Biomedical Engineering Reference
In-Depth Information
operation of atrial and ventricular leads must be evaluated or
pharmacological therapy must be considered to decrease the
retrograde conduction [ 19, 20, 32 ] .
9.10
Types of Timing
A single-chamber pacing mode triggers an appropriate EI
upon the detection of a sensed or paced event. If the pace-
maker does not sense cardiac activity until the end of the EI,
it paces the respective chamber. For example, if a ventricle is
paced, the ventricular EI is timed from one ventricular event
to another [ 69 ] .
Dual-chamber pacemakers can have atrial, ventricular, or
hybrid (combination of atrial and ventricular) timing [40].
The LRL is the rate at which the pacemaker paces the atrium,
the ventricle, or both without sensed intrinsic activity or
without sensor-driven pacing. During ventricular pacing, the
EI is timed from one ventricular event to the following event.
Any time an event is sensed in the ventricle, the timing
changes from ventricular to atrial, or vice versa, according to
the type of device. Then, the timing is ensured even with
intrinsic AV conduction.
Regarding ventricular timing, which was assumed in the
description of timing intervals, the AEI is fixed and is trig-
gered by a sensed ventricular event. If a sensed ventricular
event is detected during this interval, or rather during the
AVI, it terminates the AEI and retriggers it again. If atrial
pacing is followed by normal conduction of an impulse to the
ventricles, and the interval between the atrial pacing and a
sensed ventricular event (R wave) is shorter than the pro-
grammed AVI, the resulting pacing accelerates; that is, AV
delay during ventricular pacing is equal to the programmed
value, and AA and VV intervals are equal to the LRI. With
normal AV conduction and a sensed ventricular event, the
AV delay is shorter than the programmed value, and AA and
VV intervals are shorter than the LRI. It also follows from
the relationship between these intervals: AEI = LRI −AVI.
Hence, if the AEI is triggered during the AVI, the resulting
LRI is shorter.
As far as atrial timing is concerned, the atrial (atrio-atrial
[AA]) interval (i.e., the interval between two atrial stimuli) is
fixed. This AA interval is triggered by a sensed or paced atrial
event. If activity is sensed in the ventricle during the AVI,
ventricular pacing is inhibited, but the basic AA interval does
not change. In other words, a ventricular event does not trig-
ger the LRI. This is why atrial pacing is equal to the LRL
upon a sensed or paced ventricular event. Hence, the result-
ing rate of this single-chamber-like atrial pacing does not
change. For a premature ventricular event during the AEI, the
AA interval, not the AEI, is set again. The system deducts the
AA interval, adds the AVI, and thus attempts to cover a com-
plete compensatory pause after ventricular extrasystole [40].
In dual-chamber pacemakers, an atrial event (sensed or
paced) triggers the AVI that sustains the synchronization
between atrial and ventricular events. If the pacemaker
does not detect any sensed ventricular event until the end of
9.9
Reducing Excessive Right Ventricular
Pacing
Excessive right ventricular pacing may increase the risk of
atrial fibrillation, left ventricular dysfunction, and congestive
heart failure, in particular in patients with intact or irregular
AV conduction [19]. One of the possibilities for reducing
excessive right ventricular pacing is programming longer AV
delays. In patients with intermittent or physical activity-
dependent AV node block, intrinsic AV conduction may
improve hemodynamic efficiency and increase the device's
longevity by decreasing the number of ventricular pacing
pulses.
The AV delay search function is devised to give prefer-
ence to intrinsic AV conduction. It monitors intrinsic AV con-
duction that occurs after the end of a programmed AV delay.
If the function is on, the AV delay is periodically prolonged
(according to a programmed value) for a certain number of
consecutive paced or sensed cardiac cycles. The search for
AV delay remains active if the intrinsic PR intervals are
shorter than the maximum programmed value of AV delay.
The pacemaker returns to the programmed value of AV delay
after the first ventricular pacing pulse or after the lapse of the
search interval of certain number of cardiac cycles without
sensing intrinsic ventricular activity.
The adaptation of pacing modes to the needs of patients
with normal AV conduction is a more up-to-date solution pre-
venting excessive right ventricular pacing. Instead of using a
normal DDD(R) bradycardia pacing mode, in the case of
AVIs with normal conduction, the device works in the AAI(R)
mode at the LRL or a sensor-indicated rate [70]. Back-up VVI
pacing is slower than the LRL by a certain value. When a loss
of conduction is detected, the mode automatically switches to
the DDD(R) mode for the restoration of AV conduction syn-
chronization. The device evaluates slow ventricular contrac-
tions in a certain window as loss of conduction between
atrium and ventricle. Slow contractions are defined as ven-
tricular pacing or sensed ventricular activity that is slower
than the AAI(R) pacing interval by a certain period. After
switching into the DDD(R) mode, the device paces in the
DDD(R) mode according to programmed parameters. When
normal tracking ratios are restored, the pacing mode automat-
ically switches back to the AAI(R) mode, with back-up VVI
pacing. If reliable AV conduction is restored, ventricular pac-
ing is not delivered as the back-up VVI mode runs in the
background at a lower LRL. This function works together
with ATR and adjusts the pacing mode according to the
patient's atrial rhythm and the condition of AV conduction.
