger, at which hematocrit is to be measured. First ends

12

of electrodes

10

are at least partially carried upon an inner surface

57

of bladder

50

″ and exposed to receptacle

56

. Thus, upon at least partial insertion of a body part into receptacle

56

, first ends

12

of electrodes

10

contact the body part. Second ends

14

of electrodes

10

are exposed at and may extend beyond an outer surface

58

of bladder

50

″ so as to facilitate connection of assembly

60

″ to external electronic componentry that effects the method of the present invention. Bladder

50

″ also includes a port

51

″ that facilitates pressurization and depressurization of bladder

50

″.

Yet another apparatus

80

configured to position electrodes

10

in contact with a body part at which hematocrit is to be measured and to apply pressure to the body part in accordance with teachings of the present invention is illustrated in FIG.

13

. Apparatus

80

includes a rigid member

82

, which may be formed from, for example, a polymer, that includes a receptacle

84

configured to at least partially receive a body part at which hematocrit is to be measured. Disposed within receptacle

84

is an assembly

60

, such as that illustrated in and described with reference to FIG.

9

. Assembly

60

is disposed within receptacle

84

such that upon at least partial insertion of a body part, such as a finger, within receptacle

84

, first ends

12

of all four electrodes

10

of assembly

60

contact the body part. Preferably, bladder

50

′ of assembly

60

also contacts the body pressure in a manner that will allow for the application of pressure of the body part upon pressurization of bladder

50

′ through port

51

″. Terminal ends

15

of electrodes

10

communicate with electrical connectors

18

of a known type that are held within rigid member

82

and that facilitate connection of electrodes

10

to external electronic componentry (not shown) to effect a determination of hematocrit in accordance with the present invention.

FIG. 14

illustrates a pressurization chamber

90

that may be used to effect a hematocrit determination in accordance with the present invention. As illustrated, pressurization chamber

90

includes a generally cylindrical, hollow, rigid member

91

, a flexible, pliable member

95

therein, and a receptacle

94

within pliable member

95

. Receptacle

94

is configured to at least partially receive a body part, such as finger I, at which hematocrit is to be determined. The body part may be inserted into receptacle

94

through a first end

92

of pressurization chamber

90

. Pressurization chamber

90

also includes pressurization port

96

through which a pressurization bladder

97

, formed by pliable member

95

and rigid member

91

, may be either positively or negatively pressurized. As air or gas is introduced into pressurization bladder

97

, pliable member

95

expands into receptacle

94

, decreasing the volume thereof. Thus, as air or gas is introduced into pressurization bladder

97

, pliable member

95

is forced against a finger

1

or other body part disposed within receptacle

94

, applying pressure thereto. As shown in

FIG. 14

, pressurization port

96

is configured to be coupled with a pressurization conduit

98

, such as a tube, that communicates with an external pressure source

54

. While

FIG. 14

illustrates a finger

1

with first ends

22

of each of four electrodes

20

wrapped at least partially therearound disposed within receptacle

94

, a portion of finger

1

or another body part with fewer than four electrodes

20

contacting same may be disposed within receptacle

94

.

Another pressurization chamber

90

′ that may be used in the method of the present invention is illustrated in FIG.

15

. Pressurization chamber

90

′ includes a generally cylindrical, hollow, rigid member

91

′, and a flexible, pliable member

95

′ within rigid member

91

′. Pliable member

95

′ forms a receptacle

94

′ within pressurization chamber

90

′. A pressurization bladder

97

′ is formed between rigid member

91

′ and

95

′. Pressurization chamber

90

′ also includes two opposed, open ends

92

′ and

93

′. Ends

92

′ and

93

′ are both continuous with a receptacle

94

′ of pressurization chamber

90

′. Pressurization chamber

90

′ communicates with an external pressure source

54

by way of a conduit

98

connected to a pressurization port

96

′ of pressurization chamber

90

′. Pressurization bladder

97

′ may be pressurized by introducing air, gas, or another pressurization medium therein through pressurization port

96

′. As a positive pressure forms within pressurization bladder

97

′, pliable member

95

′ is forced toward receptacle

94

′, decreasing the volume of receptacle

94

′. Thus, as positive pressure is introduced into pressurization bladder

97

′, pliable member

95

′ is forced onto a finger

1

or other body part disposed within receptacle

94

′ and applies pressure thereto.

