U.S. patent application number 12/027905 was filed with the patent office on 2008-05-29 for sensor probe and display module.
Invention is credited to Paul D. Corl, Amos Gottlieb, James D. Mikkelsen, Harry D. Nguyen, Margaret R. Webber.
Application Number | 20080125632 12/027905 |
Document ID | / |
Family ID | 34226879 |
Filed Date | 2008-05-29 |
United States Patent
Application |
20080125632 |
Kind Code |
A1 |
Corl; Paul D. ; et
al. |
May 29, 2008 |
SENSOR PROBE AND DISPLAY MODULE
Abstract
An apparatus for use with a patient having a vessel carrying
blood to ascertain characteristics of the blood. The apparatus
includes a display module and a probe having a distal extremity
adapted to be inserted into the vessel of the patient and having a
proximal extremity coupled to the display module. The probe
includes a sensor in the distal extremity for providing an
electrical signal to the display module when the probe is disposed
in the blood. The probe can have an antithrombogenic surface
treatment for inhibiting the adhesion of blood components to the
probe when disposed in the blood.
Inventors: |
Corl; Paul D.; (Palo Alto,
CA) ; Mikkelsen; James D.; (Los Altos, CA) ;
Nguyen; Harry D.; (Garden Grove, CA) ; Gottlieb;
Amos; (San Francisco, CA) ; Webber; Margaret R.;
(Los Altos, CA) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET, FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Family ID: |
34226879 |
Appl. No.: |
12/027905 |
Filed: |
February 7, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10658926 |
Sep 9, 2003 |
|
|
|
12027905 |
|
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|
Current U.S.
Class: |
600/309 |
Current CPC
Class: |
A61B 5/14546 20130101;
A61B 5/14542 20130101; A61B 2562/227 20130101; A61B 5/01 20130101;
A61B 5/14539 20130101; A61B 5/1473 20130101; A61B 5/6852
20130101 |
Class at
Publication: |
600/309 |
International
Class: |
A61B 5/02 20060101
A61B005/02 |
Claims
1. An apparatus for use with a patient having a vessel carrying
blood to ascertain characteristics of the blood comprising a
compact display module and a probe, the probe having a proximal
extremity coupled to the display module and a distal extremity
adapted to be inserted into the vessel of the patient, the distal
extremity including a sensor for providing an electrical signal to
the display module when the probe is disposed in the blood, the
probe having calibration coefficients, the display module having a
processor for processing the electrical signal to provide a reading
and a memory for storing the calibration coefficients, the
processor being coupled to the memory to permit access by the
processor to the calibration coefficients in connection with the
processing of the electrical signal so as to enhance the accuracy
of the reading.
2. The apparatus of claim 1 further comprising a band connected to
the display module for securing the display module to the wrist of
the patient.
3. The apparatus of claim 1 wherein the sensor is selected from the
group consisting of gas sensors, oxygen sensors, carbon dioxide
sensors, pH sensors and temperature sensors.
4. The apparatus of claim 3 wherein the sensor is a gas sensor
assembly having first and second electrodes disposed in an
electrolyte solution.
5. The apparatus of claim 1 wherein the display module includes a
wireless transmitter receiver circuit coupled to the processor for
permitting wireless receipt of control signals from an external
source and wireless transmission of blood characteristics to an
external device.
6. The apparatus of claim 1 wherein the memory is a nonvolatile
memory.
7. A kit for use with a patient having a vessel carrying blood to
ascertain characteristics of the blood comprising a package, a
probe carried within the package and having a distal extremity
adapted to be inserted into the vessel of the patient and including
a sensor for providing an electrical signal, the probe having
calibration coefficients, a compact display module carried within
the package and having a processor and a nonvolatile memory coupled
to the processor, the calibration coefficients being stored in the
memory of the display module whereby when the probe is coupled to
the display module and the distal extremity inserted into the
vessel and an electrical signal is received by the display module
for providing a reading the processor accesses the memory so as to
utilize the calibration coefficients and thus enhance the accuracy
of the reading.
8. The kit of claim 7 further comprising a band connected to the
compact display module for securing the display module to the wrist
of the patient.
9. The kit of claim 7 wherein the sensor is selected from the group
consisting of gas sensors, oxygen sensors, carbon dioxide sensors,
pH sensors and temperature sensors.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 10/658,926, filed Sep. 9, 2003, the disclosure of which is
hereby incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to an apparatus for measuring
physiological parameters in an individual and, in particular, to an
apparatus and method for measurement of blood gas parameters of a
patient.
[0004] 2. Description of the Related Art
[0005] Determination of cardiac output, arterial blood gases, and
other hemodynamic or cardiovascular parameters is critically
important in the treatment and care of patients, particularly those
undergoing surgery or other complicated medical procedures and
those under intensive care. Typically, cardiac output measurements
have been made using pulmonary artery thermodilution catheters,
which can have inaccuracies of 20% or greater. It has been found
that the use of such thermodilution catheters increases hospital
costs while exposing the patient to potential infectious,
arrhythmogenic, mechanical, and therapeutic misadventure. Blood gas
measurements have also heretofore been made. Commonly used blood
gas measurement techniques require a blood sample to be removed
from the patient and transported to a lab analyzer for analysis.
The caregiver must then wait for the results to be reported by the
lab, a delay of 20 minutes being typical and longer waits not
unusual.
[0006] More recent advances in the art have provided for
"point-of-care" blood testing systems wherein testing of blood
samples is performed at a patient's bedside or in the area where
the patient is located. Such systems include portable and handheld
units and modular units which fit into a bedside monitor. While
most point-of-care systems require the removal of blood from the
patient for bedside analysis, a few do not. In such systems,
intermittent blood gas measurements are made by drawing a
sufficiently large blood sample into an arterial line to ensure an
undiluted sample at a sensor located in the line. After analysis,
the blood is returned to the patient, the line is flushed, and
results appear on the bedside monitor.
[0007] A non-invasive technology, pulse oximetry, is available for
estimating the percentage of hemoglobin in arterial blood that is
saturated with oxygen. Although pulse oximeters are capable of
estimating arterial blood oxygen content, they are not capable of
measuring carbon dioxide, pH, or venous oxygen content.
Furthermore, ex vivo pulse oximetry is commonly performed at the
fingertip and can be skewed by peripheral vasoconstriction or even
nail polish.
[0008] Unfortunately, none of the available systems or methods for
blood gas analysis provide for accurate, direct and continuous in
vivo measurements of arterial and venous oxygen partial pressures,
carbon-dioxide partial pressure, pH, and cardiac output, while
presenting minimal risk to the patient.
[0009] Coatings and their applications to medical devices have
heretofore been described. See, for example, U.S. Pat. Nos.
3,443,869, 4,673,584, 5,997,517 and 5,662,960. Coatings have been
employed to maintain lubricity while minimizing complications
arising from use of exogenous material in vivo. Certain coatings
require reapplication to maintain lubricity and certain lubricious
coatings require administration of heparinized saline to maximize
immunological tolerance. For devices such as catheters and probes,
extraction from a physiological environment for reapplication of a
lubricant increases operational costs as well as exposing the
patient to heightened risk of mechanical and therapeutic
misadventure. Furthermore, reapplication of a coating can
compromise the gas permeability of the membrane upon which the
coating is applied.
SUMMARY OF THE INVENTION
[0010] An apparatus for use with a patient having a vessel carrying
blood to ascertain characteristics of the blood is provided. The
apparatus includes a display module and a probe having a distal
extremity adapted to be inserted into the vessel of the patient and
having a proximal extremity coupled to the display module. The
probe includes a sensor in the distal extremity for providing an
electrical signal to the display module when the probe is disposed
in the blood. The probe can have an antithrombogenic surface
treatment for inhibiting the adhesion of blood components to the
probe when disposed in the blood.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] For a better understanding of the nature and details of the
invention, reference should be made to the following drawings,
which in some instances are schematic in detail and wherein like
reference numerals have been used throughout.
[0012] FIG. 1 is an isometric view of an example of an apparatus
according to the present invention having a display module and a
probe for monitoring physiological parameters.
[0013] FIG. 2 is an isometric view of the probe of FIG. 1.
[0014] FIG. 3 is an enlarged cross-sectional view of the probe of
FIG. 1 adapted for multi-parameter measurement.
