An MD&DI September 1998 Column
FIELD NOTESFiltering in pacemakers has eliminated most problems with electromagnetic interference. A new study confirms that the recommended separation distance of 6 in. is more than adequate.
This article presents the results of experimental work undertaken to determine the minimum separation distance required to eliminate electromagnetic interference (EMI) between wireless phones and cardiac pacemakers. Previous studies on interaction between these devices have reported maximum interaction distances on the order of 6 in. or greater.1 However, these figures tend, in part, to reflect the methodology used to measure separation distance. The present study employed pacemaker height testing, an alternative measurement method that yielded data suggesting maximum interaction distances are significantly lower than 6 in.
The testing described below represents the second phase of a research project begun earlier by the Center for the Study of Wireless Electromagnetic Compatibility at the University of Oklahoma (Norman). The objectives of the previous phase of testing were to verify reported EMI between wireless phones and pacemakers, to more precisely classify the modes of pacemaker interaction, and to identify the wireless-phone operating modes that produce the interaction.2 All cases examined in the follow-up study had been shown to exhibit interaction during the first phase of testing.
Testing was conducted in a closed, electromagnetically shielded room at the Lucent Technologies, Inc., test facility in Oklahoma City, OK. Functioning of the heart and pacemaker system was simulated through the use of a torso simulator and various electronic equipment that generated and monitored electrical signals. The test equipment consisted of:
- A torso simulator with saline bath and supports for the pacemaker, pacemaker leads, and wireless phones.
- Signal-monitoring equipment for acquiring the waveforms from the pacemaker and the electrocardiographic (ECG) signal when injected.
- ECG signal injection equipment.
- Various pacemakers.
- Various wireless phones.
- One wireless phone base-station simulator.
Testing was conducted under worst-case conditions. Phones were set at their highest power, and pacemakers were set to the greatest sensitivity permitted for each unit. In all tests, the pacemaker case and the phone keypad were in horizontal parallel planes, with the pacemaker 0.5 cm below the surface of the saline solution. Figure 1 shows the overall test setup.
Figure 1. Experimental apparatus.
The test factors were divided into three categories: pacemaker variables, wireless phone variables, and separation distance. The pacemaker variables consisted of the pacemaker model, pacemaker mode, lead polarity configuration, and the presence or absence of an injected ECG signal. Of the six pacemakers tested, two were single-chamber units and four were dual-chamber units.
|Name||Technology Standard||Number Tested|
|CDMA||Spread Spectrum, IS-95||1|
|TDMA-50 Hz||Dual-mode digital/analog, IS-54/55||4|
|PCS 1900||TDMA-217 Hz, J-STD-007||2|
The phone variables consisted of the phone technology or model, phone test mode, and phone orientation with respect to pacemaker lead alignment. Again, only the phone technologies that interacted with pacemakers in the first phase of the study were tested in the second phase. The eight digital wireless phones that were used in the testing are shown in Table 1. Five U.S. manufacturers provided the phones.
Except for TDMA-50 Hz phones, all phones were tested in an open-loop transmit mode. This test mode produced the typical digital pulsing format of the phone technology without communicating with an active cell site or base-station simulator. For some models, the phone's hardware was modified. For other models, keypad programming was used to configure the phone in the full-power transmit mode. TDMA-50 Hz phones were used while communicating with an NADC base-station simulator (HP8920A RF Communications Test Set with an HP83201A dual-mode cellular adapter). The TDMA-50 Hz phones were used in both ringing and talk-back communication modes. The ringing mode is achieved by registration of the phone followed by paging. Once initiated, the ringing continues for more than a minute while pacemaker testing proceeds. In the talk-back (loop-back) mode, a short message (approximately 4 seconds) is spoken into the handset. The digitally encoded form of the message is transmitted to the base station, which retransmits it continuously to the handset.
