TUBULAR PRESSURE TRANSDUCER

Abstract: In a blood pressure monitoring system, a transducer which is a conductive rubber tube (16) having conductive termi­ nals (19) at each end.

Description: This invention relates to a fluid (gas or liquid) pressure transducer, and particularly to a blood pressure transducer. Blood pressure transducers are known. See, for example, Patent No. 4,576,181 disclosing a disposable blood pressure transducer and Patent No. 4,610,256 disclosing a blood pressure transducer having a disposable dome. Such transducers are complicated, expensive, and somewhat difficult to set up to provide assurance of a complete debubbling, that is, complete removal of air bubbles in the system. Transducers of the type described in the patents referred to above employ a silicon chip forming a pressure sensor, an elastomeric diaphragm on which the chip is mounted, a temperature compensation circuit, a light shield because of the sensitivity of the silicon chip, and a housing mounting all of the foregoing elements in such a way that they can be connected in line with the tubing to which the patient's catheter is connected. The system, including the transducer and tubing, is filled with a saline solution that drips slowly through the catheter, the catheter being inserted into the patient's blood vessel. Thus, the pressure in the blood vessel is transmitted directly via the saline solution through the tubing to the transducer. The sensor is electrically connected to a blood pressure monitor presenting a visual display of the patient's blood pressure. Less complex in structure is a tubular sensor of Patent No. 4,600,855. That system, however, requires a special tube and piezoelectric film surrounding the tube. A complex electric circuit is employed to energize the piezoelectric film to cause the tube to resonate and to monitor the frequencies of resonance. U.S. Patent 3748623 describes another blood pressure transducer, one end of which is secured to a plate member which includes an opening. A diaphragm is mounted in the opening and comprises a central rigid plate or piston supported in the opening by an elastic material such that the piston is free to move in response to varying pressure on the diaphragm. The other end of the transducer is secured to the piston. When fluid pressure in a tube is to be measured, the plate member is mounted in a suitably shaped slot cut in the tube wall. French Patent 2175611 provides a method for the construction of deformable electronic components and circuits by deposition of a thin layer of electro-conductive, electro-semi-conductive, photo-electric conductive, magnetic or magnetisable material on a pliable, flexible or elastic substrate. A disposable pressure detector is described comprising a silver-coated elastic hood secured around the end of a catheter tube. An objective of the present invention has been to provide a disposable pressure transducer which is exceedingly simple in its construction, is very inexpensive, and presents virtually no debubbling set up problems. A transducer for measuring fluid pressure in accordance with the invention comprises a tube, a sensor that varies in resistance in response to variations in pressure to which the tube is subjected and conductors connected to the sensor, characterised in that the tube is a resilient insulative tube, in that semi-conductive material is provided on or in the surface of a region of the tube, conductors being connected to the semi-conductive region and in that the combination of the resilient tube and semi-conductive region creates the sensor, the variations in the resistance of which are due to expansion and contraction of the semi-conductive region when the tube expands and contracts under varying pressure. When a fluid pressure system is to be monitored, the tube is connected thereto. The conductors are connected to the monitoring system, preferably through a bridge circuit, the tube forming one part of a Wheatstone bridge. It has been found that such a tubular transducer provides an excellent monitor of the pressure variations within the fluid system. In one embodiment, a stripe or band of sensor material is deposited on the surface of the tubing to form the sensor. The sensing material could be applied by printing, vapour deposition, etc. In an alternative embodiment, a layer is created immediately below the surface of the tube that is semi-conductive and is formed by ion implantation in accordance with a well known process such as is disclosed in "High Tech Materials Alert", July 1987, page 3. In this embodiment, as well as the former embodiment, when the transducer is used as a blood pressure transducer, as contrasted to an industrial application, the tube material is a dielectric. The dielectric nature of the tubing isolates the sensing element, be it printed or ion implanted, from the fluid within the tube whose pressure is to be measured. Obviously, in blood pressure monitoring applications, the straight tubular section in series with the tube connected to the patient's catheter introduces no problem of debubbling. Thus, bubbles which could damp the electrical signals are eliminated as well as any hazardous bubbles that might enter the patient's circulatory system and cause embolism. The several features of the invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which: Fig. 1 is a diagrammatic view of a pressure monitoring system, the encircled portion being greatly enlarged for illustrative purposes; Fig. 2 is a cross section taken along lines 2-2 of Fig. 1; and Fig. 3 is a fragmantary perspective view of the transducer. Referring to Fig. 1, a conventional blood pressure monitoring system is shown. It includes a catheter 10 for insertion into a patient's blood vessel. Pressure tubing 11 connects the catheter to a transducer 12 of the present invention. A bag of saline solution 13 is connected by a tubing 14 and a flush valve 15 to the transducer. The transducer is a tubular section or tube 16 of an elastomer which is such that its resistance changes in response to pressure applied to it. The tube has, at each end, spigot-type Luer adapters 17. Each adapter has one portion inserted into the end of the tube 16 with the remaining portion projecting from the end of the tubing and adapted to be inserted into the catheter system tubing to make a liquid-tight connection therewith. Between the ends of the tubular section 12, two conductive terminals 19 are fastened to the exterior surface of the tube. The contact can simply be a surface contact as by wrapping a wire around the circumference of the tubular section. Those conductors are connected to the Wheatstone bridge 20. The transducer is a variable resistance element which is connected via a bridge 20 to a monitor circuit. The bridge and monitor circuit convert the changes in resistance to a visual display indicated at 21. Except for the specific transducer, all of the other elements of the circuit are conventional. The tubing may have an internal coating indicated at 25. The coating could be deposited on the interior of the tubing or, alternatively, could be laminated to it by means of a co-extrusion process. It is important that the internal coating provide sufficient dielectric protection to provide leakage current protection and defibrillator withstand. It also must be biocompatible with the fluids passing through it so as to avoid contamination of the patient. It must be able to withstand sterilization processes. It has been found that a short length of tubing which is about 0.250 inch (0.635 cm) outside diameter, 0.200 inches (0.508 cm) inside diameter and about 2½ inches (6.4 cm) between conductors provides a good response from the variations in blood pressure normally found in a patient. The spacing between the conductors can be increased or decreased in order to increase or decrease the resistance between them as required for the particular application. The tubing dimensions, durometer, material resistivity can also be varied. In operation, the variations in blood pressure that are transmitted to the interior of the tubular section 12 cause a pressure to be applied to the tubular section and this alters its resistance. The variations in resistance cause the output from the Wheatstone bridge to vary, thus providing the visual display of the patient's blood pressure. With suitable modifications, the device is useful with industrial applications. A transducer in accordance with the invention is illustrated in Fig. 3. There standard tubing indicated at 30 is formed of an insulating polymer. A conductive polymer is deposited in a stripe 31 on the surface of the tubing 30. The conductive polymer could be printed, coated, or vapor deposited onto the surface. When that tubing or a portion of it is placed in a fluid circuit of the type shown in Fig. 1, for example, with conductors 32 connecting the stripe 31 to the Wheatstone bridge 20, variations in pressure within the tubing will be reflected by variations in the resistance between the two conductors 32 of the stripe 31 to provide information at the monitor 21. Instead of a longitudinally extending stripe 31, the stripe may be in the form of a circumferential band extending around a substantial portion of the circumference of the tubing 30. Another alternative that consists of forming the sensing element by an ion implantation immediately beneath the surface of the tubing. The ion implantation forms a semi-conductive section whose resistance will vary with changes in pressure in the tubing. It will be in the form of a stripe or a band as in the previous embodiment. Conductors will connect the implant to a bridge as in the previous embodiments.

