U.S. patent application number 14/854926 was filed with the patent office on 2018-08-23 for carbon dioxide removal system.
This patent application is currently assigned to MAQUET CARDIOPULMONARY GMBH. The applicant listed for this patent is MAQUET CARDIOPULMONARY GmbH. Invention is credited to Ulrich HAAG, Rudolf KOBER, Oliver MOLLENBERG, Mathias NAKEL, Ralf THOLKE.
Application Number | 20180236158 14/854926 |
Document ID | / |
Family ID | 48444195 |
Filed Date | 2018-08-23 |
United States Patent
Application |
20180236158 |
Kind Code |
A9 |
HAAG; Ulrich ; et
al. |
August 23, 2018 |
CARBON DIOXIDE REMOVAL SYSTEM
Abstract
An extracorporeal blood treatment system including a gas
exchange module operatively associated with a gas supply unit and
optional pump for removing CO.sub.2 from blood. The gas exchange
module includes a plurality of short conduits that are uniquely
configured and arranged in a gas exchange mat to for efficient
CO.sub.2 diffusion under conditions of low blood flow.
Inventors: |
HAAG; Ulrich; (Bisingen,
DE) ; MOLLENBERG; Oliver; (Rastatt, DE) ;
THOLKE; Ralf; (Rastatt, DE) ; NAKEL; Mathias;
(Rastatt, DE) ; KOBER; Rudolf; (Rastatt,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MAQUET CARDIOPULMONARY GmbH |
Rastatt |
|
DE |
|
|
Assignee: |
MAQUET CARDIOPULMONARY GMBH
Rastatt
DE
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20160000989 A1 |
January 7, 2016 |
|
|
Family ID: |
48444195 |
Appl. No.: |
14/854926 |
Filed: |
September 15, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/IB2014/001600 |
Mar 14, 2014 |
|
|
|
14854926 |
|
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|
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61802335 |
Mar 15, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61M 1/1698 20130101;
A61M 2230/208 20130101; A61M 1/3666 20130101; A61M 2230/202
20130101; B01D 2313/90 20130101; B01D 19/0031 20130101; A61M 1/3627
20130101; A61M 2205/3334 20130101; B01D 63/026 20130101; A61M
2230/205 20130101; A61M 2205/3368 20130101; A61M 1/3609
20140204 |
International
Class: |
A61M 1/36 20060101
A61M001/36 |
Foreign Application Data
Date |
Code |
Application Number |
May 16, 2013 |
EP |
13168103.3 |
Claims
1. An extracorporeal blood treatment system comprising: a gas
exchange module configured to provide a passageway for blood and
remove carbon dioxide from the blood as it passes through the gas
exchange module, wherein the gas exchange module comprises a
plurality of conduits forming one or more gas exchange mats,
wherein at least one conduit is configured to provide a passageway
for gas and to allow along a first length of the at least one
conduit diffusion of carbon dioxide from the blood upon exposure of
the blood to an exterior surface of the at least one conduit,
wherein a ratio of the first length to a total thickness of the gas
exchange mats is about 1:1 or less; and wherein the extracorporeal
blood treatment system does not have a heat exchange mechanism.
2. The system of claim 1, wherein a collective average of the first
lengths of the conduits that comprise pores is about 5.8 cm or
less.
3. The system of claim 1, wherein the at least one of the plurality
of conduits has an outer diameter of about 350 .mu.m to about 410
.mu.m, or all of the conduits that comprise pores have an average
outer diameter of about 350 .mu.m to about 410 .mu.m.
4. The system of claim 1, wherein a first length of the at least
one of the conduits that comprise pores is about 76.3% or less than
a full length of the at least one of the conduits that comprise
pores, or an average of the first lengths of all of the conduits
that comprise pores is about 76.3% or less than an average of the
full length of all of the conduits that comprise pores.
5. The system of claim 1, wherein the pores of the conduits that
comprise pores are about 0.2 microns or less.
6. The system of claim 1, wherein the at least one conduit has a
microporous microstructure covered by a thick and impervious
diffusion layer membrane.
7. The system of claim 1, wherein the conduits that comprise pores
are arranged as conduit layers located between a blood inlet and a
blood outlet of the gas exchange module, the blood inlet faces the
conduit layers such that the blood flows towards the conduit layers
in a direction substantially orthogonal to the conduit layers.
8. The system of claim 7, wherein each conduit layer is comprised
of conduits that comprise pores arranged substantially parallel to
one another and knitted or woven together by a separate thread or
thread-like structure.
9. The system of claim 8, wherein the conduits of adjacent layers
are substantially perpendicular to one another.
10. The system of claim 1, wherein the gas exchange module
comprises at least 10,000 conduits that comprise pores.
11. The system of any claim 1, wherein the system further comprises
a pump operatively associated with the gas exchange module for
directing and regulating a flow of the blood to the gas exchange
module, wherein the pump is adapted to deliver the blood to the gas
exchange module at rate between about 0.2 L/min to about 0.8
L/min.
12. The system of claim 1, wherein the gas exchange module
comprises a pressure sensor in direct contact with the blood
exposed to the conduits that comprise pores for measuring blood
pressure as the blood exits the gas exchange module.
13. The system of claim 1, wherein the gas exchange module further
comprises a gas inlet and gas outlet, wherein all of the conduits
of the gas exchange module that comprise pores are in fluid
communication with the gas inlet.
14. The system of claim 1, wherein the system does not include a
heat exchange mechanism.
15. The system of claim 1, wherein the system does not regulate the
temperature of any fluid entering the gas exchange module through
any inlet including the gas inlet.
16. The system of claim 1, wherein the system further comprises a
cannula operatively associated with the gas exchange system,
wherein the cannula has a size of about 20 French (6.7 mm) or
less.
17. An extracorporeal blood treatment system comprising: a gas
exchange module configured to provide a passageway for blood and to
remove carbon dioxide from the blood as it passes through the gas
exchange module, wherein the gas exchange module comprises a
plurality of conduits, at least one conduit is configured to
provide a passageway for gas and to allow for diffusion of carbon
dioxide from the blood through a wall of the at least one conduit
and to the passageway upon exposure of the blood to an exterior
surface of the at least one conduit, wherein the at least one
conduit has a first length available for carbon dioxide diffusion
of about 5.8 centimeters or less; and wherein the extracorporeal
blood treatment system does not have a heat exchanger adapted for
regulating the temperature of the blood.
18. An extracorporeal blood treatment system comprising: a gas
exchange module configured to provide a passageway for blood and
remove carbon dioxide from the blood as it passes through the gas
exchange module, wherein the gas exchange module comprises: a
plurality of conduits, at least one conduit is configured to
provide a passageway for gas and to allow for diffusion of carbon
dioxide from the blood upon exposure of the blood to an exterior
surface of the at least one conduit, wherein the at least one
conduit has a first length available for carbon dioxide diffusion
of about 5.8 centimeters or less; and a gas inlet and gas outlet,
wherein all of the conduits of the gas exchange module are
operatively associated with the gas inlet to permit fluid
communication of the gas though the gas exchange module.
19. A method for using a blood treatment system to remove carbon
dioxide from blood, wherein the blood treatment system is an
extracorporeal blood treatment system according to any one of
claims 1-18, and wherein the method for using the blood treatment
system comprises the steps of: selecting a gas exchange module to
treat an adult human; flowing the blood into the gas exchange
module at a rate of 1 liter per minute or less; and exposing the
blood to a plurality of conduits that comprise pores to remove
carbon dioxide from the blood.
20. The method of claim 19, wherein the blood flows into the gas
exchange module at a rate of 0.51 liters per minute or less.
Description
[0001] This application is a continuation-in-part application
pursuant to 35 U.S.C. 365(c) of International Application No.
PCT/IB2014/001600, which claims benefit of priority to U.S.
Provisional Patent Application No. 61/802,335, filed Mar. 15, 2013,
and European Patent Application No. 13168103.3, filed May 16, 2013.
The disclosures of these applications are herein incorporated by
reference in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention is directed to a carbon dioxide
removal system and methods for use thereof. In particular, the
invention may be useful for treating diseases, syndromes, injuries,
defects or other conditions affecting lung function, including
chronic obstructive pulmonary disease (COPD), chronic and acute
hypercapnia, respiratory acidosis, acute lung injury (ALI), and
acute respiratory distress syndrome (ARDS).
[0004] 2. Description of the Related Technology
[0005] The primary functions of the lung are oxygenation and
elimination of carbon dioxide (CO.sub.2) from blood. Currently,
treatments for respiratory problems are primarily focused on
addressing and enhancing oxygenation. Ventilation, for example, is
the standard of care for COPD, which inhibits expiration of
CO.sub.2, and persistently elevated levels of CO.sub.2 caused by
hypercapnia. Mechanical ventilation, however, is an invasive
therapy, the associated applied pressures of which induce shear
stress, over distention, cyclic stretching, lesions of the
alveolar-capillary membrane and other forms of tissue damage. These
physiological injuries along with the increased intrathoracic
pressure associated with mechanical ventilation further impair
alveolar-capillary permeability, decrease cardiac output and impede
organ perfusion. Furthermore, mechanical ventilation increases the
risk of complications, such as ventilator associated pneumonia
(VAP), can require sedation of the patient.
[0006] Alternative protective ventilation therapies, such as
extracorporeal membrane oxygenation (ECMO), has fewer negative
side-effects than mechanical ventilation. High blood flow is
necessary to drive the low tidal oxygenation and of ECMO therapy.