[0015] FIG. 4 is an enlarged cross-sectional view of the carbon
dioxide sensor section of the probe of FIG. 1.
[0016] FIG. 5 is an enlarged cross-sectional view of the oxygen
sensor section of the probe of FIG. 1.
[0017] FIG. 6A are several views of a flexible circuit subassembly
of another embodiment of the probe of FIG. 1.
[0018] FIG. 6B is an isometric view of the probe of FIG. 1.
[0019] FIG. 7 is a flowchart of the surface treatment process for
the probe of FIG. 1.
[0020] FIG. 8 is a block diagram of the circuitry contained in the
display module of FIG. 1.
[0021] FIG. 9 is a flowchart of the processing algorithm to
translate the sensor input signals into displayable values
performed by the display module of FIG. 1.
[0022] FIG. 10 is a plan view, partially cut away, of a kit of the
present invention.
[0023] Like reference numerals refer to corresponding parts
throughout the several views of the drawings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0024] Referring to FIG. 1, an apparatus 10 according to the
present invention for making intravascular measurement of
physiological parameters or characteristics generally includes a
display module 12 and one or more probes 18. As described in more
detail herein, the display module 12 and probe 18 are particularly
adapted for accurate and continuous in vivo measurement and display
of intravascular parameters such as partial pressure of oxygen
(PO.sub.2), partial pressure of carbon dioxide (PCO.sub.2), and pH.
In addition, cardiac output (CO) can be calculated by combining two
measurements of PO.sub.2 obtained from a pair of probes, one
disposed in an artery and the other in a vein. Alternatively, or in
addition to the aforementioned sensors, the probe 18 may include
sensors for other useful blood parameters such as potassium,
sodium, bilirubin, hemoglobin, glucose, pressure, etc. Additional
features of the display module 12 and probe 18 are detailed
hereinafter and in copending U.S. patent application Ser. No.
09/956,064 filed Sep. 18, 2001 and now U.S. Pat. No. 6,616,614, the
entire content of which is incorporated herein by this
reference.
[0025] As described herein, probe 18 removably connects to and
communicates with display module 12 by way of first or module
connector 15 and mating second or probe connector 22 located at the
proximal end or extremity of probe 18. Preferably, as shown in FIG.
2, probe 18 comprises a flexible elongate probe body or cannula 20
formed of a polymer or other suitable insulating material, having a
substantially uniform diameter over its entire length. The probe
body 20 supports a number of electrical contacts, and preferably at
least two, comprising a low-profile electrical connector 22, and it
includes a sensor section 24 and a blunt tip 26 near the distal end
or extremity of the probe 18. Electrical conductors attached to the
electrodes in the sensor section 24 of the probe 18 pass through
the length of the cannula 20, preferably through a bore or lumen
provided in the tubular cannula, and attach to the connector 22.
The sensor section 24 of the probe 18 includes electrodes inside an
electrolyte-filled chamber. A gas permeable window preferably
covers at least a portion of the chamber. All of the probe elements
are dimensioned to fit substantially within the diameter of the
probe body 20, having a diameter in the range from 0.010'' to
0.035'', but preferably a diameter of 0.020'', such that the entire
probe 18, including the low-profile connector 22, may be passed
through the inner bore of a suitable introducer, such as a
hypodermic needle, of a size suitable for accessing a blood vessel
in the hand, wrist, or forearm. Depending on the diameter of the
probe body 20, a suitable hypodermic needle for this purpose could
be as small as 25-gauge having an inner diameter of at least 0.010
inch, and it could be as large as 18-gauge having an inner diameter
of at least 0.035 inch, with the preferred size of 20-gauge having
an inner diameter of at least 0.023 inch, suitable for use with a
probe body having a nominal diameter of 0.020 inch. In this
preferred embodiment, the probe 18 can have a suitable length such
as 25 centimeters, permitting the sensor section 24 near the distal
end of the probe 18 to be inserted into a blood vessel in the hand,
wrist, or forearm, while the low-profile connector 22 at the
proximal end or extremity of probe 18 is connected to the display
module 12, which can be strapped to the wrist of the patient.
[0026] The low profile connector 22 is advantageous in this
application, since it permits the use of an ordinary hypodermic
needle or other suitable introducer to introduce the probe 18 into
the blood vessel with minimal trauma to the wall of the blood
vessel. The probe 18 is introduced into the blood vessel by first
inserting the appropriately sized hypodermic needle through the
skin and into the target vessel. The extremely sharp tip of the
hypodermic needle easily penetrates the skin, the underlying
tissue, and the vessel wall, while producing minimal trauma. Once
the hypodermic introducer needle has entered the target blood
vessel, the probe 18 is inserted through the bore of the needle and
advanced into the vessel. The blunt tip 26 and the lubricious
surface treatment 38 on the probe 18 minimize the likelihood of
vessel trauma as the probe 18 is advanced within the target vessel.
Once the probe 18 is properly positioned within the target vessel,
the introducer needle is withdrawn from the artery and the skin,
and completely removed from the probe 18 by sliding it off the
proximal end of the probe 18 over the low profile connector 22,
leaving the probe 18 in place in the vessel. The small puncture
left by the hypodermic needle quickly seals around the body of the
probe 18, thereby preventing excessive bleeding. The puncture site
is covered with a bandage and tape to guard against infection and
to anchor the probe. Any blood residue on the low profile connector
22 or the exposed portion of the probe 18 is wiped away with a
moist pad or alcohol swab, and the probe connector 22 is then
attached to the mating connector 15 on the display module 12. In
contrast to the simple, minimally traumatic introduction method
facilitated by the low-profile connector, a conventional probe,
having a standard connector, requires the use of a split introducer
sheath to introduce the probe into the blood vessel. The split
introducer sheath, which is blunter and bulkier than a hypodermic
needle, is much more likely to stretch or tear the vessel wall,
thereby increasing the risk of complications such as bleeding or
prolonged healing time. Although probe 18 has been described for
use in a blood vessel, it should be appreciated that the probe can
be introduced into other vessels, lumen or tissue of a body of a
patient, by means of any suitable introducer, and be within the
scope of the present invention.
[0027] In a preferred embodiment, illustrated in FIG. 3, the probe
18 is formed from a cannula, sleeve or body 20 of a suitable
polymer material, which serves the purpose of constituting a
structural element of the probe 18. All or a portion of the body 20
can also serve as a gas permeable membrane enclosing or surrounding
at least the sensor chambers 41 and 51. The polymer sleeve material
provides strength and flexibility to serve as a structural element
of the probe 18. It also permits the passage of oxygen and carbon
dioxide gases while blocking the passage of liquid water and the
ions dissolved therein when serving as the gas permeable membrane.
The sleeve 20 defines the outer surface of a major portion of the
probe 18, and the substantial majority of the sleeve 20 is
preferably filled with a flexible polymer 33 such as
ultraviolet-cured adhesive to provide robustness to the probe body
20, to anchor the electrical conductors 34 and sensor electrode
assemblies, and to seal the ends of the sensor chambers 41 and 51.
The sleeve 20 provides a substantial portion of the probe strength,
particularly in the sensor segment 24, where the sensor chambers 41
and 51 are filled with liquid, and the sleeve 20 can also form the
circumferential windows 31 enclosing said sensor chambers when all
or a portion of the sleeve is made from a gas permeable
material.
[0028] A preferred material for the sleeve 20 shown in FIG. 3 is
plastic, preferably a polymer and more preferably
polymethylpentene. The sleeve 20 has a wall thickness in the range
from 0.001 inch to 0.003 inch and preferably 0.0015 inch. Among
commonly-used polymers suitable for extrusion as thin-walled
tubing, polymethylpentene has among the highest oxygen and carbon
dioxide permeability coefficients available. In addition, it has
great stiffness. Table 1 includes gas permeability coefficients and
the stiffness-related modulus of elasticity of a representative
selection of commonly-used polymer materials, showing the
advantages of polymethylpentene for this application.
