Biomedical Engineering Reference
In-Depth Information
Time
Stamp
Sense
N
N Tout
R Tout
Sense and no noise
R
A
Pace
A Tout
Pace
Sense and noise
A Tout
W
Figure 8.13
This enhanced version of the basic VVI state machine is capable of detecting and
responding to the presence of noise. The refractory period is now composed of an absolute refrac-
tory [R Time Out] and noise window [N]. The presence of noise is assumed if the difference in time
between sense events within [N] and [A] is equal or smaller than 100 ms, causing the machine to
move to state [W] to complete the programmed period.
[Noise Time Out] elapses. By the time the machine enters the alert state [A], the complete
refractory period will have elapsed, and variable [TS] will hold the time of the last sense
event detected within the noise window. Under no-noise conditions, [TS] will be zero.
If no event is detected within the alert state [A], the device's behavior does not change.
However, if an event is detected within [A], it is evaluated against the time of events that
may have occurred within the noise window. If the di
erence between the time of occur-
rence of a sense event within the alert window and a time-stamped event is less than
100 ms, the presence of noise is assumed (since 1/100 ms
ff
10 Hz), and the state machine
is moved to state [W] to complete the programmed period. If, on the other hand, the
di
erence in time between the event sensed and the time-stamped event is larger than
100 ms (or no sensed events occurred within the noise window), the response to the sensed
event will be to inhibit pacing and start a new cycle.
Despite the additional complexity introduced by this mechanism to the basic VVI state
machine, its realization is relatively straightforward. An e
ff
cient and clear-cut method for
implementing the
finite-state machine is to encapsulate the event detection procedure within
a routine [GetEvent()] and build an in
fi
fi
nite loop that polls a variable that keeps track of the
machine's current state.
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