This large blood flow, however, increases patient risk in the event
of blood leakage and requires the use of large, invasive cannulas
and needles causing patient trauma. Furthermore, ECMO has thus far
only been proven safe and effective for treating select respiratory
diseases.
[0007] Another type of protective ventilation therapy is provided
by a combination oxygenator and CO.sub.2 removal device. This
device is designed for low blood flow resistance and therefore does
not require a pump for arterial venous use. Additionally, the
device utilizes long gas exchange fibers that are adapted for large
mass transfer of gas, which is in efficient for CO.sub.2
removal.
[0008] In light of the above, there exists is a need to develop an
improved respiratory treatment system and therapy that is safe,
relatively non-invasive and that effectively removes CO.sub.2 from
the blood.
SUMMARY OF THE INVENTION
[0009] An extracorporeal blood treatment system according to an
example embodiment of the present invention comprises a gas
exchange module configured to provide a passageway for blood and to
remove carbon dioxide from the blood as the blood passes through
the gas exchange module. The gas exchange module comprises a
plurality of conduits, wherein each conduit comprises an exterior
surface and an interior luminal surface and wherein the interior
luminal surface defines a passageway. At least some of the conduits
comprise pores, wherein upon exposure of the blood to the exterior
surface all of the conduits comprising pores have a first length
that allows for diffusion of carbon dioxide from the blood to the
passageway.
[0010] An extracorporeal blood treatment system according to an
example embodiment of the present invention comprises a gas
exchange module configured to provide a passageway for blood and to
remove carbon dioxide from the blood as the blood passes through
the gas exchange module. The gas exchange module comprises a
plurality of conduits at least partially contained in the gas
exchange module, wherein each conduit comprises an interior luminal
surface defining a passageway and an exterior surface. At least
some of the conduits comprise pores, and all of the conduits that
comprise pores have a first length along the conduits allowing for
the diffusion of carbon dioxide from the blood contained outside
the conduits but inside the gas exchange module, to the passageway
upon exposure of the blood to the exterior surface of the conduits.
The first length of at least one of the conduits that comprise
pores is about 5.8 cm or less.
[0011] An extracorporeal blood treatment system according to an
example embodiment of the present invention comprises a gas
exchange module configured to provide a passageway for blood and to
remove carbon dioxide from the blood as it passes through the gas
exchange module. The gas exchange module comprises a plurality of
conduits at least partially contained in the gas exchange module,
wherein at least one conduit is configured to provide a passageway
for gas and to allow for diffusion of carbon dioxide from the blood
through a wall of the at least one conduit and to the passageway
upon exposure of the blood to an exterior surface of the at least
one conduit and wherein the at least one conduit has a first length
available for carbon dioxide diffusion of about 5.8 centimeters or
less. The extracorporeal blood treatment system does not have a
heat exchanger adapted for regulating the temperature of the
blood.
[0012] An extracorporeal blood treatment system according to an
example embodiment of the present invention comprises a gas
exchange module configured to provide a passageway for blood and
remove carbon dioxide from the blood as the blood passes through
the gas exchange module. The gas exchange module comprises a
plurality of conduits at least partially contained in the gas
exchange module, wherein at least one conduit is configured to
provide a passageway for gas and to allow for diffusion of carbon
dioxide from the blood contained outside the conduits but inside
the gas exchange module to the passageway upon exposure of the
blood to an exterior surface of the at least one conduit and
wherein the at least one conduit has a surface area available for
carbon dioxide diffusion of about 5.42.times.10-5 m.sup.2 to about
7.85.times.10-5 m.sup.2. The extracorporeal blood treatment system
does not have a heat exchange mechanism.
[0013] An extracorporeal blood treatment system according to an
example embodiment of the present invention comprises a gas
exchange module configured to provide a passageway for blood and
remove carbon dioxide from the blood as it passes through the gas
exchange module. The gas exchange module comprises a plurality of
conduits at least partially contained in the gas exchange module
and arranged to form a gas exchange mat, wherein at least one
conduit is configured to provide a passageway for gas and to allow
for diffusion of carbon dioxide from the blood upon exposure of the
blood to an exterior surface of the at least one conduit and
wherein a ratio of a first length of the at least one conduit to a
total thickness of the gas exchange mat is about 1:1 or less. The
extracorporeal blood treatment system does not have a heat transfer
mechanism.
[0014] An extracorporeal blood treatment system according to an
example embodiment of the present invention comprises a gas
exchange module configured to provide a passageway for blood and
remove carbon dioxide from the blood as it passes through the gas
exchange module. The gas exchange module includes a plurality of
conduits forming one or more gas exchange mats, wherein at least
one conduit is configured to provide a passageway for gas and to
allow along a first length of the at least one conduit diffusion of
carbon dioxide from the blood upon exposure of the blood to an
exterior surface of the at least one conduit. A ratio of the first
length to a total thickness of the gas exchange mats is about 3:1
or less, about 2:1 or less and about 1:1 or less.
[0015] According to an example embodiment, a collective average of
the first lengths of the conduits that comprise pores is about 5.8
cm or less.
[0016] According to an example embodiment, at least one of the
plurality of conduits has an outer diameter of about 350 .mu.m to
about 410 .mu.m.
[0017] According to an example embodiment, all of the conduits that
comprise pores have an average outer diameter of about 350 .mu.m to
about 410 .mu.m.
[0018] According to an example embodiment, a first length of at
least one of the conduits that comprise pores is about 76.3% or
less than a full length of the at least one of the conduits that
comprise pores.
[0019] According to an example embodiment, an average of the first
length of all of the conduits that comprise pores is about 76.3% or
less than an average of the full length of all of the conduits that
comprise pores.
[0020] According to an example embodiment, an exposed surface area
of the at least one conduit is about 5.71.times.10-5 m.sup.2 to
about 7.47.times.10-5 m.sup.2.
[0021] According to an example embodiment, the pores of the
conduits that comprise pores are about 0.2 microns or less.
[0022] According to an example embodiment, the at least one conduit
is constructed from polymetheylpentene.
[0023] According to an example embodiment, all of the conduits that
comprise pores are constructed from polymetheylpentene.
[0024] According to an example embodiment, the at least one conduit
has a microporous microstructure covered by a thick and impervious
diffusion layer membrane.
[0025] According to an example embodiment, the conduits that
comprise pores are arranged in a crisscrossing pattern.
[0026] According to an example embodiment, the conduits that
comprise pores are arranged to form conduit layers located between
a blood inlet and a blood outlet of the gas exchange module, the
blood inlet faces the conduit layers such that the blood flows
towards the conduit layers in a direction substantially orthogonal
to the conduit layers.
[0027] According to an example embodiment, each conduit layer is
comprised of two or more of the conduits that comprise pores
arranged substantially parallel to one another.
[0028] According to an example embodiment, the two or more of the
conduits that comprise pores and are arranged substantially
parallel to one another are knitted or woven together by a separate
thread or thread-like structure.
[0029] According to an example embodiment, the conduits of adjacent
conduit layers are oriented substantially perpendicular to one
another.
[0030] According to an example embodiment, the gas exchange module
comprises at least about 10,000 conduits that comprise pores, at
least about 12,000 conduits that comprise pores, at least about
13,000 conduits that comprise pores, or at least about 13,119
conduits that comprise pores.
[0031] According to an example embodiment, a combined surface area
available for carbon dioxide diffusion of all conduits that
comprise pores is about 0.98 m.sup.2 or more.
[0032] According to an example embodiment, a combined surface area
available for carbon dioxide diffusion of all conduits that
comprise pores is about 0.92 m.sup.2 or more, about 0.95 m.sup.2 or
more, about 0.98 m.sup.2 or more.
[0033] According to an example embodiment, the system further
comprises a pump operatively associated with the gas exchange
module for directing and regulating a flow of the blood to the gas
exchange module, wherein the pump is adapted to deliver the blood
to the gas exchange module at rate of about 1 L/min or less.
[0034] According to an example embodiment, the system further
comprises a pump operatively associated with the gas exchange
module for directing and regulating a flow of the blood to the gas
exchange module, wherein the pump is adapted to deliver the blood
to the gas exchange module at rate between about 0.2 L/min to about
0.8 L/min.
[0035] According to an example embodiment, the gas exchange module
comprises a pressure sensor positioned adjacent to the blood outlet
and in direct contact with the blood exposed to the conduits that
comprise pores for measuring blood pressure as the blood exits the
gas exchange module.
[0036] According to an example embodiment, the gas exchange module
further comprises a gas inlet and gas outlet, wherein all of the
conduits of the gas exchange module that comprise pores are in
fluid communication with the gas inlet.
[0037] According to an example embodiment, the system does not
include a heat exchange mechanism.
[0038] According to an example embodiment, the system does not
regulate the temperature of any fluid entering or leaving the gas
exchange module.
[0039] According to an example embodiment, the system further
comprises a cannula operatively associated with the gas exchange
module, wherein the cannula has a size of about 21 French (7 mm) or
less, about 19 French (6.33 mm) or less, about 16 French (5.33 mm)
or less, or about 13 French (4.33 mm) or less.
[0040] According to an example embodiment, the cannula is a
double-lumen cannula.
[0041] According to an example embodiment, a combined volume of the
conduits of the gas exchange module that comprise pores is about
0.085 liters to about 0.100 liters.
[0042] According to an example embodiment, the plurality of
conduits form one or more gas exchange mats, and the ratio of the
first length of the at least one conduit to a total thickness of
the gas exchange mat is about 3.0 or less.
