Biomedical Engineering Reference
In-Depth Information
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Total inhibition (As-Vs): occurs if the patient's intrinsic
atrial rate is faster than the LRL and the intrinsic AV con-
duction is normal, followed by an intrinsic QRS complex
before the lapse of AV delay.
Ap Vp
Ap Vs
As Vp
As Vs
Fig. 9.6
Four basic cycles in DDD mode
9.3
Blanking Periods
9.2.4
VDD Mode
Paced and sensed events are followed by blanking periods.
Blanking is the initial portion of the refractory period in
which sensed signal input amplifiers are deactivated; the
pacemaker completely ignores the events in the blanking
period. This makes a difference if compared with refractory
periods, described later, in which the device records events
but they have no direct impact on its operation. Blanking
thus keeps the pacemaker from sensing and misinterpreting
intracardial signals and pacing artifacts. Blanking periods
are triggered by sensed or paced events. They prevent far-
field sensing (crosstalk) and, as a consequence, inhibit unde-
sirable pacing; they also prevent sensing of intrinsic pacing
pulses, depolarization after pacing, T waves, or excessive
sensing of the same event. Immediately after a sensed or
paced atrial or ventricular event, sensing is blanked in the
particular chamber, mostly by a nonprogrammable period
from 50 to 100 ms.
The actual time of the blanking period can be set dynami-
cally by the device, depending on the intensity and duration
of the sensed signal [94]. The dynamic blanking avoids
repeated sensing of the same signal, thus minimizing the
blanking period. During the blanking period, the sensing cir-
cuit in one chamber also ignores possible sensed electrical
activity generated by a pacing pulse in another chamber;
ventricular pacing sensed in the atrium would lead to inade-
quate ventricular pacing because the device would make an
effort to sustain the AV synchronization. If, in contrast, atrial
pacing was sensed in the ventricle, it would result in an inhi-
bition of ventricular pacing and, as a consequence, an unde-
sirable decrease in the pacing rate. For that reason, in
DDD(R), DDI(R), and VDD dual-chamber modes, ventricu-
lar pacing triggers an atrial blanking period, and in DDD(R)
and DDI(R) modes, atrial pacing triggers a ventricular blank-
ing period. Blanking periods following paced events are
equal to or longer than blanking periods following sensed
events to prevent sensing of atrial and ventricular
depolarization.
In general, the following blanking periods can be defined
in dual-chamber modes to prevent sensing of events in the
same chamber (Fig.
9.7
):
Atrial blanking after atrial sensing. Throughout this
This mode delivers the ventricular pacing triggered by the
atria up to the MTR. Sensing is conducted in both the atrium
and in the ventricle, whereas pacing is delivered only in the
ventricle. To ensure synchronized pacing at low rates, sensed
atrial activity at the end of the LRI is followed by maximum
SAV. As a result, the ventricular LRL is prolonged. Sensed
nonrefractory ventricular events during a VV interval not
preceded by sensed atrial events can be evaluated as a pre-
mature ventricular contraction and trigger a new VV inter-
val. The advantage of the VDD mode lies in the possibility of
inserting only one lead into the ventricle, with an auxiliary
sensing electrode in the atrium. However, atrial pacing is
impossible when a low intrinsic rate occurs.
9.2.5
DDD Mode
The DDD mode is referred to as a universal pacing mode. It
provides dual-chamber pacing, dual-chamber sensing, and
dual response to sensing (inhibiting or triggering). Triggering
occurs if a sensed atrial event triggers an AVI, which is fol-
lowed by pacing in the ventricle. This process, mentioned
earlier, is referred to as tracking and sustains the synchroni-
zation of the atrium and ventricle [94].
In principle, four combinations of atrial (A) and ventricu-
lar (V), sensed (s) and paced (p) events are possible. They
constitute four basic timing cycles of dual-chamber modes
[ 69 ] , shown in Fig.
9.6
. Timing intervals in the DDD mode
may vary in each cardiac cycle according to the patient's
intrinsic heart rate and the condition of AV conduction, and
they make use of either PAV or SAV [94].
Atrial and ventricular pacing (Ap-Vp): delivered if the
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patient's intrinsic atrial rate is slower than the LR, and the
intrinsic AV conduction is either damaged (AV block) or
slower than the programmed AV delay.
Atrial pacing, ventricular sensing (Ap-Vs): delivered if
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the patient's intrinsic atrial rate is slower than the LRL
and the intrinsic AV conduction is normal, followed by an
intrinsic QRS complex before the lapse of AV delay.
Atrial sensing, ventricular pacing (As-Vp): delivered if
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period, atrial sensing after a sensed atrial event is
deactivated.
Atrial blanking after atrial pacing. Throughout this period,
the patient's intrinsic atrial rate is faster than the LRL and
the intrinsic AV conduction is either damaged (AV block)
or slower than programmed AV delay.
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atrial sensing after a paced atrial event is deactivated.
