WO2012114219A1

WO2012114219A1 - Catheter, assembly including same and method of placement thereof

Info

Publication number: WO2012114219A1
Application number: PCT/IB2012/050545
Authority: WO
Inventors: Pieter Rousseau Fourie; Cornelius Scheffer; Paul Schwartz
Original assignee: Stellenbosch University
Priority date: 2011-02-22
Filing date: 2012-02-07
Publication date: 2012-08-30

Abstract

A catheter assembly is provided in which a catheter has a removable guideline passing through it for assisting in its placement. An impedance measuring apparatus measures the impedance between at least two electrodes provided in an inner end region of the catheter assembly and communicating by way of conductors extending along at least a part of the length of the catheter assembly. The at least two electrodes are provided on the catheter or guideline, or both in the region of the operative ly inner end thereof. The impedance measuring apparatus is set up to detect a first level of impedances commensurate with the presence of the electrodes in the tissue of a patient and a second level of impedances commensurate with the presence of the electrodes within the blood in a blood vessel of the patient. Means are provided for communicating the transition from the former level to the latter level to a medical practitioner.

Description

CATHETER, ASSEMBLY INCLUDING SAME AND METHOD OF

PLACEMENT THEREOF

FIELD OF THE INVENTION

This invention relates to a catheter that is to communicate with the interior of a blood vessel, especially an artery; to a catheter assembly including same; and to a method of placement thereof.

BACKGROUND TO THE INVENTION

The placement of an intra-arterial catheter, especially in the aortal or femoral arteries, can be extremely difficult for the medical practitioner and traumatic for the patient. The difficulty is created by the fact that a medical practitioner trying to place a catheter that communicates properly with the interior of a blood vessel cannot be certain that the inner end of the catheter is properly located within the blood vessel. It is possible that the blood vessel is not easily visible in which instance it is difficult to gauge when and if the open end of the catheter is positioned within the blood vessel. The problems are particularly pronounced in the instance of an artery which contains relatively high-pressure blood. Multiple attempts to achieve proper placement of a catheter are traumatic for the patient and even injurious and also increase the risk of infection.

It is known, for example from US patent 4,852,580, that a catheter carrying electrodes can be used to measure blood flow within a blood vessel, especially an artery, using impedance that is responsive to an applied alternating or pulsed current. There is a need for a catheter, a catheter assembly and a method of placement thereof whereby placement of a catheter in a blood vessel is facilitated. SUMMARY OF THE INVENTION

In accordance with one aspect of this invention there is provided a catheter assembly comprising a catheter having a removable guideline passing through the catheter for assisting in its placement, and an impedance measuring apparatus for measuring the impedance between at least two electrodes provided in an inner end region of the catheter assembly and communicating by way of conductors extending along at least a part of the length of the catheter assembly, wherein the at least two electrodes are provided on the catheter or guideline, or both in the region of the operatively inner end thereof with conductors communicating with said at least two electrodes extending along at least an appropriate part of the length of the catheter or guideline or both, and wherein the impedance measuring apparatus is set up to detect a first level of impedances commensurate with the presence of the electrodes in the tissue of a patient and a second level of impedances commensurate with the presence of the electrodes within the blood in a blood vessel of the patient and means for communicating the transition from the former level to the latter level to a medical practitioner.

In a first variation of the invention the guideline is an electrically conductive guide wire and the wire itself may form one electrode and its associated conductor.

In a second variation of the invention said two or more electrodes could be provided on the outer surface of the catheter towards its operatively inner end and the conductors may be electrically conductive material applied onto the outer surface of the catheter with an insulating coating around the major part of the length of the electrode to allow only the tip of the electrode nearest the tip of the catheter to be exposed and therefore in electrical contact with its surroundings.

In a third variation of the invention said two or more electrodes could be provided on the outer surface of the guideline in which instance the guideline would typically be electrically insulating.

In all appropriate instances the electrodes could simply be exposed inner ends of the conductors.

In accordance with a second aspect of the invention there is provided an impedance measuring apparatus adapted to detect impedance between two electrodes of a catheter assembly as defined above and having means for communicating to an operator a transition from a first level of impedance commensurate with the presence of the electrodes of the catheter assembly in the tissue of a patient and a second level of impedance commensurate with the presence of the electrodes within the blood in a blood vessel of the patient. In respect of either aspect of the invention, the impedance measuring apparatus is preferably provided with audible or visible means for communicating to a medical practitioner a transition between a first level of impedance and a second level of impedance as indicated above. The impedance measuring apparatus may be of any commercially available type that is suitable for the purpose or that may be especially adapted for this purpose. Alternatively, an impedance measuring apparatus may be especially designed and produced for the purposes of this invention and therefore be dedicated to the practice of placement of catheters. In either event, the frequency range of the AC signal used to produce the impedance will generally be in the range of 10 to 100 kHz, while the slumping frequency used to identify the impedance [using A/D conversion] will generally be in the region of 50 MHz.