[0043] According to an example embodiment, the system is configured
for carbon dioxide removal from the blood with at most only nominal
diffusion of oxygen to the blood.
[0044] A method for removing carbon dioxide from blood according to
an example embodiment of the present invention uses a blood
treatment system comprising a gas exchange module configured to
provide a passageway for blood and remove carbon dioxide from the
blood as it passes through the gas exchange module. The gas
exchange module comprises a plurality of conduits at least
partially contained in the gas exchange module, wherein at least
one conduit is configured to provide a passageway for gas and to
allow for diffusion of carbon dioxide from the blood to pass to the
passageway upon exposure of the blood to an exterior surface of the
at least one conduit and wherein at least one conduit has a first
length available for carbon dioxide diffusion of about 5.8 cm or
less. The method for using the blood treatment system comprises:
selecting a gas exchange module to treat a human including an adult
human; flowing the blood into the gas exchange module at a rate of
1 liter per minute or less; and exposing the blood to a plurality
of conduits that comprise pores to remove carbon dioxide from the
blood.
[0045] According to an example embodiment, the method involves
flowing blood into the gas exchange module at a rate of about 0.51
liters per minute or less or between about 0.4 liters per minute to
about 0.51 liters per minute.
[0046] According to an example embodiment, the method involves
flowing gas through the conduits at a rate of 0.2 liters per minute
to 15 liters per minute.
[0047] According to an example embodiment, the method involves
flowing gas through the conduits at a rate of more than about 15
liters per minute.
[0048] According to an example embodiment, the method involves
selecting a gas that has a partial pressure of carbon dioxide that
is zero or at least lower than a partial pressure of carbon dioxide
of the blood flowing into the gas exchange module.
[0049] According to an example embodiment, the method involves
treating the blood without regulating blood temperature.
[0050] According to an example embodiment, wherein throughout the
length of the at least one conduit, there exists a carbon dioxide
gradient between a gas flowing through the at least one conduit and
the blood exposed to the exterior surface of the at least one
conduit.
[0051] According to an example embodiment, wherein the carbon
dioxide gradient is substantially constant along the length of the
at least one conduit.
[0052] According to an example embodiment, the method involves
after said step of exposing, measuring blood pressure using a
sensor of the gas exchange module in direct contact with the blood
exposed to the conduits after exposing the blood to the plurality
of conduits.
[0053] According to an example embodiment, the method involves
measuring the amount of carbon dioxide removed from the blood using
a sensor of the gas exchange module.
[0054] According to an example embodiment, the conduits are
arranged in layers and are located between a blood inlet and a
blood outlet of the gas exchange module, and wherein the blood
flows towards the conduits in a direction substantially orthogonal
to the length of the conduits.
[0055] According to an example embodiment, the method involves
obtaining from a venous blood source the blood delivered to the gas
exchange module and treating the blood such that a partial pressure
of carbon dioxide in the blood after exposure to the conduits is
about 50 mm Hg to about 70 mm Hg.
[0056] According to an example embodiment, the method involves
obtaining from a venous blood source the blood delivered to the gas
exchange module and treating the blood such that a pH value of the
blood exposed to the conduits is about 7.25 to about 7.35.
[0057] According to an example embodiment, the method involves
extracting from a venous circulatory system the blood delivered to
the gas exchange module and returning the blood treated by the gas
exchange module to the venous circulatory system.
[0058] According to an example embodiment, the blood is treated by
the gas exchange module for a period of about 6 hours to about 30
days.
[0059] According to an example embodiment, the method involves
using the blood treatment system to remediate a respiratory
condition in the adult human selected from the group consisting of
chronic obstructive pulmonary disease, acute lung injury, acute
respiratory distress syndrome and hypercapnia.
[0060] According to an example embodiment, the method involves
treating the blood by removing carbon dioxide from the blood with
no or at most only nominal diffusion of oxygen.
[0061] According to an example embodiment, the method for removing
carbon dioxide is performed using any of the above described
example blood treatment system embodiments.
[0062] An extracorporeal blood treatment system according to an
example embodiment of the present invention comprises a gas
exchange module configured to provide a passageway for blood and
remove carbon dioxide from the blood as it passes through the gas
exchange module. The gas exchange module comprises a plurality of
conduits at least partially contained in the gas exchange module,
wherein at least one conduit is configured to provide a passageway
for gas and to allow for diffusion of carbon dioxide from the blood
upon exposure of the blood to an exterior surface of the at least
one conduit and wherein the at least one conduit has a length
available for carbon dioxide diffusion of about 5.8 centimeters or
less. The blood treatment system further includes a gas inlet and
gas outlet, wherein all of the conduits of the gas exchange module
are operatively associated with the gas inlet to permit fluid
communication of the gas though the gas exchange module.
BRIEF DESCRIPTION OF THE DRAWINGS
[0063] FIG. 1 is a diagram showing an exemplary blood treatment
system attached to a patient's jugular vein using a dual lumen
catheter and including a gas exchange module.
[0064] FIG. 2 is a diagram showing an exemplary blood treatment
system attached to a patient's jugular vein using a dual lumen
catheter and including a gas exchange module operatively associated
with a gas supply unit and a pump.
[0065] FIG. 3(a) is a perspective view of an exemplary gas exchange
module of the blood treatment system.
[0066] FIG. 3(b) is a perspective view showing the internal housing
components of the gas exchange module of FIG. 3(a), including a
single blood treatment chamber without the gas exchange
conduits.
[0067] FIG. 3(c) is a perspective view of another embodiment of the
gas exchange module of FIG. 3(b) showing a frame dividing the blood
treatment chamber into two compartments, each adapted for receiving
a gas exchange mat.
[0068] FIG. 3(d) is a front view of the gas exchange module of FIG.
3(a).
[0069] FIG. 3(e) is a cross-sectional view of the gas exchange
module of FIG. 3(d) taken at line A-A, showing a single, empty
blood treatment chamber.
[0070] FIG. 3(f) is a cross-sectional view of the gas exchange
module of FIG. 3(d) taken at line A-A, showing a blood treatment
chamber with a gas exchange mat situated within the blood treatment
chamber and illustrating the flow of gas through the gas exchange
module.
[0071] FIG. 3(g) is a cross-sectional view of another embodiment of
the gas exchange module of FIG. 3(e) corresponding to the
embodiment of FIG. 3(c), showing a frame dividing the blood
treatment chamber into two compartments that fluidly communicate
with one another as best shown in FIG. 3(c), each compartment
containing a gas exchange mat.
[0072] FIG. 3(h) is an overhead view of the blood treatment system
of FIG. 3(a).
[0073] FIG. 3(i) is a cross-sectional view of the gas exchange
module of FIG. 3(h) at line B-B illustrating the flow of gas
through the gas exchange module.
[0074] FIG. 3(j) is a cross-sectional view of the gas exchange
module of FIG. 3(i) at line D-D.
[0075] FIG. 3(k) is a two dimensional schematic diagram of two
adjoining conduit layers of the gas exchange mat showing the
perpendicular orientation of the conduit layers.
[0076] FIG. 3(l) is a three dimensional diagram of a portion of a
conduit layer showing a plurality of parallel conduits.
[0077] FIG. 3(m) is a two dimensional schematic diagram of two
adjoining conduit layers of the gas exchange mat showing the
relative perpendicular orientation of the conduit layers.
[0078] FIG. 3(n) is a cross-sectional view of the gas exchange
module of FIG. 3(h) at line C-C.
[0079] FIG. 4 shows an exemplary blood treatment system attached to
a patient's jugular and femoral veins using two small single lumen
catheters and including a gas exchange module and integral pump
operatively associated with a gas supply unit.
[0080] FIG. 5 shows a flow chart of describing an exemplary blood
treatment method of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0081] For illustrative purposes, the principles of the present
invention are described by referencing various exemplary
embodiments. Although certain embodiments of the invention are
specifically described herein, one of ordinary skill in the art
will readily recognize that the same principles are equally
applicable to, and can be employed in other systems and methods.
Before explaining the disclosed embodiments of the present
invention in detail, it is to be understood that the invention is
not limited in its application to the details of any particular
embodiment shown. Additionally, the terminology used herein is for
the purpose of description and not of limitation. Furthermore,
although certain methods are described with reference to steps that
are presented herein in a certain order, in many instances, these
steps may be performed in any order as may be appreciated by one
skilled in the art; the novel method is therefore not limited to
the particular arrangement of steps disclosed herein.
[0082] It must be noted that as used herein and in the appended
claims, the singular forms "a", "an", and "the" include plural
references unless the context clearly dictates otherwise. Thus, for
example, reference to "a conduit" may include a plurality of
conduits and equivalents thereof known to those skilled in the art,
and so forth. As well, the terms "a" (or "an"), "one or more" and
"at least one" can be used interchangeably herein. It is also to be
noted that the terms "comprising," "including," "composed of," and
"having" can be used interchangeably.
[0083] For purposes of the present invention, the "active length"
or "active portion" of a conduit refers to the collective lengths
or portions of a conduit having a surface area that allows for
passage of gas through the conduit, particularly CO.sub.2
diffusion. For example, the active length or active portion may be
the total lengths or portions of a conduit membrane having pores
that are at least substantially unimpeded and allow for gas
exchange through the conduit via the pores.
