Biomedical Engineering Reference
In-Depth Information
4.10 Cross-section of steroid eluting electrode (courtesy of Medtronic, Inc.).
reaction at the electrode tip insertion site. In particular, formation of the foreign
body fibrous capsule surrounding the electrode resulted in an increased
impedance of the electrode±tissue interface.
The steroid-eluting electrode, invented by Stokes in early 1980s, was the
breakthrough technology that solves this problem. The steroid was combined
with silicone to form a plug that was positioned inside the electrode shank
behind the porous tip (Fig. 4.10). The steroid elution through the porous material
thus mitigated the foreign body response at the electrode tip. This combined
electrode technology was seen to result in a minimal-to-no threshold rise as a
function of implant time. Like the introduction of thinner biostable
polyurethanes, the introduction of the steroid-releasing lead reshaped electrical
stimulation device technology, in this case, by allowing more reliable pacing,
longer battery life, and more accurate sensing.
Electrode materials used in early transthoracic temporary pacemakers
included tantalum, silver plated copper, and stainless steels. Silver-plated
copper and stainless steel gave way to more corrosion-resistant materials such as
platinum, platinum±iridium alloys, and activated carbon. Titanium has also been
used. Under controlled conditions, oxides of varying structures can be grown on
titanium electrode surfaces. These oxides are stable in the body, even when
charged. Titanium oxide electrodes have been used successfully in Europe for
more than 25 years. The negative aspect of titanium is its poor radiopacity,
which makes implant placement difficult because of poor tip visibility with X-
ray imaging methods.
4.3
Defibrillators
An implantable cardioverter defibrillator (ICD) detects tachycardia and fibril-
lation, and shocks the heart to restore the normal rhythm. ICDs provide a tiered
therapy that escalates in energy to defibrillate the heart. If the device detects a
tachyarrhythmia, or fibrillation, it first attempts to pace the heart out of
fibrillation (anti-tachycardia pacing). If pacing fails, the ICD escalates to deliver
a low energy shock. If the low energy shock fails to stop the tachyarrhythmia,
