Abstrict Combination of a flow meter and a bubble trap for blood which has
been extracted for extra-corporeal treatment, in particular for
dialysis. Said combination comprises a container divided by a vertical
partition wall into a vertically elongated inlet chamber having
an inlet opening receiving the blood flow to be measured and treated,
and an outlet chamber of substantially greater cross-section than
the inlet chamber and in liquid communication with an outlet opening.
According to the invention, a narrow passage with a substantially
smaller cross-section than the inlet opening is provided through
the partition wall, preferably near the lower end of the same, causing
the liquid level in the elongated inlet chamber to rise substantially
above the level of said narrow passage. The difference between the
liquid levels in the respective chambers will be an indication of
the blood flow rate through the container, a pressure equalization
opening being provided through the wall above said liquid levels.
Both chambers are serving as efficient bubble traps, the influx
of blood to the inlet chamber taking place below the liquid level
and preferably in a vertical direction, and the output chamber providing
the advantageous combination of an extended free surface and a reduced
blood flow rate.
Claims I claim:
1. An apparatus for measuring the flow rate of and for releasing
entrapped air bubbles in a blood stream which has been extracted
temporarily from a patient for extracorporeal treatment comprising
a closed container having an inlet opening and an outlet opening
therein so that the blood stream can flow therethrough,
a vertically disposed partition means for dividing the container
into two chambers, respective ones of which are in direct communication
with each of the openings to thereby form inlet and outlet chambers,
first opening means in said partition means above the level of
said inlet opening and said outlet opening said first opening means
being of a smaller cross-section than the inlet opening so that
blood accumulates in said inlet chamber to a depth at which said
first opening is sufficiently below the surface of the blood in
the inlet chamber that increased pressure equalizes the blood flow
rate through said first opening means thereby controlling the flow
of the blood stream therethrough from said inlet chamber to said
outlet chamber so that the height of blood in said inlet chamber
is a function of the blood flow rate, and
second opening means in said partition means adjacent the top thereof
for equalizing the air pressure in said closed container between
said inlet and outlet chambers.
2. An apparatus according to claim 1 wherein said first opening
means directs the blood stream down from said inlet chamber to said
outlet chamber.
3. An apparatus according to claim 1 wherein the horizontal cross-sectional
area of the outlet chamber is larger than the horizontal cross-sectional
area of the inlet chamber.
4. An apparatus according to claim 3 wherein the inlet opening
is in the bottom of the container so that inflowing blood is directed
vertically upwardly in the inlet chamber.
5. An apparatus according to claim 4 wherein the outlet opening
is in the bottom of the container.
6. An apparatus according to claim 1 wherein said partition means
comprises a top section and a bottom section horizontally spaced
and vertically overlapping and said first opening means is the opening
formed by the separation of said top and bottom wall sections.
7. An apparatus according to claim 6 wherein said bottom section
is horizontally spaced towards said inlet opening and said top section
is horizontally spaced towards said outlet opening.
Description The present invention concerns a combination of a flow meter and
a bubble trap for blood which has been extracted for extracorporeal
treatment, in particular for dialysis.
When performing such extra-corporeal treatment, for example, dialysis
of blood from patients with kidney diseases, monitoring the flow
of blood through the external circuit is very important. The usual
and simplest method for measuring this flow of blood consists in
timing the transit of an air bubble from one permanent mark to another
on a transparent tubing. Better blood and more accurate flow meters
have, of course, also been developed, but their use with routine
treatments is too costly, due to the large number of patients being
treated simultaneously, and due to the requirement for technically
competent personnel to operate such flow meters properly.
The extra-corporeal blood flow circuit should also contain one
or more bubble traps for removing air bubbles from the blood, since
it is well known that such bubbles may be detrimental and even lethal
to the patient, if they are allowed to enter his circulatory system.
Bubble traps employed at the present time for this purpose are designed
according to the principle shown in FIG. 1 of the accompanying drawing.
As shown by an arrow in FIG. 1 the blood is introduced through
an inlet tube A to a chamber D, which is only partially filled with
blood, the inlet tube A opening in the air above the liquid level
V in chamber D. As equally shown by an arrow in the Figure, the
blood leaves the bubble trap through an outlet U for a further circulation
through the extra-corporeal circuit.
Bubble traps of the above design appear, however, in practice often
to behave more like bubble generators than bubble traps, since a
heavy flow of blood tends to whisk additional air into the blood,
and this newly entrapped air is unlikely to rise into the air pocket
above the surface V due to the small cross-section of the trap and
the concomitant downwardly directed rapid blood flow velocity.