[0084] As used herein the "inactive length" or "inactive portion"
of a conduit refers to the collective lengths or portions of the
conduit incapable of passage of gas through the conduit,
particularly incapable of CO.sub.2 diffusion. For example, the
inactive length or inactive portion may be the total lengths or
portions of a conduit potted within a matrix such that any pores of
the potted length or portion are blocked or otherwise prevented
from the transfer of gas through the conduit wall.
[0085] As used herein, "non-physiological values" of the partial
pressure of CO.sub.2 in blood or of blood pH refers to values of
CO.sub.2 partial pressure or blood pH that are not within the
standard accepted physiological range. For example, for blood taken
from the arterial system, normal physiological values of pCO.sub.2
of the blood typically may be about 32-46 mm Hg and normal value of
pH may be about 7.45; for blood taken from the venous system,
normal physiological values of pCO.sub.2 of the blood typically may
be about 38-54 mm Hg and normal values of pH may be about 7.35.
[0086] As used herein, "property of blood" refers to a
physiological characteristic or component of blood. Exemplary
properties include temperature, composition and partial pressure or
CO.sub.2 content.
[0087] Furthermore, "treating" as used herein refers to improving,
alleviating or remedying a disease, syndrome, injury and defect,
other condition, or an associated symptom thereof.
[0088] The present invention is directed to a novel extracorporeal
blood treatment system and therapeutic method for efficiently,
effectively and safely removing CO.sub.2 from a patient's blood
stream in a minimally invasive manner. In an exemplary embodiment,
the invention is adapted to directly access a patient's vascular
system, the extracorporeal blood treatment system is specifically
designed to remove substantially all the CO.sub.2from a flow of a
patient's blood passing through a gas exchange module of the system
at a low flow rate in a single pass. The invention may be used for
various applications, including treating respiratory conditions,
such as COPD, chronic and acute hypercapnia, respiratory acidosis,
acute lung injury, acute respiratory distress syndrome and
hypercapnia, by substantially eliminating CO.sub.2 from blood
circulating in a patient.
Blood Treatment System
[0089] FIGS. 1-2 illustrate exemplary embodiments of the
extracorporeal blood treatment system 1 of the present invention,
which include a gas exchange module 10 having a plurality of
conduits 30, at least some or all of which are configured to alter
a property of blood flowing through gas exchange module 10. In
particular, gas exchange module 10 includes a plurality of short
conduits 30 each having a gas permeable membrane, wherein conduits
30 are uniquely configured and arranged in one or more gas exchange
mats 34 for efficient gas diffusion, such as efficient CO.sub.2
diffusion. In an exemplary embodiment, gas exchange module 10 is
configured as a gas transfer device having a plurality of gas
permeable conduits 30 specifically designed and adapted for
CO.sub.2 diffusion. Blood treatment system 1 may optionally further
include a gas supply unit 50 that delivers a stream of gas through
the lumens of conduits 30 while blood passes through gas exchange
module 10 contacting and flowing past an exterior surface of
conduits 30 at a low flow rate. Gas diffusion, specifically
CO.sub.2 diffusion, from the blood and through the gas permeable
membrane of conduits 30 is driven by the difference in gas partial
pressure, e.g. CO.sub.2 partial pressure, between the gas flowing
through conduits 30 and the gas partial pressure, e.g. CO.sub.2
partial pressure, of the patient's blood exposed to and flowing
around conduits 30. Gas supply unit 50 supplies gas at a high
velocity gas flow rate through conduits 30 to maximize and maintain
the driving force of gas diffusion, e.g. CO.sub.2 diffusion, along
the length active length of conduit 30. Blood treatment system 1
may optionally further include a pump 60 and an integral or
otherwise operatively associated control unit 62 for regulating the
flow of blood through gas exchange module 10. In an exemplary
embodiment, extracorporeal blood treatment system 1 is not designed
to oxygenate the patient's blood and/or does not include a heat
exchanger for heating or cooling blood delivered to, flowing
through or exiting from the gas exchange module 10 or otherwise
seek to change or regulate blood temperature.
[0090] FIGS. 3(a)-3(m) show an exemplary gas exchange module 10
having a housing 12 defining an internal cavity 13 through which
blood is flowed and for at least partially containing a plurality
of conduits 30 adapted for gas diffusion, in particular CO.sub.2
diffusion. As shown in FIGS. 3(a)-3(b) and 3(d)-3(f), gas exchange
module 10 includes a blood inlet port 14 and blood outlet port 16
spaced apart from one another and located on opposing faces of
housing 12. A plurality of gas exchange conduits 30 are arranged
between blood inlet and outlet ports 14, 16 such that blood
entering blood inlet port 14 flows towards conduits 30 in a
direction substantially orthogonal to the length of one or more, or
all of conduits 30. The opening and length of elongated blood inlet
and/or outlet ports 14, 16 may also be oriented substantially
orthogonal to the length of one or more conduits 30. Gas exchange
module 10 further includes a gas inlet port 18 and gas outlet port
20 for delivering gas to and from the plurality of conduits 30. Gas
inlet and outlet ports 18, 20 are spaced apart from one another and
may be aligned in the same plane as conduits 30 and oriented
substantially orthogonal to blood inlet and outlet ports 14, 16
such that gas flowing through gas exchange module 10 is
substantially orthogonal to blood flow through gas exchange module
10.
[0091] Conduits 30 may be configured as hollow, thin fibers or
other tubules with a central lumen for gas passage, best shown in
FIGS. 3(k)-3(m). These lumens provide a passageway through which
gas is transported to induce CO.sub.2 diffusion from blood
contacting an exterior surface of conduit 30, through the conduit
walls and into the conduit lumens. Conduits 30 may be configured to
have a gas permeable membrane, such as a porous membrane including
a plurality of pores adapted for gas diffusion, particularly
CO.sub.2 diffusion. In one embodiment, conduits 30 may have a
microporous membrane, such as the microporous polypropylene hollow
fiber manufactured by Polypore and marketed under the trade name
OXYPAHAN having a maximum 2 micron pore size, or alternatively a
diffusive membrane, such as the diffusive polymethylpentene hollow
fiber membrane having a 55% porosity manufactured by Polypore and
marketed under the trade name OXYPLUS. Conduits 30 may have the
same or different degree of porosity and/or pore size. In one
embodiment, the gas permeable membrane of conduit 30 may have a
pore size or diameter of about 0.2 microns or smaller, which may be
construed as a maximum pore size of about 0.2 microns. In one
embodiment, the gas permeable membrane of conduits 30 may be
configured to only permit gas passage, specifically CO.sub.2
diffusion, inhibiting diffusion of liquids or solids. The gas
permeable membrane may also be configured to inhibit blood plasma
leakage. In one embodiment, conduits 30 may be configured to allow
for CO.sub.2 diffusion while preventing blood plasma leakage for up
to at least 30 days under normal operating pressures and flow
rates. Conduits 30 may be constructed from any gas permeable
material that optionally also inhibits blood plasma leakage. In an
exemplary embodiment, polymetheylpentene may be used to construct
conduits 30.
[0092] While gas exchange module 10 may include other types of
conduits different than gas exchange conduits 30, such as conduits
which do not affect a property of blood, non-porous conduits which
affect the property of blood, gas impermeable conduits which affect
the property of blood, and/or porous conduits which allow for
diffusion of gases other than CO.sub.2, in one embodiment all the
conduits of gas exchange module 10, inclusive of all the gas
exchange conduits 30, are adapted for gas diffusion, such as
CO.sub.2 diffusion. In another embodiment, all the conduits of gas
exchange module 10 that are configured to alter a property of
blood, inclusive of gas exchange conduits 30, may be gas permeable
and/or have a microporous membrane with pores adapted for gas
diffusion, such as CO.sub.2 diffusion.
[0093] Efficient removal of CO.sub.2 using gas exchange module 10
under low blood flow rate conditions is achieved by the uniquely
configured conduits 30 and/or the arrangement of a plurality of
these conduits 30 to form one or more gas exchange mats 34 having a
sufficient collective thickness to effectively diffuse CO.sub.2from
blood. In an exemplary embodiment, gas exchange conduits 30 may
have a short length that allows for decreased fluidic resistance of
the gas, such as a high velocity stream of gas, flowing through the
lumen, and therefore minimizes any pressure drop within and across
conduit 30. Consequently, the low back pressure conditions within
conduit 30 inhibits formation of potentially dangerous microbubbles
on the exterior blood contacting surface of conduit 30 thereby
preventing formation of an emboli in the blood. In one embodiment,
conduit 30 may have a sufficiently short length that substantially
prevents formation of microbubbles on an exterior of conduit 30
and/or a drop in gas flow pressures within and along the length of
conduit 30 as gas is flowed through the conduit lumen at a
predetermined, constant gas flow rate. In an exemplary embodiment,
conduits 30 have a full length, illustrated as dimension X in FIGS.
3(i) and 3(j), of about 71 cm to about 81, about 71 cm to about 76
cm, or about 76 cm to about 81 cm.
