Note: Descriptions are shown in the official language in which they were submitted.
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The invention has utility ln medical systems
whenever an objective is to prevent gas from entering -
the human blood system from an external supply system~ :
A few of the examples in which the invention has utility
are infusion, transfusion, intravenous feeding and ~
hemodialysis procedures. The object in any of these ~:
systems is to set an alarm and cease the procedure upon
the detection of the transportation of even small amounts
of gas through the external system to prevent its entry
into the patient.
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Prior Art
In view of the criticality of preventing gas from
entering the blood system of a patient, any failure of
the system will often result in the death of the-patient.
The detection of gas in such procedures has been of concern
for quite some time to the medical profession. Various
detection means have been used and are now in use. One
attempt to solve the problem has been the use of conductivity
probes immersed in the body entry line. In such systems, a
bubble between the probes will cause a reduction in
electrical current which in turn causes flow shut-off and
the alarm condition. While this approach is practical ;
for detecting bubbles of a relatively large size, when
bubbles are significantly smaller ~han the inner diameter
oi ~n~ ~uoing at tne detection area, the ef~ect o~ such a
bubble is negligible on overall electrical conductivity.
This means that apparatus adjustment for every application is
required and there is also the psychological disadvantages
of having electrical contact with the blood circuit.
Optical bubble detectors have also been used. This
type of detector is not effective when bubbles are significantly
smaller than the inner diameter of the conduit since the
blood or other liquid surrounding the bubble may conceal the
bubble from the optics of the system. In commercial
embodiments of optical systems, as much as 2 cc of gas has
been necessary before it could be detected.
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Rate of change detection has been used in various
applications and may be used to advantage in automatic
monitoring of liquid level, as for example, in drip chambers.
In U.S. Patent 3,500,366 - Chesney et al, a monitoring system
for fluid flow in drop form is disclosed which comprises a -
radio frequency oscillator having a resonant circuit with
two electrodes spaced apart axially along the outside of the
drip chamber and coupled to the resonant circuit. Downward
.
passage of a drop, frequency modulates the oscillator output
and a detector demodulates the frequency modulation to
produce electrical pulses as a measure of the drip rate. -
Upon an alarm condition, shut-off mechanism will stop the
flow. While such a system bas advantages for use with drop
monitors, there is not disclosed a means of detection of
gases through flow lines which is the principal object of
the present invention.
Summary of the Invention
Accordingly, it is an object of the subject
invention to provide reliable gas detection means for use
in medical systems to detect the passage of gas through
body feed lines.
It is a further object of the subject invention
to provide gas detection means in which the flow path is
of such configuration that a gas bubble passing there-
through will be detected of a smaller size than previously
practical.
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It is still another object of the subject invention
to provide gas detection means which may be used with transparent
and/or opaque liquids.
It is yet a further object of the subject invention
to provide a detection means which can be used to detect a
variation in the flow of a fluid passing through a flow line
whether the fluid be liquid or gas and thus may be used to
detect impurities in the fluid. -
The subject invention is directed to an apparatus for
detecting change in medium within a confined fluid path. A
controlled restriction is disposed within the confined fluid
path. Detectors are disposed adjacent to but externally of
the controlled restriction. Included are monitoring means that
are responsi~e to the detectors to monitor changes in medium
within the controlled restriction. Also included are signal
meanQ that are responsive to the monitoring means to signal
changes in the medium flowing through the controlled restriction.
Accordingly, the subject invention is specifically
directed to a means of detecting gas bubbles flowing through
a liquid medium within extra-corporeal tubing prior to entry
into the human blood system. The detection unit in a preferred-
embodiment may comprise a venturi tube which has a mean diameter
to ensure the detection of bubbles of the smallest desired size,
and sensing elements which comprise electrical detectors
~ which are placed in very close proximity to the restricted flow
; line of the detection area. The sensing elements may be coupled
with a radio frequency proximity detector which is operated at
a high enough frequency to eliminate the need for direct immer- -
sion probes. A rate of change circuitry follows the proximity -
detector and upon a small signal from the detector, the change
will be apparent to detect a bubble of any desired size.
