Note: Descriptions are shown in the official language in which they were submitted.
TITLE OF THE INVENTION
~ollow Fiber Type Oxygenator
BACKGROUND OF THE INVENTION
This invention relates to a hollow fiber type
oxygenator for use in extracorporeal blood circulation
which oxygenates blood passing therethrough while removing
carbon dioxide.
Conventional oxygenators may be divided into two
classes, that is, a bubble type oxygenator and a membrane
type oxygenator. The membrane type oxygenators including
flat membrane type and hollow fiber type oxygenators cause
less damages to blood such as hemolysis, denaturation of
proteins, blood coagulation, blood adhesion and the like as
compared with the bubble type oxygenators, and are
generally recognized to have a me~hanism much closer to a
living lung. In spite of the superiority of the membrane
type oxygenators to the bubble type oxygenators, the bubble
type oxygenators have been widely used in open heart
surgery.
In membrane type oxygenators now available, blood
stream layers must be thin to obtain a higher oxygenating
capacity. Narrow ~low paths cause high flow resistance to
blood. These factors prevent blood perfusion by gravity
wherein blood is perfused through an oxygenator by the head
between the patient and the oxygenator. Then, an
extracorporeal blood circuit having a membrane type
oxygenator must employ a pump upperstream of the
oxygenator, namely on the venous side of the oxygenator.
This invites a problem that the pressure near the outlet of
the pump exceeds the sum of pressure losses across the
blood return catheter and the oxygenator, which results in
an increased pressure in the blood-returning line. If the
pressure is extremely increased, a tube in a roller pump
can be expanded to burst.
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An additional problem is that two oxygenators are
required when extracorporeal circulation is separately
carried out for the brain and the lower half body during an
open heart sugery.
To abate postoperative complicationsv the use of a
pulsatile pump is recently recommended for the
extracorporeal circulation of blood which is close to the
blood in vivo. The pressure Ioss across conventional
oxygenators are too high to provide pulsatile flow.
1 a European Patent Publication No. 103899 proposes a
process for exchanging oxygen with carbon dioxide in a
hollow fiber type oxygenator by passing oxygen through the
interior of the hollow fiber membranes and blood along the
exterior of the hollow fiber membranes, that is, the space
defined by the housing and the hollow fiber membranes.
However, there is no oxygenator which allows for blood
drainage by gravity with a suficient gas exchangeability.
SUMM~RY OF THE INVENTION
It is, therefore, an object of the invention to provide
an improved hollow fiber type oxygenator of the type
wherein blood is passed between the inner wall of a housing
and the outer surface of hollow fibers and oxygen is passed
through the interior of the hollow fibers, which allows
blood to reach the oxygenator only by the head between the
patient and the oxygenator, while providing a sufficient
gas exchangeability even with a reduced membrane area as
compared with oxygenators of the other type wherein blood
is passed through the interior of hollow fiber membranes.
It is another object of the invention to provide a
compact hollow fiber type oxygenator as mentioned above by
determining optimum dimensional parameters to achieve
smooth blood passage effective gas exchange.
6'1
According to the present invention, there is provided
a hollow fiber type oxygenator comprising a housing having
opposite end portions, a bundle of a plurality of hollow fibers
axially extending through the housing and each presenting a
gas-exchange membrane, partitions engaged in the housing at its
opposite end portions and fluid tightly retaining t~e opposite
ends of the hollow fibers without blocking the opening of the
fibers, a gas inlet and a gas outlet both in fluid
communication with the interior space o~ said hollow fibers,
a blood chamber defined by the partitions, the outer surface
of said hollow fibers, and the inner surface of said housing,
and a blood inlet and a blood outlet located in the housing
wall at the opposite end portions in fluid communication with
said blood chamber, characterised in that the bundle of hollow
fibers has an end packing density d1 of from 20 to 30% near the
partitions and a central packing density d2 of 40 to 50~ near
an axial mid-point, the ratio of d1~d2 ranging from 4/10 to
6/10, thereby providing a pressure loss of blood of up to 60
mmHg.
Several preferred embodiments of the presant invention
are described below~
(i) The hollow fiber membrane comprises a porous hollow
fiber membrane made of polyolefin having an inner diameter of
about 100 to about 1,000 ~m, a wall thickness of about 10 to
about 200 ~, an average pore diameter of about 200 to about
2,000 A, and a porosity of 20 to 80~. Polypropylene is the
preferred polyole~in.
~ii) The wall of the housing is tapered toward an axial
mid-point such that the wall has a minimum inner diameter
approximately at the mid-point and is flared therefrom toward
the opposite end portions.