[0094] Each conduit 30 has an elongated body including a proximal
end 36 and distal end 38. As will be described in further detail
below, when conduits 30, are positioned within, potted in, affixed
to, attached to or otherwise disposed within a blood treatment
chamber 24 located in the internal cavity 13 of housing 12,
portions of conduit 30, particularly proximal and distal ends 36,
38, may be rendered incapable of gas transfer by virtue of the
manner in which conduit 30 is attached to blood treatment chamber
walls 26. This inactive portion or inactive length of a conduit 30,
shown in FIG. 3(i) by the collective dimensions W of the total
length X of one conduit 30, have pores that may be blocked and
otherwise prevented from allowing localized gas diffusion,
specifically CO.sub.2 diffusion. In the illustrated embodiment, the
inactive length of a conduit 30 is the sum of the two W dimensions
of a total length X of one conduit 30. The remaining active portion
or active length of conduit 30, shown in FIG. 3(i) as dimension Y,
may allow for gas diffusion, in particular CO.sub.2 diffusion. In
the illustrated embodiment, one Y reference denotes the active
length of the conduits 30 of a first conduit layer 32a (labeled as
32), and the other Y reference denotes the active length of the
conduits 30 of an adjoining perpendicularly oriented second conduit
layer 32b (not labeled) beneath conduit layer 32a. In an exemplary
embodiment, the active length of the conduit 30 available for
CO.sub.2 diffusion may be about 5 cm to about 6 cm, about 5.2 cm to
about 5.8 cm, about 5.2 cm to about 5.5 cm, or about 5.5 cm to
about 5.8 cm. The percent of the active length of conduit 30 to the
overall length of conduit 30 may be about 76.3% or less, about 40%
to about 76.3%, about 68.4% to about 76.3%, or about 68.4% to about
72.4%. In another embodiment, the ratio of conduit active length to
the total length of conduit 30 is about 0.724:1.+-.5%, about 0.79:1
or less, about 0.77:1 or less. The ratio of conduit active length
to conduit inactive length may be about 2.12: 1 to about 3.22:1,
about 2.12:1 to about 2.62:1, or about 2.62:1 to about 3.22:1. The
available active surface area for CO.sub.2 diffusion of a single
conduit 30 may be 5.42.times.10.sup.-5 m.sup.2, about
5.42.times.10.sup.-5 m.sup.2 to about 7.85.times.10.sup.-5 m.sup.2,
about 5.60.times.10.sup.-5 m.sup.2to about 7.85.times.10.sup.-5
m.sup.2, about 5.71.times.10.sup.-5 m.sup.2 to about
7.47.times.10.sup.-5 m.sup.2, or about 5.71.times.10.sup.-5
m.sup.2to about 7.01.times.10.sup.-5 m.sup.2. In an exemplary
embodiment, the active surface area of a single conduit 30 may be
about 5.71.times.10.sup.-5 m.sup.2.+-.5% to about
7.47.times.10.sup.-5 m.sup.2.+-.5%. The outer diameter of conduit
30 may be about 350 um to about 410 um. Additionally, the volume of
conduit 30 may be about 0.085 L to about 0.100 L.
[0095] As best shown in FIGS. 3(k)-3(m), conduits 30 are arranged
in a set and positioned substantially parallel to one another,
wherein conduits 30 are bound to, attached to or otherwise
connected to one another in order to form a thin conduit layer 32.
Conduit layer 32 may be constructed from conduits 30 of the same or
different configuration and/or dimensions, such as length and
diameter. Conduits 30 and conduit layer 32 form a plurality of
passages for gas to pass from one side of the gas exchange module
10 to an opposite side of gas exchange module 10. Additionally,
conduit layer 32 is configured to allow for blood to pass between
and around adjacent conduits 30. In one embodiment, the conduits 30
within a conduit layer 32 are knitted together with a filament,
such as thread, yarn or other suitable material, so as not to
substantially or at most only minimally impede and interfere with
gas diffusion. This is best illustrated in FIG. 3(l), wherein
conduits 30 are arranged in a conduit layer 32 and fixed to one
another by intermittently knitting one or more filaments along the
length of conduit layer 32 to connect adjoining conduits 30.
[0096] In an exemplary embodiment, a plurality of conduit layers 32
are stacked on top of one another and oriented parallel to one
another in order to form a gas exchange mat 34, as shown in FIGS.
3(k) and 3(m). Gas exchange mat 34 may be constructed from conduit
layers 32 of the same or different dimensions and/or configuration.
The length of conduits 30 of two adjoining conduit layers 32 are
oriented in the same plane and offset from one another. In one
embodiment, the two adjoining conduit layers 32 are oriented
substantially perpendicular to one another such that the conduit
length of and direction of gas passing through a first conduit
layer 32a is substantially perpendicular to the conduit length of
and direction of gas passing through an adjoining second conduit
layer 32b. In one embodiment, the conduits 30 of two adjoining
conduit layers 32a, 32b are oriented substantially about 45.degree.
to about 135.degree., about 65.degree. to about 115.degree., about
75.degree. to about 105.degree., or about 85.degree. to about
95.degree. out of phase with and relative to one another. By way of
example, there may be about 120 to about 160, about 130 to about
155, about 135 to about 153, or about 140 to about 148 conduits
layers 32 in gas exchange mat 34. In another embodiment, there may
be about 95 to about 115, about 100 to about 108, or about 102 to
about 106 conduits layers 32 in gas exchange mat 34. In one
embodiment, there may be about 13,119 to about 11,712 conduits 30
in gas exchange mat 34. The resultant gas exchange mat 34 can have
any configuration fitted to and/or positionable within blood
treatment chamber 24, such as a cuboid or cylinder. This layered
arrangement of conduit layers 32 creates a dense network of gas
exchange conduits 30 designed to maximize the available surface
area for gas transfer and thereby enhance CO.sub.2 diffusion
efficiency, while still allowing for sufficient flow of blood
between the blood inlet and outlet ports 14,16.
[0097] Gas exchange module 10 may include one or more gas exchange
mats 34. In one embodiment, gas exchange module 10 may have a
single gas exchange mat 34. In another embodiment, as best shown in
FIG. 3(g), two adjacent gas exchange mats 34a, 34b, each composed
of a plurality of stacked conduit layers 32, may be potted within
blood treatment component 24. These gas exchange mats 34a, 34b may
adjoin and be positioned in a stacked orientation such that blood
passing through blood treatment chamber 24 flows in a direction
substantially orthogonal to both gas exchange mats 34. As
illustrated in the exemplary embodiment of FIGS. 3(c) and 3(g),
first and second gas exchange mats 34a, 34b may be spaced apart
from one another by a frame 28 having a plurality of openings to
allow blood to pass from first gas exchange mat 34a to second gas
exchange mat 34b with no to minimal impedance.
[0098] To further improve CO.sub.2 diffusion efficiency, the
collective thickness of one or more gas exchange mats 34 is may be
sufficient to effectively remove in a single pass through the one
or more gas exchange mat 34 and/or in a single pass through gas
exchange module 10 substantially all the CO.sub.2 from the
patient's blood that is passed therethrough. A suitable total
thickness of the adjoining one or more gas exchange mats 34,
identified in the exemplary embodiment of FIG. 3(j) as dimension Z,
may be described in terms of the length of conduits 30. In one
embodiment, the ratio of the active length of conduit 30 to the
total thickness of one or more gas exchange mats 34 of gas exchange
module 10 may be about 3:1 to about 0.5:1, about 2:1 to about
0.8:1, about 2:1 to about 0.9:1, or about 1:1.1 to about 0.9:1. In
another embodiment, the ratio of the active length of conduit 30 to
the thickness of gas exchange mat 34 is about 3.5:1 or less; about
3:1 or less, about 2:1 or less, about 1:1 or less or about 1.1:1 or
less. In one embodiment, a ratio of about 1:1 suggests that the
blood flow path and gas flow path are designed to allow for maximum
exposure, processing and filtration of the blood by conduits 30 and
to facilitate CO.sub.2 diffusion by reducing the relative
differences in blood flow and gas flow resistance. In another
embodiment, the aforementioned ratio values may also represent the
ratio of the active length of conduits 30 to the shortest path of
blood flow through the blood treatment chamber. In an exemplary
embodiment, the total thickness of the one or more gas exchange mat
34 may have a thickness of about 54.7 mm. In one embodiment, the
overall available gas exchange surface area of the gas exchange mat
34 is about 0.5 m.sup.2 to about 1.3 m.sup.2, 0.5 m.sup.2 to about
1.2 m.sup.2, about 0.5 m.sup.2 to 0.98 m.sup.2, or 0.98 m.sup.2 to
1.3 m.sup.2. The gas exchange mat 34 may include at least about
10,000, at least about 12,000 conduits, at least about 13,000
conduits, at least about 13,119 conduits, or at least about 13,300
conduits. Alternatively or additionally, the gas exchange mat 34
may have at least 13,300 conduits per square meter of gas exchange
surface area.
[0099] As shown in FIGS. 3(f) and 3(i)-3(j), gas exchange mats 34
are potted within, disposed within, affixed to or otherwise
attached to one or more blood treatment chamber 24 positioned
within housing internal cavity 13. Blood treatment chamber 24,
which is in fluid communication with and connects blood inlet port
14 and blood outlet port 16, is designed to process the blood so as
to filter CO.sub.2 from the blood circulated through blood
treatment chamber 24 by exposure to the gas exchange surface area
of one or more gas exchange mats 34, e.g. unpotted surface area of
gas exchange mats 34 capable of CO.sub.2 diffusion. One or more gas
exchange mats 34 may be potted using any suitable material, such as
an epoxy resin, within blood treatment chamber 24 so that opposing
proximal and distal ends of each conduit layer 32 and the proximal
and distal ends 36, 38 of their respective conduits 30 extend
across blood treatment chamber 24 and through the walls 26 of blood
treatment chamber 24, such that the blood treatment chamber walls
26 form a liquid impermeable and sealed perimeter of the blood
treatment chamber 24. The proximal and distal ends 36, 38 of
conduits 30 in each conduit layer 32 of gas exchange mat 34 extend
outwardly beyond the potted portions of gas exchange mats 34 and
blood treatment chamber 24, so as to be in fluid communication with
and open to a space exterior to blood treatment chamber 24, namely
gas passageways 41a, 41b. The lumens of conduits 30 are therefore
in fluid communication with gas passageways 41a, 41b as well as gas
inlet and outlet ports 18, 20 as described in further detail
below.
