14th Annual Meeting of the
North American Neuromodulation Society
Impedance Variances over Time
for Depth and Strip Electrodes
for Implanted Responsive
Neurostimulation Devices
Chengyuan Wu, MD
Ashwini Sharan, MD
Las Vegas, NV
Friday, December 3, 2010
Disclosures
• Part of ongoing project conducted by
Neuropace® to characterize long-term
impedance changes across all subjects
participating in the RNS studies
Outline
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Background
Methods
Results
Discussion
Conclusions
http://www.neuropace.com/graphics/trials-fig1.gif
Background
• Responsive neurostimulation (Neuropace® - RNS®) is a
potential treatment modality for intractable partial epilepsy
– Detection and interruption of epileptiform activity prior to propagation
• Intraparenchymal depth electrodes
vs. Subdural strip electrodes
• FDA-approved investigational device
http://www.neuropace.com/graphics/rns.jpg
• Stimulation characteristics over time are important
– Analysis of changes in electrode impedance
Methods
• Single-center, retrospective review of impedance
– October 2007 – September 2010
• 7 patients
– 7 intraparenchymal depth electrodes
– 7 subdural strip electrodes
• Impedance of all 4 leads of each electrode
measured in Ω and plotted over time
Patient Data
Patient Data
Patient Data
Patient Data
Results
Mean
Impedance
(σ)
Impedance
Range
Mean
Impedance at
Implantation
Mean
Impedance at
Last Follow-Up
Subdural Strip
Electrodes
860 Ω
(432)
250-3812 Ω
730 Ω
818 Ω
Intraparenchymal
Depth Electrodes
530 Ω
(148)
48-934 Ω
557 Ω
535 Ω
Discussion
• Subdural strip electrodes have higher mean
impedance
• Subdural strip electrodes have greater variability
over time
• Intraparenchymal depth electrodes have lower
mean impedance and more stability
• Net change in impedance values is not
statistically significant for either type of electrode
Conclusions
• Stability of implanted device is of utmost importance
– Changes in impedance alters stimulus delivered to brain
• Contributing factors to changes in impedance:
V=I×Ω
– Thickness/conductivity of encapsulation capsule
– Surface contact area
– Lead damage
• Further analysis on greater number of patients required
Summary
• Stability of implanted medical devices is of
the utmost importance
• Changes in impedance are not necessarily
secondary to encapsulation by scar tissue
• Depth electrodes demonstrate greater
stability with less variability over time
Questions?
Thank You