Apparatus and method for testing the response of cardiac pacemakers to electromagnetic interference

• US 5,246,000 A
• Filed: 04/27/1992
• Issued: 09/21/1993
• Est. Priority Date: 04/27/1992
• Status: Expired due to Fees

• Current Assignee: The United States Of America As Represented By The Secretary Of The Army
• Original Assignee: The United States Of America As Represented By The Secretary Of The Army
• Inventors: Brown, Charles L., Ellis, Vincent J.
• Primary Examiner(s): Kamm, William E.
• Application Number:
  US07/874,320
• Time in Patent Office: 512 Days
• Field of Search: 128/419.0 PT
• US Class Current: 607/27
• CPC Class Codes: A61N 1/37 Monitoring; Protecting
• CPC Class Codes: A61N 1/3718 Monitoring of or protection against external electromagnetic fields or currents
• CPC Class Codes: G09B 23/28 for medicine

Abstract

A phantom test cell and method for testing the response of a cardiac pacemaker to electromagnetic fields. The phantom comprises a substantially rigid shell which has a torso-like section and two leg-like appendages. The entire shell is filled with a tissue-equivalent material, which in one embodiment represents an upper bound approximation of human whole body averages. The phantom'"'"'s torso is equipped with a plurality of access ports to which a current probe containment vessel may attach when immersed in the tissue-equivalent material. The containment vessel houses a current probe and flooded tube through which an implanted pacemaker'"'"'s leads may pass, allowing induced currents to be monitored. In operation, lead current measurements are taken within the phantom and are normalized using measurements taken without the presence of a phantom. A transfer function is computed by taking the ratio of the induced current measured and a reference E-field and/or H-field measurement. A source of interest is then convolved with the transfer function to determine what the induced current would be in an implanted pacemaker. In one embodiment, the induced current which is arrived at analytically is then physically injected into a cardiac pacemaker outside the test phantom to reproduce EMI effects.

Claims

1. A phantom for testing the response of a cardiac pacemaker to electromagnetic fields, said phantom comprising a substantially rigid shell provided with a torso-like section and two leg-like appendages attached thereto;

a tissue-equivalent material disposed within said rigid shell;

a plurality of ports disposed along one side of said torso-like section;

a probe containment device comprising a water-tight housing, tubular extension, and tubular passage through said housing;

a clip-on current probe disposed within said containment device and around said tubular passage;

means to connect said tubular extension to any of said ports; and

means to position a cardiac pacemaker within said rigid shell such that any leads emanating from said pacemaker are made to pass through said tubular passage.

2. A phantom for testing the response of a cardiac pacemaker to electromagnetic fields, said phantom comprising:

a substantially rigid shell comprising a rectangular tank and two right circular cylinders, said tank having one open side and a side provided with two holes, said cylinders each having one open end which is attached to said tank concentric with one of said holes;

a tissue-equivalent material disposed within said shell;

a plurality of ports along one side of said rectangular tank;

a probe containment device comprising a water-tight housing, a tubular conduit extending from said housing, and a tubular passage through the center of said housing;

a clip-on current probe disposed within said containment device and around said tubular passage, said current probe having one or more lead wires, and said lead wires being disposed within and extending from said tubular conduit;

means to connect said tubular conduit to any of said ports in such a manner as to maintain substantial perpendicularity of said conduit to the side of said rectangular tank which is provided with ports;

means to hold a cardiac pacemaker within said phantom;

wherein a pacemaker may be positioned within said phantom, and any leads emanating therefrom can be made to pass through said tubular passage so that induced currents may be monitored at different positions along said pacemaker lead.

3. The invention of claim 2 wherein the open side of said rectangular tank is adjacent to the side provided with said holes, whereby said phantom may be laid such that said right circular cylinders are parallel to the ground plane and said open side will contain said tissue-equivalent material, thereby allowing EMI tests to be conducted in a horizontal position.

4. The invention of claim 2 wherein the open side of said rectangular tank is opposite the side provided with said holes, whereby said phantom may be positioned upon said right circular cylinders allowing EMI tests to be conducted in a vertical position.