[0100] FIGS. 3(e)-3(f) and 3(i) show a plurality of gas passageways
41a, 41b, each having two interconnected first and second sections
42a, 42b and 42c, 42d, respectively. As shown, gas passageways 41a,
41b may be configured as channels positioned within the housing
internal cavity 13 around and along the perimeter of blood
treatment chamber 24. As previously described, gas passageways 41a,
41b are in fluid communication with conduits 30 for delivering gas
to and receiving gas from conduits 30. In the embodiment
illustrated in FIG. 3(i), each section 42a, 42b, 42c, 42d,
configured as compartments of gas passageways 41a, 41b, is defined
by a corresponding housing sidewall 22a, 22b, 22c, 22d of housing
12 and a corresponding opposing blood treatment chamber wall 26a,
26b, 26c, 26d spaced apart relative to one another to form the gas
passageways 41a, 41b. The length of each section 42a, 42b, 42c, 42d
is oriented in the same plane as and is substantially perpendicular
to a length of conduits 30 in fluid communication with the
respective sections 42a, 42b, 42c, 42d. In one embodiment, all of
conduits 30 are in fluid communication with gas inlet port 18
and/or gas outlet port 20 via a gas passageway 41a, 41b. Gas inlet
port 18 may be positioned between and connected to first and second
interconnected sections 42a, 42b of gas passageway 41a and is
defined by respective adjoining housing sidewalls 22a, 22b and
blood treatment chamber walls 26a, 26b, forming a forked gas
passage. Upon entering gas inlet port 18, gas travels through one
of the two diverging sections 42a, 42b of gas passageway 41a and
through the proximal ends 36 of conduits 30 of alternating conduit
layers 32. For example, gas flowing through section 42a passes
through a plurality of the alternating conduit layers (i.e. conduit
layer 32a) in a first direction while gas flowing through section
42b passes through adjoining intervening conduit layers (i.e.
conduit layer 32b) in a second direction perpendicular to the first
direction, as illustrated in FIGS. 3(f) and 3(i). CO.sub.2
diffusion occurs upon exposure of and contact between the blood and
conduits 30, flowing blood over, around and between the porous
membranes of conduits 30 while a gas, such as a gas substantially
free of CO.sub.2, is flowed through conduits 30. Blood may flow
through the interstices of one or more gas exchange mats 34 over,
between and around conduits 30 in a direction that is substantially
orthogonal to the direction of the gas flow within conduits 30 and
substantially orthogonal to a length of conduits 30. Gas exiting a
distal end 38 of conduits 30 flow into first and second sections
42c, 42d of gas passageway 41b which converge and deliver the gas
to gas outlet port 20. Gas outlet port 20 may be located between
and connected to first and second sections 42c, 42d of gas
passageway 41b, defined by adjoining housing sidewalls 22c, 22d and
blood treatment chamber walls 26c, 26d.
[0101] In an exemplary embodiment, gas exchange module 10 may
optionally further include one or more sensors 44 for detecting a
physiological parameter of blood or gas flowing through gas
exchange module 10. For example, sensor 44 may be in direct contact
with blood entering or exiting gas exchange module 10 and is
adapted for detecting and measuring blood pressure, blood flow
rate, CO.sub.2 content, or O.sub.2 content. In the exemplary
embodiment shown in FIG. 3(h), at least one sensor 44 is located
within or otherwise disposed at blood outlet port 16, adjacent to
the passageway through which blood flows through blood outlet port
16. A second sensor 44 may also or alternatively be attached to and
extend from an internal surface of blood inlet port 14. Optionally,
one or more sensors 44 may be in direct contact with gas flowing
through gas exchange module 10. For example, sensor 44 may be
attached to and/or extend from an internal surface of gas inlet
port 18 and/or gas outlet port 20. Each of the above described
sensors 44 may be operatively associated with control unit 62 and
used to confirm blood flow, blood pressure or CO.sub.2partial
pressure within gas exchange module 10; detect the presence of gas
or blood leakage through gas exchange module 10; and/or provide
information based on which the user may set, change and/or modify
the blood and gas flow rates through gas exchange module 10 may be
adjusted to achieve efficient or otherwise the desired degree or
rate of CO.sub.2 diffusion.
[0102] Blood treatment system 1 may optionally further include a
gas supply unit 50 operatively associated with gas exchange module
10 to provide a continuous stream of gas at a controlled, high
velocity flow rate to gas inlet port 18. As shown in FIGS. 1-2, gas
supply unit 50 delivers gas directly to gas inlet port 18 of gas
exchange module 10 through one or more tubing. In an exemplary
embodiment, gas supply unit 50 may be adapted to controls gas flow
through conduits 30 such that the gas flow rate through the lumens
of conduits 30 is about 0.2 L/min to about 15 L/min, about 1 L/min
to about 15 L/min, about 2 L/min to about 15 L/min, or about 5
L/min to about 15 L/min. Gas supply unit 50 may also be used to
control the gas pressure within conduits 30. In one embodiment,
there is substantially no change in gas pressure across conduit
30.
[0103] The gas delivered to conduits 30 may be non-toxic,
biocompatible and substantially free from CO.sub.2 and may be
administered in toxicological safe amounts. In one embodiment, the
partial pressure of CO.sub.2 in the gas is either negligible or
there is no CO.sub.2 in the gas. In an exemplary embodiment, the
gas may be oxygen, mixtures of oxygen with air, nitrogen or any
suitable noble gas. Optionally, gas supply unit 50 may further
include one or more gas blending functionalities for mixing or
otherwise preparing the gas to be delivered to gas exchange module
10.
[0104] Optionally, blood treatment system 1 may further include a
blood pump 60 and/or control unit 62 that are operatively
associated with gas exchange module 10 for regulating the flow rate
of blood through blood treatment chamber 24. In the embodiments
shown in FIGS. 2 and 4, blood pump 60 is fluidly connected to a
venous access point and gas exchange module through one or more
tubing. In one embodiment, pump 60 may be an occlusive (i.e.
peristaltic) pump or centrifugal pump, such as the centrifugal pump
manufactured by Maquet Cardiopulmonary of Rastatt, Germany and
marketed under the trade name ROTASSIST, or a roller pump. A
control unit 62 may be integrated in or otherwise operatively
associated with pump 60 to regulate blood flow through pump 60 and
through blood treatment chamber 24. Pump 60, as instructed by
control unit 62, may control and regulate blood flow through gas
exchange module 10, specifically through blood treatment chamber
24, at a rate of about 1.2 L/min or less, about 1 L/min or less,
about 0.8 L/min or less, about 0.51 L/min or less, about 0.5 L/min
or less, about 0.4 L/min to about 0.51 L/min, about 0.4L/min to
about 1L/min, or about 0.51 L/min to about 1.2 L/min. A user may,
as desired, interface with control unit 62 to change the rate of
blood flow within a designated low blood flow range.
[0105] In an exemplary embodiment, blood treatment system 1 does
not have a heat exchanger. In such embodiments, gas exchange module
10 does not have any substantially water impermeable fibers adapted
for passing a thermally managed flow of water to heat or cool blood
within gas exchange module 10. Additionally, in these embodiments
blood treatment system 1 is not designed to provide oxygenation and
therefore regulation of blood temperature is not required. Blood
treatment system 1 may therefore be configured as a dedicated
CO.sub.2 removal system adapted specifically and/or only for
CO.sub.2 diffusion.
[0106] Blood treatment system 1 may optionally further includes a
catheter providing vascular access to the patient. Since blood
treatment system 1 can be operated under conditions of low blood
flow, it is possible to work with small-lumen cannulas or
dual-lumen cannulas which provide for less invasive vascular access
and improved safety, and thus requires fewer monitoring controls
and potential complications. In one embodiment, the size of a
single lumen cannula may be about 21 French (7 mm) or less, about
13 French (4.33 mm) or less. In another embodiment, the size of a
double lumen cannula may be about 24 French (8 mm) or less or about
19 French (6.33 mm) or less. In an embodiment, the size of the
single lumen cannula may be selected from those ranging between 21
French to 19 French.
[0107] In an exemplary embodiment, the blood contacting lumens
(e.g. cannula and tubing lumens), chambers (e.g. blood treatment
chamber), components and portions of extracorporeal blood treatment
system 1, including those lumens, chambers, compartments and
surfaces of gas exchange module 10, optional pump 60, access
catheters as well as all connective tubings of system 1 may be
coated with a material that improves the biocompatibility of the
extracorporeal circulation system and may also be
thromboresistant.
[0108] While the above described embodiments of blood treatment
system 1 describe in particular a CO.sub.2 removal system, one
skilled in the art would appreciate that blood treatment system 1,
gas exchange module 10, particularly conduits 30, and all other
described system components may be designed, adapted and configured
for the removal, diffusion, extraction or exchange of other gases,
in addition to or in place of CO.sub.2. In particularly, the gas
permeable membrane of conduit 30 and selection of gas to be flowed
through conduits 30 may be designed and selected for the transfer
of these other gases.