TABLE-US-00001 CO2 Tensile permeability 02 permeability Modulus
Material (Barrer.sup.1) (Barrer.sup.1) (GPa) Polymethylpentene 80
27 1.5 _Low Density Polyethylene 13 10 0.1-0.3
Polytetrafluoroethylene 10 4.3 0.3-0.8 Polypropylene 8 2.3 0.9-1.5
Polycarbonate 6.4 1.4 2.3-2.4 Polyimide 0.3 0.15 2-3 Polyester 0.13
0.05 2-4 Nylon 0.09 0.04 2.6-3.0 .sup.1The Barrer is a unit of gas
permeability, equivalent to 10.sup.-10 (cm.sup.3 per second of gas
at standard temperature and pressure) (cm of membrane thickness)
per (cm.sup.2 of membrane area) per (cmHg of pressure)
[0029] A cylindrical sleeve 20 of gas permeable membrane material
is particularly advantageous as the covering for the blood gas
sensor chamber 41 or 51, since it creates a complete
circumferential window 31, thereby maximizing the membrane area for
a given sensor length. In addition to maximizing the membrane area,
the circumferential window 31 inhibits the "wall effect" artifact
seen in previous blood gas sensor probes, wherein the gas permeable
membrane on the tip or one side of a blood gas sensor probe is
fully or partially blocked from exposure to the blood when the
probe is inadvertently positioned against a vessel wall. The
circumferential window of the present invention precludes the
possibility for a substantial portion of the membrane to be blocked
by close proximity of the probe to the wall of the blood vessel.
For the carbon dioxide sensor, the flow of gas through the membrane
mainly affects the response time of the sensor. The electrolyte or
other solution inside the carbon dioxide sensor chamber eventually
reaches carbon dioxide equilibrium with the surrounding blood, as
long as there is a reasonable rate of diffusion through the
membrane. However, the oxygen sensor relies on a continuous flow of
oxygen through the membrane to be consumed at the platinum sensing
electrode, therefore, any significant obstruction to the flow of
oxygen to the sensing electrode can affect the accuracy of the
sensor. The sensitivity of the oxygen sensor to the "wall effect"
is minimized by making the membrane permeability so high that the
reaction rate is limited primarily by the rate of consumption of
oxygen at the sensing electrode, which is then determined by the
exposed area of platinum catalyst. In this case, any effect on the
probe due to a partial blockage of the circumferential window 31 by
close proximity to the wall of a blood vessel is minimized.
[0030] The probe body 20 supports electrical contacts 32
constituting the low-profile electrical connector 22 and it
contains the electrical conductors 34 and the sensor section 24 of
the probe 18. The electrical contacts 32 consist of gold bands or
the like, soldered or welded to the electrical conductors 34, which
are electrically coupled to the one or more sensors in the sensor
section 24 of the probe by any suitable conductors so as to carry
the electrical signals from multiple sensors and thus permit
electrical access to the probe from outside the patient's body. The
multiple sensors can include a carbon dioxide sensor 40, an oxygen
sensor 50, a thermocouple 47 and a pH-sensing electrode 58, or any
combination thereof or other sensors. Preferably, at least the
portion of the sleeve 20 that is placed inside the blood vessel,
including the sensor section 24, is provided with a surface
treatment 38 to inhibit the accumulation of thrombus, protein, or
other blood components which might otherwise impair the blood flow
in the vessel or impede the diffusion of oxygen or carbon dioxide
into the sensing chambers 41 and 51. A preferred method for the
application of such surface treatment is hereinafter described.
[0031] FIG. 4 provides a detailed view of one embodiment of the
carbon dioxide sensor 40 contained within the sensor section 24 of
probe 18. The carbon dioxide sensor 40 includes a chamber 41
containing an electrolyte solution and first and second electrodes
43 and 44. The sleeve 20 and the ultraviolet-cured adhesive 33,
which seals each end of the chamber 41, define the chamber volume.
The chamber 41 is preferably filled with an electrolyte solution
such as 0.154 Molar NaCl (normal saline) with 0.001 Molar
NaHCO.sub.3 (sodium bicarbonate). The pH of this solution varies
with the partial pressure of carbon dioxide, and the electrodes 43
and 44 generate an electrical potential in response to this pH. The
reference electrode 43 for the carbon dioxide sensor preferably can
be formed from a silver wire coated with silver chloride, produced
by dipping a silver wire into molten silver chloride, or
alternatively by a known electrochemical process. The sensing
electrode 44 for the carbon dioxide sensor is a platinum wire
coated with platinum dioxide, produced by sintering platinum
dioxide powder onto the surface of a platinum wire, or
alternatively, by an electrochemical or vapor deposition process.
The electrodes 43 and 44 are attached or otherwise coupled to
respective first and second electrical conductors 45 or 46, such as
insulated copper wires, by soldering or welding.
[0032] Ideally, the carbon dioxide sensor 40 occupies a small axial
length of the probe 18 in the range of 1 mm to 10 mm, but
preferably 4 mm, so that the sensor section 24 of the probe 18 is
short enough, such as less than 20 mm, but preferably less than 13
mm, to be easily advanced into a tortuous vessel. While occupying a
small axial length of the probe 18, the carbon dioxide sensor
design provides large electrode areas and maintains a large
physical separation between the electrodes. Additionally, the
carbon dioxide sensor provides a conduit for passage of the
electrical conductors to the more distal electrodes of a
multi-sensor probe, electrically isolated from the electrolyte
solution inside the carbon dioxide sensor chamber 41. In the
embodiment shown in FIG. 4, both the reference electrode 43 and the
sensing electrode 44 are coiled around a tube 42, such as a
polyimide tube having an outer diameter of 0.011 inch, an inner
diameter of 0.009 inch, and a length of 8 mm. The coiled electrodes
43 and 44 provide large electrode surface areas in a small volume,
and the two electrodes 43 and 44 are physically separated from each
other by coiling the reference electrode 43 around the proximal
half of the tube 42, while the sensing electrode 44 is coiled
around the distal half of the tube 42 with a relatively large axial
separation, such as 1 mm, between the two coils. Additionally, the
inner lumen of the polyimide tube 42 provides a conduit for passage
of the conductors for the more distal electrodes, electrically and
physically isolated from the electrolyte solution in the sensor
chamber 41 by multiple layers of insulation including the
insulation on the electrical conductors themselves, the polyimide
tubing, and the air or adhesive that fills the inner lumen of the
polyimide tube 42. The polyimide tube 42 is anchored in the
adhesive 33, which seals the ends of the sensor chamber 41, thereby
providing additional mechanical strength to the carbon dioxide
sensor section of the probe 18, beyond that provided by the sleeve
20 alone. The electrolyte solution of the carbon dioxide sensor 40
is contained in the annular space between the polyimide tube 42 and
the sleeve or body 20 of the probe 18. The sleeve 20 can form a
large surface area circumferential window 31 for the carbon dioxide
sensor 40, which is not easily blocked by close proximity to a
blood vessel wall.
[0033] FIG. 4 also shows a temperature sensing thermocouple 47
contained within the sensor section 24 of probe 18. The
thermocouple 47 can include a pair of conductors 48 and 49 of
dissimilar materials, electrically connected to each other by
soldering or welding. The conductors are chosen from known pairs of
materials, such as copper and constantan, with known responses to
temperature. The thermocouple junction is electrically insulated
from the other sensor electrodes, and it is embedded within the
sensor section 24 of probe 18 in proximity to the other sensors
where it will accurately reflect the temperature of the surrounding
blood.
[0034] FIG. 5 provides a detailed view of one embodiment of the
oxygen sensor 50, which is contained within the sensor section 24
of probe 18. The oxygen sensor 50 can include a chamber 51
containing an electrolyte solution and third and fourth electrodes
53 and 54. The chamber 51 is defined by the sleeve 20 and
ultraviolet-cured adhesive 33, which seals each end of the chamber.
The chamber is preferably filled with an electrolyte solution, such
as 0.154 Molar NaCl (normal saline) buffered with 0.120 Molar
NaHCO.sub.3 (sodium bicarbonate). With an appropriate electrical
potential biasing the electrodes, such as 0.70 volts, a platinum
electrode 54 serves as the catalyst for a chemical reaction that
consumes oxygen and generates an electrical current in proportion
to the rate of consumption of oxygen at the platinum electrode,
which is in turn dependent on the partial pressure of oxygen in the
blood surrounding the sensor 50. The sodium bicarbonate buffer
stabilizes the pH of the electrolyte solution against changes that
would be otherwise induced by the chemical reaction that consumes
oxygen at the platinum electrode 54. When the buffer or electrolyte
solution is exhausted, or when the sensor chamber 51 becomes filled
with excessive silver chloride precipitate, the oxygen sensor
response will change, and the sensor will no longer be viable.
