US2976865A - Cylindrical strain gauge - Google Patents

Description

March 28, 1961 R. E. sHlPLEY 2,976,865

CYLINDRICAL STRAIN GAUGE Filed oct. 21, 1958 Fl 6 l INVENToR. @cf/wp I 5H/PL fr United vStates Patent O CYLINDRICAL STRAIN GAUGE Richard Edwin Shipley, 400 Hazel Ave., Millbrae, Calif.

Filed Oct. 21, 1958, Ser. No. 768,644

Claims. (Cl. 12S-2.05)

This invention relates to transducers or pressure sensing devices, and more particularly to an intracardiac manometer capable of insertion into the heart and blood vessels in either infants or adults.

In the care of patients with acquired or. congenital heart disease, it is frequently essential to perform right heart catheterization in order to establish the diagnosis or evaluate the severity of physiologic disturbance resulting from varied cardiac pathology. During this procedure, intracardiac pressure changes or pressure-pulse contours are routinely recorded for the above purposes. In addition, blood samples from the catheterized heart chambers or great vessels entered are obtained for purposes of oxygen content analysis in order to determine cardiac output or localize structural defects causing intracardiac shunting of blood.

At the present time, such examination is carried out by inserting a long, narrow bore, plastic catheter via an appropriate vein (as for example, in the arm of an adult through the median basilic vein or in the groin of an infant through the saphenous vein). The tip of the catheter is maneuvered under uoroscopic control into the accessible intracardiac chambers or great vessels. The blood samples are withdrawn from the tip through the catheter, and pulse-pressures at the catheter tip are transmitted via the long fluid column to an externally placed pressure pick-up and appropriately recorded.

Although this technique enables proper blood sampling, it does not permit the registration of accurate, artefactfree, intracardiac or great vessel pressure changes. These limitations are, of course, due to the long iluid column in the catheter between the externally placed transducer and the area of the heart or great vessels being examined. The effective mass and viscous damping due to this iiuid column markedly impair the frequency response of the over-all system. These eifects are also enhanced by the lack of catheter rigidity and the presence at times of air bubbles anywhere in the fluid system. The pressure curves are seriously distorted, lparticularly in infants or children with faster heart rates, by artefact produced by pressures generated in the catheter lumen due to oscillation of the fluid column induced by cardiac contraction or other source of catheter movement.

Wetterer, in 1943, and Gauer and Gienapp, in 1950, pointed out that the deciencies regarding frequency response and artefact could be overcome by a manometer of small mass attached to the catheter tip with lead wires extending via the catheter wall to the amplifierrecorder assembly. Gauer and Gienapp designed a differential transformer manometer with relative coupling of the two coils effected by a movable iron core mounted on a piston actuated by pressure changes. In spite of their efforts, the space requirements of the internal components of this manometer made it impossible to construct a manometer having an external diameter less than the diameter of a No. 8 French catheter, which limited the use of such an instrument to adults having veins suiciently large to accommodate this size catheter. Further- 2,976,865 Patented Mar. 28, 1961 ICC more, being of solid construction, blood samples could not be taken at the point of pressure measurement at the tip of the catheter-manometer assembly. Thus, the above noted limitations of size and solid construction have prevented this transducer from receiving more than limited clinical trails.

In order to be used in infants (aged two years and under), a No. 6 French catheter, having an external diameter of only 0.079 inch, must be employed in over 90% of infant catheterizations. This diameter, of course, cannot be exceeded by the external diameter of the transducer, and this severe size limitation has heretofore prevented the development of a practical instrument. It is apparent that if a transducer could be made to t within an infants veins (on the end of a No. 6 French catheter), then the dimensions could easily be enlarged so that it could be used with a No. 8 French catheter for use in adults, but the opposite is not necessarily true. A transducer might be made sufficiently small for use on a large catheter, but this would not mean that it would be possible from a manufacturing standpoint to reduce the size at will to obtain an instrument which would work with the smallest catheter to be used. With this in mind the present invention was made in view of the smallest dimensions to be encountered in use.