[0109] The unique configuration of blood treatment system 1 of the
present invention provides numerous operational and therapeutic
advantages. Designed to accommodate a low rate of blood flow
through gas exchange module 10, the blood treatment system 1
enables the use of minimally invasive small-lumen or dual-lumen
cannulas to provide minimally traumatic vascular access. The low
blood flow rate also results in low blood pressure conditions
within the lumen of conduit 30, which reduces the potential for
blood leakage from blood treatment system 1 as well as reduces the
severity of the risk associated with blood leakage. Consequently,
blood treatment system 1 need not require any or a plurality of
highly sensitive, highly restrictive blood pressure and/or blood
flow monitors for accessing the possibility of leakages, thereby
simplifying the overall system.
[0110] Another advantageous feature of the exemplary embodiments of
the invention is the configuration and arrangement of gas exchange
conduits 30. The relatively short length of conduits 30 decreases
fluidic resistance of the gas flowing through conduit 30, which
consequently reduces fluidic back-pressure for gases passing
through the lumen of conduit 30. The short length of conduit 30
thereby inhibits the formation of microbubbles on an exterior blood
contacting surface of the conduit 30 membrane, which can obstruct
blood flow in capillaries, cause tissue ischemia and form blood
embolisms leading to further vascular and tissue damage. By
contrast, oxygenators are designed with long fibers that are few in
number in order to achieve mass transfer of gas.
[0111] By including a large number of conduits 30 in gas exchange
mat 30, no efficiency in the gas exchange module is lost by virtue
of the short length of conduits 30. To the contrary, due to the
relatively short length of conduits 30 and high gas flow rate
therethrough, the difference in the partial pressure of CO.sub.2 of
the gas and of the patient's blood is greater at the distal end
(i.e. gas exiting end) of conduit 30 than a distal end (i.e. gas
exiting end) of a longer conduit. Consequently, CO.sub.2 diffusion
driving force and efficiency is greater as a result of using a
plurality of shorter conduits 30.
[0112] Additionally, exemplary embodiments of the invention further
enhances CO.sub.2 removal efficiency by arranging the plurality of
parallel conduits 30 in layers 32 to form one or more gas exchange
mats 34, such that the conduits 30 of adjoining layers 32 are
oriented substantially perpendicular to one another, thereby
providing a maximum surface area available for CO.sub.2 diffusion.
The efficiency of CO.sub.2 diffusion is further improved by
dictating that the combined thickness of the one or more gas
exchange mat 34 is such that the ratio of the active length of a
conduit 30 to the total thickness of the one or more gas exchange
mats 34 is about 3:1 to about 0.5:1, thereby enabling efficient
removal of CO.sub.2 from blood flowed through gas exchange module
10 at a low blood low flow rate. In an exemplary embodiment, the
thickness of the gas exchange mat may be about 2.6 cm to about 5.4
cm.
[0113] Furthermore, an exemplary embodiment of blood treatment
system 1 and all its components, including gas exchange module 10
may be compact, light-weight and portable, enabling a patient to
remain mobile while being treated. In one embodiment, the various
components of system 1 may be integrated into a single device that
is either hand-held or otherwise portable, as shown in FIGS. 1-2
and 4. In one embodiment, all the components of system 1 may be
removably positioned on, hung on or otherwise attached to a wheeled
cart or stand, enabling a patient to easily roll system 1 to a
desired location with minimal hindrance, thereby allowing system 1
to move with the patient.
Blood Treatment Method
[0114] The present invention is further directed to a novel method
for removing CO.sub.2 from blood circulated through extracorporeal
blood treatment system 1. In one embodiment, the method involves
accessing a patient's circulatory system, directing blood through a
circuit of the extracorporeal blood treatment system so as to
remove substantially all the CO.sub.2 from the blood upon passage
through gas exchange module 10 and returning the substantially
CO.sub.2free blood to the patient's circulatory system. This
therapeutic method may be used to treat a variety of respiratory
conditions associated with impaired lung functionality,
particularly health problems associated with excess CO.sub.2
concentration in the blood or inhibited ability to remove CO.sub.2
from the blood. Exemplary conditions that may be treated with the
present method include diseases, syndromes, injuries or defects
affecting lung function including but not limited to COPD, chronic
and acute hypercapnia, respiratory acidosis, ALI and ARDS.
[0115] In the exemplary embodiment set forth in FIG. 5, the method
involves diagnosing a patient with or otherwise
accessing/determining the likelihood that a patient has a
respiratory condition and applying the blood treatment system 1 to
the patient for the purpose of decreasing CO.sub.2 concentration in
a patient's blood or otherwise treating the respiratory condition.
In particular, a physician may select and apply any one of the
aforementioned embodiments of blood treatment system 1, including
any gas exchange module 10, optional gas supply unit 50, optional
pump 60, or combinations thereof that is adapted for treating a
patient, in particular for treating an adult human. The physician
may also select the gas flow, blood flow and/or the gas to be
delivered to the conduits in order to optimize CO.sub.2 diffusion.
In one embodiment, the parameters set by the physician for gas
flow, blood flow and/or gas selection are not optimized for
transfer of O.sub.2 for patient oxygenation.
[0116] Vascular access is achieved by percutaneous cannulation of
the jugular vein, subclavian vein, femoral vein or any combinations
thereof using two small single lumen catheters or a double lumen
catheter. The tip of the catheter or a separate needle positioned
within a cannula of the catheter may be used to create a small
vascular puncture site, connecting the catheter to the patient's
circulatory system. When using a needle, upon puncture, the needle
may be retracted and/or the catheter may be advanced to secure the
catheter to the vein. In an exemplary embodiment, only a single
puncture site is necessary to provide vascular access, such as
venous-venous access using a small double lumen catheter.
[0117] A tubing attached to a proximal port of the catheter may be
used to transport blood from the vascular access site to and from
gas exchange module 10 at a low flow rate. In exemplary embodiments
of blood treatment system 1 that include optional pump 60, blood is
transported to pump 60 which directs and delivers the blood to
blood treatment chamber 24 of gas exchange module 10 at a
controlled rate. Control unit 62, operatively associated with pump
60, instructs pump 60 to regulate blood flow through blood
treatment chamber 24 at a predetermined low flow rate. If desired,
the user may instruct controller 62 and/or pump 60 to change the
rate of blood flow through gas exchange module 10 within a
designated low flow rate range. In one embodiment, blood is
delivered to blood inlet port 14 and through blood treatment
chamber 24 at a positive, non-zero low flow rate, such as about 0.5
L/min or less.
[0118] As blood is delivered to gas exchange module 10, optional
gas supply unit 50 supplies a continuous stream of gas
substantially free of CO.sub.2 to gas inlet port 18 of gas exchange
module 10. Best shown in FIGS. 3(f) and 3(i), gas flows through gas
inlet port 18 and diverges into one of two compartments or sections
42a, 42b of gas passageway 41a which are in fluid communication
with open conduit ends of the conduits 30 of alternating conduit
layers 32 that form gas exchange mat 34. The gas then flows through
the conduits 30 of respective conduit layers 32 in a direction
substantially perpendicular to the length of the corresponding
sections 42a, 42b, as illustrated by the arrows in FIG. 3(i). In
one embodiment, gas supply unit 50 controls and regulates the flow
rate of gas such that the gas flow rate through conduits 30 is
maintained at a high velocity of about 15 L/min. Additionally, the
gas pressure within conduits 30 may be kept low and regulated so
that it does not exceed a level at which conditions would induce
microbubble formation, i.e. bubble point.
[0119] When blood enters blood inlet port 14 and flows into blood
treatment chamber 24, the flow of blood is oriented in a direction
substantially orthogonal to the one or more gas exchange mats 34,
conduit layers 32 and the respective lengths of conduits 30. Blood
passes through the interstices of and contacts the one or more gas
exchange mats 34 so as to pass over, around and between the
exterior surface of individual conduits 30 forming conduit layers
32 and one or more gas exchange mats 34. Upon contact with and
exposing the flow of blood to the porous membrane of conduits 30,
through which a constant supply of gas substantially free of
CO.sub.2 is flowed, CO.sub.2 diffuses from the blood, through the
porous membrane of conduit 30 and is swept along and through the
lumen of conduit 30 by the high velocity gas flowing through
conduit 30. The difference in the partial pressure of CO.sub.2 in
the patient's blood introduced into gas exchange module 10 and any
partial pressure of CO.sub.2 in the gas circulated through conduits
30 drives the diffusion of CO.sub.2 from the blood and into the
lumen of conduit 30. In an exemplary embodiment, this difference in
the partial pressure of CO.sub.2 may be about 45 mm Hg to about 70
mm Hg, about 45 mm Hg to about 50 mm Hg, or about 40 mm Hg to about
50 mm Hg. By providing a high velocity stream of gas through
conduit 30, the exposure and contact time between the blood and gas
flowing through conduits 30 is relatively short. Consequently, the
partial pressure of CO.sub.2in the blood and the partial pressure
of CO.sub.2 in the gas, or lack thereof, is prevented from
equilibrating, thereby maintaining a continuous driving force of
CO.sub.2 diffusion created by the blood and gas CO.sub.2 partial
pressure differential. The gradient of the high pCO.sub.2
concentration in blood in comparison to the low pCO.sub.2 gradient
in the gas is therefore maintained by the high velocity of gas
flowing through conduits 30; gas carrying diffused CO.sub.2 from
blood is quickly purged and replaced with new gas having
substantially no CO.sub.2. The gradient is further maintained as
only small amounts of pCO.sub.2 are diffused from the blood and
into each conduit lumens. As discussed above, near complete removal
of pCO.sub.2 from the blood, however, may be accomplished by
including a plurality of such short conduits 30 within gas exchange
mat 34.
