Biomedical Engineering Reference
In-Depth Information
then high energy shocks (30±35 J) are delivered at about 800 V to stop the
tachyarrhythmia. Additionally, ICDs can deliver `back-up' pacing if bradycardia
occurs.
An ICD can be thought of as a derivative of the pacemaker. Both share basic
structures, working principles, materials, biocompatibility, and implantation
procedures. Like the pacemaker, an implantable defibrillator device includes a
pulse generator and leads. The pulse generator is made of titanium, the
connector module is made of rigid polyurethane, its lead insulation is made of
soft polyurethane or silicone, and its electrodes are made with platinum or
platinum-plated metals.
These defibrillators, however, have more functions than pacemakers. They
deliver a much higher energy (30±35 J in less than 1 second) at a much higher
voltage than pacemakers. No battery in a current implantable device can have a
discharge rate high enough for this fast, high energy delivery. Therefore, ICD
pulse generators have capacitors to form and store high energy packages for
delivery within a short period of time, as needed. The ICD battery and capacitor
occupy about half to two-thirds of its volume. The ICD lead has one or two long
defibrillation coils to deliver cardioversion (low energy) or defibrillation (high
energy) shocks. These coils have large surface areas to increase defibrillation
efficacy and, at the same time, reduce failure risk from corrosion. One coil is
always in the right ventricle. A second coil is in the superior vena cava.
ICD cans are an integral portion of the circuit, similar to unipolar pacemakers
whose titanium cans on the devices that become part of the electrical circuit
formed between the device and the human body. The shock electrical field is
between the device and the right ventricle coil, when one coil is used. An
electric field between the superior vena cava coil and the right ventricle coil is
added when a second coil is used. The device is used as one electrode,
permitting more efficient delivery of high energy from the ICD to the cardiac
muscle. Without the can in the defibrillation circuit, the device would require a
great deal more energy and/or different, possibly less comfortable device and
lead configurations within the body to supply the high energy shocks across the
heart needed to stop a life-threatening arrhythmia.
4.4
Neurostimulators
Researchers and engineers developing neurostimulation devices borrowed
extensively from cardiac stimulation technology since many fundamental
design requirements of both technologies are similar, and numerous cardiac
pacing technologies were already standard-of-care therapies. Neurostimulation
applications do, however, include a number of different requirements for the
various therapies. The sections that follow describe some well-developed
neurostimulation applications that emerged from pacing technologies.
