Pacemakers and ICDs: Still a Problem?
E. O
CCHETTA, L. P
LEBANI, M. B
ORTNIK, P. M
ARINOElectromagnetic interference (EMI) may affect the behaviour of some med- ical electrical equipment, including cardiac pacemakers and implanted car- dioverter-defibrillators (ICDs) [1]. EMI occurs when an electronic device is subjected to any electromagnetic field with an amplitude higher than the interference threshold. The effects on the device depend on the energy of the electromagnetic signal interfering with the normal function of the electronic circuit, with the following possibilities:
− A temporary malfunction
− An alteration of the device during exposure to interference
− A permanent alteration of the circuit due to the interference
Electromagnetic fields may be of various natures, and their ability to interfere with the electronic device depends mainly upon two factors:
− The frequency range of the electromagnetic field – either the carrier or its modulation – which may fall within the operative range of the device
− The power of the electromagnetic wave, which, when relevant, can modify the behaviour of the active device
Nowadays there are multiple sources of environmental EMI signals, e.g.
metal detector gates (airports, supermarkets, jewellers, banks, etc.), which may generate dangerous electromagnetic fields; however, while some authors [2, 3] have reported no interference with pacemaker functioning, other stud- ies [4–7] have found interference, sometimes significant.
In the Spiced Teas study [7] interactions between acoustomagnetic elec- tronic article surveillance gates and pacemakers have been reported for vir-
Divisione Clinicizzata di Cardiologia, Facoltà di Medicina e Chirurgia, Università degli Studi del Piemonte Orientale ‘A. Avogadro’, Novara, Italy
tually all the pacemakers tested (50 patients, 6 different types of pacemak- ers). Interactions included asynchronous pacing, atrial and ventricular over- sensing, and even paced beats resulting from the direct induction of current in the pacemaker. Other modern applications have also been investigated, such as automatic toll payment radiofrequency systems: in this case no inter- ference effect on pacemakers was found [8].
Although the effects of electromagnetic interference on cardiac pacemak- ers have been widely studied, there are still many perplexities about the con- sequences on more sophisticated devices such as ICDs. Fetter et al. [9] evalu- ated the electromagnetic interference generated in a working environment by large welding machines and motors on patients with implanted ICDs;
they did not observe any interference with normal ICD functional operation.
The Spiced Teas study [7] has reported the absence of interference between electronic article surveillance devices and correct functioning of ICDs. The authors suppose that this immunity to electromagnetic interfer- ence is due to complex detection algorithms and programmable rate cut-off parameters, which of course are not present in pacemaker sensing systems.
Mathew et al. [10] and McIvor [11], on the other hand, have reported inappropriate discharge of ICDs as a result of exposure to electromagnetic interference from electronic article surveillance devices, perhaps because of complex interference between arrhythmia detection and automatic gain control.
In 1989, Karson et al. [12] reported a case of inadvertent ICD activation by a magnet located in the loudspeaker of a stereo system. Other anecdotal reports have been published of inappropriate ICD shocks caused by EMI from a magnetic bingo wand [13], remote controls of toys [14], slot machines [15], and an electric razor [16]; in this last case, EMI was caused by defective insulation of the electrode and could not be evoked with an intact electrode.
Most of the previous reports of EMI in ICDs involved earlier, less sophisti- cated devices with epicardial screw-in sensing leads. Theoretically, endocar- dial leads should be better protected from EMI, because of both the spatial orientation of the endocardial bipole and the endocardial location itself [17].
Another common medical source of EMI is transcutaneous electric nerve stimulation (TENS), a widely used method of relieving various muscu- loskeletal pains. Several well-documented cases of spurious shocks triggered by TENS application in patients with a variety of lead configurations and sensing algorithms have been described [18, 19].
An ever more common source of electromagnetic waves in today’s envi-
ronment is cellular phones; they may be clinically relevant sources of EMI
and may affect pacemaker and ICD function [20–23]. Patients with an
implanted pacemaker or ICD are so alarmed at the interference phenomenon
that there have been urgent calls for studies to evaluate the problem [24–29].
Currently there are no standards prescribing limits for pacemaker com- patibility. The IEC standard 601–1-2 [30] is not adequate, recommending testing with an electric field of 3 V/m modulated sinusoidally with 1 KHz when digital mobile phones with 2 W output may produce up to an ampli- tude of 120 V/m at a 2 cm distance [20]. There is an European standard to which pacemakers designers conform, named EN 50061/A1 [31]. This norm specifies voltages as a function of frequency below which (30 MHz) pace- makers must function undisturbed. Although the standard of 30 MHz does not cover the mobile phone frequencies, Irnich [20] found that most pace- makers were resistant to 2 W digital GSM (Global System for Mobile Communication) fields, suggesting that most manufacturers have developed resistant models without having mobile phone compatibility as a design cri- terion.
Cellular phones may be divided in two different systems utilised in Europe and North America:
1. Analogue transmission: transmission passes through a continuous wave which has modulation variations in frequency. The carrier frequency in this system may be about 450 MHz (NMT 450) or around 900 MHz (NMT 900, European TACS, and American FDMA).
2. Digital transmission: several simultaneous conversations are allowed on the same line; to do this, a carrier frequency of around 900 MHz for the European GSM system and 800–900 MHz for the American NADC, TDMA, and CDMA systems is coded. In digital transmission systems the signal is coded and modulated at a frequency which on the GSM system is around 217 Hz. The system also has an energy saving mechanism, called DTX, which is activated when the user is in the listening mode, with an additional modulation of 2 Hz and 8 Hz.