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This invention is specifically directed to an
apparatus for detecting change in a medium flowing within a
confined fluid path comprising: ;
a controlled restriction having an external wall and :
disposed within the confined fluid path;
a plurality of annular sensing elements peripherally
disposed around the external wall and spaced axially along -~
the fluid path;
the controlled restriction extending between the ~ ~:
sensing elements;
monitoring means responsive to the sensing elements
to monitor changes in the medium within the controlled
restriction; and
signal means responsive to the monitoring means
to signal changes in the medium flowing through the controlled
restriction.
In the apparatus above, the monitoring means can
include: .
an oscillator to determine the frequency from the
capacitance of the sensing elements;
a frequency discriminator for providing a signal
proportional to the oscillator frequency;
a rate of change detector responsive to the signal
for producing an output when the signal changes by a given
magnitude in a short interval of time.;
a high pass amplifier for amplifying fine frequency
changes;
a threshold detector to be tripped by the fine
frequency changes; and
a latch to control the signal means.
In the apparatus, the cross-sectional area of the
central portion of the controlled restriction can be determined
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by the cross-sectional area of the smallest unrestrained
globule for which detection is desired.
The invention is also specifically directed to a
detection unit for detecting a change in a medium flowing in
a confined fluid path comprising:
a venturi restriction through which the medium flows;
a plurality of substantially annular sensing elements
disposed about axially-spaced points in the venturi restriction; '
monitoring means to detect changes in impedence
between ones of the elements during the flow of medium through
the venturi restriction; and
signal means responsive to the monitoring means to
signal discontinuities in the flow of the medium as manifested
by changes in capacity. ~ 'r
The invention also specifically relates to an ~A '.
apparatus for use with a system to detect discontinuities in
a medium flowing through a confined fluid path comprising:
a housing unit having ingress and egress portions
; adapted to be connected in fluid communication with a flow
line, the housing unit further including a central portion
having a controlled restriction with which the ingress and
egress portions communicate; and
a plurality of sensing elements which penetrate
the housing unit and substantially externally encircle the
control restriction, the elements being separated from the
internal control restriction by a wall member.
In the apparatus above described, three sensing
elements can be used, the outer two of which are maintained
at a common reference potential to permit the sensing of a -
liquid flowing from either direction.
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srief Descriptions of the Drawings
.
Fig. 1 is an isometric view of ~he bubble detector
showing the outer cover structure of the venturi tube with the
sensing rings protruding therefrom;
Fig. 2 is a side cross-sectional view of the detector
taken along the lines 2-2 of Fig. 1 and showing by block diagram
means, the general circuitry for the rate of change detection;
and
Fig~ 3 is an end cross-sectional view of the detector
taken along the line 3-3 of Fig. 1.
Detailed Description
The bubble detector comprises generally the detection
means and the circuitry to measure a rate of change in the
liquid flow caused by a bubble of gas flowing through one of the
various medical systems described above such as that used for
hemodialysis.
More particularly, with respect to Fig. 1, the
detector means comprises a housing unit 10 with ingress 11 and
egress 12 portions which are to be connected to extra-corporeal
tubing (not shown) which is part of a flow line leading to the
patient and which may be for example a tubing of .092 inches
(inner diameter). The tubing may be slipped over the ingress
and egress portions 11, 12 and is well known for in line con- -
nections such as this. Sensing elements comprising a center
element 1~ and two outer elements 18 and 20 are inserted or
molded into the housing unit 10 as shown. A portion of housing
10 comprises a central section 13 of increased diameter in ~-
which is included the restricted sensing area such as the ~
venturi tube 14 of Fig. 2. ;
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With reference to Figs. 2 and 3, it can be seen
; that the sensing elements 16, 18, 20 are sensing rings
having a rounded portion 22, 24 and 26 each of which
encircle the venturi tube 14 at their respective positions.
As seen in Fig. 2, the sensing elements 16, 18, 20 are -
placed in clcse proximity to the flow path in the venturi
area.
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Shown passing through venturi 14 is bubble 28
w~ich by means of the constricted venturi is brought into
close proximity to the sensing elements 16, 18, 20. It
. ~8 to be noted that the sensing elements do not probe within
the flow line and are electr~cally insulated from the fluid
I to be monitored. The thickness of the venturi wall may be
on the order of .015 inches with the sensing elements abutting
! the wall.