In the development of the present invention, the following basic criteria dictated the construction of the manometer:

(l) The blood sampling must be permitted from the exact point of pressure measurement, that is, at the distal end of the catheter-manometer assembly.

(2) The manometer must be the most distal portion of the catheter-manometer system, and blood sampling must be permitted through the distal end of the manometer.

(3) The external diameter of the manometer must not exceed in size the outer diameter of a No. 6 French catheter and should have the same cross-sectional configuration thereas.

(4) The lumen of the catheter must not be constricted in any Way by the lumen of the manometer and the lumens of the catheter and manometer should be similar in cross-sectional configuration.

(5 The length of the manometer should be sutliciently long to enable a sensing device to be incorporated therein but must Abe suiiiciently short so as not to impair the flexibility of the catheter tip in such manner as would hinder the maneuvering of the catheter through the veins.

(6) The manometer must 'be constructed so as not to cause undue trauma to a patient.

(7) The manometer, and/or manometer-catheter assembly, should be constructed in such manner as to permit the cleaning and sterilizing thereof by normal techniques used in the art.

It is the chief object of this invention to provide an intracardiac manometer capable of meeting the above requirements.

Another object is to provide an intracardiac manometer which is relatively inexpensive to manufacture.

Another object is to provide a cylindrical strain gauge for pressure sensing employing two concentrically disposed tubes with -a sealed annular chamber therebetween in which is disposed a strain gauge wire, and with or Fig. 2 is a sectional View taken on line 2-2 of Fig. l and at the end of hub 15.

Fig. 3 is a generally schematic view' of the invention in use in a heart to record the pressures therein and to permit blood samples tot be taken therefrom.

Fig. 4 is a schematic representation of an'electrical circuit' embodying the invention.

Referring now to the drawings, and particularly to Figs. l and 2 thereof, the cylindrical strain gauge, designed for use as an intracardiac manometer, and generally indicated by .the reference numeral 10, comprises inner and outer tubular members 11 and 12 coaxially disposed one within the other and spaced radially from one another to formv an elongated annular chamber 13 therebetween. These tubular members are preferably machined from molded nylon blanks. The* inner tubular member has radially enlarged hubs 14 and 15 on each end thereof which are bonded to the outer tubular member 11 by a suitable resin or adhesive 16, thus sealing each end of chamber 13. The sealed chamber 13 may be iilled with air, or the like, or may be evacuated, if desired.

A first length of strain gauge wire 17 is disposed within chamber 13 and is spirally wound under tension around the inner tubular member 12. The wire 17 is bonded throughout its length by suitable adhesive to the inner tubular member so as to move integrally therewith.

As will be seen in Fig. l, the wall thickness of the outer tubular member 11 is much greater than that of the inner tubular member and consequently the inner tubular member will be much more exible in a lateral direction than will be the outer tubular member. If the manometer is immersed in a fluid, the pressure inside the inner tubular member will be equal to the pressure outside the outer tubular member. If this pressure is increased, the inner tubular member will flex outwardly while the outer tubular member will be comparatively unfiexed. Similarly, if the pressure decreases, the diameter of the inner tubular member will decrease slightly. In other words, the inner tubular member behaves as a cylindrical diaphragm disposed within the outer tubular member. In either direction of flexing movement, the magnitude of movement will be greater towards the middle of the inner tubular member. It will also be appreciated that as the inner tubular member tiexes, the length of the strain gauge wire 17 will vary in accordance therewith, and the `resistance of the wire will vary in direct accordance with its change in length.

A second length of strain gauge wire 1S is disposed in chamber 13 and is preferably placed near hub 15, so as to be unaffected by flexing movement of the inner tubular member due to pressure changes. To further insure that the wire 18 will be unaffected by flexing movement of the inner tubular member 12, it is encased in the cement 16. The two pieces of strain gauge wire 17 and 18 are preferably approximately equal in length and diameter.

