Biomedical Engineering Reference
In-Depth Information
brillation on the order of 200 to 300 J is successful in 85% of the cases. A combination
of antiarrhythmic drugs and de
de
fi
fi
brillation has a 95% success rate. In contrast, internal de
fi
b-
rillation requires far less energy. Depending on electrode con
fi
guration, energy require-
ments for de
brillators can deliver
a maximum of about 30 J per shock. Of course, the electrodes used for internal de
fi
brillation can be less than 5 J. Typical implantable de
fi
fi
brillation,
especially those used with implantable de
fi
brillators, are di
ff
erent than the gelled paddles
common for external de
brillators.
Originally, open-chest surgery was required to implant the large,
fi
flat patch electrodes
that were sewn to the outer surface of the heart. However, the advent of the
fl
rst transvenous
lead systems in the early 1990s meant that physicians could maneuver the leads through a
vein into the heart, eliminating the need to open the chest. Today, most implantable
de
fi
brillator's titanium can is implanted in
the upper chest, and it acts as the return electrode for the de
fi
brillators use a single de
fi
brillation lead. The de
fi
fi
brillation current.
SHOCK BOX PROTOTYPE
Modern implantable de
brillators are true marvels of microelectronic packaging. Figure 8.33
shows the innards of one such device. This level of miniaturization is achieved using pack-
aging technologies that are outside of the typical hobbyist's budget. In fact, many startup
companies developing implantable devices often chose not to pursue the technologies
required for miniaturization (e.g., custom ICs, chip-level packaging, ceramic substrates), and
instead, use o
fi
-the-shelf components and inexpensive manufacturing technologies (e.g.,
surface-mounted components, low-power commercial ICs, printed circuit boards) so they
can invest their e
ff
erentiate them from the rest
of the pack. For this reason it is not easy to build an experimental implantable de
ff
orts into developing the technologies that di
ff
brillator.
Instead, we chose to present the instrument of Figure 8.34 only as a demonstrator of the inter-
nal workings of an implantable de
fi
brillator. It shows the considerations included in imple-
mentation of the various modules of a
shock box
, the circuitry responsible for generating
high-voltage de
fi
fi
brillation pulses. This instrument does not include simulation of the parts of
an automatic de
brillator that are responsible for detecting ventricular arrhythmias.
As shown in Figure 8.35, power for the circuit is obtained through a power line-
operated medical-grade power supply. The
fi
15-V line is used to power an isolated dc/dc
converter that yields isolated 30 V dc. The 30 V is used to operate a smart gel-cell battery
charger which charges two 12-V, 1.2-Ah gel-cell batteries in series. The battery powers the
module's microcontroller constantly. Whenever the de
brillation module is enabled, the
battery is made to power a high-voltage power supply which charges the energy-storage
capacitor bank (165
fi
F) to a programmable level (up to 50 J).
The level of charge to be stored in the capacitor bank is selected through a digital-to-
analog converter controlled by the microcontroller. The actual voltage across the capacitor
bank is monitored by the microcontroller through an analog-to-digital converter which
samples the voltage divider internal to the high-voltage power supply. Once charged to the
desired level, the de
µ
fi
brillation pulse is generated by commuting the capacitor bank onto
the de
brillation load through an H-bridge switch matrix. The switches in the H-bridge are
under the control of the microcontroller.
Internal discharge of the capacitor bank is possible through a circuit that dumps stored
charge into a dummy load. This makes it possible to discharge capacitor banks after a
capacitor reform procedure
1
as well as to disarm the de
fi
fi
brillation module after an aborted
1
The capacitance of electrolytic capacitors changes as a function of use and other factors. Whenever they are not
used for some time, they require “
reforming
” such that they can be made to store the full desired charge.
Reforming is accomplished by periodic charging of the capacitors to their full capacity.
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