Probe 18 therefore advantageously provides a sufficiently large
chamber volume filled with buffered electrolyte to provide the
required lifetime for the oxygen sensor. The reference electrode 53
for the oxygen sensor 50 preferably consists of a silver wire
coated with silver chloride, produced by dipping a silver wire into
molten silver chloride, or alternatively by a known electrochemical
process. The sensing electrode 54 for the oxygen sensor 50 is a
platinum wire. The electrodes are attached or otherwise coupled to
respective third and fourth electrical conductors 55 or 56, such as
insulated copper wires, by soldering or welding.
[0035] Preferably, the oxygen sensor 50 occupies a small axial
length of the probe 18 in the range of 1 mm to 10 mm, but
preferably 4 mm, so that the sensor section 24 of the probe 18 is
short enough, such as less than 20 mm, but preferably less than 13
mm, to be easily advanced into a tortuous artery. While occupying a
small axial length of the probe 18, the oxygen sensor design should
provide a large reference electrode area, maintain a large physical
separation between the electrodes, and provide a large volume of
electrolyte solution. Additionally, the sensing electrode 54
exposes only a small and well-defined surface area to the
electrolyte solution. Additionally, the oxygen sensor provides a
conduit for passage of the electrical conductors to the more distal
electrodes of a multi-sensor probe, electrically isolated from the
electrolyte solution inside the oxygen sensor chamber 51. In the
embodiment shown in FIG. 5, the reference electrode 53 is coiled
around a tube 52, such as a polyimide tube having an outer diameter
of 0.007 inch, an inner diameter of 0.005 inch, and a length of 5
mm. The coiled reference electrode 53 provides a large electrode
surface area in a small volume. The sensing electrode 54 is
preferably formed from a short exposed length of a small diameter
platinum wire, in the range from 0.001 inch to 0.008 inch, but
preferably 0.002 inch in diameter.
[0036] Preferably, the sensing electrode 54 is formed by first
oxidizing the surface of a small diameter platinum wire by heating
in a furnace with an oxygen atmosphere, then fusing a bead 57 of
sealing glass onto the platinum wire. The sealing glass is chosen
to provide a coefficient of thermal expansion in the range from 8.0
to 9.2.times.10.sup.-6/.degree. K, but preferably
8.6.times.10.sup.-6/.degree. K, closely approximating or matched to
the coefficient of thermal expansion for platinum,
9.0.times.10.sup.-6/.degree. K. The glass forms a strong bond to
the platinum oxide on the surface of the platinum wire, and the
matched thermal expansion coefficients minimize the thermal stress
during cooling of the glass and platinum, thereby inhibiting
cracking of the glass or separation of the glass from the electrode
that could lead to drift in the oxygen sensor as the exposed
platinum electrode area changes. The glass bead 57 forms a reliable
seal to the platinum wire electrode 54, ensuring a stable platinum
electrode area for drift-free operation of the device. The bond
between the sealing glass and the oxidized platinum wire is much
more tenacious and fluid-resistant than the bond formed by an
adhesive used in prior oxygen sensor designs, rendering the present
invention much more stable than a design based on an adhesive seal.
Gluing the glass bead 57 into the end of the tube 52 and trimming
the distal end of the platinum wire flush, or within one wire
diameter of the tip of the glass bead 57 completes the oxygen
electrode assembly. The two electrodes 53 and 54 are physically
separated from each other because the reference electrode 53 is
coiled around the tube 52 and the sensing electrode 54 is exposed
only at the tip of the glass bead 57, separated from the reference
electrode 53 by a relatively large axial separation such as 1 mm.
Additionally, the oxygen sensor 50 includes a conduit 59A,
preferably formed from polyimide or other insulating tubing, for
passage of the conductor 59 leading to the more distal pH-sensing
electrode 58. The conductor 59 is electrically and physically
isolated from the electrolyte solution in the sensor chamber 51 by
multiple layers of insulation including the insulation on the
electrical conductor 59, the insulating tubing conduit 59A, and the
air or adhesive that fills the inner lumen of the conduit 59A. The
electrolyte solution of the oxygen sensor 50 is contained in the
annular space between the polyimide tube 52 and the sleeve 20, and
in the cylindrical space beyond the tip of the glass bead 57 and
the platinum sensing electrode 54. The sleeve 20 preferably forms a
large surface area circumferential window 31 for the oxygen sensor
50, which is not easily blocked by close proximity to a blood
vessel wall.
[0037] FIG. 5 also shows a detailed view of the pH sensor contained
within the sensor section 24 of probe 18. The pH sensor includes a
noble metal electrode 58, such as a gold or platinum band, mounted
on the external surface of the probe 18 where it is directly
exposed to the blood, and a reference electrode 43 or 53. The
reference electrode for the pH sensor preferably consists of a
silver wire coated with silver chloride, produced by dipping a
silver wire into molten silver chloride, or alternatively by a
known electrochemical process. The reference electrode 43 or 53 can
be shared with the oxygen sensor 40 or the carbon dioxide sensor
50. The pH-sensing electrode 58 is attached to an electrical
conductor 59, such as an insulated copper wire, by soldering or
welding.
[0038] As hereinbefore described, the probe is generally
constructed from various wires, tubes, and electrodes, inserted
into a bore of a tubular sleeve 20, which is subsequently filled
with adhesive and electrolyte solutions to form the sensors. In an
alternative embodiment, a flexible circuit replaces the wires,
tubes, and electrodes. The flexible circuit can be mass-produced in
a batch process at low cost, thereby minimizing the cost of the
multi-sensor probe. FIG. 6A shows a flexible circuit 60 which
incorporates all of the electrical elements of a multi-sensor blood
gas sensor probe, including electrical contact pads 62 comprising a
low profile electrical connector 22, electrical conductors 61, and
sensing electrodes 63 through 68 of various types, all fabricated
on a flexible planar substrate having three layers of circuitry
separated by two layers of flexible insulating substrate such as
polyimide. Such a flexible circuit can be manufactured using a
known batch process wherein successive layers of conducting
materials on insulating substrates are deposited by electroplating,
vapor deposition, or other methods, then patterned by
photolithography, laser ablation, or other methods. The patterned
layers are bonded together with an insulating adhesive to complete
the multi-layer flexible circuit. Once the processing steps have
been completed, individual circuits are cut into narrow strips
having a width such as 0.015 inch, such that the circuit can be
inserted into a sleeve 20 and filled with adhesive 33 and
electrolyte solutions to form the sensor chambers 41 and 51 over
the electrode sections of the flexible circuit 60.
[0039] The flexible circuit 60 has a length, such as 25 cm,
appropriate for the circuit to be situated longitudinally within
the lumen of a sleeve and can have a width ranging from 0.008 inch
to 0.030 inch and preferably 0.015 inch. The proximal end or
portion of the flexible circuit 60 preferably has at least two pads
62, and more preferably seven pads 62, which serve as the
electrical contacts 32 for a low profile electrical connector 22.
The connector pads 62 are plated with gold to provide reliable
electrical contact with the mating connector 15 of the display
module 12. The contact pads are connected to traces or conductors
61, sandwiched or disposed between first and second insulating
layers 161 and 162 of the flexible circuit substrate and more
specifically formed one or both of the inner surfaces 163 and 164
of respective layers 161 and 162. The traces 61 are in turn
connected to a plurality of pads 63-66 and 68 near the distal end
or portion of the flexible circuit 60, which serve as electrodes
for the various sensors. The pads and traces of the flexible
circuit 60 are primarily formed of copper, and the pads are plated
with various metals including silver, platinum, and gold to create
the electrodes of the various sensors. The pads 62, 63-66 and 68 on
one or both of the exposed outer surfaces 166 and 167 of the flex
circuit are connected to traces 61 by feedthrough vias 69 or any
other suitable means. The reference electrodes 63 and 65 for the
oxygen, carbon dioxide, and pH sensors are preferably formed by
subjecting silver-plated pads to a known electrochemical process
wherein the silver is reacted with chloride ions in a solution to
form a layer of silver chloride on the surface of the silver. The
sensing electrode 64 for the carbon dioxide sensor is preferably
formed by subjecting a platinum-plated pad to a known
electrochemical process wherein the platinum metal is reacted in a
platinum chloride solution to form a platinum dioxide layer on the
surface of the platinum. The sensing electrode 66 for the oxygen
sensor is preferably formed by masking a platinum-plated pad
electrode with an insulating material to define a small exposed
area of platinum metal in the range from 0.001 inch to 0.008 inch
in diameter, but preferably 0.002 inch in diameter. The pH-sensing
electrode 68 is preferably a gold-plated or platinum-plated pad,
directly exposed to the blood. The flexible circuit 60 can further
accommodate a temperature sensor in the form of a patterned thin
film of known material, forming a temperature-sensitive resistor 67
on inner surface 163 of first layer 161. Alternatively, the
temperature sensor can be a diode, thermistor, or thermocouple,
bonded to one of the flexible circuit substrate layers 161 and
162.