The hub is longitudinally slotted, as at 19, to accommodate lead wires 20, 21, 22 and 23, which are cemented therein. Lead wires 20 and 21 are connected electrically to each end of strain gauge wire 17, and lead wires 22 and 23 are electrically connected to each end of strain gauge wire 1S.

A plastic catheter 25 has its end 26 reduced in diameter so as to iit into the end of .the outer tubular member 11. A plurality of lead wires 27 (four in all) are embedded longitudinally within the wall of catheter 25. In connecting the manometer 10 to the catheter 25, the lead wires 20, 21, 22 and 23 are each electrically connected or soldered, as at 2S, to one of the catheter lead wires 27. A forming mandrel (not shown) is inserted through the lumen of the manometer and into the lumen 29 of the catheter. The reduced end of the catheter is coated with adhesive 30. The manometer and catheter are then twisted as they are pushed together so that the lead wires 20, 21, 22 and 23 will spiral around the forming mandrel. A reverse twisting action applied to the manometer before adhesive 30 hardens will cause the lead wires to move laterally outward from the forming mandrel and into contact with the inner wall of tubular member 11. When so assembled, adhesive 30 fills the space between the end of hub 15 and the end of the catheter, thereby embedding lthe lead wires. After the adhesive 30 is cured, the forming mandrel is removed, leaving the lead wires bonded in a spiral around the inner wall of tubular member 11 and disposed in a positiony where they will not impede the lumen of catheter 25 or make contact with iiuid passing therethrough.

If later desired, the manometer 10 may be removed from the catheter 25 by dissolving the adhesive 30 by a suitable solvent. The manometer and catheter are then untwisted and pulled apart, and the catheter leads 27 are disconnected from the manometer leads 20, 21, 22 and 23. As is apparent, the adhesives 16 and 30 should be of different material snch that a solvent may be used which will be effective to dissolve adhesive 30 without dissolving the adhesive 16.

The catheter 25 has standard dimensions which dictate the dimension of the manometer 10. yAs has been mentioned above, a No, 6 French catheter is necessary in most instances, where young infants (up to two years of age) must be examined. Such a catheter has an outside diameter of 0.079 inch and a lumen of between 0.036 and 0.038 inch. As a consequence, the manometer 10 must have an external diameter of not more than 0.079 inch, and the internal diameter, or lumen, thereof must be at least equal to the lumen of the catheter. In a successful model of the present invention, the outside diameter is 0.079 inch and the lumen is 0.040 inch. In addition, this Wall thickness of the outer and inner tubular members is 0.010 and 0.001 inch, respectively. If a larger catheter is used, for example, a No. 7 French or a No. 8 French in instances where adults are examined, `the inner and outer diameter of the manometer 10 may be increased in accordance therewith.

In the use of the invention, the catheter 25, with the manometer 10 secured to its end, is inserted in an appropriate vein 31 of the patient. The catheter is then worked through the vein until the manometer 10 is situated at the desired point, as in chamber 32 of heart 33. The catheter lead wires 27 are connected to a suitable recording instrument 34.

Fig. 4 is merely representative of an electrical system which may be used to obtain an indication of the pressures within the heart 33. The two strain gauge wires 17 and 18 are connected into a bridge circuit, together with fixed resistances 35 and 36. As is well known, when the bridge circuit is energized by battery 37, the current through the current sensing device 38 will be governed by the relative value of the resistances 17, 18, 35 and 36. The current sensing device 38 may be used to control the movement of pen 39 which records the heart pressures on the moving chart 40 in a conventional manner.

As the heart beats, the pressure therein will Vary. These variations in pressure cause the inner tubular member 12 to expand or contract therewith, in turn causing the length of the strain gauge Wire 17 to increase or decrease therewith and the resistance of the wire to correspondingly increase or decrease. The strain gauge wire 1S does not respond to the lateral excursions of the inner tubular member and thus does not change in resistance and is thus unaffected by pressure variations. The above variations in resistance of Wire 17 create imbalances in .the bridge circuit and cause changes in the current iiow through the sensing device 38 in correspondence with the variation of the pressures in the heart. These then are recorded by pen 39 on chart 40.