On this background, it is an object of the present invention to
provide a simple and inexpensive combination of a flow meter and
a bubble trap for blood, allowing both a more easily determined
flow reading than the commonly utilised flow meters and overcoming
the above-noted disadvantages of known bubble traps. This combination
comprises a container through which the blood flows between an inlet
opening and an outlet opening, said container being, according to
the present invention, vertically divided into two chambers by means
of a partition wall, namely an inlet chamber in communication with
the inlet opening and an outlet chamber in communication with the
outlet opening. A narrow flow passage of substantially smaller cross-section
than the inlet opening is provided between said chambers through
the partition wall, preferably near its lower end, and said wall
has an opening above the liquid levels of the respective chambers
for air pressure equalization.
Since said narrow passage between the chambers is of substantially
smaller cross-section than the inlet opening, the liquid level in
the inlet chamber rises substantially above the level of the narrow
passage. The difference between liquid levels in the respective
chambers is thus an indication of the blood flow through the narrow
passage, which is identical to the blood flow in all parts of the
extra-corporeal circuit.
Due to said pressure equalization opening, the air pressure in
the chambers is equal. Said difference between liquid levels in
the chambers is then approximately proportional to the square of
the liquid flow rate in the circuit. According to the invention,
the outlet chamber is preferably designed with a substantially larger
horizontal cross-section than the inlet chamber, which means that
the liquid level in the outlet chamber changes much less than the
liquid level in the inlet chamber with blood flow variations. Such
variations in the blood flow may then easily be monitored by keeping
an eye on the liquid level in the inlet chamber only, and ignoring
the small variations of the liquid level in the outlet chamber.
The present invention will now be described by way of an example
of an embodiment, with reference to the accompanying drawings, wherein:
FIG. 1 is a schematic diagram of a common design of a known bubble
trap, and
FIG. 2 shows a vertical cross-section through a combination of
a flow meter and a bubble trap according to the present invention.
FIG. 1 has been described in the preamble of the present specification
and will not be the subject of further explanation. Corresponding
parts in FIGS. 1 and 2 have been given identical designations.
FIG. 2 shows a vertical cross-section through a container, which,
by means of a partition wall S, has been divided vertically into
two chambers, namely an inlet chamber C and an outlet chamber D.
The inlet chamber C is in communication with an inlet opening A
at the bottom of the container, as indicated by an inwardly directed
arrow, while the outlet chamber D is in communication with an outlet
opening U, also at the bottom of the container and indicated by
an arrow.
FIG. 2 also shows a narrow passage E with considerably smaller
cross-section than the inlet opening A, allowing for blood flow
through the wall S near its lower end. Near the upper end of the
wall an opening F is provided for air pressure equalization between
the chambers.
In operation, the present combination is connected in series with
an extra corporeal blood treatment circuit, which, e.g., may contain
a dialysis apparatus. The blood flowing in this circuit then flows
through the inlet opening A and into chamber C. Since the passage
E has a considerably smaller cross-section than the inlet A, the
liquid level V1 in the inlet chamber rises to a certain level above
the passage E to provide the extra pressure required for forcing
the blood through the narrow passage E.
Having passed through the passage E, the blood enters the outlet
chamber D which has a substantially larger cross-section than the
inlet chamber C. Finally the blood leaves the container through
the outlet opening U for further circulation through the rest of
the extra-corporeal circuit. The difference between the liquid levels
V1 and V2 is then an indication of the liquid flow rate the passage
E which is identical to the flow rate everywhere in the extra-corporeal
circuit. Since the cross-section of the outlet chamber D is substantially
larger than that of the inlet chamber C, the level V2 changes only
slightly as the flow rate varies. This means that the blood flow
rate for all practical purposes is indicated by the level V1. The
outer wall of the inlet chamber C may conveniently be designed with
an elongated transparent window, graduated for easy reading of the
liquid level V1 and thus the blood flow rate at all times.
The embodiment shown in FIG. 2 furthermore, serves as a bubble
trap in a much more efficient manner than the known design of FIG.
1. As indicated in FIG. 2 the blood is introduced into the inlet
chamber C below the blood level V1 in the chamber which means that
the influx of blood will not in any way whisk the blood to absorb
additional air bubbles. FIG. 2 shows, furthermore, that the blood
is introduced into chamber C in an upward direction as indicated
by the arrow in the inlet opening which coincides with the ascent
of any possible air bubbles trapped in the new blood. Practically
all entrapped air bubbles escape from the blood already in the chamber
C, and any possible remaining bubbles have an excellent opportunity
to escape through the surface V2 in chamber D due to the large cross-section
and concomitant small flow rate in this chamber.
The combination flow meter and bubble trap of the present invention
may be manufactured as a very compact unit, e.g., cast from a plastic
material, and at small expense. Thus, it may be feasible to combine
the device of this invention with the tubing of the extra-corporeal
flow circuit to provide an inexpensive unit which may be discarded
after use to save costly sterilization. |