NeuroTherm NT1100 Operator's Manual page 84

to inconsistent results, and resulted in the acknowledgment of the equilibrium lesion size as being
the optimum methodology.
It is noteworthy that impedance monitoring has a great value in assessing the progress of a heat
lesion. The impedance seen by the electrode tip depends on tissue interfaces and this property
has been used very effectively to distinguish between the interface of electrolytic fluids and
tissue. For example this has been used with percutaneous cordotomy electrodes to clearly tell
when the electrode has traversed from the cerebral spinal fluid to a position of contact with the
spinal cord. Impedance monitoring has also been used to identify when an advancing electrode
has progressed from the annulus of the disk into the nucleus pulposus. The change of
impedance during the heating process is dramatic. It is been shown as the tissue or medium
heats up, the impedance will drop. This is very much related to the phenomena that the engine
oil in an automobile will become less viscous as the temperature of the engine increases. There
is a point, however, as the temperature at the lesion tip approaches 100 C, when the impedance
will cease to decrease and, in fact, will rise precipitously as the temperature approaches the
boiling point. The reason for this is that the protein coagulation has a rapid onset in this
temperature range causing a decrease in Ionic mobility. Near the boiling point, gas suddenly
forms around the electrode tip, acting as an electrically insulating barrier thus sending the
impedance to very high levels. At the onset of boiling, the impedance rises very rapidly. In
summary, it is clear that the monitoring of temperature and impedance are both of great
significance.
10.1.2 Pulsed radiofrequency
Historically, radiofrequency was neuroablation. This was true for percutaneous cordotomy, the
treatment of trigeminal neuralgia, and the destruction of the medial branch nerve for facet pain.
Mysteries remained however. It was not understood why RF lesions were so often followed by
long periods of discomfort before any beneficial clinical effect appeared. In the 1990s, additional
unanswered questions were added. The mode of action of RF lesions of the lumbar sympathetic
change (other than for vascular disease) was not understood since there were acceptable
success rates, though the results did not correlate with the degree of sympathetic block.This led
to the hypothesis by Sluijter that heat might not be the element causing the clinical effect of an
RF lesion. The next obvious steps were to define a method to apply radiofrequency at high
intensity without allowing the tip temperature to rise to neurodestructive levels. The method that
was chosen by Sluijter was placing the output setting of the RF generator in the same range as
was customary for making heat lesions but interrupting the output, thus allowing for sufficient time
for the generated heat to be washed away by thermoconductivity and circulation.
This method has been commonly referred to as pulsed radio frequency (PRF). Pulsed radio
frequency is a relatively new technique that
applies short pulses of radiofrequency (20 ms) at
a high voltage of 45 to 60 volts to neural tissue.
Figure 10.5 shows the currently accepted
paradigm of 20 milliseconds of RF followed by
480 milliseconds of off time. In this way high
intensity radiofrequency is delivered but with a
short enough on time so as not to cause heating
above 42 C.
Document 109.00
NEUROTHERM RADIO FREQUENCY LESION GENERATOR
MODEL NT1100
OPERATORS MANUAL
®
Figure 10.5
10September2012 Issue 06
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