LCM:mls
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_RIEF DESCRIPTION OF THE DRAWINGS
These and other objects, features, and advantages of
the present invention will be readily understood from the
following description taken in conjunction with the
accompanying drawings, in which:
FIG. l is an elevational cross section of a hollow
fiber type oxygenator according to one embodiment of the
present invention;
FIGS. 2, 3 and 4 illustrates different circuit
arrangements wherein the oxygenator of the present
invention is used;
FIG. 5 is a diagram showing the volume of oxygen
migràted and the pressure loss ~P in relation to the end
packing density dl;
FIG. 6 is a diagram showlng the volume of oxygen
.~ migrated and the pressure loss ~P in relation to the
'?~ central packing density ~;
FIG. 7 is a diagram showing the volume of oxygen
migrated and the pressure loss ~P-in relation to the ratio
of dl/d2;
FIG. 8 is an elevational cross section of a hollow
fiber type oxygenator accordlng to another embodiment of
the present invention; and
FIG. 9 is a perspective view of a hollow fiber type
oxygenator similar to that shown in FIG. 8.
DESCRIPTION OF THE_PREFERRED EMBODIMENTS
Referring to FIG. l, there is schematically lllustrated
a hollow fiber type oxygenator according to one embodiment
of the present invention. The oxygenator generally
designated at l includes a hollow cylindrical housing 2
defining an interior space and being open at axially
opposite end portions. The housing 2 receives therein a
bundle or assembly 35 of a plurality of axially
~ZS49~67
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extending hollow fibers 3 each presenting a gas exchange
membrane. The opposite ends of the hollow fibers 3 are
fluid tightly retained by opposed partitions 41 and 42
which are engaged in the housing 2 at its opposite ends,
respectively. For brevity of description, the partitions
41 and 42 and the corresponding housing ends are referred
to as upper and lower partitions and upper and lower
housing ends.
Upper and lower conical headers 21 and 22 are fixedly
1~ secured to the upper and lower ends of the housing 2. The
inner surface of the upper header 21 and the outer surface
of the upper partition 41 define a gas incoming chamber 23
in fluid communication with the interior space of hollow
fibers 3. The header 21 has a gas inlet 24 at its apex.
The inner surface of the lower header 22 and the outer
surface of the lower partition 42 define a gas outgoinig
chamber 25 in fluid communicat.ion with the interior space
of hollow fibers 3. The header 22 has a gas outlet 26 at
its apex.
In the oxygenator 1 of this arrangement, a gas such as
oxygen and air can be passed from the inlet 24 through the
interior space of the hollow fibers 3 to the outlet 26.
The lower header 22 may not necessarily be provided,
that is, the outgoing chamber 25 and outlet 26 may be
omitted. Then the outer side of the lower partition 42
becomes a gas discharge end and the gas coming out of the
hollow fibers 3 is directly released into the atmosphere.
A blood chamber 5 is defined by the inner surfaces of
the partitions 41, 42, the inner wall of the housing 2, and
outer wall of the hollow fibers 3. The housing 2 is
provided near its lower and upper ends with a blood inlet
61 and a blood outlet 62 both in fluid communication with
the blood chamber 5. The oxygenator permits blood to pass
through the blood chamber 5 in a turbulent flow around
hollow fiber membranes 3.
S446~
That portion of the wall of the housing 2 where the
blood inlet 61 i5 provided is radially dilated as compared
with the intermediate portion. An annular space is thus
defined between the outer periphery of the bundle 35 of
hollow fibers 3 and the radially dilated housing wall,
providing an annular blood flow path. The annular flow
path circumscribes the bundle 35 of hollow fibers 3 so that
blood is smoothly and evenly distributed among all the
hollow fibers 3. Also, that portion of the wall of the
' 10 housing 2 where the blood outlet 62 is ~rovided is radially
dilated as comparea with the intermediate portion. An
annular space is thus defined between the outer periphery
of the bundle 35 of hollow fibers 3 and the radially
dilated housing wall, providing an annular blood flow path.
The annular ~low path circumscribes the bundle 35 of hollow
fibers 3 so that blood flowing around the hollow fibers 3
is smoothly directed to the blood outlet 62 from the entire
periphery of the fiber bundle 35.
The wall of the housing 2 is tapered toward an axial
mid-point. The housing wall has a minimum inner diameter
approximately at the mid-point and is flared or divergent
therefrom toward the upper and lower ends.
The wall of the housing is tapered toward the
mid-point to squeeze or throttle the bundle or assembly 35
of hollow fibers at the axial mid-point. The throttling of
the fiber assembly 35 by the housing 2 ensures a uniform
blood flow in a transverse cross-section of the assembly 35
; and changes the flow velocity of blood in axial direction
of the fiber assembly 35, promoting the occurrence of
turbulent flow to improve the gas exchangeability of the
oxygenator.