In accordance with a third aspect of the invention there is provided a method of placing a catheter comprising progressively introducing a catheter into a patient and monitoring the output from impedance measuring apparatus such that placement can be considered to have been achieved once an output from the impedance measuring apparatus corresponds to a second level of impedance as indicated above.

The invention also provides catheters that are especially configured to form part of an assembly as defined above and to be used in a method as defined above. Of course, such catheters could also be provided with other sensors and detectors for monitoring other conditions such as blood flow, temperature, oxygen saturation and the like.

In order that the invention may be more fully understood, one embodiment thereof will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:-

Figure 1 is a schematic isometric illustration of one form of catheter assembly according to the invention;

Figure 2 is a schematic isometric illustration of another form of catheter assembly according to the invention; Figure 3 is a schematic diagram illustrating the composite apparatus of the invention ;

Figure 4 is a schematic section taken through a blood vessel on a patient and showing the catheter in use; and,

Figure 5 is a schematic circuit diagram of one arrangement of catheter monitoring circuit that has been used in tests of the functioning of the invention.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

In one embodiment of the invention a catheter assembly, as illustrated in Figure 1 and indicated generally by numeral [1 ], comprises a catheter [2] having a removable guideline in the form of a hypodermic style of needle [3] passing through the catheter for assisting in its placement.

In this embodiment of the invention two spaced electrodes [4] are provided at the inner end region of the catheter with conductors [5] that communicate with them extending along at least part of the length of the catheter assembly and electrically insulated from each other and from the outside along the major length of the catheter so that only the spaced electrodes are exposed to their surroundings. In the instance of the embodiment of the invention illustrated in Figure 2 of the drawings, the catheter (1 1 ) has a single electrode (12) with its associated conductor (13) extending along the catheter and wherein the electrically conductive guideline that may again be in the form of a hypodermic style of needle (14) serves as the second electrode and its associated conductor. In this instance the hypodermic style of needle has an electrical conductor (15) in electrical contact with it so that the two conductors (13, 15) can be connected to the impedance measuring apparatus that is further described below.

Impedance measuring apparatus [21 ] is provided for measuring the impedance between the two electrodes, in each instance, with the apparatus being adapted to detect a first level of impedances commensurate with the presence of the electrodes in the tissue of a patient and a second level of impedances commensurate with the presence of the electrodes within the blood in a blood vessel of the patient.

The impedance measuring apparatus is preferably provided with both audible (22) and visible (23, 24) indicators of the fact when the impedance measured changes from the first level to the second level. Such a change in impedance level occurs with placement of the catheter, in practice, when the electrodes firstly contact a patient's tissue (25) and then become interconnected by blood (26) in a blood vessel (27) rather than by the tissue (see Figure 4).

A visible indicator could simply be a suitable LED arrangement, such as a red LED [23] and a green LED [24] with the red LED being illuminated when the electrodes are positioned within the tissue of the patient and the green LED becoming illuminated when the electrodes are positioned in the blood vessel. The apparatus may also have a screen [28] enabling a graphic output of impedance and changes therein to be displayed and visually observed.

In one test that was carried out, a catheter (1 1 ) according to Figure 2 was fabricated from a 0,4mm (diamater) standard intra arterial catheter with a conductor (13) in the form of a copper thread wound around the canule (exterior part) of the arterial catheter. The copper thread was held in place by an electrically insulating non-toxic resin with the last distal 0,5mm being left un-insulated by the resin so as to leave it exposed in order to serve as the electrode (12). The proximal end of the cannula was attached to a conductor (16) by means of silver adhesive paint and the conductor was attached to the signal processing device that is further described below. The hypodermic needle (14) of the arterial catheter assembly was electrically connected by means of silver adhesive paint to the conductor (15), at the proximal end of the needle and this conductor was also connected to the signal processing device.

The device that was the subject of testing had a signal processing device in the form of a combination of a signal generator (31 ) (to produce the reference signal and the signal to induce the impedance), a PC oscilloscope (32) (used to analyze the reference and output signals) and a laptop style of processor (33), running MS Excel 2010, used to interpret the data from the PC oscilloscope (taking A/D measurements, sampling at 50MHz), record the information and to give feedback during the testing.

The signal generator used produced a signal with a peak-to-peak amplitude of 8V at a frequency of 30kHz. A reference resistor (34) of 1 ,2kQ was used to calculate the measured impedance, using the reference signal, the output signal and the reference resistor. The equation used was Z_L = (V_T / (V_T - V|_))^*Z_S, where Z_L is the impedance; V_T is the reference voltage; V_L is the measured output voltage and Z_s is the reference resistor.

