User's Manual

70
Antitachycardia Functions
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The implant behavior thus resembles a 2:1 block. The implant paces in
the ventricle at a rate that corresponds to one-half of the atrial rate. At
very high atrial rates, this can produce high ventricular rates that are
physiologically unsuitable.
Example: If atrial flutter at a rate of 280 bpm takes place, then the
pacemaker paces with a ventricular rate of 140 ppm.
This phenomenon is called 2:1 lock-in and can cause the patient severe
problems in cases of long atrial flutter episodes.
Effects on Mode
Switching
In such a 2:1 situation, the Mode Switching function may not start at
all or only start at a very high rate, even though the function is
necessary. Therefore, the purpose of this function is to ensure the
effective use of mode switching.
To terminate 2:1 lock-in behavior, the AV delay is extended by a value
equal to the far-field blanking period, and the device may switch to a
ventricular-controlled pacing mode. The algorithms for the 2:1 lock-in
behavior have been designed as follows:
— A phase where the behavior is suspected
— Confirmation of such
— Termination
When 2:1 Lock-In Behavior is Suspected
The following criteria must be fulfilled in order for there to be a 2:1
situation:
— Eight (8) consecutive V
p
A
s
intervals must occur
— The actual ventricular rate must be higher than 100 ppm
— The average deviation of the 8 V
p
A
s
intervals must lie within the
tolerance limit of the 2:1 lock-in stability criterion
When these three conditions are met, the 2:1 lock-in situation is
considered confirmed.