In additon to the tapering of the housing wall toward
the mid-point as previously described, preferably the
tapered wall of the housing 2 is connected to the radially
dilated wall defining the annular blood flow path by a
tapered surface (not shown). Then air to be purged upon
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priming is smoothly driven off along the inner surEace of
the housing 2 and released into the atmosphere without
remaining in the blood chamber 5.
The hollow fiber membranes 3 used in the oxygenator of
the invention may be microporous membranes. The porous
hollow fibers may preferably be formed of a polyolefin such
as polypropylene and polyethylene. Polypropylene is most
preferred. Each hollow fiber (membrane) used in the
present invention has many micropores which communicate the
interior and the exterior of the membrane. The fiber has
an inner diameter of about lO0 to about l,000 um, a wall
thickness of about lO to 200 um, an average pore diameter
of about 200 to about 2,000 A, and porosity of 20 to 80~.
When the hollow fibers 3 of the microporous membrane
type are used, gas migration takes places as a volume flow.
The resistance of the membrane to gas migration is reduced
and the gas exchangeability of the membrane is improved.
Silicone membranes which provide for gas migration
through disolution and diffusion process may also be
employed as well as the microporous membranes.
The partitions 41 and 42 serve for the important
funciton of isolating the interior from the exterior of the
hollow fibers 3. In general, the partitions 41 and 42 are
formed by casting a highly polar, high molecular weight
potting compound, for example, polyurethane, silicone and
epoxy resins by a centrifugal casting process to the inner
wall surface of the housing end portions where it is cured.
More illustratively r first a number of hollow fibers 3
longer than the axial length of the housing 2 are prepared,
opposite open ends of the fibers are sealed with a viscous
resin, and then the fibers are placed in the housing 2 in
mutually juxtaposed relationship. The opposite ends of the
fiber bundle are completely covered. The housing 2 is
rotated about its longitudinal axis while a polymeric
potting compound is introduced from the sides of blood
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inlet 61 and outlet 62 where it is cured. The edge
portions of the fiber bundle bonded with the potting
compound are severed with a sharp knife to expose freshly
cut openings of the hollow fibers 3 at their opposite ends.
The partitions 41 and 42 are thus formed~ Those surfaces
of the partitions 41 and 42 facing the blood chamber 5 are
circular and concave as shown in FIG. l.
In order that the oxygenator thus prepared may exert
satisfactory performance, the bundle 35 of hollow fibers
must meet certain limitations with respect to fiber packing
density. First, the hollow fiber bundle 35 must have a
packing density dl of not more than 30% at the location of
partitions 41 and 42. The term packing density used in the
present invention is the sum of the outer diameter areas of
the hollow fibers 3 divided by the area defined by the
envelope of the fiber bundle 35 as expressed in percentage.
With a packing density dl of above 30%, the amount of
oxygen permeated through the membranes is reduced below a
practically required level. Also an increased pressure
loss prevents passage of blood by gravity.
More illustratively, a head dH of 90 to 120 cm,
preferably lO0 cm, is set between an operating table O and
a floor level F in order to enable passage of blood by
gravity as shown in FIG. 2. This means that the passage of
blood by gravity may not be conducted unless the pressure
loss is kept less than 60 mmHg.
The oxygenator must accommodate a maximum blood flow
rate of approximately 6.0 l/min. The oxygenator must have
such an oxygenating capacity that the amount of oxygen
migrated through the membranes be at least 240 ml/min. at a
blood flow rate of 6.0 l/min.
These requirements are satisfied when the packing
density dl is equal to or less than 30%. More preferable
results are obtained when the packin~ density dl is from
~2~
20% to 30%. When the packing density dl is less than 20%,
the oxygenator is no longer compact and undesirably
increases the volume of blood contained therein.
Secondly, the hollow fiber bundle 35 must have a
packing density d2 of 40% to 50% near its axial mid-point
or the throat o~ the housing 2.
A packing density d2 of more than 50% makes impossible
the passage of blood by the head or gravity, whereas a
packing density d2 of less than 40% produces a practically
unacceptable oxygenating capacity.
The ratio of fiber packing density dl/d2 should be kept
equal to or below 0.6. Ratios of dl/d2 in excess of 0.6
make impossible the passage of blood by the head or gravity
and result in a low oxygenating capacity. For the
compactness of the oxygenator, the ratio of dl/d2 may
preferably be between 0.4 and 0.6.
Further, the radius r2 of the hollow fiber bundle 35 at
the housing throat may preferably be not less than 60 mm,
when the blood flow rate QB is 6.0 l/min. Then the blood
flow rate solely caused by the head is increased~ For
compactness purpose, r2 may preferably be less than about
90 mm.