[0040] FIG. 6B shows the flexible circuit 60, including various
electrodes, inserted into the lumen or bore of the sleeve 20, which
is preferably sealed with adhesive 33 and filled with electrolyte
solutions to form the internal chambers 41 and 51 of the carbon
dioxide and oxygen sensors. The proximal end or portion of the
flexible circuit 60 includes buried traces, which serve as
electrical conductors 61, and gold-plated pads, which serve as
electrical contacts 62 for the electrical connector 22. The buried
traces conduct electrical signals from the sensor electrodes 63
through 68 to the electrical contacts pads 62, which serve as a low
profile electrical connector 22 that can be coupled to the mating
connector 15 of the display module 12.
[0041] As hereinbefore described, at least the portion of the
polymer sleeve 20 that forms the external surface of the probe 18
is preferably provided with a durable surface treatment 38 to
inhibit the accumulation of thrombus, protein, or other blood
components, which might otherwise impair blood flow in the artery
or impede the transport of oxygen or carbon dioxide through the
circumferential window 31 into the sensing chamber 41 or 51 (see
FIG. 3). One preferred method for treating the surface of the
sleeve 20 is photoinduced graft polymerization with
N-vinylpyrrolidone to form a dense multitude of microscopic
polymerized strands of polyvinylpyrrolidone, covalently bonded to
the probe outer surface. This surface treatment 38 is durable, due
to the strong covalent bonds, which anchor the polymer strands to
the underlying substrate. The surface treatment 38 adds only a
sub-micron thickness to the probe body 20, yet it provides a
hydrophilic character to the probe surface, rendering it highly
lubricious when hydrated by contact with blood or water, thereby
facilitating the smooth passage of the probe 18 through the blood
vessel. This hydrophilic surface treatment 38 also inhibits the
adsorption of protein onto the surface of the underlying polymer
substrate, thereby minimizing the accumulation of thrombus,
protein, or other blood components on the probe 18. Although the
dense multitude of polyvinylpyrrolidone polymer strands shields the
underlying outer wall of the sleeve or cannula 20 from large
protein molecules, it does not significantly impede the migration
of small molecules such as oxygen or carbon dioxide through the
wall of the cannula. Therefore, the surface treatment 38 of the
polymethylpentene sleeve 20 facilitates consistent, reliable
communication of the gases in the blood, such as oxygen and carbon
dioxide, through the circumferential window 31 into the oxygen and
carbon dioxide sensor chambers 41 and 51, even after prolonged
residence time up to three days in the bloodstream of a
patient.
[0042] One procedure for surface treatment of the polymer sleeve
material is described hereinafter and is shown as a flowchart in
FIG. 7. In preparation for the surface treatment process, two
solutions are prepared, the sensitizing dilution 76 and the coating
solution 79. The sensitizing dilution 76 is prepared in two phases.
In a first phase or step 74, performed under room light
illumination, a blanket of nitrogen gas is applied to a volume of
acetone, preferably 90 ml of acetone, after the acetone has been
purged with nitrogen gas for a duration such as five minutes. In a
second phase or step 75, performed under red light illumination, a
mass of benzophenone, preferably 1.0 g of benzophenone, is
dissolved in the acetone, with additional acetone added to the
solution to make up a total volume of 100 ml. The coating solution
79 is prepared in two phases, both of which are carried out under
room light illumination. In the first phase or step 77, a blanket
of nitrogen gas is applied to a volume of distilled water in a
flask, preferably 80 ml of distilled water, after the distilled
water has been purged with nitrogen gas for a duration such as five
minutes. In the second phase or step 78, while the nitrogen gas is
still being applied, a mass of N-vinylpyrrolidone, preferably 11.4
grams of N-vinylpyrrolidone, is dissolved in the distilled water.
The flask is capped and the coating solution 79 is ready for
storage or application.
[0043] A membrane tubing assembly is prepared for surface treatment
in step 70 by placing a mandrel inside a polymethylpentene tube of
the proper length and sealing one end of the tube. In a preliminary
phase or step 71 of the surface treatment procedure, performed
under room light illumination, the membrane tubing assembly is
immersed in methanol and sonicated for five minutes to thoroughly
clean the outer surface, then allowed to air dry for five minutes.
In a second phase or step 72 of the surface treatment procedure,
performed under red light illumination, the membrane tubing
assembly is dipped into a sensitizing dilution 76 of benzophenone
in acetone for 30 seconds, under a nitrogen purge. The sensitized
membrane tubing assembly is then removed and placed in a
dessicator, still under red light illumination, dried for a
duration such as five minutes under partial vacuum such as 28 mmHg,
and stored in an amber vial with a nitrogen blanket. In a third
phase or step 73 of the surface treatment procedure, performed
under room light illumination, the sensitized membrane tubing
assembly is dipped in a volume of the N-vinylpyrrolidone coating
solution 79, such as 30 ml of solution, that has been heated to
60.degree. C. The coating is cured by exposure to ultraviolet
curing lights for a period such as 90 seconds, during which time
the N-vinylpyrrolidone is polymerized to form a multitude of
polyvinylpyrrolidone strands, covalently bonded to the membrane
tubing substrate. The membrane tubing assembly is rinsed with
copious amounts of distilled water, then placed in a dessicator to
be dried under vacuum such as 28 mmHg for a period such as two
hours to complete the preparation of the surface treated membrane
tubing.
[0044] The surface treated polymethylpentene tubing may be used as
the sleeve 20 in the manufacture of a complete probe assembly,
which will then retain the beneficial properties of the
N-vinylpyrrolidone surface treatment. Alternatively, the probe
assembly 18 can be manufactured using untreated tubing, and the
surface treatment can be subsequently applied to the completed
probe 18 using substantially the same method as describe
hereinbefore.
[0045] The display module 12, as shown in FIG. 1, includes a
housing 17 formed of a suitable material such as plastic and which
is sized so that it can be worn on the patient, such as on the
patient's wrist, arm or other limb, sometimes referred to herein as
the subject, with the probe(s) 18 inserted into vessel(s) in the
hand, wrist, or forearm. The module 12 also includes a display 13
such as a liquid crystal display (LCD) for displaying measured
parameters and other information, and adapted to be readily visible
to the attending medical professional, sometimes referred to herein
as the user. The display 13 may include backlighting or other
features that enhance the visibility of the display. The band 14
attached to the housing 17 is adapted to secure the display module
12 to the subjects wrist. Alternatively, the module 12 may be
attached to the subject's arm or to a location near the subject.
Optionally, in the case the subject is a newborn infant (neonate),
the module 12 may be strapped to the subject's torso, with the
probe(s) 18 inserted into umbilical vessel(s). The band 14 is
comprised of any suitable material, such as Velcro, elastic, or the
like. Buttons 16 or keys facilitate entry of data and permit the
user to affect the display 13 and other features of the module 12.
While FIG. 1 shows three buttons, any number or type of buttons,
keypads, switches or the like may be used to permit entry of
parameters or commands, or to otherwise interface with the
apparatus 10. The module 12 may also include wireless
communications capability to facilitate display of physiologic
parameters on a remote monitor or computer system, and/or to
facilitate the entry of patient parameters or other information
into the module 12 from a remote control panel or computer system.
The module 12 also includes one or more connectors 15 that provide
physical connection and communication with one or more probes 18.
Preferably, each connector 15 includes a receptacle adapted to
receive, secure, and communicate with a corresponding connector 22
on the proximal end of the probe 18.
