Catheter Apparatus

Abstract

A catheter having walls of which at least a portion is electrically conductive and has an electrical resistivity approximately equal to the electrical resistivity of blood.

Description

BACKGROUND OF THE INVENTION

This invention is concerned with an improved catheter. Catheters constructed as hollow tubes are well known in the art for their use in the practice of medicine as well as in medical research. Catheters are used, for example, to measure pressures in body organs and in the circulatory system, to withdraw samples of body fluids, and to introduce drugs, intravenous fluids, X-ray contrast media, dyes for cardiac output measurements, and nasogastric feeding.

In performing functions such as those mentioned above, the catheter or tube is normally filled with solutions which are electrically conductive. In many of these applications, one end of the catheter terminates inside the body in close proximity to the heart while the other end is attached to an electrical device or electronic instrumentation. As the catheter is normally made of an electrically insulating material, the solution within the catheter acts as an isolated circuit for electrical current into the body, and close proximity to the heart causes the possibility of accidental death due to accidentally applied electrical power which will be felt near the heart and may cause ventricular fibrillation.

The above-mentioned danger is now well known to those skilled in the art, and various studies are being conducted to overcome the problem. The apparatus of the present invention does overcome the problem by providing a catheter having a conductive wall so that electrical current is disbursed through the blood and body tissues rather than being concentrated at or near the heart.

SUMMARY OF THE INVENTION

Briefly described, the apparatus of this invention comprises a tube having walls at least a portion of which is electrically conductive or semiconductive material and has an electrical resistivity sufficient to prevent harmful current flow to organs within the body when the tube is used as a catheter for intracorporeal applications.

IN THE DRAWINGS

FIG. 1a is a representation of the catheter apparatus of this invention;

FIG. 1b is a representation of the apparatus of this invention as it is in the process of being assembled;

FIG. 2 is a schematicized drawing representative of the current density pattern in the body of a human being between a pair of skin electrodes;

FIG. 3 is a schematicized drawing of the current density pattern between a prior art intravascular catheter and a skin electrode; and

FIG. 4 is a schematicized diagram of the current density pattern between an intravascular catheter of this invention and a skin electrode.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

It will be understood that the drawings are intended to be merely representative of the general configuration of the parts they show and are not drawn to scale, or to relative scale, in the interests of clarity.

As used throughout this specification and the claims, "electrical resistivity" means volume resistivity in ohm-centimeters, according to the equation,

which equations is known to those skilled in the art.

Referring to FIG. 1a, it should be understood that the catheter apparatus of this invention comprises a hollow tube or catheter 10 with electrically conductive or semiconductive walls 14 which have electrical resistivity. The catheter apparatus must be durable and preferably flexible. In FIG. 1b, there is shown in the stage of construction a catheter of this invention 10 which is being made by winding a base material 11, such as a Dacron mesh, around a rod 12, preferably a stainless steel rod.

The apparatus of this invention has been made by first coating rod 12 with wax and then cutting a base material 11, such as Dacron or nylon mesh, into 3-foot long strips, 1 centimeter wide. Mesh 11 is then wound on rod 12 in a helical pattern with overlapping windings. A solution is then applied to the base material 11 in sufficient quantities to flow through the base material, the solution being one which will harden into a durable, flexible, electrically conductive or semiconductive coating when cured. The solution is then cured, and when dry, further coatings are applied and dried as necessary to form walls 14. A solution which has been used in the preferred embodiment of the apparatus of this invention is a flexible carbon impregnated elastomer, known as Eccocoat No. 258, which has a measured volume resistivity of about 133.5 ohm-cm. This material was thinned out with a solvent and painted on the base material 11, using enough solvent to cause the material to flow through mesh 11. It was than dried in a 150.degree. F. oven for 15 minutes. A second coat was then painted on using a thicker solution of the material than in the previous step, and it was again dried in the oven. Two further coats of the material were added as in the latter step. The completed catheter was then removed from rod 12. This catheter, comprising the apparatus of this invention, was tested in a manner to be more fully described below. Other materials which could be used to fabricate the apparatus of this invention include carbon-filled polyethylene, carbon-filled polyvinylchloride, carbon-filled rubber, and carbon-filled silicones. Catheters can also be fabricated from such materials alone without the need for the mesh base as described previously.

To best understand the operation of the apparatus of this invention, reference should be made to FIGS. 2, 3, and 4 of the drawings.

As stated above, electrical shock hazards resulting from defective medical electrical equipment are well known in the art. It is common knowledge that 60 cycle alternating current greater than 100 milliamperes can be dangerous when flowing through the human body between two points on the body surface. The heart tissue is most susceptible to such current, and electrocution deaths are most commonly the result of ventricular fibrillation of the heart.

