User's Manual

73
Antitachycardia Functions
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An atrial refractory period extension might be necessary in the case of
a short atrial refractory period in conjunction with a long VA
conduction period in order to prevent the triggering of a PMT by
asynchronous ventricular depolarizations.
PMT Protection
Pacemaker-mediated tachycardias can also be caused by artifacts and
atrial extrasystoles. In such cases, the PMT protection algorithm
provides functions for both reliable detection as well as termination of
PMTs. In this way the hemodynamically more favorable AV
synchronization can rapidly be reestablished.
PMT Detection
The period between a ventricular event and the sensing of a retrograde
P wave is designated as the VA delay or retrograde conduction: V
p
-A
s
interval (V
p
= ventricular pace, A
s
= a sensed atrial event). The VA delay
is a programmable parameter (VA criterion) and can be set between
250 and 500 ms.
A pacemaker-mediated tachycardia is recognized by the sensing
algorithm when the following criteria are satisfied:
• Eight consecutive V
p
-A
s
intervals must be shorter than the
programmed VA delay.
• The average standard deviation of the eight V
p
-A
s
intervals must lie
within the tolerance limits of the PMT stability criterion.