US 3898637 A
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 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 (OCR text may contain errors)
United States Patent [1 1 Wolstenholme Aug. 5, 1975  DETECTION MEANS FOR GAS ENTERING 3,390,326 6/1968 lmadate 324/71 CP X HUMAN BLOOD SY FROM 3,469,157 9/1969 Rhodes 324/71 CP X 3,500,366 3/1970 Chesney et a1. 340/222 EXTRA-CORPOREAL TUBING Eugene B. Wolstenholme, 365 W. Bristol Rd., Northampton Twp., Southampton, Pa.
Filed: July 27, 1973 Appl. No.: 383,284
References Cited UNITED STATES PATENTS 2/1963 Clark 324/61 P UX 12/1963 Foster et a1. 324/71 CP UX Primary Examiner-John W. Caldwell Assistant Examiner-Daniel Myer Attorney, Agent, or FirmWoodcocl Washburn, Kurtz & Mackiewicz [5 7 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 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.
8 Claims, 3 Drawing Figures FREQUENCY DISCRIMINATOR OSCILLATOR RATE OF CHANGE DETECTOR AMPLIFIER THRESHOLD DETECTOR ALARM AND LATCH BCUTOFF RELAY PATENTEDAUG 5|975 IO i J v FREQUENCY DISCRIMINATOR L TOR RATE; OF CHANGE DETECTO F 'lg Z AMPLIFIER THRESHOLD DETECTOR ALARM AND LATCH cuToFF RELAY 1 DETECTION MEANS FOR GAS ENTERING HUMAN BLOOD SYSTEM FROM EXTRA-CORPOREAL TUBING BACKGROUND OF THE INVENTION l. Field of the Invention The invention has utility in 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.
2. 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 than the inner diameter of the tubing at the detection area, the effect of 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 disadvantage 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.
Rate of change detection has been used in various applications and may be used to advantage in automatic monitoring ofliquid level, as for example, in drip chambers. In US. Pat. No. 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 has 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 therethrough will be detected of a smaller size than previously practical.
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.
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 may 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 immersion 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.
BRIEF DESCRIPTIONS OF THE DRAWINGS FIG. 3 is an end cross-sectional view of the detector taken along the line 33 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 extracorporeal tubing (not shown) which is part of a flow line leading to the patient and which may be for example a tubing of 0.092 inches (inner diameter). The tubing may be slipped over the ingress and egress portions ll, 12 and is well known for in line connections such as this. Sensing elements comprising a center element 16 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 asthe venturi tube 14 of FIG. 2.
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 l6, 18, 20 are placed in close proximity to the flow path in the venturi area.
Shown passing through venturi 14 is bubble 28 which by means of the constricted venturi is brought into close proximity to the sensing elements 16, 18, 20. It is to be noted that the sensing elements do not probe within the flow line and are electrically insulated from the fluid to be monitored. The thickness of the venturi wall may be on the order of 0.015 inches with the sensing elements abutting the wall.
As seen in FIG. 2, a preferred embodiment of the proximity detector system is shown by means of the block diagram. The diagram represents, in functional form, a circuit such as that shown in FIG. 311 of the publication How to Build Proximity Detectors and Metal Locators" by .l. P. Shields, published by Bobbs- Merrill Inc. which represents a circuit used in a successfully-tested embodiment. 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 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 frequency changes which will in turn trip the threshold detector following it. A latch following the threshold detector locks an alarm cut-off relay which will stay locked until manually reset.
Circuitry for the above system may be along the lines of that I used in aforementioned U.S. Pat. No. 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 US. Pat. No. 3,500,366. In addition, still other examples of alternative constructions of the various stages may be found in the prior art. A capacitance-tuned oscillator appropriate for such a system is set forth at FIG. 22.4, page 308 of Transistor Circuit Design, published by McGraw-l-Iill. An adaptable discriminator is exemplitied at FIG. 2 A Color TV Sound System Using the p. A703 by L. Blaser Fairchild Semiconductor Corp.; and a typical differentiating circuit is shown at FIG. 13-6 of Electronic Measurements by Terman and Pettit, published by McGraw-Hill. A transistorized amplifier suitable for use with the disclosed invention is shown at ,FIG. 40 of the Fairchild Semiconductor Linear Integrated Circuit Application Handbook published by the Fairchild Semiconductor Corp. and a latching circuit is illustrated at 109, Volume 38; 23 of Electronics see Unit Junction Transistor turns off Latching Relay by D. P. Lynch.