The study examined four relative orientations of the pacemaker and phone. The pacemaker was oriented in the tank such that the leads exited the header to the right (east) when viewed from above, with the long axis of the tank going from left to right (west to east). The orientation of the phone antenna from base to tip was either south to north (90°), east to west (180°), northeast to southwest (225°), or southeast to northwest (135°). Four additional orientations (0°, 45°, 270°, 315°) were examined during the first phase of testing, but produced redundant results. Therefore, the orientations of 0°, 45°, 270°, and 315° were eliminated from future tests.
Various combinations of these factors defined a single test run (for example, pacemaker 01, unipolar, VVT [ventricle sensing, ventricle pacing, ventricle pulse delivered after sensing], with injected ECG signal, in conjunction with phone Y, talk-back mode, or at a 90° orientation). The run itself consisted of individual tests at appropriate grid points. All runs in which interaction was observed in the first phase of the study were tested.
To determine the distance at which interaction between the wireless phone and the pacemaker no longer occurs, the phone was raised in discrete steps of approximately 1/8 in. starting from the lowest height, until the interaction stopped. Testing then proceeded by reducing the height by one discrete step. This height was the maximum vertical planar separation distance at which interaction occurred. Planar separation distance is the distance between the following two planes: the plane representing the top of the saline solution, approximately 1 cm above the pacemaker; and the parallel test plane defined by the top of a plastic grid upon which the phone is suspended and over which it is moved to test interaction. This parallel test plane is separated from the saline solution at various increasing distances until interaction ceases. The surrounding grid where interaction occurred in the first phase was tested thoroughly, and new interaction points were identified.
A second interaction distance measure, called the Euclidean distance, is shown in Figure 1. This distance was obtained by identifying the farthest x-y grid point from the pacemaker header and the base of the phone antenna at which an interaction event was observed. This x-y grid point was converted to a Euclidean distance and used as a measure of interaction distance susceptibility. This distance measure was used in the first phase.
RESULTS AND CONCLUSION
Figure 2 shows the overall results of the second phase of the study. Interaction was minimal at planar separation distances less than 50% of the AAMI-recommended separation distance of 6 in. Additionally, half of the pacemakers tested did not interact at more than the minimal separation distance. This is reflected in the figure as zero distance.
Figure 2. Height separation effects: maximum planar distance between the pacemaker header and B antenna of the wireless phone. The height of the bars indicates the distance. The two-dimensional rectangles indicate that there was no interaction beyond the zero separation distance.
This study was conducted with each phone operating at its highest power level and each pacemaker programmed to its maximum sensitivity setting. All tests were conducted with the phone in close proximity to the pacemaker, representing a phone being carried in clothing pockets or held adjacent to the body, in the vicinity of an implanted pacemaker. Caution must be exercised in using these results to directly contrast one phone technology with another due to differences in the frequency bands used and possible differences in the implementation of these technologies.
The following general conclusions can be drawn from the study. The planar separation distance measured in this study is significantly less than the Euclidean distance measured in the first phase of the study. The maximum planar separation distance obtained was 3 in., as opposed to a maximum Euclidean distance of 7.6 in. The study also showed that the maximum field strength may be concentrated over the case, the antenna of the phone, or both. Approximately 50% of the phones tested had the maximum field strength concentrated over the phone body.
Generally, the results of the study are encouraging when compared with the recommended guideline of a 6-in. separation between wireless phones and pacemakers. Additional security can be derived from the fact that many medical device companies that manufacture pacemakers now install feed-through capacitors to filter radio-frequency emissions that might affect the pacemaker's operation. Such filtering virtually eliminates EMI.
1. Barbaro V, Bartolini P, Donato A, et al., "GSM Cellular Phone Interference with Implantable Pacemakers: In Vitro and In Vivo Observations," presented at the BEMS conference, Stockholm, 1994.
2. Schlegel RE, Raman S, Grant FH, et al., In Vitro Study of the Interaction of Wireless Phones with Cardiac Pacemakers. EMC Report 19963, Norman, OK, Center for the Study of Wireless EMC, University of Oklahoma, 1996.
F. Hank Grant is director and Robert E. Schlegel is a professor of industrial engineering and associate director for research at the Center for the Study of Wireless EMC at the University of Oklahoma (Norman).