CA1060561A

CA1060561A - Detection means for gas entering human blood system from extra-corporeal tubing

Info

Publication number: CA1060561A
Application number: CA205,718A
Authority: CA
Inventors: Eugene B. Wolstenholme
Original assignee: Individual
Current assignee: Individual
Priority date: 1973-07-27
Filing date: 1974-07-26
Publication date: 1979-08-14
Also published as: JPS5071190A; GB1466594A; BE817955A; US3898637A; DE2436140A1

Abstract

ABSTRACT
Apparatus for detecting the flow of gas through tubing which is suited for use during medical procedures in which liquids are pumped into the blood system of a patient. The gas or bubble detector comprises conductivity detectors positioned along a controlled flow path such as a detecting means such as a venturi tube so as to be in close proximity to the flow through the tube. In a preferred embodiment, the conductivity detectors are connected to rate of change of flow detector circuitry which is in turn hooked to alarm and shut-off means to be actuated upon detection of gas bubbles.

Description

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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 ~ - 4a -10~05~
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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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.
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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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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for detecting change in a medium flowing within a confined fluid path comprising:
a controlled restriction having an external wall and disposed within said confined fluid path;
a plurality of annular sensing elements peripherally disposed around said external wall, and spaced axially along said fluid path;
said controlled restriction extending between said sensing elements;
monitoring means responsive to said sensing elements to monitor changes in the medium within said controlled restriction; and signal means responsive to said monitoring means to signal changes in the medium flowing through the controlled restriction.

2. The apparatus of claim 1 wherein said monitoring means includes:
an oscillator to determine the frequency from the capacitance of the sensing elements;
a frequency discriminator for providing a signal pro-portional to the oscillator frequency;
a rate of change detector responsive to said signal for producing an output when said 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 said fine frequency changes; and a latch to control said signal means.

3. The apparatus of claim 1 in which the cross-sectional area of the central portion of said controlled restriction is determined by the cross-sectional area of the smallest unrestrained globule for which detection is desired.

4. A detection unit for detecting a change in a medium flowing in a confined fluid path comprising:
a venturi restriction through which said medium flows;
a plurality of substantially annular sensing elements disposed about axially-spaced points in said venturi restriction;
monitoring means to detect changes in impedence between ones of said elements during said flow of medium through said venturi restriction; and signal means responsive to said monitoring means to signal discontinuities in the flow of the medium as manifested by changes in capacity.

5. Apparatus for use with a system to detect discon-tinuities 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, said housing unit further including a central portion having a controlled restriction with which said ingress and egress portions communicate; and a plurality of sensing elements which penetrate said housing unit and substantially externally encircle said control restriction, said elements being separated from the internal control restriction by a wall member.

6. The apparatus of claim 5 wherein three sensing elements are used, the outer two of which are maintained at a common reference potential to permit the sensing of a liquid flowing from either direction.