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~' As seen in Fig. 2, a preferred embodiment of the
proximity detector system is shown by means of the block
diagram. First, there is a radio frequency oscillator whose
frequency is determined by the capacitance of the sensing
elements 16, 18, 20. Following the oscillator is a frequency
discriminator which produces a voltage proportional to the
oscillator frequency. Following the frequency discriminator
is a rate of change detector. This is necessary since very
, low voltages are involved and the frequency discriminator
only puts out an output of about one millivolt in the presence
of a bubble. Over a long period of time, the oscillator,
due to changes of temperature or supply voltage, will drift
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M-484 i~056~ ~
and cause a change of output exceeding this magnitude. It -
is necessary that these long term changes in voltage be
disregarded. The rate of change detector which follows
the frequency discriminator responds to a voltage level
change approximating a millivolt in a short time such as
a tenth of a second. However, it will not respond to
changes of the same magnitude which occur over a long time
period. ~
. ' ' ' '..... . . The output of the rate of change detector is
applied to or is integral to a high pass amplifier which
rejects slow drift in the radio circuits but amplifies fine
$requency changes which will in turn trip the threshold
detector following it. A latch following the threshold
detector locks an alarm cut-off reiay which will stav
locked until manually reset. ;
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` Circuitry for the above system may be along the
nes of that used in aforementioned U.S. Patent 3,500,366.
The subject invention is used with kidney hemodialysis
machines and the like and the interest is limited to monitoring
and not time control as in U.S. Patent 3,500,366.
.
In operation, when a globule passes the sensing
elements, a shift in the oscillator frequency will occur
giving a higher signal which will be amplified and will
cause the threshold detector to be tripped, thus causing
the latch to cause an alarm condition. ~
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It will be noted that detection involves impedance
monitoring which includes fluid dielectric constant as well
as conductivity and thus, the system has utility for all -~
fluids. While a small bubble may cause only a very small
signal from the proximity detector such as 0.1 per cent of
full output there will nevertheless be a change in signal
sufficient to actuate the rate of change detector and set
off an alarm condition with the flow being terminated.
Bubble detection on an even higher order is made
possible by the venturi tube 14 which brings the bubble in
close proximity to the sensing elements 16, 18, 20 making
it possible to detect bubbles, smaller than the inside
diameter of the tubing. The venturi 14 also causes high
fluid velocity through the venturi which improves the
o~eration of the rate of change detector.
The sensing rings 16, 18 and 20 are made of an
electrical conductive material such as beryllium, copper
or could even be stainless steel or aluminum. As shown in
Figs. 2 and 3, they encircle the venturi tube 14 and may be
molded in placed if a mold is used for the housing structure.
The extra-corporeal tubing generally will be PVC, but of
course may be any tubing such as rubber, silicone, or
polyurethane.
With respect to the positioning of the sensing
elements 16, 18, 20, it is advantageous to position them
close together since the rate of change starts to actuate
as the bubble front passes the first element 18 as indicated
by the flow arrows in Fig. 2. Sensing elements 18 and 20
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are ground elements and since they are on both sides of
the central element 16, the flow may be in either direction.
Only one ground element would be necessary but this would
restrict proper operation of the detector to one flow
direction. The sensing elements 16, 18, 20 can be placed
at a distance from each other on the order of .01 inch.
The closer together they are, the more sensitive is the
instrument. A ground is not absolutely necessary since a
balanced oscillator may be used in lieu thereof. The sensing
elements 16, 18, 20 do not necessarily need to be placed
longitudinally to the flow path nor do they need to radially
encompass the sensing area.
With respect to the venturi 14, the orifice diameter
is as small as practical for a given flow rate. A diameter
on the order of .030 inch is desirable to detect a bubble
of a diameter of approximately .01 inch.
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The subject invention may be used for detecting
either changes in a liquid medium or a gas medium, an example
of the latter being utilized in an air line to detect the
presence o~ impurities such as liquid particles.
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While particular embodiments of the invention have
been shown and described, it will of course be understood ~
that various modifications may be made without departing . .
from the principle of the invention. The appended claims
are, therefore, intended to cover any such moaifications,
within the spirit and scope of the invention.
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