In forthcoming clinical trials, it is proposed to make the following changes. The copper thread wrapped around the canule is to be replaced by a gold or silver thread which is to be embedded into the canule by heating the canule appropriately. The 0,5mm distal end of the thread is to be exposed to form the one electrode (12). A plastic housing (37) fitted on the proximal end of the arterial line is to keep all conductors and electronics together and function as a grip for a practitioner using the device. The housing may also house the LED's responsible for user feedback, possibly an RGB LED displaying colours that depend on impedance levels detected, preferably avoiding the colours red and green to accommodate users who are colour blind. The signal generator may be one that produces a 5V peak-to-peak voltage at 30kHz. An ICL 8038 chip may be used to produce this signal and a series of op amps may be used to modify the signal around 2,5V, giving it a maximum of 5V and a minimum of 0V. The PC oscilloscope and laptop will be replaced with an Arduino Mega 2560 board, which takes A/D Conversions (at a sampling rate of 130kHz) of the reference signal and the output signal and processes the results using an iterative algorithm to solve the equation Z_L = (V_T / (V_T -V_L))^*Z_S. The Arduno board will compare the present impedance with stored impedances and determine under which group the present impedance falls; that is, the associated impedance level for tissue or blood. It follows by using three onboard digital outputs to set the RGB LED to an appropriate colour, that which represents the level of the measured impedance. This process is repeated during the entirety of the insertion process of the modified arterial catheter. Data accumulated is stored on a 64K EEPROM built into the Arduino board for later analysis.

It will therefore be understood that unlike prior impedance measuring apparatus, the apparatus of this invention is intended to be active whilst the catheter is being placed rather than only after the catheter has been properly placed.

In another variation of the invention, both electrodes could be carried by the guideline provided they are adequately electrically insulated from each other. Numerous other variations may be made to the embodiments described above without departing from the scope hereof.

Claims

A catheter assembly comprising a catheter having a removable guideline passing through the catheter for assisting in its placement, and an impedance measuring apparatus for measuring the impedance between at least two electrodes provided in an inner end region of the catheter assembly and communicating by way of conductors extending along at least a part of the length of the catheter assembly, wherein the at least two electrodes are provided on the catheter or guideline, or both in the region of the operatively inner end thereof with conductors communicating with said at least two electrodes extending along at least an appropriate part of the length of the catheter or guideline or both, and wherein the impedance measuring apparatus is set up to detect a first level of impedances commensurate with the presence of the electrodes in the tissue of a patient and a second level of impedances commensurate with the presence of the electrodes within the blood in a blood vessel of the patient and means for communicating the transition from the former level to the latter level to a medical practitioner.

A catheter assembly as claimed in claim 1 in which the guideline is an electrically conductive guide wire and the wire itself forms one electrode and its associated conductor.

A catheter assembly as claimed in claim 1 in which said two or more electrodes are provided on the outer surface of the catheter towards its operatively inner (distal) end.

A catheter assembly as claimed in claim 1 in which said two or more

electrodes are provided on the outer surface of the guideline in which instance the guideline is electrically insulating. A catheter assembly as claimed in any one of the preceding claims in which a conductor is electrically conductive material applied onto the outer surface of the catheter or guide wire with an electrically insulating coating around the major part of the length of the conductor to allow only a tip of the conductor nearest a tip of the catheter to be exposed and therefore serve as an electrode in electrical contact with its surroundings.

An impedance measuring apparatus adapted to detect impedance between two electrodes of a catheter assembly as defined in any one of claims 1 to 5, the impedance measuring apparatus having means for communicating to an operator a transition from a first level of impedance commensurate with the presence of the electrodes of the catheter assembly in the tissue of a patient and a second level of impedance commensurate with the presence of the electrodes within the blood in a blood vessel of the patient.

An impedance measuring apparatus as claimed in claim 6 in which there is provided audible or visible means for communicating to a user a transition between a first level of impedance and a second level of impedance.

An impedance measuring apparatus as claimed in either one of claims 6 or 7 in which a frequency range of an AC signal to produce the impedance is in the range of 10 to 100 kHz and the slumping frequency used to identify the impedance [using A/D conversion] is in the region of 50 MHz.

A method of placing a catheter as claimed in any one of claimsl to 5 comprising progressively introducing a catheter into a patient and monitoring the output from impedance measuring apparatus such that placement can be considered to have been achieved once an output from the impedance measuring apparatus corresponds to a second level of impedance as defined above.

A catheter suitable for use in a catheter assembly as claimed in any one of claims 1 to 5 wherein the catheter has a removable guideline passing through it for assisting in its placement with at least two electrodes provided on the surface of the catheter or guideline in an operatively inner end region of the catheter and communicating by way of conductors extending along at least a part of the length of the catheter.