5. A phantom for testing the response of a cardiac pacemaker to electromagnetic fields, said phantom comprising:

a substantially rigid shell comprising a rectangular tank and two right circular cylinders, said tank having one open side and a side provided with two holes, said cylinders each having one open end which is attached to said tank concentric with one of said holes;

a tissue-equivalent material disposed within said shell, said tissue-equivalent material comprising a saline solution;

means to introduce gas bubbles near the closed ends of said right circular cylinders to agitate said saline solution;

a plurality of ports along one side of said rectangular tank;

a probe containment device comprising a water-tight housing, a tubular conduit extending from said housing, and a tubular passage through the center of said housing;

a clip-on current probe disposed within said containment device and around said tubular passage, said current probe having one or more lead wires, and said lead wires being disposed within and extending from said tubular conduit;

means to connect said tubular conduit to any of said ports in such a manner as to maintain substantial perpendicularity of said conduit to the side of said rectangular tank which is provided with ports;

means to hold a cardiac pacemaker within said phantom;

wherein a pacemaker may be positioned within said phantom, and any leads emanating therefrom can be made to pass through said tubular passage so that induced currents may be monitored at different positions along said pacemaker lead.

6. The invention of claim 5 wherein said saline solution has a conductivity of 0.45 S/m.

7. A method for testing the response of a cardiac pacemaker to electromagnetic fields, said method comprising the steps of:

(a) generating an electromagnetic field with a source and an antenna;

(b) varying the output frequency of said source;

(c) measuring the amplitude and orientation of the E-field or H-field at a point in space relative to said antenna where the pacemaker is to be positioned during the test, wherein said measurement is taken in such a way as to characterize the field in three coordinate planes and over the frequency range of interest;

(d) placing the cardiac pacemaker within a phantom comprising a substantially rigid shell filled with a tissue equivalent material, wherein said phantom is provided with leg-like appendages to account for ground plane interaction;

(e) positioning said phantom so that the center of the pacemaker lead resides at the point in space where previous measurements were taken;

(f) generating an electromagnetic field as in step (a);

(g) measuring the induced current in the pacemaker lead over the frequency range of interest;

(h) computing the transfer function produced by the ratio of the induced current measured and the field characterized in step (c), whereby it is possible to determine the induced current in an implanted pacemaker given any frequency and field intensity by analytical means and without resort to further testing.

9. The method of claims 7 or 8 wherein said phantom is filled with a saline solution and provided with means to introduce gas bubbles at the base of said legs to agitate the solution and achieve a substantially uniform distribution of particles throughout the phantom, thereby improving the accuracy and repeatability of said method.

10. The method of claim 9 wherein said saline solution has a conductivity of 0.45 S/m.

8. A method for testing the response of a cardiac pacemaker to electromagnetic fields, said method comprising the steps of:

(a) generating an electromagnetic field with a source and an antenna;

(b) varying the output frequency of said source;

(c) measuring the amplitude and orientation of the E-field or H-field at a point in space relative to said antenna where the pacemaker is to be positioned during the test, wherein said measurement is taken in such a way as to characterize the field in three coordinate planes and over the frequency range of interest;

(d) placing the cardiac pacemaker within a phantom comprising a substantially rigid shell filled with a tissue equivalent material, wherein said phantom is provided with leg-like appendages to account for ground plane interaction;

(e) positioning said phantom so that the center of the pacemaker lead resides at the point in space where previous measurements were taken;

(f) generating an electromagnetic field as in step (a);

(g) measuring the induced current in the pacemaker lead over the frequency range of interest;

(h) computing the transfer function produced by the ratio of the induced current measured and the field characterized in step (c);

(i) convolving a source of interest with said transfer function to determine what the induced current would be in an implanted pacemaker;

(j) injecting the current arrived at in step (i) into a free-standing pacemaker outside the phantom cavity;

(k) monitoring the voltage output of said pacemaker to determine the effect of said source on said pacemaker performance, whereby the response of said pacemaker to an arbitrary electromagnetic source while implanted may be evaluated without an actual source or phantom.