Over the past decade, an increased awareness of the susceptibility of the heart to 60 cycle alternating electrical currents has been developed. When a current flows between two points on the surface of the body, it is distributed more or less uniformly throughout the tissues, and the density of current at any single point deep inside the body is comparatively small. This may be seen by reference to FIG. 2, where a body 15 has placed thereon a pair of electrodes 16 and 17 between which is flowing a current indicated by current density lines 20. A review of the current density lines will indicate that when electrical current flows through the body from two points on or near the body surface, the current becomes distributed more or less uniformly in the body blood and tissues, and the current density at any one point inside the body, such as the heart, is relatively small.

The heart is highly sensitive to 60 cycle alternating current, and currents as low as 10 to 20 microamperes can cause fibrillation when injected over 1 square millimeter of heart muscle. However, to generate 10 microamperes per square millimeter of current density at the heart surface from electrical contact on either arm, such as electrodes 16 and 17 on body 15 of FIG. 2, it is necessary to provide a current flow of 100 milliamperes. This is because the current is effectively disbursed through the volume of the blood and tissues in the body.

However, when a catheter containing electrically conductive liquid is introduced near the heart, the current density situation is significantly altered. In FIG. 3, there is again shown body 15 and electrode 16, but the current density lines 21 now indicate the flow of current between electrode 16 and an intravascular catheter 18 here shown to be a prior art hollow catheter having a nonconductive wall.

As can be seen in FIG. 3, catheter 18 terminates within the heart, and because of the conductive effect of the solution or liquid within catheter 18, virtually all of the current passing through catheter 18 exits at a small area near the heart muscle. That is, the current density at the end of the catheter is sufficiently great so that as little as 20 microamperes of current can be lethal. Such small currents can result from many accidental causes. For example, if the catheter is connected to a grounded pressure transducer or fluid injector, as is often the case, a small electrical potential introduced at any point on the body can be dangerous. As a specific example, a nurse who may adjust a properly functioning bedlamp having an exposed metal part which is not grounded can kill the patient by simply touching him.

It is, therefore, desirable that the walls of the catheter or at least a portion thereof be of a conductive material so that electrical currents impressed on the catheter may pass through the catheter walls into the body and be disbursed to avoid density at the heart. If the walls of the catheter are substantially better electrical conductors than the surrounding blood and tissue, then the electrical current will pass out uniformly into the blood and tissue along the length of the catheter. However, if the catheter walls are highly conductive along its length as well as through its walls, it will provide a low impedance between the source of power and the heart and be even more dangerous than the above-mentioned prior art catheter because very low voltages can generate high currents. Obviously then, a highly conductive catheter is even more dangerous than a highly insulated catheter.

The catheter of the apparatus of this invention is selected to have a conductive or semiconductive surface with an electrical resistivity approximately in the range from 12 to 1,200 ohm-centimeters, preferably approximately equal to the electrical resistivity of blood, 120 ohm-centimeters. That is, the catheter apparatus of this invention has a wall with at least a portion having an electrical resistivity of one-tenth to 10 times the electrical resistivity of blood.

Referring now to FIG. 4, there is again shown the body 15 and electrode 16 and a plurality of current density lines 22 flowing between electrode 16 and an intravascular catheter 10 comprised of the apparatus of this invention. In FIG. 4, it is apparent that the current begins to disburse at the site at which catheter 10 penetrates the skin of body 15. Thus, the pattern of dispersion is similar to that which would be present if the catheter 10 were not used and the current were injected from a surface electrode such as 17 in FIG. 2. It has been proven by experimentation that greatly increased currents can be applied without causing ventricular fibrillation of the heart when the apparatus of this invention, as shown in FIGS. 1a, 1b, and 4, is used in place of prior art catheters.

From the above, and with reference to FIG. 4, it will be apparent that catheter 10 need not be electrically conductive or semiconductive along its entire length, but that it must include a part of its wall which is conductive or semiconductive at the point adapted to be in contact with the skin after intracorporeal application of the improved catheter.

The improved catheter apparatus of this invention was built and tested in a live, intact dog under conditions normally present in human cardiac catheterization. Similar tests were performed on a conventional catheter normally used in human diagnostic cardiac studies, in this case a standard No. 7 Lehman cardiac catheter.

A catheter of the apparatus of this invention was passed to the right ventricle of the heart through the femoral vein and wedged between muscular trabeculations. The prior art catheter of identical length and internal diameter was passed to the right ventricle through another femoral vein and also wedged between muscular trabeculations. Sixty cycle AC current was applied to the catheter of this invention beginning with 6 microamperes and increasing in 15 steps to 10,000 microamperes. Each current step was applied for 5 seconds. Electrocardiogram and intraventricular pressures were monitored continuously. No effect on the heart was observed. At the 10,000 microamperes level, the animal's leg was seen to twitch where the catheter entered the skin. Results of this test are shown in Table I below. --------------------------------------------------------------------------- TABLE I

Improved Catheter Apparatus

Current-ua Time applied-sec. Response __________________________________________________________________________ 6 5 none 12 5 none 23 5 none 55 5 none 105 5 none 175 5 none 240 5 none 325 5 none 530 5 none 740 5 none 1000 5 none 1450 5 none 2300 5 none 2950 5 none 6200 5 none 10,000 5 leg muscle twitch __________________________________________________________________________

