Study of the Interaction of
Wireless Phones and Cardiac Pacemakers
This report summarizes the results of the large-scale in vitro investigation of the interaction between wireless phones and cardiac pacemakers conducted by the Center for the Study of Wireless Electromagnetic Compatibility at the University of Oklahoma. The research focused on testing 29 pacemaker models with 5 different phone standards to (1) evaluate the amount of interaction between wireless phones and pacemakers, and (2) identify those factors which had a significant influence on the level of interaction. The phone standards included analog, CDMA, PCS 1900, TDMA-11 Hz, and TDMA-50 Hz. TDMA-50 Hz phones were operated in conjunction with a base station simulator in both ringing and talkback modes. Pacemakers included single-chamber and dual-chamber models tested in AAT, VVT and DDD modes. Tests were conducted with unipolar and bipolar lead configurations, and in the presence and absence of a simulated ECG signal. Relative orientation and coplanar separation distance of the phone and pacemaker were also considered.
This project was undertaken to verify the reported electromagnetic interference (EMI) between cellular phones and cardiac pacemakers and to extend those results to wireless phone technologies not previously tested. In addition, the project was designed to more precisely classify the modes of pacemaker interaction and to identify the wireless phone operating modes that produce the interaction. Finally, the study investigated the conditions which might promote interactions in order to increase current understanding of the mechanism of the interaction.
A peer review team, named the Pacemaker Study Design Group, helped define the scope of the project and provided input with respect to the test protocol, design of the torso simulator, and other parameters of the study. The group consists of representatives from pacemaker manufacturers, wireless phone manufacturers, wireless carriers, government agencies, and other researchers.
Testing was conducted in a closed, electromagnetically shielded room at the Lucent Technologies Inc. Test Facility in Oklahoma City, Oklahoma. Simulated functioning of the heart/pacemaker system was accomplished through the use of a torso simulator and various electronic equipment items which generated and monitored electrical signals. The test equipment consisted of (1) a torso simulator with saline bath and supports for the pacemaker, pacemaker leads, and wireless phones, (2) signal monitoring equipment for acquiring the waveforms from the pacemaker and the ECG signal when injected, (3) ECG signal injection equipment, (4) various pacemakers, (5) various phones, and (6) a wireless phone base station simulator.
All testing was conducted under worst-case conditions with the phones at their highest power and pacemakers set to the greatest sensitivity permitted for each unit. In all tests, the pacemaker case and the phone keypad were in the horizontal plane, with the pacemaker 0.5 cm below the surface of the saline solution. For various phones, the centerline of the longitudinal axis of the antenna ranged from 1.77 cm to 2.77 cm above the saline surface.
The test factors were divided into two categories: pacemaker variables and wireless phone variables. The pacemaker variables consisted of the pacemaker model, pacemaker mode, lead polarity configuration, and presence or absence of an injected ECG signal. Twenty-nine pacemaker models were provided by the five major manufacturers of pacemakers which supply more than 90% of the U.S. market. Among the 29 pacemakers, 13 were single-chamber units and 16 were dual-chamber units. This fact, along with other model features, determined the available modes for testing each unit. All units were tested in the AAT and VVT modes if these modes were available. If not available, the AAI and/or VVI modes were used. Dual-chamber units were also tested in the DDD mode. For both single-chamber and dual-chamber units, the triggered, single-chamber modes were selected as a result of pilot testing in which it was determined that interaction events were more easily identified in a trigger mode (AAT or VVT) than in an inhibit mode (AAI or VVI). In general, single-chamber units were only tested in the atrial configuration.
Twenty-three of the twenty-nine pacemakers under test could be operated in either the unipolar or bipolar mode, and were tested in both modes. One unit was strictly unipolar and five units would only accept bipolar leads. With one exception, the same set of IS-1 BIpolar leads (ISO/DIS5841-3) was used for all tests (both in the unipolar and bipolar configurations). All single-chamber tests (AAT and VVT) were conducted twice, once with no injected ECG signal (typical asynchronous pacing) and once with an injected ECG signal (typical inhibition). Testing in the DDD mode was restricted to the asynchronous pacing mode without an injected signal since the torso simulator provides only a single saline chamber.
The phone variables consisted of the phone technology/model, phone test mode, and phone orientation with respect to pacemaker lead alignment. Nine test phones provided by six major manufacturers of U.S. digital wireless phones were used in the testing as follows:
- Analog (AMPS FM; EIA-553; dual-mode digital/analog; 4 mfrs.)
- CDMA (spread spectrum; IS-95; 2 mfrs.)
- TDMA-50 Hz (IS-54/55; dual-mode digital/analog; 4 mfrs.)
- TDMA-217 Hz (J-STD-007; 2 mfrs.)
- TDMA-11 Hz (ITU-R M.[8A/XB]; 1 mfr.).
Except for TDMA-50 Hz phones, all phones were used in an open-loop transmit mode, that is, producing the typical pulsing format of that technology without being in communication with an active cell site or base station simulator. For some models, this was accomplished through hardware modification of the phone. For other models, keypad programming was used to configure the phone in the full-power transmit mode. TDMA-50 Hz phones were used while in communication with an NADC base station simulator (HP 8920A RF Communications Test Set with an HP83201A Dual-Mode Cellular Adapter). Two TDMA-50 Hz communication modes were used: ringing and talkback. The ringing mode is achieved by "registration" of the phone followed by "paging." Once initiated, the ringing continues for more than one minute while pacemaker testing proceeds. In the talkback (loopback) 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 re-transmits it continuously to the handset where it is decoded. Finally, the TDMA-50 Hz dual-mode phones were also tested in the analog ringing mode to examine any interaction with the analog format.
