CARDIAC PACING AND DEFIBRILLATION - Design and Development of Medical Electronic Instrumentation - page 386

pulse width selection is accomplished through rotary switches that select a resistor used
within a voltage-divider circuit fed from the battery voltage. The output of the resistive
divider is measured by one of IC4's analog inputs. Di erent voltages are mapped by the

microcontroller to the various parameter value selections.
Power for the circuit is obtained from a single nonrechargeable 3-V lithium battery
(e.g., a Panasonic lithium carbon mono

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uoride battery). Please note that pacing pulse
amplitude and sensing sensitivity vary as a function of battery voltage. Although two reg-
ular alkaline batteries in series could be used to power the circuit, the lithium carbon
mono

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flat discharge curve which minimizes the shift in the
sensing threshold as battery capacity is used.
Almost all commercially available implantable pacemakers designed in the last 20 years
use lithium-iodide cells (Li/I 2 ). These cells are designed to deliver current drains in the
microampere range, reliably over long periods of time. They are available from Wilson Great-
batch Technologies, Inc. in a variety of sizes, shapes, and capacities. Lately, implantable-grade
lithium carbon mono

fl uoride chemistry has an almost fl

uoride (Li/CFx) are being used more and more in pacemakers and other
implantable devices. The internal impedance of the CFx cell is much lower than that of the
Li/I 2 cell throughout its entire life, allowing more

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flexibility in circuit design and performance.
Wilson Greatbatch Technologies, Inc. now has Li/CFx batteries, which feature a titanium
case, making it weigh half of a Li/I 2 cell of the same capacity.

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Firmware for the VVI Pacemaker

The microcontroller runs algorithms that implement the state machine as well as stimulus
routines. Firmware for pacemakers is usually coded in assembly language due to reliabil-
ity concerns as well as real-time and power consumption issues. For clarity in this exam-
ple, however, programming was done in C. Despite this, power consumption and real-time
performance are reasonable, and use of a high-level language could be used to develop
code for an implantable device.
The basic state machine for a VVI pacemaker was shown in Figure 8.6. However,
enhancements are required to enable the logic to discriminate true intrinsic cardiac events
from interference, such that pacing therapy is inhibited only when true ventricular activity
occurs. A possible way of implementing a discrimination mechanism is to use dedicated
hardware to prevent interfering signals from triggering a sense event at the microprocessor's
input. For example, a retriggerable monostable together with edge-triggered sensing by the
microprocessor would be able to cope with noise. However, this implementation requires
additional circuitry and does not lend itself to real-time reporting of noise detection. Instead,
this pacemaker design incorporates software mechanisms to detect noise and change the
device's behavior to prevent noise from inappropriately inhibiting pacing therapy.
International standards that de ne the minimum requirements for pacemakers establish

that devices must consider events detected repeating at more than 10 Hz to be noise. When
such a condition is detected, a VVI pacemaker must automatically switch the mode to
VOO. The device should remain in this asynchronous mode until normal sensing is
resumed. Events detected at a rate below 10 Hz cannot be distinguished by simple circuitry
from real cardiac events and may occasionally give rise to uncertain responses.
The state machine of Figure 8.13 is an enhanced version of the basic VVI state machine
capable of detecting and responding to the presence of noise. Two new states [N] and [W]
have been added. These states a

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ect the sense condition, as well as the way in which the
machine returns from the [R] state to the [A] state. The refractory period is now split in
two: [R Time Out], which is an absolute refractory, which then proceeds to state [N]-a
noise window within which events are sensed but not reported to the VVI state machine.
Whenever a sense event occurs within state [N], the moment of occurrence is stored in
time stamp variable [TS], but the machine remains in state [N] until a 100-ms timeout

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