The prior art catheter was then tested in a similar manner. However, when the current reached 70 microamperes, ventricular fibrillation of the heart occurred. The dog was restored immediately to normal sinus rhythm using DC cardioversion at 50 watt-seconds, in a manner known to those skilled in the art. It is also interesting to note that at 50 microamperes the dog began to have occasional ventricular premature beats. The results of this test are shown in Table II below. --------------------------------------------------------------------------- TABLE II

Prior Art Catheter

Current-ua Time applied-sec. Response __________________________________________________________________________ 6 5 none 12 5 none 17.5 5 none 22 5 none 34 5 none 50 5 Occ. VPB'S* 70 5 Ventricular fibrillation __________________________________________________________________________ *(ventricular premature beats)

The prior art catheter was withdrawn to the vena cava, and the catheter of this invention was repositioned in the right ventricle. The above-described test was repeated, and again, no cardiac disturbances were observed. Again, the 10,000 microamperes current produced leg twitching. Reference is made to Table III for the results of this test.

TABLE III

Improved Catheter Apparatus

Current-ua Time applied-sec. Response __________________________________________________________________________ 6 5 none 23 5 none 80 5 none 240 5 none 740 5 none 1450 5 none 3000 5 none 4500 5 none 6300 5 none 8000 5 none 10,000 5 leg muscle twitch __________________________________________________________________________

The catheter of this invention was then withdrawn to the vena cava, and the prior art catheter was repositioned in the right ventricle. The above test was again repeated, and again, the dog had ventricular fibrillation of the heart at 70 microamperes of input current. Cardioversion again restored normal sinus rhythm. The results of this test are shown in Table IV.

TABLE IV

Prior Art Catheter

Current-ua Time applied-sec. Response __________________________________________________________________________ 6 5 none 12 5 none 17 5 none 22 5 none 34 5 none 50 5 70 5 Ventricular fibrillation __________________________________________________________________________

The two sets of the last above two paragraphs were repeated in sequence, with the same results. However, when using the prior art catheter, the animal fibrillated at 50 microamperes but stopped after the current was turned off. When 70 microamperes was again reached, ventricular fibrillation occurred but was irreversible. The results of these further tests can be seen in Tables V and VI below.

TABLE V

Improved Catheter Apparatus

Current-ua Time applied-sec. Response __________________________________________________________________________ 12 5 none 80 5 none 530 5 none 740 5 none 1450 5 none 2950 5 none 6250 5 none 10,000 5 leg twitch __________________________________________________________________________ --------------------------------------------------------------------------- TABLE VI

Prior Art Catheter

Current-ua Time applied-sec. Response __________________________________________________________________________ 6 5 none 12 5 none 17 5 none 22 5 none 34 5 none 50 5 Ventricular fibrillation (Stopped after current turned off) 70 5 Ventricular fibrillation (Irreversible) __________________________________________________________________________

From the above, it will be apparent that there is a great advantage in the use of a catheter having at least a portion of its walls electrically conductive or semiconductive and having an electrical resistivity which is comparable to that of the surrounding blood and tissues during an intracorporeal application to allow current flow through the walls into the blood and tissue, but to provide a significant impedance between the source of current and the end of the inserted catheter. As explained above, this electrical resistivity lies approximately in the range of 12 to 1,200 ohm-centimeters, and is preferably approximately at the electrical resistivity of the surrounding blood and tissue.

It will be apparent that construction of the catheter of this invention can be other than that described above with respect to the preferred embodiment. For example, rings or strips of conductive material may be embedded in an insulative material, a portion rather than all of the catheter wall may be conductive, and in some cases only the portion of the catheter adapted to be adjacent to the skin during an intracorporeal application may be conductive.

Claims

What is claimed is:

1. Catheter apparatus comprising tubular means having walls, at least a portion of said walls being electrically conductive and having an electrical resistivity of approximately from 12 ohm-centimeters to 1,200 ohm-centimeters.

2. The apparatus of claim 1 in which all of said walls are electrically conductive and have an electrical resistivity of approximately from 12 ohm-centimeters to 1,200 ohm-centimeters.

3. The apparatus of claim 1 in which the electrical resistivity of said portion of said walls is approximately equal to the electrical resistivity of blood.

4. The apparatus of claim 3 in which the electrical resistivity of said portion of said walls is approximately from one-tenth to 10 times the electrical resistivity of blood.

5. The apparatus of claim 1 in which said portion of said walls comprises a part of said walls adapted to be adjacent to the skin of an animal during intracorporeal application of said catheter apparatus.

6. The apparatus of claim 1 in which said portion of said walls is electrically semiconductive.

7. Improved catheter apparatus comprising: tubular means defining a lumen for the passage of fluids into an animal's body; and said means being electrically semiconductive and having a substantially uniform electrical resistivity selected in the range from approximately 12 ohm-centimeters to 1,200 ohm-centimeters.