Biomedical Engineering Reference
In-Depth Information
designed in the United States by Cordis. In the early 1990s, however, CCC formed an in-
house design team. Today, CCC o
ff
ers highly reliable pacemakers to markets that cannot
a
ord the prices of devices manufactured in the United States or Europe.
In addition, CCC caters its design and manufacturing capabilities to companies interested
in developing medical devices. Their
ff
field of expertise is in the design, prototyping, and man-
ufacture of low-power circuitry for implantable and other critical-use medical devices.
In the sections that follow we describe the basic logic and circuitry of pacemakers.
These were kindly contributed by CCC's engineering team: Fernando Brum designed the
software architecture, hardware was developed by Pedro Arzuaga and Julio Arzuaga, and
Oscar Sanz was responsible for the
fi
fi
rmware.
Pacemaker State Machines
The
first pacemakers were simple devices that generated a pacing pulse at a constant inter-
val. Figure 8.2 shows a
fi
finite-state machine that represents the operation of such a pace-
maker. This state machine has a single state [S]; [Time Out] is the event that causes the
state machine to evolve; and [Pace] is the action that occurs as the state transition occurs.
An arrowed line indicates the direction of a state transition and separates the event from
actions taken during the transition.
In a simple pacemaker, the implementation of such state machine would consist of a
timer with a
fi
fixed period. Every time that a length of time [Time Out] elapses, the state
machine exits state [S], generates a pacing pulse (as described by the action [Pace]), and
returns to state [S]. In early pacemakers, the timer's period, as well as the pacing pulse
characteristics (amplitude, waveshape, and duration) were solely a function of the circuit.
Take for example the circuit of Figure 8.3, which has been set up for PSpice simulation.
The circuit is a replica of a 1960s design by Wilson Greatbatch. The story goes that around
1956, Greatbatch was designing a transistorized 1-kHz marker oscillator circuit to help
record fast heart sounds. By mistake, he grabbed the wrong resistor from a box and
plugged it into the circuit that he was making. Instead of producing the tone he expected,
the circuit pulsed for 1.8 ms, stopped for 1 s, then repeated the cycle. Greatbatch recog-
nized the “lub-dub” rhythm and the potential of the circuit for driving a sick human heart.
On May 7, 1958, Greatbatch brought what would become the world's
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first implantable car-
diac pacemaker to William Chardack and Andrew Gage. The three connected the oscilla-
tor circuit to the exposed heart of a dog. The device took control of the rate.
The blocking oscillator of the circuit is conceptually similar to that used in Greatbatch's
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first pacemaker. The circuit is self-starting and its output waveshape (pulse width and inter-
val between pulses) remains almost constant despite drops in battery voltage. The circuit
consumes almost no power between pulses. The original pacemaker used 10 zinc-mercury
Time out
S
Pace
Figure 8.2
This finite-state machine represents the operation of an early pacemaker that generated
pacing pulses at a constant interval. This state machine has a single state [S], [Time Out] is the event
that causes the state machine to evolve, and [Pace] is the action that occurs as the state transition
occurs. An arrowed line indicates the direction of a state transition and separates the event from
actions taken during the transition.
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