User's Manual

64
Antitachycardia Functions
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Tachycardia Mode
The resulting tachycardia mode (either 2:1 or Wenckebach) is
automatically displayed, depending on the combination of selected
parameters.
A response similar to Wenckebach block (the WRL mode) results if the
selected upper tracking rate is lower than the rate corresponding to the
atrial refractory period. If the upper tracking rate is exceeded in the
WRL mode, the AV delay is continually prolonged so that the
ventricular pacing rate does not exceed the programmed upper
tracking rate.
Extension of the AV delay is interrupted as soon as a P wave occurs
before the end of the extended AV delay initiated by the preceding P
wave. In this case, the corresponding ventricular pulse is inhibited. If
the atrial rate is only slightly above the upper rate, then a 6:5 block, for
example, is the result.
Higher atrial rates produce higher degree blocks. If the length of the
atrial cycle eventually becomes shorter than the programmed atrial
refractory period, then a 2:1, 3:1, etc. block results.
Figure 12: Wenckebach-typical pacing behavior
If the selected upper tracking rate exceeds the rate corresponding to
the atrial refractory period, the maximum P-wave-triggered ventricular
rate results exclusively from the atrial refractory period, not from the
programmed upper tracking rate. If the length of the atrial cycle is
shorter than the programmed atrial refractory period, a 2:1 block, then
a 3:1 block, etc., will result before the upper tracking rate is reached in
the ventricle (DDD mode, 2:1 mode).