The strain gauge wire 18 is used for temperature compensationpurposes. As the temperature of the fluid in which the manometer 10 is immersed varies, the strain gauge wires 17 and 18 Will be equally atected. The thermal expansions and contractions in length, and the resistance changes, of the two wires 17 and 18 will be equal, and the bridge will remain in balance.

As will be apparent, the resistance changes in the strain gauge wires 17 and 18 are relatively small, due to the small dimensions of the manometer, but with the use of suitable amplifying equipment the signals may be amplified so as to be used in conventional recording apparatus.

At any time, blood may be taken from the tip of the manometer, through catheter lumen 29, by means of the syringe 41. Thus, blood sampling may be made by the invention at the precise point at which the pressure measurements are being made.

In addiiton, desired solutions may be forced by the syringe 41- through the catheter and out through the manometer into the circulatory system. The invention has an important aspect in such use, namely, that the lumen of the manometer is at least the same in size as that of the catheter. Also, the cross-sectional shapes of the lumensof the catheter and manometer are both circular, and, therefore, the catheter lumen is unrestricted by the manometer'lumen. Thus, there is no danger that the force necessary to force theiiuid from the syringe through the catheter will cause the manometer to break oi of the end of the catheter and become lodged in the patients circulatory system.

By the use of the present invention it is now possible to obtain pressure indications directly within the heart and to take blood samples from the exact location under examination. The present invention eliminates the inherent undesirable features of externally located pressure indicators by the elimination of the long blood column in the catheter as a part of the pressure sensing system. The outer tubular member of the manometer is perfectly smooth and will not cause any more trauma in a patient than will normal catheterization. The manometer is completely sealed and can be cleaned and sterilized by normal techniques.

It Will, of course, be appreciated that, although the cylindrical strain gauge 10 has been illustrated as an intracardiac manometer and has important advantages in such use, the cylindrical strain gauge may be used for other purposes wherein the continuity of the fluid vessel, containing the fluid whose pressure is to be determined, is to be maintained. As, for example, the strain gauge may be employed as part 'of a fluid conduit through which uid ows with the gauge being used to measure the pres sure of such uid. It is also apparent that this gauge is well adapted for remote indications of pressure since there is only an electrical connection lbetween the gauge and the recording, or indicating, instrument. Other uses of such a device will be apparent to those in the art.

Although the strain gauge wire has been described as being Ibonded by adhesive throughout its length to the outer circumference of the inner tubular member, it is possible that the wire may be so secured at only its ends, or may be secured by other than adhesive bonding. For example, the strain gauge wire might `be embedded in plastic and then disposed around the inner tubular member. The important thing is that the strain gauge wire be varied in length by ilexure of the inner tubular member so that variations in resistance of the wire will result from pressure variations which induce flexure of the inner tubular member.

It should be noted that the cylindrical strain gauge 10 is insensitive to position and movement. That is, the device may be disposed with its axis horizontal or vertical or in any other direction, and the amount of ilexure of the inner tubular member by pressure changes acting thereon will be equal. Similarly, movements of the entire device, whether gradual, abrupt, or sustained will not be sensed by the device. For example, suppose the device were abruptly moved in a direction normal toits axis. Theoretically, the inertia of the inner tubular member would cause it to bow slightly ina direction opposite to the direction of movement, but such bowing will not affect the circumference of the member nor the length of the strain gauge wire therearound. As is apparent, the insensitivity of the device to position yand 4movement greatly enhances the reliability of the measurements of pressure variation to which the device is sensitive.

It is to be further realized that the form of the invention herewith shown and described is to be taken as a preferred embodiment of the same, and that various changes in the shape, size and arrangement of parts may be resorted to without departing from the spirit of the invention or the scope of the attached claims.

