Pacemaker Phase II:

Height of Separation Study Results




Phase I of the pacemaker study was a large- scale study designed to determine the significant factors and the extent of interaction between five wireless phone technologies (CDMA, PCS 1900, TDMA-11 Hz, TDMA-22 Hz and TDMA-50 Hz) and 29 pacemakers. The results were published in October 1996 in EMC Report 1996-3, In Vitro Study of the Interaction of Wireless Phones with Cardiac Pacemakers

Phase II of the pacemaker study builds upon the previous study by investigating the planar separation distance for those pacemaker-phone combinations that exhibited interaction. This resulted in testing combinations of six pacemakers with five phone technologies (TDMA-11 Hz, TDMA-22 Hz, TDMA-50 Hz, TDMA-217 Hz, and CDMA). The results of the Phase II study were published in June 1998.

The test methodology consisted of testing for interactions between the phone and pacemaker at various heights of separation from 0 inches to 3 inches. Testing was begun at the lowest level, and continued throughout subsequent levels until the interaction disappeared. The highest level with interaction was scanned in detail, and all interaction points were charted. A diagram illustrating the test setup for measuring the interaction distance is shown in Figure 1.

The researchers found that, for interaction to occur, the planar separation between the base of the antenna and the pacemaker header had to be significantly less than the Euclidean separation distance reported in Phase I (Figure 2). This may provide a more realistic recommendation for required minimum separation during phone use. Additionally, the researchers found that the maximum fields were often concentrated over the body of the phone rather than the antenna, as might be expected.

1

 

2

 

In Vitro Study of the Interaction of
Wireless Phones with Cardiac Pacemakers

Phase II: Planar Separation Effects of Air


 

Background

This report summarizes the results of the phone-pacemaker planar separation study which is an extension to earlier research conducted by the Center for the Study of Wireless Electromagnetic Compatibility at the University of Oklahoma. The initial project was undertaken to quantify the reported electromagnetic interaction between wireless phones and cardiac pacemakers. In addition, the original project was designed to more precisely classify the modes of pacemaker interaction and to identify the factors that influence the extent of the interaction. This second phase of testing was conducted to further define the maximum planar separation distance of the phone and pacemaker needed to avoid interaction.

Method

Testing was conducted in a closed, electromagnetically shielded room at the Lucent Technologies Inc. Test Facility in Oklahoma City, Oklahoma. The functionality of the heart/pacemaker system was simulated through the use of a torso simulator and various electronic equipment items which generated and monitored electrical signals. The test equipment comprised:

  1. a torso simulator consisting of a saline bath and grid 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 a horizontal plane, with the pacemaker 0.5 cm below the surface of the saline solution.

Testing was conducted only for those combinations of pacemakers and phones that elicited interaction in the original study, and only under the specific test conditions where interaction was observed. These test conditions included specific combinations of pacemaker mode, lead polarity configuration, ECG signal injection, phone mode, and phone orientation with respect to pacemaker lead alignment. Results were classified by pacemaker unit, phone unit, and wireless phone technology. Among the six pacemakers tested, two were single-chamber units and four were dual-chamber units. Nine test phones representing CDMA (cellular), TDMA-11 Hz, TDMA-22 Hz, TDMA-50 Hz (cellular) and TDMA-217 Hz(PCS) were provided by five major manufacturers of U.S. digital wireless phones.

All phones except TDMA-50 Hz were tested 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 (HP8920A 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.

Four different relative orientations of the pacemaker and phone under test were possible. 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°).

Specific levels of the above factors were combined to define a single test run (e.g., pacemaker 01, unipolar lead configuration, 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. All runs in which interaction was observed in the original study were tested. The run itself consisted of individual tests at all grid points determined by the decision rules of the test.

The primary dependent variable was the maximum Euclidean distance of interaction for each test run. This Euclidean distance was composed of two components, the planar separation distance and the X-Y coordinate location on the test plane. Planar separation is defined as the incremental separation distance above the baseline surface height used in the original study. This baseline height was defined such that the top surface of the phone support grid was 1.0 cm above grid point from pacemaker header at which interaction event was observed. This identified the location of the base of the phone antenna during the test without regard to the relative phone orientation or the corresponding point of maximum field strength along the antenna axis. In the original study, this X-Y grid point was converted to a two-dimensionalEuclidean distance. In this study, a three-dimensional Euclidean distance was computed by inluding the planar separation distance.

To determine the incremental separation distance in air at which interaction ceased, the phone was raised in discrete steps starting from the smallest separation of 1.0 cm (the baseline distance used in the original study) until the "No Interaction" height was reached. The testing continued by decreasing the separation distance by one discrete step. This height represented the maximum vertical planar separation distance where interaction occured. A thorough scan was conducted of the surrounding area of the grid where interaction was observed in the original tests. All interaction points were recorded on the data sheet.

Conclusions

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. 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.

Twenty pacemaker-phone combinations involving six pacemakers and nine phones were tested in 208 test runs. The selected test conditions had all resulted in interaction in the EMC Center's prior study. Small increases in the planar separation distance resulted in a dramatic reduction in interaction. Approximately half of the original 208 test runs exhibiting interaction no longer resulted in interaction when the plane of the phone was further separated by only 0.375 inches. At an incremental planar separation above baseline of 2.5 inches, the percentages of runs with interaction decreased to approximately 1.4% of the original number of runs with interaction. Keep in mind that this is a further reduction to the number of runs with interaction in the original study which represented less than 5% of all test runs. Therefore, the overall interaction rate, considering all 8,926 test runs from the original study, was less than 0.07% (seven hundredths of 1%) for a total planar separation distance of 3 inches.

The planar separation threshold of interaction measured in this study was significantly less than the two-dimensionalEucliden distance reported in the original study. The maximum planar separation distance at which interaction occured was 3 inches compared with 7.6 inches for the maximum three-dimensional Euclidean distance in this study and 7.6 inches for the maximum two-dimensional Euclidean distance across the same set of tests in the original study (from the base of the antenna to the pacemaker header with an air gap of only 1.0 cm between the top surface of the saline and the closest plane of the phone).

These two methods of measuring distance represent two different but important performance measures. The baseline Euclidean distance measure (original study) is important since it reflects effects that would occur if the phone were positioned and moved directly on the chest of the pacemaker wearer. The planar separation measure reflects the effects of the phone when brought in close proximity to the pacemaker wearer, but separated by air and not placed directly on the chest. Alternatively, this measure represents the closest distance that any portion of the hand-held phone may be placed with any possibility of interaction. By keeping all parts of the phone at least 3 inches from the pacemaker, the probability of an interaction approaches zero. With the improved electromagnetic immunity of current production pacemakers, this probability has been further reduced.

Note: A summary article in Medical Device & Diagnostic Industry can be found on Medical Device Link,http://www.devicelink.com/mddi/98/09/008.html

   
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