[0046] In a preferred embodiment of the display module 12, the
module is designed to be low in cost so that it can be packaged
together with a probe(s) 18 and accessories as a disposable kit
100, with all of the components of the kit packaged together in a
sterile pouch or other container 101, as illustrated in FIG. 10. In
addition to the display module 12 and probe(s) 18, the kit would
optionally include a probe holder 102 to protect the probe from
damage or degradation, a wrist band 14 or other means for attaching
the display module to a patient, a needle or other introducer 103,
alcohol swabs 104 for cleaning the skin prior to cannulating the
vessel and for cleaning blood or other residue from the probe
connector prior to attaching the probe to the module, a bandage 105
to cover the puncture site and anchor the probe in place, and any
other items that may be utilized for preparing and using the probe
18 and display module 12. The display module 12 is further designed
to require low power so that it can operate for the expected
lifetime of the device, such as 72 hours, on battery power without
the need for battery replacement or connection to an external power
source. The probe 18 is preferably suited to be a single-use,
disposable device, since it has a limited operational lifetime and
is used in direct contact with the subject's blood. The module 12
is durable enough to be used many times, however, the advantage of
a disposable module is that it eliminates the expense and the
infection hazard associated with cleaning, replacing batteries, and
reusing a single module for multiple patients. An additional
advantage of a disposable module 12 packaged together with its
associated probe(s) 18 is that the calibration data can be stored
in the module at the time of manufacture, greatly simplifying the
use of the apparatus 10 by eliminating the need for the user to
enter calibration data into the module prior to using the probe 18.
An additional advantage of a disposable module 12 packaged together
with its associated probe(s) 18 is that the calibration data stored
in the module at the time of manufacture can account for all of the
monitor and probe inaccuracies and artifacts in a single set of
calibration coefficients, thereby avoiding the accumulation of
inaccuracies that can occur with separate calibrations of the probe
18 and the module 12. In a preferred embodiment of the module, no
user inputs at all are required, eliminating the need for buttons,
keypads, switches, and the like. The display module 12 is
automatically energized upon connection of the probe 18 to the
module 12, and all of the calibration data and other needed
information is pre-programmed into the module at the time of
manufacture.
[0047] One embodiment of the electronic circuitry 80 included in
the display module 12 is shown in block diagram form in FIG. 8. As
shown therein, signals from the one or more sensors provided on the
one or more probe(s) 18 arrive at the display module 12 via
connector(s) 15. The sensor signals are received by a respective
plurality of analog signal conditioning circuits 82, one for each
sensor in the associated probe(s) 18. The outputs from the analog
signal conditioning circuits 82 are directed to a microcontroller
81, such as the Texas Instruments MSP430F435, which includes many
of the circuit elements required by the display module 12. In
particular, the microcontroller 81 includes an analog multiplexer
and an analog-to-digital converter to digitize the analog signals
from the plurality of analog signal conditioning circuits 82, as
well as analog support circuitry including a voltage reference, a
temperature sensor, and power supply monitoring circuitry. In one
preferred embodiment, the algorithm for processing the signals,
together with the sensor and module calibration coefficients, is
embedded in software stored in non-volatile memory included in the
microcontroller 81. The microcontroller 81 further includes a
central processing unit to execute the software algorithm and other
peripheral functions including clock circuitry, serial and parallel
input/output interfaces, timers, and the LCD driver circuitry. The
LCD driver circuitry supplies the waveforms for the liquid crystal
display 13, and the display module 12 can also communicate with an
external computer or module over a serial data link via an optional
wireless interface circuit 83 or other suitable means. The
integration of most of the required functions of the display module
circuitry into a single, inexpensive, low-power component, that is
the microcontroller, makes it feasible to manufacture the module as
a low cost, battery-powered disposable unit.
[0048] Each of the analog signal conditioning circuits 82 is
adapted to the particular type of sensor to which it is connected.
For the oxygen sensor, the analog signal conditioning circuit can
be a current-to-voltage converter with a full-scale input current
that includes the maximum full-scale current expected for the
oxygen sensor, such as 100 nanoamps, and a full-scale output
voltage matched to the analog-to-digital converter input range. The
input bias current for the oxygen sensor circuit is preferably much
less than the normal sensor operating current, such as an input
bias current of less than 100 picoamps. For the carbon dioxide or
pH sensor, the analog signal conditioning circuit can be a voltage
amplifier with very high input impedance, such as greater than
10.sup.12 ohms, and very low input bias and input offset currents,
such as less than 100 femtoamps. The circuit can include a fixed
gain and offset voltage chosen to translate the full-scale sensor
voltage range to match the analog-to digital converter input range.
The carbon dioxide or pH sensor circuit requirements may be
satisfied by an instrumentation amplifier or by a simpler
operational amplifier circuit, with the amplifier selected to
provide the required low input bias and offset currents. For the
thermocouple temperature sensor, the analog signal conditioning
circuit can be a high gain voltage amplifier with an input voltage
range of zero to 2 millivolts over the expected temperature range,
and an output voltage to match the analog-to-digital converter
input range. For the required high gain thermocouple signal
conditioning circuit, the amplifier is preferably chosen to provide
an input offset voltage much less than the signal voltage, such as
an input offset voltage of less than 10 microvolts.
[0049] One processing algorithm 90 that can be performed by the
microcontroller 81 to convert the digitized sensor signals into
displayable numeric values is shown in block diagram form in FIG.
9. The processing algorithm includes the steps of digitizing the
sampled sensor outputs in step 91, temporal filtering or averaging
to reduce the noise from external interference or other sources in
step 92, correcting for gain or offset errors in the analog signal
conditioning circuitry in step 93, incorporating gain, offset, and
linearity corrections from the sensor calibration data in step 94,
compensating for the temperature dependence of the gain, offset and
linearity of the sensor according to the measured probe temperature
in step 95, and translating the value into the desired units for
display on the LCD in step 96. In practice, if the module 12 and
probe 18 are calibrated together as a single disposable apparatus,
then all of the gain, offset, nonlinearity, temperature, and unit
conversion factors from steps 93, 94, 95, and 96 can be
incorporated into a single set of calibration functions that permit
the direct translation of filtered analog inputs into displayable
values without the need to calculate any intermediate corrections,
and without the accumulation of errors from separate calibrations
of the individual components of the apparatus. Optionally, the
algorithm may include step 97 of calculating other physiologic
parameters according to known formulas, possibly combining readings
from multiple sensors, or combining multiple readings from a single
sensor to provide additional useful information.
[0050] An example of a calculation based on a single reading from a
single sensor is the estimation of arterial or venous oxygen
saturation (SaO.sub.2 or SvO.sub.2) from the corresponding measured
partial pressure of oxygen (PaO.sub.2 or PvO.sub.2). There is a
known nonlinear relationship between the oxygen saturation and the
partial pressure of oxygen in blood, but the saturation value is
useful for calculating cardiac output and for other assessments of
patient status.
[0051] An example of a calculation based on multiple readings from
a single sensor is the determination of the trend in the associated
blood gas parameter, that is, whether the value is increasing,
decreasing, or stable. The trend in the blood gas parameter can be
symbolically indicated on the display, making it easier for the
user to quickly assess patient status.
[0052] An example of a calculation based on combined readings from
multiple sensors is the use of the carbon dioxide reading and the
pH measurement to calculate the bicarbonate level. According to a
known relationship, the log of the bicarbonate concentration is
equal to the pH, plus the log of the partial pressure of CO.sub.2,
minus the constant 7.608. This equation is appropriate for blood at
37.degree. C., and it may be further compensated for temperature
deviation from normal.
[0053] An example of a calculation based on combined readings from
multiple sensors on multiple probes is the use of an arterial
oxygen reading and a venous oxygen reading to estimate the cardiac
output using a modified version of the Fick oxygen consumption
method. According to the Fick method, cardiac output
(liters/minute) is calculated as the oxygen consumption
(milliliters/minute), divided by the arteriovenous oxygen
concentration difference (milliliters of O.sub.2 per liter of
blood). For the present invention, oxygen consumption is estimated
as 3 milliliters/kilogram times the subject's weight, which can be
entered into the module by way of the buttons or keys, or by way of
a wireless communication from an external computer or control
panel. Assuming standard values for hemoglobin (12.5
grams/deciliter) and the oxygen carrying capacity of hemoglobin
(1.36 milliliters of O.sub.2 per gram of hemoglobin), the
arteriovenous oxygen concentration difference can be calculated as
the difference between the arterial oxygen saturation and the
venous oxygen saturation (SaO.sub.2-SvO.sub.2) times the standard
value of 170 milliliters of O.sub.2 per liter of blood. In this
calculation, the value of the venous oxygen saturation may be
adjusted to compensate for the experimentally determined
discrepancy between the pulmonary artery oxygen saturation and the
forearm venous oxygen saturation.
