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on implanted leads (Bassen et al, 2007). We
compared this model with an actual PM lead. First
of all, we compared the temperature increase
measured at the tip of an insulated metal wire with
its equivalent numerical model: very good
agreement between simulated and experimental
results was found, for all the frequencies of interest
(10kHz-128MHz). In the LF range, temperature
increases measured at the tip of the PM lead are
much higher than for the insulated wire (more than
three times higher), for the same excitation
conditions. Numerical simulations show that an
improvement of the lead model can be achieved
considering the actual resistivity of the lead: when
modelling a wire with a finite-conductivity tip, the
gap from the real PM lead becomes significantly
lower (<28% respect to the experimental value). The
lead impedance is a characteristic that can sensibly
vary from lead to lead, mainly depending on the
properties of the tip material, When it is made of
ceramic components (such us pyrolitic carbon), the
resistivity is close to 50 Ohm, whereas, in case of
metallic materials, it goes down to few Ohm or even
less. In any case the lead manufacturers should
indicate the value of the impedance in the technical
notes of their products; thus, it should be an easy
element to take into account. At higher frequencies
(32MHz, 64MHz, 128MHz), marked differences
between the PM lead and its numerical model are
observed, even when modelling the finite-
conductivity tip wire. In particular, at 128MHz, the
temperature increase at the PM lead tip, which up to
64MHz is always higher than the insulated wire,
becomes lower. It suggests that at higher frequencies
other mechanisms play an important role in the heat
generation process, that the simple model of an
insulated wire cannot take into account. To better
understand this aspect, we measured the current
peak distribution along the lead. The higher is the
frequency, the closer is the length of the lead to the
theoretical resonance value inside the gel. At
128MHz the theoretical resonance length is about
23cm for a dipole inside a dielectric mean with a
permittivity of about 25. In such conditions, a kind
of resonant wave can be supposed inside the lead. A
resonance phenomenon in various kinds of linear
structures (e.g. catheters used in interventional
radiology) has been hypothesized also by other
groups (Nitz et al, 2001). Heating induced by the
same current measured at the section of the tip
shows opposite trend with increasing frequency in
the insulated wire and in the PM lead. This is a
further confirmation that, at high frequencies, the
simplified model of the insulated wire moves far
from the actual behaviour of the PM lead. Thus, in
order to obtain reliable results new and more
realistic models must be developed. For example, as
already proposed by some groups (Helfer et al,
2006, Neufeld et al, 2009), a model able to
reproduce the actual inductance of the lead may
represent a substantial improvement of the analysis.
5 CONCLUSIONS
Our study reveals that at frequency below 1 MHz,
the RF–induced heating on endocardial leads can be
properly evaluated by modelling the lead as an
insulated wire. It is however necessary to take into
account the resistivity of the lead, in particular the
resistivity of the tip. At higher frequencies this
model becomes less reliable and new solutions
which consider also other aspects, such as the lead
inductance, must be adopted.
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