Pacemaker Timing - Implantable Cardiac Devices Technology - page 69

The pacemaker in the fallback mode gradually decrements
the ventricular pacing to, for example, the value of the fall-
back LRL, a sensor-indicated rate, or an algorithm of ven-
tricular rhythm stabilization, whichever is higher.

9.8 Pacemaker-Mediated Tachycardia

In DDD(R) and VDD dual-chamber modes, the pacemaker
may sense retrograde conducted P waves falling outside the
PVARP, which may result in triggered ventricular pacing
reaching MTR. That being so, it is called PMT, also referred
to as endless loop tachycardia. PMT often starts with prema-
ture ventricular contraction, which may be either intrinsic or
paced. The electric impulse is conducted retrogradely
through a cardiac conduction system to the atrium. If this
retrograde P wave occurs after the end of a PVARP, it is
sensed by the pacemaker (Fig. 9.14 ). This triggers an AVI,
after the lapse of which the ventricle will be paced. The cycle
then starts again [ 94 ] .
The detection of PMT depends on the manufacturer. In
one system, the PMT condition is met by counting 16 con-
secutive ventricular pacing pulses at a rate equal to the MRT
following sensed atrial events. During these 16 intervals, the
VA interval is also monitored, and it is evaluated whether
PMT has occurred or whether the intrinsic atrial rate reaches
or exceeds the MTR. The VA intervals are compared with the
initial VA interval measured during 16 paced ventricular
events. If any of the subsequent intervals is shorter or longer
than the initial interval by more than 32 ms, the rhythm is
declared an Wenckebach event; the counter of Wenckebach
events counts the event, and the algorithm continues moni-
toring subsequent ventricular stimuli to detect possible PMT.
If all VA intervals meet the 32-ms criterion, the rhythm is
declared to be PMT. Another system defines the PMT as
eight consecutive Vp-As intervals shorter than 400 ms.
When the pacemaker detects the PMT, according to set rules,
it prolongs the PVARP parameter in one cardiac cycle. As
the following event sensed in the atrium falls into the refrac-
tory period, the PMT is interrupted. The prolonged PVARP
is up to 500 ms long.
Acute PMT may be dealt with by reprogramming
(e.g., into the DDI mode) or by applying a magnet to turn off
atrial sensing. If PMT episodes occur despite algorithms
being enabled to inhibit the PMT, it is recommended that
the PVARP be adjusted. If inefficient, the position and

9.7.1 Regulation of Ventricular Rhythm

The purpose of regulation of ventricular rhythm through pac-
ing is to attenuate the symptoms of tracked atrial fibrillation
or atrial flutter by the regulation of ventricular pacing during
episodes of tachycardia. The variability of the VV interval
must be decreased during tracked atrial arrhythmias: very
long VV intervals must be eliminated, and very short VV
interval occurrence must be reduced. This option is suitable
in the DDD(R) dual-chamber mode for patients with parox-
ysmal atrial fibrillation, and in the VVI(R) single-chamber
mode for patients with permanent atrial fibrillation.
In a dual-chamber mode, the regulation of ventricular
pacing is activated upon the detection of onset of an episode
of atrial tachycardia. The pacemaker stabilizes ventricular
pacing either by setting the pacing slightly below the level of
average ventricular action, or, in another manufacturer's sys-
tem, based on the weighted sum of a relevant VV cycle and
preceding pacing intervals. In the former, the pacing slightly
accelerates after each sensed ventricular event but never
exceeds the maximum set rate. If no ventricular event is
sensed, the pacemaker decelerates the pacing until another
ventricular event is sensed or the LRL is reached. If the pace-
maker detects the end of an episode of atrial tachycardia, the
ventricular pacing regulation is turned off. In the latter, paced
intervals have more influence than sensed intervals, so the
paced events cause a decrease in pacing rate. This is because
of the weighted-sum methodology stated above. The indi-
cated pacing rate also depends on the LRL and MPR. As
soon as a tracking mode is restored after termination of the
arrhythmia, the algorithm is turned off.
If the ventricular rhythm regulation is on, it is permanently
active in the VVI(R) single-chamber mode. The VVI(R)
mode does not include atrial sensing; hence, permanent atrial
tachycardia is assumed.

Retrograde P-waves

PVC

AVI
PVARP

Fig. 9.14 Pacemaker-
mediated tachycardia