Notwithstanding improved interference protection, the current generation o f demand pacemakers remains sensitive to interference from radio-frequency sources. The studies reported in this chapter have shown inhibition and failure in many devices at field strengths that are likely to be encountered on aircraft under normal operating conditions. Sensitivity appears to be greatest in the HF and VHF range and is increased by modulation and pulsing. In the microwave band, interference is less common but may occur at higher field strengths at the lower end of the frequency range, particularly with pulsing. The decreased susceptibility in the higher frequency ranges may partly be due to more effective filtering and noise recognition by the pacemakers' sensing circuitry. It may also be relevant that lead reactance increases with frequency (table 5.4) and at high frequencies, the greater attenuation renders inductive effects and pacemaker interference less likely.
The interference caused in radio-frequency fields is thought to be mediated almost entirely by current induction in the sensing electrode. This is supported by occasional spot tests carried out during these studies, in which short-circuiting the aerial of the test apparatus during interference, promptly restored normal pacing. The possibility of additional direct effects on the pacemaker circuitry due to time-varying magnetic fields, cannot be ruled out and may be of particular relevance in the response to rapid changes in field strength and frequency. Efforts were made to minimise the possible generation of heat within the pacemaker circuitry by switching off the power and pausing between exposures but local heating effects of discrete components may explain some of the effects seen, such as hysteresis and delayed recovery.
Pacing systems with bipolar leads, although much less sensitive, were also affected to some extent at higher field strengths over a narrow frequency range. The capacity of a bipolar lead to minimise the risk of EMI due to the smaller distance between electrodes is well recognised but it does not confer immunity (Irnich, 1984).
The test apparatus used was deliberately designed to expose "worst case" effects. The area encompassed by the aerial was equivalent to the maximum that might be delineated by a 58cm pacing lead, a condition unlikely to be matched in practice. The impedance o f the aerial was also lower than that of a standard pacing electrode and it was therefore more sensitive to inductive effects. The magnitude of the difference in reactance, however, was relatively small when compared to the variation with changing frequency. The orientation of the test apparatus to the incident field was also selected to maximise electromagnetic coupling.
The field strengths at which interference and failure occurred were generally lower than in previous studies (Mitchell et al, 1975; Hardy, 1979). Although this may partly be due to changes in the devices and their interference protection circuitry, it is likely that it also reflects the use of a more rigorous protocol. Previous studies assessed only a small number of discrete frequencies within each band, selected either arbitrarily or to assess sensitivity to particular radio-frequency emitters. In the present study, each band was scanned either continuously or in small frequency increments. The occurrence of 'windows' of susceptibility, both for frequency and field strength, suggests that the selective approach previously used, may miss or underestimate important effects. This has important implications for safety standards and manufacturer's perceptions of EMI susceptibility. The safety standards in the UK are currently under review but the approach in the USA and that adopted by many manufacturers is based on recommendations of the Association for the Advancement of Medical Instrumentation (AAMI, 1975), which limit testing to a small number of discrete frequencies. High field strengths are applied (up to 200Vm^) but these may simply serve to provide inappropriate reassurance regarding the risk of EMI, based on negative results at insensitive frequencies. The field strengths at which EMI occurred in these studies were well below the reference levels for environmental exposure proposed by the National Radiological Protection Board (NRPB, 1989) and the International Non-Ionizing Radiation Committee (INIRC, 1988).
In these studies, free-field exposure was used, in preference to a saline-filled test chamber of the type recommended by the AAMI (1975), in order to disclose 'worst case' responses that reflect the true EMI susceptibility of the devices. It is recognised
that implantation may attenuate the risk of EMI but the attenuation provided by a saline model is arbitrary and represents neither the 'worst case' nor the true response of an implant. Attenuation in saline models has been shown to vary between a factor o f ~3 at 450MHz and ~5 at 3.1GHz (Mitchell et al, 1975). In determining EMI susceptibility, it may be more appropriate to determine 'worst case' responses in a free- field exposure and then apply the attenuation factors to predict the response o f an implant.
It is clear that there is scope for further improvement in pacemaker interference protection (Irnich, 1984). The devices used in this study were from a variety of manufacturers, none of whom appear to have resolved the potential problem of EMI. Details of the interference protection methods used by each manufacturer are not in the public domain and no attempt was made to correlate the findings in this study with the technical characteristics of the devices. The low prevalence of clinically significant EMI may discourage manufacturers from investing heavily in efforts to further improve the interference protection of their devices. If meaningful national and international safety standards are to be developed and applied, the limitations of the current methods of interference protection will need to be recognised and more rigorous test methods than those presently applied will be required.
CONCLUSIONS
The studies reported in this chapter have demonstrated that explanted unipolar pacemakers are susceptible to EMI in radio-frequency fields at levels likely to be encountered in aircraft. Susceptibility varies in different devices and is related to frequency and field strength. It is greatest in the HP and VHP bands and it is increased by modulation and pulsing. In the microwave band, EMI is less evident but it may occur towards the lower end of the frequency range at high field strengths, particularly in pulsed fields. Bipolar systems are less susceptible but they are not immune to EMI and may be affected at higher field strengths. There is considerable scope for improvements in interference protection. The test methods previously applied may underestimate EMI susceptibility and their limitations should be recognised by appropriate revision of the national and international safety standards.
Table 5.1:
Pacemakers studied
M anufacturer Model Polarity
Biotronik Neos Unipolar
Cordis Multicor Gamma (SN337 A) Unipolar
Cordis Stanicor S (SN342 A7) Unipolar
Intermedics Quantum Unipolar
Medtronic Activitrax (8403) Unipolar
Medtronic Spectrax (8423) Unipolar
Sorin Lit 610A Unipolar
Sorin Orion 30 Unipolar
Telectronics Optima Unipolar
Telectronics Optima MF Unipolar
Medtronic Activitrax (8400) Bipolar