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.
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 percent 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 l6, 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 operation of the rate of change detector.
The sensing rings l6, l8 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 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 0.01 inch. The close 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. I
With respect to the venturi 14, the orifice diameter is as small as practial for a given flow rate. A diameter on the order of 0.030 inch is desirable to detect a bubble of a diameter of approximately .01 inch.
Up to this point the discussion has been directed to the use of a radio frequency proximity detector coupled with the sensing elements 16, 18, 20. It is to be understood that in place of the radio frequency detector, an optical or ultrasonic detector system may as well be used where the venturi will enhance the effectiveness of detection.
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 of impurities such as liquid particles.
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 modifications, within the spirit and scope of the invention.
1. Apparatus for use with a system to detect discontinuities in a medium in a 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 substantially annular sensing elements which externally encircle said controlled restriction, at least one of said elements being disposed along said controlled restriction intermediate the ends thereof, said elements being separated from the area within the controlled restriction by a wall of predetermined thickness.
2. The apparatus of claim 1 wherein three sensing elements are used, the outer two of which are at a reference potential to permit the sensing of discontinuities in a flow of liquid from either direction.
3. Apparatus for use with a system to detect discontinuties in a medium in a 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;
a plurality of substantially annular sensing elements which externally encircle said controlled restriction, at least one of said elements being disposed along said controlled restriction intermediate the ends thereof, said elements being separated from the area within the controlled restriction by a wall of predetermined thickness;
monitoring means coupled to said sensing elements for detecting changes in the capacitance therebetween occasioned by the passage of a discontinuity through said controlled restriction; and
alarm means coupled to said monitoring means to signal and alarm condition upon the detection of a discontinuity in the medium.
4. The apparatus of claim 3 wherein said monitoring means includes:
an oscillator whose frequency is determined at least in part by the capacitance of the detectors;
a frequency discriminator for providing a signal which is a function of the oscillator frequency;
a rate of change detector responsive to said signal for producing an output when the rate of change of said signal exceeds a given magnitude;
a high pass amplifier for amplifying signals outputted by said detector representing fine frequency changes;
a threshold detector adapted to be tripped by said amplified outputs; and
a latch to control said alarm means.
5. The apparatus of claim 3 wherein said sensing elements are circumferentially disposed around the internal wall of said controlled restriction.
6. The apparatus of claim 3 in which the diameter of the central portion of said controlled restriction is determined by the diameter of the smallest discontinuity for which detection is desired.
7. The apparatus of claim 3 in which the conductivity detectors comprise three sensing elements.
8. Apparatus for use with a system to detect discontinuities in a medium in a 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 venturi with which said ingress and egress portions communicate.
a plurality of substantially annular sensing elements at least one of which encircles the narrowest portion of said venturi another of said elements being axially disposed along said venturi and spaced from said first element, said elements being separated from the area within said venturi by walls of aipredetermined thickness;
monitoring means to detect changes in capacitance between said elements during the flow of fluid through said venturi restriction; and
signal means coupled to said monitoring means to signal the presence of gaseous bubbles in the flow of fluid as manifested by changes in said capaci- UNl'l'l'Il) S'l'A'l'l-IS I'A'I'I'JNT OFFICE CILR'IIl'ICAIJL Ol' (JOIUULCHON Patent No. 3,898,637 Dated August 5,' 1975 Invcntor(s) Eugene B. Wolstenholme It is certified that: error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Col. 2, line 36: the word crosssetional" between "side" and "view" should be deleted.
Col. 3, line 66: "Unit Junction" should be "Unijunction".
Col. -4, line 41: "close" should be "closer".
Claim 8, line 30: the period [.1 should be replaced with a semicolon Signed and Scaled this I twenty-third D 3) Of December I 9 75 [SEAL] Attest:
RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner nfPaIents and Trademarks