Having thus described my invention, what I claim and desire to secure `by Letters Patent is:

l. A cylindrical strain gauge comprising a first tubular member consisting of a unitary self-supporting relatively rigid but flexible insulating element capable of circumferential expansion and contraction and adapted to have uid pressure exerted on the interior thereof, a length of strain gauge wire disposed around the exterior of said rst tubular member, means securing said strain gauge wire throughout its length to said tubular member so that circumferential expansion and contraction of said tubular member Will vary the length of said strain gauge Iwire, a relatively rigid second tubular member disposed coaxially -around said rst tubular member and spaced therefrom Ito form an annular space therebetween, and means sealing the ends of said first tubular member to said second tubular member.

2. An intracardiac manometer comprising first and second plastic tubular members coaxially disposed one within the other and spaced radially from one another to form an elongated annular chamber therebetween, means sealing the ends of one of said tubular members to the `other tubular member to seal the ends of said chamber, said rst tubular member being relatively stiff and said second tubular member consisting of a unitary self-supporting relatively rigid but tiexible insulating material which is relatively flexible in a lateral direction as compared to said first tubular member, a length of strain gauge wire spirally wound in said chamber'along the length of said second tubular member, and means bonding said length of strain gauge wire throughout its length to said second tubular member so that lateral flexing of said second member will vary 'the length of said wire, the outermost of said tubular members having an external diameter sufficiently small to pass through a humans veins into a chamber of the heart thereof and the innermost of said tubular members having an internal diam-` eter sufficiently great to permit the passage of fluids therethrough.

3. A cylindrical strain gauge comprising rst and second tubular members coaxially disposed one Within the other and spaced radially from one another to form an elongated annular chamber therebetween, means sealing the ends of one of said tubular members to the other tubular member to seal the ends of said chamber, said first tubular member being relatively stiff and a major portion of said second tubular member being relatively flexible in a lateral direction as compared to said first tubular member, a first length of strain gauge wire wound in said chamber around the relatively flexible portion of said second tubular member, means bonding said rst length of strain gauge wire to said second member so that lateral ilexing of said second member will vary the length of said wire, and a second length of strain gauge wire wound around said second tubular member on a relatively rigid portion thereof, said second length being approximately equal to the length of said first length of strain gauge wire but insensitive to lateral exing of said second member.

4. A11 intracardiac manometer comprising first and secon'd plastic tubular members coaxially disposed one within the other and spaced radially from one another to form an elongated annular chamber therebetween, means sealing the ends of one of said tubular members to the other tubular member to seal the ends of said chamber, said rst tubular member having a greater wallthickness than said second tubular member, a rst length of strain gauge wire spirally wound in said chamber along the length of said second tubular member, means bondting said rst length of strain gauge wire throughout its length to said second tubular member, and a second length of strain gauge wire wound around said second tubular member and disposed in said sealing means at one end, said second length being approximately equal to the length of said rst length of strain gauge wire but insensitive to lateral ilexing of said second member, the outermost of said tubular members having an external diameter sufciently small to pass through an infants veins into a chamber of the heart thereof and the innermost of said tubular members having an internal diameter suiciently great to permit the passage of fluids therethrough.

5. Apparatus of the character described comprising rst and second tubular members coaxially disposed one within the other and spaced radially from one another to form an elongated annular chamber therebetween, means sealing the endsk of one of said tubular members to the other tubular member to seal the ends of said chamber, said first tubular member being relatively stiff and said second tubular member being relatively flexible in a lateral direction as compared to one another, a rst length of strain gauge wire wound in said chamber around said second tubular member, means bonding said first length of strain gauge wire to said second member so that lateral flexing of said second member will vary the length of said wire, a second length of strain gauge wire wound around said second tubular member and disposed in said sealing means at one end, said second length being approximately equal to the length of said irst length of strain gauge wire but insensitive to lateral exing of said second member.

Article on Barium Titanate Microphone, Final Report on Atmospheric Physics, Penn State College, 1950, pages 19-22, Figure 1.38. Copy in 12S-2.05.

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