In ordçr to obtain an improved oxygenating capacity, it
is desirable that the total membrane surface area may
preferably be at least l.5 square meter as calculated on
the basis of the inner diameter of the hollow fibers. A
total membrane area of about 2.0 to 3.0 m is recommened
for compactness.
Another embodiment of the hollow fiber type oxygenator
according to the present invention is shown in FIG. 8.
FIG. 8 shows an oxygenator similar to the previously
described oxygenator shown in FIG. l with the exception
that the blood outlet 62 is replaced with blood drain means
in the form of a plurality of blood drain openings 65 and
blood reserving chamber 7 is further provided in fluid
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communication with the blood drain means. The blood
reserving chamber 7 is provided at the bottom with an
outlet 75.
The oxygenator l shown in FIG. 8 permits the line
between the blood outlet and a blood reservoir to be
omitted so as to reduce the volume of blood retained in the
overall circuit. This is particularly effective when the
oxygenator is used for a neonate with a low absolute volume
of blood. Another advantage is ease of handling due to
simplified circuit.
As shown in FIG. 9, the blood reserving chamber 7 may
be provided with a heat exchanger 85, with the advantage of
omitting a line connecting the blood storage chamber to a
heat exchanger.
EFFECT OF THE INVENTION
FIG. 2 shows a circuit arrangement wherein the
oxygenator l of the invention is used in combination with a
roller pump 7 placed downstream of the oxygenator, whereby
blood is passed through the oxygenator by gravity or the
head b tween the patient and the oxygenator.
A pre~sure loss of less than 60 mmHg is achieved with
the present oxygenator at a head aH of approximately l00 cm
so that a sufficient blood flow rate is secured. The
amount of oxygen migrated can be at least 240 l/min. at a
blood flow rate of 6.0 l/min. The oxygenator of the
invention has another benefit over prior art oxygenators of
the type wherein blood is passed inside hollow fiber
membranes, that an equivalent oxygenating capaclty is
obtained with an about one half membrane area, which
3~ results in size, weight and cost reductions.
FIG. 3 shows another circuit arrangement wherein the
oxygenator l of the invention is used in combination with
two roller pumps 71 and 75 placed downstream of the
oxygenator~ Separate extracorporeal circulation can be
smoothly carried out, because of the capacity of the
oxygenator.
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FIG. 4 shows a further circuit arrangement wherein the
oxygenator l of the invention is used in combination with a
pulsatile pump 8.
According to embodiments (i) and tii), the size of the
oxygenator is reduced and the volume of blood retained
therein is accordingly reduced. An oxygenator according to
embodiment (iii) achieves a remarkably hlgh gas
exchangeability. An oxygenator according to embodiment
(iv) offers a uniform flow of blood resulting in an
improved gas exchangeability.
The oxygenator thus constructed was tested to confirm
the effect of the invention. One of such experiments ls
described hereinafter.
Example
An oxygenator as shown in FIG. l was manufactured using
hollow microporous fibers of polypropylene which were
formed by axially stretching to the following
specifications: length 85 mm, inner diameter 200 ~m,
outer diameter 240 ~m, average pore diameter 650 A, and
porosity 45%. The fiber bundle had a total membrane area `
of 2.5 m2 as calculated on the basis of the inner diameter
of hollow fibers, and a radius r2 at the throat of 76 mm.
The fiber bundles having varying fiber packing
densities dl and d2 were assembled in housings. Bovine
blood having a Hematocrit value Ht of 35% at 37C was
passed through the oxygenators at a blood flow rate QB of 6
l/min., while oxygen gas was passed at a flow rate V of 6
l/min. As described above, the lower limit of the volume
of oxygen migrated through the membranes is 240 ml/min. and
the upper limit of the pressure loss aP during passage of
blood by gravity is 60 mmHg.
FIG. 5 shows the volume of oxygen migrated and the
pressure loss aP in relation to the end packing density d
-- ~Z~i~46~
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when the central packing density d2 is set to 46~. The end
packing density dl should be 30~ or less in order to clear
the critical points.
FIG. 6 shows the volume of oxygen migrated and the
pressure loss ~P in relation to the central packing density
d2 when the end packing density dl is set to 24~. The
central packing density d2 should range between 40~ and 50
% (about 53% under the experimented conditions).
FIG. 7 shows the volume of oxygen migrated and the
pressure loss ~P in relation to the ratio of packing
density dl/d2. The ratio dl/d2 should be 0.6 or less.
The data in FIGS. 5~ 6 and 7 reveal that the packing
densities defined in the present invention are critical.
