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Portable cellular (mobile) phones represent an
important source of electromagnetic interference for patients with an implanted pacemaker,
essentially because they can generate signals in the very low frequency range (10-100 Hz)
to which pacemakers are responsive. It is very important that these patients and
physicians be aware of the risks and consequences of exposure to such interference, and
understand the ways to avoid such problems, also related to non-clinical concerns, such as
legal consequences.
In recent literature there is also considerable difference of opinion concerning the
ability of cellular phones to interfere with implanted devices and several studies have
been performed, both in vitro and in vivo, mostly with phones operating under the GSM
protocol. On the contrary, there are very few reports on the way phones using TACS
protocol will influence the pacemaker operation.
TACS (Total Access Communications Standard) protocol operates on analog basis: the
carrier frequency (450 or 900 MHz) modulated within 25 kHz according to the voice signal.
Both, carrier and modulation frequencies, are totally outside the spectrum of cardiac
signals recognized by a pacemaker, but modulation may occur at a repetition rate
comparable to such signals, both at the beginning and at the end of a call, when the
carrier frequency is switched on or off.
GSM (Global System for Mobile telecommunications) protocol is based on digital
techniques: the voice signal is digitized and then transmitted from the phone to the
nearest base station. The digital code is sent at fixed intervals. The phone uses radio
frequency carriers in the 890-960 MHz range modulated by pulses at a repetition rate of
217 Hz. The protocol employs a transmission phase, called DTX, for "energy
saving" purposes. During this phase all data are sent within short time slots of 0.5
ms at periods of 4 ms or shorter, when there is no answer. The DTX consists of two
separate components: the Silence Descriptor (SID), at 2.08 Hz, and the Slow Associated
Control Channel (SACCH), operating with a repetition frequency of 8.33 Hz. These two last
frequencies are compatible with the band-pass of the pacemaker sensing amplifiers.
To assess the potential for interference with normal pacemaker function, we tested the
two transmission protocols for their pro-arrhythmic effect on patients implanted with
various pacemakers, who attend our hospital for routine follow-up.
From July, 1995 to October, 1996, 142 patients (86 males, 46 females, age range: 26-95
yr) were included in this study with 33 pacemaker models from 9 manufacturers. All 33
different device models (59 DDD, 29 VDD, 51 VVI and 3 AAI) were tested. All were implanted
at our Center from 1992, and each one was programmed to different operating modalities
before to be tested. Tests for interference were performed under continuous ECG monitoring
while each pacemaker model was temporarily programmed to its highest sensitivity level and
while the phone antenna was positioned over the pacemaker connector. Phone antenna was
than driven away in order to measure the maximum interfering distance.
We tested each patient with two different models of phone: GSM - Motorola, model:
8200, 2 W power, and TACS - Motorola, model: Microtac Gold, 0.6 W power. Absence of
interactions between test site and measuring instruments (ECG recorders: Marquette model
MAC PC and MAC 15, Pacemaker Function Analyzer, and each specific pacemaker programmer)
was previously established by means of a calibrated Spectrum Analyzer HP model 8596E.
While testing the GSM protocol, reproducible interference was detected in 37/142
patients, ie in 19 pacemakers models (Intermedics, Inc. mod. Relay 294-03; LEM Biomedica
mod. Turinal 30 and Turinal 305; Medtronic, Inc. mod. Thera D 7940, Thera D 7944, Thera D
7945, Thera VDD 8948 and Minix 8340; Vitatron BV mod. Harmony 701). Inhibition of pacing
and inappropriate ventricular tracking were observed singly or in combination. Some models
switched to an asynchronous mode while the phone was operating. Unintentional
reprogramming of the pacemaker was observed in some patients, but the pacemaker could be
reprogrammed to its previous settings after the tests. In figures 1 and 2 two
"phone-generated" arrhythmic behaviors are shown.
In figure 1 the interference irregularly triggers the ventricular channel at the phone
ringing. In this case, the absence of spontaneous atrial activity facilitates the
retrograde ventriculo-atrial nodal conduction (arrows) of most of the paced ventricular
complexes. This phenomenon ends when the phone is switched off.
Fig. 1: GSM mode interference in a pacemaker Intermedics model relay 294-03 (mode DDD,
basic rate 50 bpm, upper rate limit 100 bpm, post ventricular atrial refractory period 300
ms, A-V delay 170 ms, atrial sensitivity 0.5 mV, ventricular sensitivity 1.0 mV.
In figure 2 the high ventricular pacing rate, induced by phone interference, do not
decrease even after the phone is removed. The magnet application only causes a reduction
in the ventricular pacing rate with a period equal to the pacemaker's "magnet
mode." Consequently, it is necessary to reprogram the pacemaker in order to restore
its original function. From these experiences it appears that interferences, generated by
the GSM protocol, can, on account of its complex frequencies of operation, cause the
pacemaker to operate inappropriately. The interaction may result in unusual, chaotic and,
sometimes even in dangerous pacing conditions.
Fig. 2: GSM mode interference in a pacemaker Medtronic model Thera VDD 8948 (mode VDD,
basic rate 50 bpm, upper rate limit 120 bpm, post ventricular atrial refractory period 310
ms, A-V delay 120 ms, atrial sensitivity 0.5 mV, ventricular sensitivity 1.0 mV).
While testing the TACS mode, interference was observed in 16/142 patients, ie 4
pacemaker models (Medtronic, Inc. mod. Thera D 7944, Thera VDD 8948 and Minix 8340;
Vitatron BV mod. Harmony 701). These pacemakers were inhibited only briefly (< 3 s)
while the carrier for the transmission was switched either on or off.
Figure 3 shows the typical pacemaker response to the interference generated by a TACS
phone. The first two cycles show proper atrial sensing and normal ventricular output,
according to the marker channel. During ringing, artifacts appear on the marker line
(middle trace) and the ventricular output is inhibited, for few pulses only, as manifested
by both the top and bottom strips. TACS mobile cellular phones represent a potential, but
limited, hazard for pacing, only for those patients who are totally dependent on their
pacemakers.
Fig. 3: TACS mode interference in a pacemaker Medtronic model Thera VDD 8948 (mode
VDD, basic rate 50 bpm, upper rate limit 120 bpm, post ventricular atrial refractory
period 310 ms, A-V delay 120 ms, atrial sensitivity 0.5 mV, ventricular sensitivity 1.0
mV).
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