Determining EMC Test Levels for Implantable Devices in Bipolar Lead Configuration

Implantable cardiac pacemakers are used in millions of patients to regulate or reproduce normal heart rhythm. Patients are candidates for a pacemaker when the heart's natural rhythm is too slow or if a conduction block is present in the heart's electrical system. An implantable cardioverter defibrillator (ICD) functions like a pacemaker, with the added function of being able to deliver a strong electrical shock to treat life-threatening arrhythmias. Both pacemakers and ICDs have lead(s) that extend from the metallic case under the skin, usually in the pectoral region, to the interior of the heart. Each lead has a distal tip electrode at the far end and one or more ring electrodes spaced a small distance closer to the metallic case (Figure 1). These electrodes are placed in one or more chambers of the heart. The implanted devices detect the heart's intrinsic electrical activity via these electrodes to determine what stimulation to deliver. A voltage can be sensed between the tip electrode and the metallic case (unipolar) or between the tip and a ring electrode (bipolar). The choice of sensing is up to the clinician.

Pacemakers and ICDs are continuously sensing the heart's electrical activity and are susceptible to electromagnetic interference (EMI), which may be interpreted as cardiac signals. EMI is a disturbance generated by an electrical source, such as a cell phone. EMI to pacemakers and ICDs is well known.¹  Because pacemakers and ICDs sense frequencies between 1 and 500 Hz, they are most susceptible to low-frequency magnetic fields. The Food and Drug Administration recognizes ISO 14117:2019²  as describing an electromagnetic compatibility (EMC) test method for pacemakers and ICDs.

The Active Implants Joint Working Group 1 (ISO/TC 150 SC 6 JWG 1)³  is the standards group that authors the EMC standard ISO 14117. While writing the second edition of this standard, the group questioned the basis for the material published in the first edition. One topic of discussion was the requirement and lack of rationale in determining the appropriate test levels for bipolar lead configurations. The test levels described for a unipolar configuration are reasonably supported based partly on the reference levels in the European Commission Recommendation 519⁴  under certain assumptions of magnetic fields inducing a voltage in leads. However, the normative requirements of ISO 14117:2012 simply state, “Bipolar differential mode performance shall be tested using the test signal reduced to one-tenth amplitude” (referring to one-tenth of the test signal specified for devices with unipolar leads).⁵  The only rationale provided in ISO 14117:2012 is as follows: “Because of the close proximity of tip and ring electrodes, the applicable test signal is reduced to 10% of the common mode test signal amplitude.” No documented scientific basis exists for this 90% reduction for bipolar differential tests.

The objectives of the current study were to determine the appropriate test levels below 10 MHz for the bipolar lead configuration and to provide a clear rationale for those levels. All implantable pacemakers and ICDs are tested to the ISO 14117 standard, and the large majority that are implanted are programmed to a bipolar lead configuration. The current study sought to improve understanding regarding whether implantable pacemakers and ICDs are tested to an adequate level.

We performed computational modeling of bipolar and unipolar leads using finite element, three-dimensional electromagnetic simulation software (Ansys HFSS; ANSYS, Canonsburg, PA). Two insulated leads were placed adjacent to one another in a simulated plastic tank (a cylinder with a dielectric constant of 2.0, a diameter of 30 cm, and a height of 30 cm). The cylinder contained a simulated liquid that had electrical properties similar to human soft tissues. The liquid had various conductivities for different simulations. Among the materials used were normal saline with a conductivity of 1.39 S/m and a saline mixture with a conductivity of 0.139 S/m. Saline of various conductivities often is used for simulating human body tissues in simple electromagnetics problems.^2,5  A magnetic flux density (B) was generated by a simulated circular Helmholtz coil pair (60 cm radius, 60 cm separation) that was driven by a simulated current generator over various frequencies in the 10 Hz to 10 kHz frequency range. The 1.2-G flux density produced by the Helmholtz coil was highly uniform throughout the volume of the plastic cylinder containing the saline and the leads. The vector of the B field was parallel to the long axis of the cylinder or perpendicular to the surface of the liquid (Figures 4 and 5).

The leads in this simulation were a pair of adjacent, insulated wires, each forming a semicircular arc. The leads had an inner conducting wire (perfect electrical conductor) with a diameter of 1.0 mm and an insulation thickness surrounding the wire of 0.5 mm (dielectric constant = 2.0). Each wire had a 2-mm bare length at each end. The longer lead, representing the portion of the lead containing the tip, was 37.6 cm long and formed a 180° semicircular arc (24 cm diameter). The shorter lead, representing the portion of the lead containing the ring, was above and parallel to the longer lead. The length of this lead was varied for each simulation performed (Table 1). The two leads were separated by 0.4 mm in the Z direction (Figure 5). The shorter lead formed a semicircular arc, with its angular arc being less than 180° depending on which one of several simulations was performed for different tip-to-ring spacings. The induced voltage on each lead was computed between the distal and the proximal ends of the leads. Among other modeling situations, we performed three separate simulations for the above parameters, with one lead being 10, 20, or 30 mm shorter in length than the longer lead. We used these configurations to determine the ratio of bipolar to unipolar sensing by computing the induced voltages for the longer lead versus the shorter lead, according to the procedure performed for the analytical study.

Table 1 presents data for the computational and analytical results. The table shows the computational results in columns 2 through 4. Columns 5 and 6 present the computational and analytical ratios for the induced voltage in the bipolar configuration versus the induced voltage in the unipolar configuration, respectively. The final column is the ratio between the two methods (computational and analytical). All results are presented as a function of the tip-to-ring spacing.

The last column in Table 1 shows the general agreement between the analytical solution and the computational solution, where a ratio of 1 would be exact agreement. The values between 0.82 and 0.95 give us reasonable confidence in each solution (though not statistically significant due to sample size). Figure 6 shows how the ratio of bipolar to unipolar induced voltages change for each solution based on different tip-to-ring spacing. The longest tip-to-ring spacing found via a search of the literature was 16 mm, but we chose to display potential large spacings up to 30 mm.

When great differences in conductivities were used in the computational modeling, significant differences in the tip voltage and in the ring voltage were observed. However, the ratio of bipolar to unipolar voltages did not change.

Our test data show that for tip-to-ring spacings less than 19 mm, the induced voltage into a bipolar lead configuration is less than 10% of the induced voltage from a unipolar configuration. This means the normative requirements of ISO 14117:2012 are conservative for tip-to-ring spacings under 19 mm.⁵  Conservative indicates that pacemakers and ICDs are adequately tested (or appropriately over tested) for leads with tip-to-ring spacing less than 19 mm. The rationale for the ratio of bipolar to unipolar induced voltages has been updated in ISO 14117:2019.²  If future devices use larger tip-to-ring spacing (>19 mm), then higher test levels may be warranted. Currently, we believe that testing to one-tenth of the unipolar test levels, as described in ISO 14117, is appropriate for bipolar differential lead configurations.