[0054] The incorporation of wireless interface circuitry into the
display module is advantageous in preserving the electrical safety
and freedom of movement of the patient afforded by the
self-contained, battery-powered display unit, while providing the
benefits of an integrated system in terms of centralized data
collection. The compact display module of the present invention
makes the most of the wireless communications by freeing the
subject from the tubes and cables that normally tether them to
their bed, and by eliminating the need for additional bulky
instrumentation at the already crowded bedside.
[0055] From the foregoing it can be seen that the apparatus 10 and
method of the present invention makes it possible to measure blood
gases of a subject, such as oxygen and carbon dioxide, as well as
other blood parameters including temperature and pH. As
hereinbefore described, a single probe may include more than one
sensor, e.g., an oxygen sensor, a carbon dioxide sensor, a
temperature sensor, and a pH sensor. The sensors are included in a
probe body, having a small diameter of less than 0.023'' so that it
can be readily inserted through a 20-gauge needle into a blood
vessel in the hand, wrist, or forearm. This probe includes at least
one sensor with a window 31 having a large surface area and high
permeability to the target gas molecules, which facilitates the
rapid diffusion of blood gases into or out of the sensor chamber to
ensure a fast response to changes in the blood gas concentration.
The probes utilized are preferably blunt tipped and atraumatic to
the vessel wall and are preferably provided with an
antithrombogenic surface treatment to inhibit the formation of
thrombus or the adhesion of protein or other blood components,
ensuring consistent performance of the blood gas sensors and
minimizing the need for continuous infusion of heparin to maintain
a clot-free environment. The probe carries electrical signals from
the sensors, through electrical conductors, to a low profile or
other connector removably attached to a mating connector on the
display module. The low profile of the preferred connector
facilitates the removal of the hypodermic needle or other
introducer used to most simply introduce the probe into the lumen
of a vein or artery, thereby eliminating the need for using a split
sheath introducer or other more complex technique for introducing
the probe into the vessel. The display module is small and
inexpensive, and it is particularly suited for attachment to the
patient's wrist. The apparatus and method herein described may be
adapted to the particular requirements of a variety of different
medical applications, several of which are outlined below.
[0056] For patients in the intensive care unit (ICU) or coronary
care unit (CCU), there is typically the need for monitoring
arterial blood gases (oxygen and carbon dioxide) and pH. Currently,
this monitoring is performed on an intermittent basis, typically
three to twelve times per day, by drawing a blood sample from an
arterial line in the patient's forearm, and delivering the blood
sample to a blood gas analyzer. A multi-sensor probe as described
herein, providing continuous oxygen, carbon dioxide, and pH
measurements, can eliminate the need and the associated expense and
risks of placing and maintaining an arterial line and repeatedly
drawing blood samples therefrom. Furthermore, the continuous
monitoring provided by the present invention gives rapid feedback
regarding the effects of any interventions such as adjustments to
the ventilator settings or administration of drugs. The timely
feedback on the effects of the medical interventions permits the
subject to be more quickly weaned from the ventilator and released
from the ICU/CCU, a benefit to the both the patient and the
healthcare system.
[0057] In a subset of ICU/CCU patients, where there is a need to
monitor cardiac output, the addition of a venous oxygen sensor
probe to the previously described multi-sensor arterial probe,
makes it possible for the present invention to estimate the cardiac
output using a modified arteriovenous oxygen concentration
difference equation (the Fick method) as hereinbefore described.
Currently, cardiac output is most frequently monitored using the
thermodilution technique, which requires placement of a Swan-Ganz
catheter in the jugular vein, through the right atrium and right
ventricle, and into a branch of the pulmonary artery. The
thermodilution technique requires injections of cold saline boluses
at intervals, whenever a cardiac output reading is desired. The
replacement of the right heart catheter with the present invention
greatly reduces the risk to the patient by eliminating the right
heart catheterization procedure, and it provides greater utility by
providing on demand cardiac output readings without cumbersome
injections of cold saline.
[0058] In another subset of ICU/CCU patients, where there is a need
to frequently monitor cardiac output but not arterial blood gases,
a simpler apparatus is a single venous oxygen probe used to monitor
the venous oxygen content. This value is combined with independent
measurements of arterial oxygen saturation from a noninvasive pulse
oximeter, hemoglobin density from a daily blood sample, and
calculated oxygen consumption according to the standard
approximation based on weight and height, to calculate cardiac
output according to the Fick method. The probe is placed in a vein
in the hand, using an experimentally determined compensation factor
to account for the expected difference between the oxygen
saturation in the right atrium and the oxygen saturation in a vein
of the hand. Alternatively, the oxygen probe can be inserted
directly through the jugular vein in the neck, into the vena cava
or the right atrium of the heart to provide a direct measurement of
the oxygen saturation of the mixed venous blood without the need
for a compensation factor. Besides its utility for estimating
cardiac output, the venous oxygen content is a valuable parameter
on its own for assessing the status of the patient.
[0059] In neonates, there is frequently the need for arterial and
venous blood gas monitoring, along with the measurement of cardiac
output and other blood parameters. The present invention is
particularly suitable for neonates, since it minimizes if not
eliminates the need for drawing blood from the neonate subject with
a small blood volume to draw from. The addition of hemoglobin,
bilirubin, electrolyte, or glucose sensors to the blood gas and pH
sensors increases the utility of the multi-sensor probe for this
application. The probes are conveniently inserted into umbilical
arteries and veins, and the display module is appropriate in size
to be strapped around the abdomen of a neonate.
[0060] In diagnosing congenital heart defects in neonate and
pediatric patients, there is often a need to sample the oxygen
saturation in a variety of locations throughout the chambers of the
heart and in the great vessels. This oxygen saturation data is
normally collected in conjunction with an angiographic study of the
heart, and it permits the operation of a malformed heart to be more
accurately diagnosed, thereby resulting in more appropriate
treatment for the patient. Currently, oxygen saturation data is
collected by drawing multiple blood samples through a small
catheter from a variety of locations throughout the heart and the
great vessels. These blood samples are sequentially transferred to
a blood gas analyzer to obtain an oxygen saturation reading for
each sample. Using the technology of the present invention, a small
oxygen sensor mounted on a probe or guidewire of suitable size such
as less than 0.023'' in diameter and 50 to 150 cm in length can be
advanced through a guiding catheter to various locations in the
heart and the great vessels to sample the oxygen saturation in
vivo, thereby reducing the risk to the patient by eliminating the
need to draw a large number of blood samples from a small subject
and by reducing the time for the procedure.
[0061] In one aspect of the invention, an apparatus for use with a
patient having a vessel carrying blood to ascertain characteristics
of the blood is provided. The apparatus comprises a display module
and a probe having a distal extremity adapted to be inserted into
the vessel of the patient and having a proximal extremity coupled
to the display module. The probe includes a gas sensor assembly
mounted in the distal extremity for providing an electrical signal
to the display module when the probe is disposed in the blood. The
probe has an antithrombogenic surface treatment for inhibiting the
adhesion of blood components to the probe when disposed in the
blood.
[0062] In another aspect of the invention, a probe for use in a
patient having a vessel carrying blood to ascertain characteristics
of the blood is provided. The probe comprises a cannula adapted to
be inserted into the vessel of the patient and a gas sensor
assembly mounted inside the cannula. The cannula has an
antithrombogenic surface treatment for inhibiting the adhesion of
blood components to the cannula when disposed in the blood.
[0063] In a further aspect of the invention, a probe for use in a
patient having a vessel carrying blood to ascertain characteristics
of the blood is provided. The probe comprises a cannula having
proximal and distal extremities, the distal extremity being adapted
to be inserted into the vessel of the patient. A gas sensor
assembly is mounted inside the distal extremity of the cannula. The
cannula has an annular window of a gas permeable material extending
around the gas sensor assembly.