Four different relative orientations of the pacemaker and phone under test were examined. 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 (1) south to north (90°), (2) east to west (180°), (3) northeast to southwest (225°), or (4) southeast to northwest (135°). Four additional orientations (0°, 45°, 270°, 315°) were examined during initial testing and determined to produce redundant results. Therefore, the orientations of 0°, 45°, 270°, and 315° were eliminated from future tests.
Various levels of the above factors were combined to define a single test run (e.g., pacemaker 01, unipolar, VVT, with injected ECG signal, in conjunction with phone Y, talkback mode, at a 90° orientation). The run itself consisted of individual tests at all grid points determined by the decision rules of the test. Not every sample of a given technology was tested with every pacemaker in every mode. In general, at least two different phone models of a given technology (TDMA, CDMA, PCS, etc.) were tested with each pacemaker to allow comparison of model differences within each standard. The results of 8,296 test runs are reported here.
At any test point on the grid, a simple determination was made as to whether an interaction event occurred or did not occur. If an event occurred, the event was classified as to interaction mode and regularity based on the identification of 14 different modes of interaction (e.g., inhibition, asynchronous tracking, transient response) and whether the occurrence of the interaction was repetitive or sporadic. Meaningful information can be derived from the data through various methods of summarization. The total number of interaction events can be computed as a function of the pacemaker unit, phone unit, lead polarity, etc. This measure has the distinct disadvantage of not accounting for varying opportunity for an event to occur, that is, the number of test runs involving a specific level of a factor.
To properly adjust for the differing opportunity, the total number of interaction events within a given category must be adjusted by the number of test runs conducted in that category. This adjusted measure has been termed theIncidence of interaction. This Incidence measure can be further decomposed into two elements, Rate and Density.Rate refers to the proportion of runs in a given category in which at least one interaction event was observed. In other words, Rate is the ratio of the number of runs with at least one observed interaction to the total number of runs in that category. Density refers to the number of interaction events divided by the number of runs with any observed interaction. Thus, Density provides a measure of the average number of interactions for a test run that had interaction. Note that Incidence = Rate x Density.
Finally, the maximum distance of interaction was obtained by identifying the farthest X-Y grid point from the pacemaker header at which an interaction event was observed. This identified the location of the base of the phone antenna without regard to the relative phone orientation or the point of maximum field strength along the antenna axis. This X-Y grid point was converted to a Euclidean distance and used as a measure of interaction susceptibility.
This study was conducted under worst-case conditions with phones operating at their highest power levels 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. Hence, the results should be interpreted carefully.
The following general conclusions can be drawn from the study:
1. Throughout all testing, there was no instance of pacemaker re-programming.
2. Whenever interaction was observed during a test, the pacemaker returned to its normal operation as soon as the phone was turned off.
3. A relatively small number of pacemakers was responsible for a disproportionately large number of interaction events, while many models possessed a perfect record of no interaction.
4. No interaction events were observed during the analog tests (0 of 1184 runs).
5. Under worst-case conditions there was more interaction of pacemakers with the TDMA-11 Hz phone, with 21.4% of the test runs resulting in some interaction.
6. There was comparatively little interaction with the other digital standards as follows:
- CDMA - 2.8%
- PCS 1900 - 0.6%
- TDMA-50 Hz - 4.7%
7. The Ringing and Talkback modes tested for TDMA-50 Hz phones produced similar interaction events at comparable Rates
8. The majority of interaction events in the single-chamber test conditions were either asynchronous pacing (a designed noise reversion response) or a transient response in which atrial or ventricular pulses were missing or occurred with continuous rate fluctuation.
9. Except for minor variations in a few pacemakers, there was essentially no difference in the Incidence
of interaction between unipolar and bipolar operation.
10. For the majority of units, the atrial side of the pacemaker appeared more susceptible to interaction than the ventricular side. For the single-chamber units, this was evidenced by a lower Density
of interaction points in the ventricular mode. For the dual-chamber units, a lower Rate
of interaction resulted in the ventricular mode.
11. The injection of an ECG trigger signal provided a better opportunity to observe some forms of interaction. Any test protocol must include an injected ECG signal to reduce the possibility of false negatives.
12. The relative orientation of the phone and the pacemaker case was a significant factor in terms of the amount of interaction that occurred.
13. The maximum coplanar (X-Y) distance between the pacemaker header and the base of the phone antenna at which interaction occurred for any pacemaker was 8.7 inches for the highest power TDMA-11 Hz phone (6 inches for the overwhelming majority of pacemakers which exhibited interaction). If measured from the pacemaker header to the tip of the antenna or to the point of maximum RF field strength, the maximum distance of interaction is substantially less. These results support HIMA-recommended pacemaker labeling to "maintain a minimum separation of 6 inches."
14. When compared with previous in vitro
and in vivo
data, the results of this study show a lower rate of interaction, but provide similar relative rates of interaction across the various phone standards.
15. The test results were repeatable to the level of identifying the specific location of interaction points and the mode of the interaction. Further comparisons with clinical data are needed to determine the value of in vitro
testing for predicting in vivo