Biomedical Engineering Reference
In-Depth Information
Pacing Mode This parameter selects the state machine to be used to deliver therapy. A
three-letter code is used in medical nomenclature to specify a pacemaker's behavior. The
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first letter represents which chamber can be stimulated by the device (A, atrium; V, ven-
tricle; D, both atrium and ventricle; O, no pacing enabled), the second represents the
sensed chamber (A, atrium; V, ventricle; D, both atrium and ventricle; O, no sensing avail-
able or enabled), and the third the behavior of the device upon detecting a sensed event (I,
inhibit pacing; T, trigger pacing; D, both inhibit or pace, depending on the current state of
the state machine; O, disregards sensed events).
For example, the
first pacemakers were asynchronous ventricular stimulators, also
known as VOO pacemakers :
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• V
Pacemaker can stimulate the ventricle.
• O
No sensing capabilities are present.
• O
There is no response to sensed events (since no sensing means are available).
The simpli
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ed state machine of Figure 8.6 corresponds to the VVI mode:
• V
Pacemaker can stimulate the ventricle.
• V
Pacemaker can sense the ventricle.
I
Ventricular pacing is inhibited whenever the pacemaker detects a timely intrinsic
ventricular event.
The simpli
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ed dual-chamber state machine of Figure 8.7 implements a simple DDD
pacemaker:
• D
Both atrium and ventricle can be stimulated by the pacemaker.
• D
Both atrial and ventricular intrinsic signals can be detected by the pacemaker.
• D
Whenever timely intrinsic activity is present in both atrium and ventricle, the
device inhibits pacing (I). However, when ventricular intrinsic activity does not
follow the atrial activity in a timely manner, the device triggers (T) pacing on the
ventricle in sequentially after the atrium.
Another mode which is commonly included in pacemakers is a ventricular trigger
(VVT):
• V
Pacemaker can stimulate the ventricle.
• V
Pacemaker can sense intrinsic activity from the ventricle.
• T
Pacemaker stimulates the ventricle immediately following ventricular sensing.
COMMUNICATING WITH AN IMPLANTABLE DEVICE
Figure 8.8 shows how pacemakers have changed since their inception in the 1960s. The
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first pacemakers had a predetermined set of operating parameters. Later, the pacing rate of
some early user-settable pacemakers could be changed using a
fine needle screwdriver to
change the setting of a potentiometer embedded within the material (e.g., epoxy and/or sil-
icone rubber) encapsulating the pacemaker's circuitry. Today, pacemakers are program-
mable in a noninvasive manner using a bidirectional RF link that permits an external
programmer to communicate with the implanted device's microprocessor. Operating
parameters for external (temporary) pacemakers are usually selected through switches and
dials.
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