As illustrated in

FIG. 15

, pressurization chamber

90

′ is configured to partially receive finger

1

, with a base

3

of finger

1

extending through end

92

′ and a tip

2

of finger

1

extending through end

93

′.

FIG. 15

also shows that first ends

22

of all of four electrodes

20

that are wrapped around finger

1

are located within chamber

94

′. Although

FIG. 15

depicts all four electrodes

20

contacting finger

1

as being disposed at least partially within chamber

94

′, as shown in

FIG. 16

, finger

1

or another body part at which hematocrit is to be measured may be positioned within a receptacle

94

″ of a pressurization chamber

90

″ such that fewer than four electrodes

10

are located within chamber

94

″. As illustrated, electrodes

10

extend through rigid member

91

″ and pliable member

95

″ of pressurization chamber

90

″.

System and Methods for Noninvasively Measuring Hematocrit

Overview

An overview of one exemplary structure of a system incorporating teachings of the present invention will be followed by a discussion of its methods for hematocrit measurement.

An embodiment of the present invention is shown in FIG.

17

. In this embodiment, the pulsatile impedance and pulsatile pressure are measured in a chamber surrounding a patient's finger. Although a finger has been chosen as an exemplary body part to illustrate this embodiment, it is important to note that other body parts may be used. In addition, although

FIG. 17

illustrates some specific components, any assemblage of electrodes, pressurization apparatus, and other devices that incorporate teachings of the present invention may be used in the system to effect the hematocrit determination method of the invention.

An array of electrodes

10

is placed on a finger

1

with the outer electrodes

10

O separated as widely as possible and the inner electrodes

10

I each separated by approximately 5.0 mm from the closest of outer electrodes

10

O and separated from each other by a distance as far as possible but necessarily limited by the length of finger

1

.

The outer electrodes

10

O are driven by an alternating current generator

5

which may be set to deliver a constant current at a frequency in the approximate range of 10 kHz to 200 kHz.

The optimum frequency for this system likely lies in the previously-mentioned range to achieve the goals of maximum current with no discernible neuromuscular stimulation and low phase difference between the current and voltage applied to the electrodes. While test results on an early prototype of this invention indicate that 100 kHz is appropriate for achieving satisfactory results, it is possible that another frequency may result in a more practical implementation.

The inner electrodes

10

I are connected to the input of a high impedance voltage amplifier

6

which senses the voltage between these electrodes. Both the current generator

5

and the amplifier

6

are connected to a computing system

7

, which combines these electrical signals with other signals from the pressure sensing apparatus (described below) to compute and display hematocrit. The computing system

7

also has the purpose of controlling the automated operation of the measurement apparatus. The details of the computing system

7

are not depicted in

FIG. 17

or in the description of this invention because any of a number of configurations, including, but not limited to, an appropriately programmed PC-type computer with an analog-to-digital converter and an output port control interface, or a dedicated monitor, will suffice for performance of this function.

A sealed pressure chamber

8

surrounds the finger with air tight, pressure-withstanding seals at its distal and proximal. These seals are located over, or in very close proximity to, the inner electrodes

10

I. The first purpose of this chamber

8

is to contain the air in the closed volume of the chamber around the finger so that small pulse pressure rises occur in the chamber

8

when blood from the arterial system of the patient's circulation is pulsed into the finger. The second purpose of this chamber

8

is to contain an externally applied bias pressure which causes blood vessels in the part of the finger that is enclosed by the chamber

8

to partially collapse. This partial collapse of the vessels results in greater blood flow into the finger on each cardiac pulse. In turn, the larger blood flow pulses, created as a result of the bias pressure, cause greater pressure pulses to occur within the chamber and greater voltage pulses to occur between the inner impedance sensing electrodes

10

I. It should be understood that the electrodes

10

may be integral with the chamber

8

, or may be separate from the chamber

8

and applied to the finger independently of its insertion into the chamber.

A pressure transducer

9

is pneumatically connected to the pressure chamber