76-year-old man; ischemic cardiomyopathy with wide QRS; implanted with a triple-chamber defibrillator
Summary
episode diagnosed in VT zone with atrial rate = ventricular rate
programmed discrimination: Onset/stability
EGM layout
rapid ventricular rate with A/V ratio 1/1, on the edge of the VT zone (RVS marker)
slight acceleration of the rhythm into the VT zone (VT zone limit: 375 ms)
criterion 8/10 met; analysis of onset of arrhythmia favors sinus tachycardia (Gradl: gradual onset)
at the end of the duration, the rhythm is considered stable; no therapy delivered in the presence of a stable rhythm with gradual onset
cycle-by-cycle analysis of stability continues; the rhythm is still considered stable and therapies are inhibited
Take home message
this episode is diagnosed as sinus tachycardia in the presence of a stable rhythm with gradual onset; therapies are therefore inhibited
the purpose of the Onset parameter is to inhibit ventricular therapies for the lowest tachycardia zone when the heart rate increase is gradual; its application is limited to the lowest rate zone of a multi-zone configuration; when a detection window is satisfied, the device starts Sudden Onset calculations in two steps; step 1 measures the ventricular intervals preceding the start of the episode and locates the pair of adjacent intervals (pivot point) where the cycle length has decreased the most; if the decrease in cycle length is equal to or greater than the programmed Onset value, step 1 declares the onset to be sudden; step 2 then compares other intervals; if the difference between the average interval preceding the pivot point and 3 of the first 4 intervals following the pivot point is equal to or greater than the programmed Onset threshold, step 2 declares the onset to be sudden.
the Onset parameter can be programmed as a percentage of cycle duration or as an interval duration (in ms); the programmed Onset value represents the minimum difference that must exist between intervals above and below the lowest programmed rate threshold.
this episode corresponds to an episode of sustained atrial tachycardia with 1/1 AV conduction; the tachycardia frequency oscillates around the lower limit of the VT zone, which explains why the onset is considered gradual; in fact, the ventricular rate varies very little but oscillates between the sinus and VT zones; this patient’s rhythm initially went from its resting rate (60 beats/minute) to a rate of 165 beats/minute; gradually, the tachycardia rate then accelerated to reach the VT zone; there was therefore a sudden onset that was not diagnosed by the device; paradoxically, the fact that the sudden onset parameter was implemented avoided the patient having to receive inappropriate therapies
1/1 atrial tachycardia is not correctly discriminated by the combination of V>A (atrial rate is equal to ventricular rate in 1/1 atrial tachycardia as in VT with 1/1 retrograde conduction), abrupt onset (onset is usually abrupt in atrial tachycardia as in VT) and stability (ventricular rate is usually stable in atrial tachycardia as in VT).
in this patient, it is advisable to raise the lower limit of the VT zone to avoid overlap between the diagnostic zone and the zones of his clinical atrial tachycardia; similarly, it may be advantageous to use discrimination based on the Rhythm ID