[0120] In an exemplary embodiment, substantially all the CO.sub.2
may be removed from the blood introduced into gas exchange module
10 upon a single pass of the blood through gas exchange module 10,
specifically through blood treatment chamber 24 and gas exchange
mats 34. In one embodiment, the percent of CO.sub.2removed from
blood after a single pass through gas exchange module 10 may be
about 10% to about 95%, about 20% to about 90%, about 40% to about
90%, and 60% to about 90%. The partial pressure of CO.sub.2 in the
blood after a single pass through the gas exchange module 10 may be
about 60 mm Hg to about 5 mm Hg, about 40 mm Hg or less, about 30mm
Hg to about 10 mm Hg, or about 25 mm Hg to about 5 mm Hg. In an
exemplary embodiment, the pH of blood after a single pass through
gas exchange module 10 may be about 7.45 or more, about 7.6 or
more, about 7.8 or more, about 7.5 to about 8.2, about 7.6 to about
8.2, or about 7.7 to about 8.2.
[0121] A fresh supply of gas may be constantly streamed through gas
exchange module 10, and the patient's blood may be recirculated
through extracorporeal blood treatment system 1 as desired until
all or substantially all the CO.sub.2is removed. In an exemplary
embodiment, the method of the present invention allows for the
complete or substantially complete depletion of all CO.sub.2 from
the treated blood.
[0122] The gas containing CO.sub.2 leaving conduits 30 is collected
in first and second sections 42c, 42d of gas passageway 41b and
pushed out through gas outlet port 20 of gas exchange module 10 by
the high velocity flow of gas in gas passageways 41a, 41b and
conduits 30. This gas may be subsequently vented to atmosphere or
collected in a reservoir. In one embodiment, gas outlet port 20 may
optionally connected to a vacuum source to further control the rate
of gas flow through conduits 30.
[0123] The overall duration of the therapy may be up to about 30
days, about 6 hours to about 30 days. In another embodiment, the
therapy may last for a period of time up to about 5 days or about 6
hours to about 5 days. Additionally, the therapy may be
continuously or intermittently administered as needed to achieve
the desired degree of CO.sub.2 removal.
[0124] The same or similar method of use of other embodiments blood
treatment system 1 may be used to remove, extract, transfer or
exchange other gases from the blood. Again, blood treatment system
1, inclusive of gas exchange module 10, particularly conduits 30
and the selection of gas to be flowed through conduits 30, as well
as all other described system components may be designed, adapted
and configured for the removal, diffusion, extraction or exchange
of other gases in addition to or in place of CO.sub.2.
[0125] The CO.sub.2removal method of the present invention has a
number of therapeutic advantages. For example, low blood flow makes
it possible to decrease the invasiveness of the procedure by
reducing the size of the vascular access point, permitting usage of
a small-lumen or small dual-lumen cannulas which causes less stress
and trauma to the vessels during cannulation. Moreover, the
veno-venous cannulation, low blood flow and corresponding low blood
pressure reduces the risk of death or consequences associated with
the patient bleeding out due to blood leakage from blood treatment
system 1.
[0126] Additionally, the high velocity stream of gas through
conduits 30 maintains a stable and maximized driving force of
CO.sub.2 diffusion created by the difference in the CO.sub.2
partial pressure of the patient's blood and in the gas. Microbubble
formation on the blood contacting outer surface of the conduit 30
membrane is also inhibited by maintaining a low gas pressure in
conduits 30.
[0127] Furthermore, in one embodiment, the method enables efficient
CO.sub.2 diffusion by endeavoring to substantially remove all
CO.sub.2from blood in a single pass through gas exchange module 10
and seeking to achieve non-physiological values of the partial
pressure of CO.sub.2 in blood and non-physiological values of blood
pH. For example, the pCO.sub.2 of the treated arterial blood may be
about 32 mm Hg or less, about 25 mm Hg or less, about 15 mm Hg or
less, and the pH of treated arterial blood may be about 7.45 or
more, about 7.6 or more, or about 7.8 or more, representative of
respiratory alkalosis. In one embodiment, the pCO.sub.2 value of
the treated arterial blood may be about 10 to about 15 mm Hg and
the pH value may be about 7.8. In one embodiment, the pCO.sub.2
value of the treated arterial blood may be about 10 mmHg to about
32 mm Hg and the pH value may be about 7.45 to about 7.8. In these
embodiments, the method may involve targeting and managing therapy
conditions to these atypical values that are not within standard
acceptable physiological ranges. In contrast, oxygenators are
optimized to maintain normal physiological partial pressures of
gas, inclusive of CO.sub.2; mass transfer of gas is thus only
achievable by requiring a high blood flow through the oxygenator
and complete elimination of CO.sub.2 would not be possible.
Surprisingly, the blood treatment system 1 of the present invention
is as or more effective than large gas exchange modules that
require high blood flow and whose gas exchange conduits have
greater gas exchange surface areas.
EXAMPLES
Example 1
[0128] In one embodiment, gas exchange module 10 of the present
invention has the same configuration as shown in FIGS. 3(a)-3(b),
3(d)-3(f) and 3(h)-3(m). Gas exchange module 10 included a gas
exchange mat 34 constructed from 13,834 or more microporous gas
permeable conduits 30 adapted for carbon dioxide diffusion.
Conduits 30 were positioned parallel to one another to form conduit
layers 32. The conduit layers 32 were stacked on top of one another
to form gas exchange mat 34, each layer oriented perpendicular to
an adjoining layer. All of the conduits 30 had an active length of
about 5.5 cm and a total conduit length of about 7.6 cm. The active
length percentage of conduit 30 capable of gas transfer was at most
about 72.4%. Gas exchange mat 34 had a total gas exchange surface
area of about 0.98 m.sup.2 and a conduit density of about 14,116
conduits per m.sup.2. The ratio of a maximum conduit active length
to the 5.4 cm thickness of the gas exchange mat 34 (which can also
be expressed here as the minimum distance of the blood flow
passageway through blood treatment chamber 24) is about 1.02:1. The
blood and gas flow paths through gas exchange mat 34 and blood
treatment chamber 24 were designed to expose the blood to conduits
30 and ensure comprehensive treatment and processing of the blood
passing therethrough. This configuration also facilitates CO.sub.2
diffusion by virtue of the relative blood flow resistance and gas
flow resistance.
Example 2
[0129] In one embodiment, gas exchange module 10 of the present
invention has the same configuration as shown in FIGS. 3(a)-3(b),
3(d)-3(f) and 3(h)-3(m). Gas exchange module 10 included a gas
exchange mat 34 constructed from 13,119 or more microporous gas
permeable conduits 30 adapted for carbon dioxide diffusion.
Conduits 30 were positioned parallel to one another to form conduit
layers 32. The conduit layers 32 were stacked on top of one another
to form gas exchange mat 34, each layer oriented perpendicular to
an adjoining layer. All of the conduits 30 had an active length of
about 5.8 cm and a total conduit length of about 7.6 cm. The active
length percentage of conduit 30 capable of gas transfer was at most
about 76.3%. Gas exchange mat 34 had a total gas exchange surface
area of about 0.98 m.sup.2 and a conduit density of about 13,300
conduits per m.sup.2. The ratio of a maximum conduit active length
to the 5.4 cm thickness of the gas exchange mat 34 (which can also
be expressed here as the minimum distance of the blood flow
passageway through blood treatment chamber 24) is about 1.07:1. The
blood and gas flow paths through gas exchange mat 34 and blood
treatment chamber 24 were designed to expose the blood to conduits
30 and ensure comprehensive treatment and processing of the blood
passing therethrough. This configuration also facilitates CO.sub.2
diffusion by virtue of the relative blood flow resistance and gas
flow resistance.
Example 3
[0130] In one embodiment, gas exchange module 10 of the present
invention has the same configuration as shown in FIGS. 3(a)-3(b),
3(d)-3(f) and 3(h)-3(m). Gas exchange module 10 included a gas
exchange mat 34 constructed from 17,148 or more microporous gas
permeable conduits 30 having an outer diameter of about 0.35 mm and
is adapted for carbon dioxide diffusion. Conduits 30 were
positioned parallel to one another to form conduit layers 32. The
conduit layers 32 were stacked on top of one another to form gas
exchange mat 34, each layer oriented perpendicular to an adjoining
layer. All of the conduits 30 had an active length of about 5.2 cm
and a total conduit length of about 7.6 cm. The active length
percentage of conduit 30 capable of gas transfer was at most about
68.4%. Gas exchange mat 34 had a total gas exchange surface area of
about 0.98 m.sup.2 and a conduit density of about 17,497 conduits
per m.sup.2. The ratio of a maximum conduit active length to the
5.4 cm thickness of the gas exchange mat 34 (which can also be
expressed here as the minimum distance of the blood flow passageway
through blood treatment chamber 24) is about 0.963:1. The blood and
gas flow paths through gas exchange mat 34 and blood treatment
chamber 24 were designed to expose the blood to conduits 30 and
ensure comprehensive treatment and processing of the blood passing
therethrough. This configuration also facilitates CO.sub.2
diffusion by virtue of the relative blood flow resistance and gas
flow resistance.
[0131] The foregoing description of the invention has been
presented for the purpose of illustration and description only and
is not to be construed as limiting the scope of the invention in
any way. The scope of the invention is to be determined from the
claims appended hereto.
* * * * *