All these systems operate on a power of 0.6–1 W in the USA and from 20
mW to 2 W in Europe, with a mean value of 0.8 W. It is, therefore, theoreti-
cally possible that a mobile phone operating near enough to a pacemaker may
cause interference in its working condition. Numerous studies have been car-
ried out on this matter both in vitro [25, 32] and in vivo [27, 28, 33–36], which
discuss the question of the significance and nature of the interference: briefly,
it was shown that interference was around 20–30%, always with the telephone
in close proximity, within 20 cm. In the majority of cases the interference was
considered insignificant, with brief inhibition, conversion to asynchronous
mode, and synchronisation of atrial-triggered stimulation; in each case inter-
ference took place when the phone was placed on the pacemaker itself, the
sensitivity of the pacemaker was at its maximum, the device used GSM digital
technology, and interference occurred almost exclusively when the phone was
in the calling or receiving mode.
Less clear is the question of interference by cellular telephones in the operation of more sophisticated devices such as the ICDs [23, 24, 26, 29]. An electromagnetic field interfering with an ICD may alter its working mecha- nisms in the following ways:
− By not allowing for correct recognition of an arrhythmia (undersensing), resulting in failure to deliver cardioversion or defibrillation therapy
− By generating false recognition of an arrhythmia (oversensing), which may induce inappropriate delivery (most uncomfortable for the patient), or may even induce a serious arrhythmia such as ventricular fibrillation
− By inhibition of pacing, which of course may be very dangerous after shock delivery.
The few data presently available indicate that shielding by the body is an important element of protection from external interference [26]. Bassen et al. [37] documented an in vitro inappropriate delivery of therapy induced by digital telephone signals. In vivo studies by Chiladakis et al. (36 patients) [26], Stanton et al. (25 patients) [38], Wilke et al. (50 patients) [39], and Altamura et al. (200 patients) [40] investigated different types of telephones, analogue and digital. They found that no interference was produced when the telephone antenna was kept directly over the ICD, and there was no inter- ference effect on recognition of induced ventricular fibrillation, pacing/sens- ing functions, or telemetry data.
Notwithstanding these findings, some ICD today are equipped with spe- cial filters which may reduce EMI from mobile telephones. Interesting results emerged when these filters were tested on pacemakers and ICDs [41]. A pre- vious study by ourselves [29] evaluated quite a large cohort of ICD models on today’s market. Using either analogue (TACS) or digital (GSM) European phone systems, patients were monitored by means of surface ECG and tele- metric intracardiac electrogram with sensing markers, using an identical protocol for each group (Fig. 1):
1. Telephone in direct contact with both the ICD and the programming head of the programmer
2. Telephone in direct contact with the ICD alone (head of the programmer moved away)
3. Telephone positioned in the area of the pacing/sensing ventricular lead 4. Telephone held in patient’s hands
Following manoeuvres were carried out for each position:
1. Telephone in receiving and calling mode (frequency around 900 MHz) 2. Telephone in active conversation (speaking, frequency around 900 MHz
modulated at 217 Hz)
3. Telephone in passive conversation mode (listening, with activation for
the digital transmission phone of the discontinuous transmission mode –
DTX – frequency modulated at 2–8 Hz)
In no patient there was any interference induced by the electromagnetic field from a cellular phone causing false arrhythmia recognition, even under the maximum limit condition, with the telephones placed in direct contact with the ICD. Conversely, for both the analogue and the digital system, con- stant interference on telemetric transmission between the programmer and the ICD was observed when the phone in receiving or calling mode was in direct contact with the telemetric head. Sometimes the interference was so great to cause loss of telemetric monitoring of the intracardiac ECG, even if there was no inappropriate reprogramming.
In a subgroup of five patients, we also evaluated the presence/absence of interference caused by cellular phones during ventricular arrhythmia induc- tion and recognition. There was no evidence of interference with arrhythmia detection. The implanted system recognised the arrhythmia correctly and appropriately delivered a defibrillation shock. No difference was observed when we compared timing of ICD discharge with cellular phones operating versus inactive (Fig. 2).
Fig. 1.Protocol of cellular phone evaluation: 1 the telephone in direct contact with both the ICD and the programming head of the programmer; 2 the telephone in direct con- tact with the ICD alone (head of the programmer moved away); 3 the telephone posi- tioned in the area of pacing/sensing ventricular lead; 4 the telephone placed in the hands of the patient
Conclusions
1. Patients with an implanted pacemaker or ICD may safely use a cellular phone, provided they do not use it in closer range than 20 cm to the device when in receiving and conversation mode. This means: it is advis- able for the patient not to keep the cellular telephone in the breast pocket of his/her jacket, on the same side as the device.
2. Use of a cellular telephone could be permitted, even in cardiac environ- ments where programming and telemetric checking up of pacemakers and defibrillators is being carried out, if it is kept at least 1 m away from electronic devices.
3. If the patient has a cellular telephone, it would be useful to determine any possible interference effect on his/her own pacemaker/ICD in order to give psychological reassurance and allow resumption of work in industri- al facilities [42].
Fig. 2a, b.Test performed during Defender ICD (Ela Medical) implantation. a During the first ventricular fibrillation induction a GSM phone was active (ringing in receiv- ing mode): the time before the 13 J shock was 7 s. b During a second ventricular fib- rillation induction without GSM (phone OFF) the time before the shock was very similar (8 s)
a
b
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