[0064] Another aspect of the invention provides a probe for use in
a patient having a vessel carrying blood to ascertain
characteristics of the blood. The probe comprises a cannula having
proximal and distal extremities, the distal extremity being adapted
to be inserted into the vessel of the patient. An electrolyte
solution is disposed in the cannula. A gas sensor assembly is
mounted in the distal extremity of the cannula and includes an
electrode disposed in the electrolyte solution. A conductor extends
to the electrode and a sealing glass extends around the conductor.
The conductor has a coefficient of thermal expansion and the
sealing glass has a coefficient of thermal expansion approximating
the coefficient of thermal expansion of the conductor for
inhibiting separation of the conductor from the sealing glass and
thus inhibiting the electrolyte solution from creeping between the
conductor and the sealing glass.
[0065] A further aspect of the invention provides an apparatus for
use with a patient having a vessel carrying blood to ascertain
characteristics of the blood. The apparatus comprises a display
module and a probe, the probe having proximal and distal
extremities. The distal extremity of the probe is adapted to be
inserted into the vessel of the patient and has a gas sensor
assembly for providing an electrical signal when the probe is
disposed in the blood. The display module has a first connector and
the proximal extremity of the probe has a second connector for
mating with the first connector. The second connector has a
cylindrical portion and an electrical contact extending around at
least a portion of the cylindrical portion. A conductor extends
though the probe for electrically coupling the gas sensor assembly
with the electrical contact. The electrical contact is seated flush
with the cylindrical portion so as to provide the second connector
with a substantially smooth cylindrical surface. The first and
second connectors permit connection and disconnection between the
probe and the display module.
[0066] In yet another aspect of the invention, a probe for use with
an introducer in a patient having a vessel carrying blood to
ascertain characteristics of the blood is provided. The probe
comprises a cannula having proximal and distal extremities. The
distal extremity of the cannula is adapted to be inserted into the
vessel of the patient. A gas sensor assembly is disposed in the
distal extremity of the cannula for providing an electrical signal
when the cannula is disposed in the blood. A connector is provided
on the proximal extremity of the cannula. The distal extremity of
the cannula is adapted for slidable travel through the introducer
when inserting the cannula into the vessel. The cannula and
connector have a size which permits the introducer to be slid off
of the proximal extremity of the cannula and the connector after
the distal extremity of the cannula has been inserted into the
vessel.
[0067] An aspect of the invention also provides an apparatus for
use with a patient having a vessel carrying blood to ascertain
characteristics of the blood. The apparatus comprises a compact
display module and a probe, the probe having a proximal extremity
coupled to the display module and a distal extremity adapted to be
inserted into the vessel of the patient. The distal extremity
includes a sensor for providing an electrical signal to the display
module when the probe is disposed in the blood. The probe has
calibration coefficients. The display module has a processor for
processing the electrical signal to provide a reading and a memory
for storing the calibration coefficients. The processor is coupled
to the memory to permit access by the processor to the calibration
coefficients in connection with the processing of the electrical
signal so as to enhance the accuracy of the reading.
[0068] A kit for use with a patient having a vessel carrying blood
to ascertain characteristics of the blood is provided in another
aspect of the invention. The kit comprises a package. A probe is
carried within the package. The probe has a distal extremity
adapted to be inserted into the vessel of the patient and includes
a sensor for providing an electrical signal. The probe has
calibration coefficients. A compact display module is carried
within the package and has a processor and a nonvolatile memory
coupled to the processor. The calibration coefficients are stored
in the memory of the display module. When the probe is coupled to
the display module and the distal extremity inserted into the
vessel and an electrical signal is received by the display module
for providing a reading, the processor accesses the memory so as to
utilize the calibration coefficients and thus enhance the accuracy
of the reading.
[0069] A further aspect of the invention provides a probe for use
in a patient having a vessel carrying blood to ascertain
characteristics of the blood. The probe comprises a cannula adapted
to be inserted into the vessel of the patient and having proximal
and distal extremities. An electrolyte solution is disposed in the
distal extremity of the cannula. A gas sensor assembly is mounted
in the distal extremity of the cannula and is disposed in the
electrolyte solution. The gas sensor assembly has a tube with a
distal portion and a first electrode coiled around the tube. A
second electrode is carried by the distal portion of the tube.
First and second conductors extend from the proximal extremity of
the cannula to the gas sensor assembly, the first conductor being
coupled to the first electrode and the second conductor extending
through the tube and being coupled to the second electrode. The
tube serves as support for the first electrode and as a conduit for
the second conductor.
[0070] A probe for use in a patient having a vessel carrying blood
to ascertain characteristics of the blood is also provided. The
probe comprises a cannula having proximal and distal extremities.
The distal extremity is adapted to be inserted into the vessel of
the patient. A flex circuit extends through at least a portion of
the cannula. The flex circuit has proximal and distal portions with
first and second electrodes formed on the distal portion and first
and second conductors extending from the proximal portion to the
first and second electrodes. An electrolyte solution is disposed in
the distal extremity of the cannula in the vicinity of the first
and second electrodes.
EXAMPLES OF THE INVENTION
[0071] Numerous probes and display modules according to the present
invention have been fabricated and tested to demonstrate the
feasibility and performance of the device.
[0072] The following experimental data illustrates the typical
performance of the invention under experimental conditions.
[0073] Chart 1 shows the performance of a representative example of
an oxygen sensor probe over a range of dissolved oxygen
concentrations from zero to 150 mmHg partial pressure of oxygen.
The response is linear over the range of interest, making the
calibration to 5% accuracy a simple process.
Besides exhibiting accuracy and linearity, the oxygen sensor
provides rapid response to changes in the dissolved oxygen
concentration. Chart 2 shows the time response of a representative
oxygen sensor probe to a sequence of step changes in oxygen partial
pressure, demonstrating a settling time of less than 3 minutes to a
within 5% of the final value. Besides demonstrating accuracy,
linearity, and rapid response, the oxygen sensor provides greater
than 72 hours of longevity to satisfy the requirements of the
ICU/CCU monitoring application. Chart 3 shows the stability of the
oxygen sensor output over the course of a 90-hour longevity study.
With a constant, room air, partial pressure of oxygen of 150 mmHg,
the output of the sensor remains nearly constant for greater than
72 hours except for the expected small variations in output due to
temperature fluctuations and noise.
[0074] Chart 4 shows the performance of a representative example of
a carbon dioxide sensor probe over a range of dissolved carbon
dioxide concentrations from 10 to 100 mmHg partial pressure of
carbon dioxide. The response shows the classic logarithmic
performance expected for this type of pH-responsive sensor, making
calibration to 5% accuracy a simple process.
Besides exhibiting accuracy and linearity, the carbon dioxide
sensor provides rapid response to changes in the dissolved carbon
dioxide concentration. Chart 5 shows the time response of a
representative carbon dioxide sensor probe to a sequence of step
changes in carbon dioxide partial pressure, demonstrating a
settling time of less than three minutes to a within 5% of the
final value.
Besides demonstrating accuracy, linearity, and rapid response, the
carbon dioxide sensor has an inherently long lifetime, since it
does not consume the electrodes or the electrolyte solution as the
oxygen sensor does.
[0075] Chart 6 shows the performance of a representative pH sensor
output over a range of pH from 4 to 10. This pH sensor is mounted
in a multi-sensor probe that also includes oxygen, carbon dioxide,
and temperature sensors. The response shows the classic linear
voltage response to the logarithmic pH parameter. The standard
deviation for repeated measurements at a single pH value is
approximately 0.02 pH, demonstrating that calibration to the
required 0.05 pH accuracy over the physiological range of pH from 7
to 8 is feasible.
The response time of the pH sensor is fast, with a settling time of
approximately 10 seconds to a step change in the pH value.
[0076] This sample data shows that the oxygen, carbon dioxide, and
pH sensors according to the present invention provide the accuracy,
response time, and longevity to meet the needs of the medical
monitoring applications for which it is intended. All of the sample
probes have outside diameters of 0.020'' as described in the
preferred embodiment, and a single probe includes the four oxygen,
carbon dioxide, temperature, and pH sensors. While the invention is
susceptible to various modifications and alternative forms,
specific examples thereof have been shown in the drawings and are
herein described in detail. It should be understood, however, that
the invention is not to be limited to the particular forms or
methods disclosed, but rather the invention is to cover all
modifications, equivalents and alternatives falling within the
spirit and scope of the appended claims.
* * * * *