Claims: A transducer for measuring fluid pressure comprising a tube, a sensor that varies in resistance in response to variations in pressure to which the tube is subjected and conductors connected to the sensor, characterised in that the tube is a resilient insulative tube (30), in that semi-conductive material is provided on or in the surface of a region (31) of the tube (30), the conductors (32) being connected to the semi-conductive region (31) and in that the combination of the resilient tube (30) and semi-conductive region (31) creates the sensor, the variations in the resistance of which are due to expansion and contraction of the semi-conductive region (31) when the tube (30) expands and contracts under varying pressure. A transducer as claimed in Claim 1, wherein the semi-conductive region (31) comprises a semi-conductive stripe deposited on the tube (30). A transducer as claimed in Claim 1, wherein the semi-conductive region (31) comprises a semi-conductive band deposited on and partially surrounding a section of the tube (30) and wherein the conductors (32) connected to the band (31) are circumferentially spaced. A transducer as claimed in Claim 1, wherein the semi-conductive region (31) comprises a portion of tube (30) in the surface of which ions are implanted. Apparatus for measuring fluid pressure comprising means (17) for connecting a transducer (12) as claimed in any one of Claims 1 to 4 to a fluid system (10, 11, 13, 14) whose pressure is to be measured so that fluid thereof flows through the tube (30) and means (20, 21) connected to the conductors (32) for measuring changes in the resistance of the sensor as the pressure of the fluid in the tube (30) changes. Apparatus as claimed in Claim 5, wherein the fluid system is a blood pressure measuring apparatus comprising a catheter (10) for insertion into a patient's blood vessel, tubing (11) connected to the catheter and to the transducer tube, means (13, 14, 15) for supplying a saline solution to the catheter (10), tubing (11) and transducer tube (30) and monitor means (20, 21) connected to the transducer conductors (32).

Inventors: FRANK THOMAS P, FRANK, THOMAS, P.
Applicants: MEDEX INC, MEDEX, INC.
Application year: 1987
Main classification: A61B5
IPC codes: A61B5