Biomedical Engineering Reference
In-Depth Information
Time out
Sense
S
Pace
Time out
Figure 8.5 An important development was the inclusion of circuitry that could detect a patient's
intrinsic heart activity and pace only when the heart's rate fell below a predefined rate. To do so
requires the addition of a sense event to the state machine. In its simplest form, when the pacemaker
detects an intrinsic cardiac event, the timer in charge of issuing [Time Out] is retriggered.
Sense
A
R
R T o u t
A Tout
Pace
Figure 8.6 The practical implementation of the state machine of Figure 8.5 requires the inclusion
of a refractory period to “debounce” the sense event detected by a simple threshold comparator that
is generally used in the sense amplifier circuit. In this state machine, [A] is the “alert” state in which
the pacemaker attempts to detect the heart's intrinsic electrical activity, while [R] is the refractory
state in which the pacemaker ignores any external signal. The stimulus period is the sum of the time-
out periods ([A Time Out] [R Time Out]).
artifact and evoked potentials). To do so, the state machine is redesigned to incorporate the
concept of a refractory period . In the state machine of Figure 8.6, [A] is the “alert” state
in which the pacemaker attempts to detect the heart's intrinsic electrical activity, while [R]
is the refractory state in which the pacemaker ignores any external signal. The stimulus
period is now de
[R Time Out].
An additional problem of using a simple signal detection scheme is the possibility that
interfering signals, either from a source internal to the patient's body (e.g., potentials from
the arm and chest muscles) or an external source (e.g., electromagnetic interference) will
falsely trip the sense ampli
fi
ned by the sum of two time-out periods [A Time Out]
fi
er. One possible solution to this problem is considered later in
the chapter.
Most pacemakers of the type described so far were used to stimulate the heart's ventricles
in patients in whom intrinsic atrial signals would not propagate to the ventricles, most com-
monly due to a block in the atrioventricular conduction system. An obvious limitation of
these pacemakers is their inability to make use of atrial function to enhance hemodynamic
performance (the heart's capability to pump blood to the body) when the ventricles are paced.
Improved physiological response of the pacemaker can be achieved by expanding the state
machine to synchronize the activation of the ventricles to atrial activity. Figure 8.7 displays
the state diagram of a dual-chamber pacemaker.
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