Sudden onset criterion and sinus tachycardia - Evera MRI XT VR

Patient

45-year-old male implanted with a single-chamber defibrillator (Evera MRI XT VR) for hypertrophic cardiomyopathy with episodes of VT at 160 beats/minute.

Trace

1- What is the diagnosis made by the defibrillator for this episode?
This episode was classified as SVT by based on the sudden onset criterion.

2- How many detection zones are programmed?
Three detection zones have been programmed with a VT zone of 450 to 350 ms.

3- What diagnosis does the interval plot suggest?
The graph shows an appearance compatible with a sinus tachycardia oscillating around the lower limit of the VT zone; the tachycardia appears regular with progressive acceleration and then deceleration at the end of the episode.

4- How many discrimination parameters are programmed?
Two discrimination criteria are programmed: sudden onset (81%) and Wavelet (70% similarity).

5-What does the term ‘Reset Onset’ mean?
The QRS complexes visualised on the shock channel are narrow and compatible with supra-ventricular tachycardia; the sudden onset criterion is not fulfilled, the acceleration and re-entry into the VT zone being very gradual; as long as the sudden onset criterion is not fulfilled, the sudden onset algorithm is reset and Wavelet is not analysed.

Take home message

The patients who benefit most from discrimination are those where the heart rates of ventricular tachycardias and supraventricular tachycardias overlap (patients with slow VT, patients likely to have rapid AF, young patients with rapid sinus tachycardia on exertion).
The percentage of inappropriate therapies has fallen considerably since the very first defibrillator studies and now stands at between 1 and 5% per annum. The reasons for this significant reduction are multifactorial (programming of longer detection times, programming of detection zones for tachycardias > 187 beats/minute in the first instance, programming of discrimination algorithms); it is therefore difficult to precisely isolate the effect of programming discrimination algorithms.
This episode corresponds to a sinus tachycardia correctly discriminated by the defibrillator; typically, in sinus tachycardia, 2 out of 3 criteria indicate SVT; in fact, there is usually an absence of sudden onset, a stable rhythm and a similar morphology; in this example, stability had not been programmed.
The sudden onset criterion is based on the principle that a sinus tachycardia starts gradually whereas a ventricular tachycardia starts abruptly; a gradually accelerating tachycardia (sudden onset criterion not met) is not considered VT by the device and is therefore not treated.
In practice, the device compares the average duration of 4 consecutive cycles with the average duration of the 4 preceding cycles; a programmable minimum percentage of acceleration between these 2 series of 4 cycles defines a sudden onset; as long as the onset is considered to be gradual, even if the cycle length corresponds to the VT zone, detection is delayed, the event is marked VS and the annotation «Reset Onset» appears on the trace.
The sudden start criterion applies – for VT and FVT via VT zones but not for VF or FVT via VF zones, – during the initial detection phase, but not during the re-detection phase.
The sudden onset criterion may be misleading in certain scenarios and lead to an erroneous diagnosis in 2 types of patients: 1) patients with VT during exertion (ischaemic cardiomyopathy, hypertrophic cardiomyopathy, long QT or right ventricular dysplasia) where there may be no clear break between the rate of the sinus tachycardia preceding the VT and the VT rate; 2) patients where the VT rate oscillates around the lower limit of the VT zone.
The stability criterion is based on the principle that atrio-ventricular conduction during an episode of conducted atrial fibrillation is generally irregular and gives rise to an unstable ventricular rate, whereas an episode of ventricular tachycardia is most often associated with a stable ventricular rate; an irregular tachycardia (stability criterion not met) is not considered VT by the device and is therefore not treated.
In practice, stability analysis begins when the VT counter reaches at least 3; the device compares the ventricular cycle length with the 3 previous cycle lengths; when the difference between a ventricular cycle and one of the 3 previous cycle lengths is greater than the programmed stability interval, the rhythm is deemed unstable, the cycle is classified as VS, the VT counter is reset to 0 and the annotation «Reset: Stability» appears on the trace.
The stability criterion applies – for VT and FVT via VT zones but not for VF or FVT via VF zones (same as for sudden onset) – during the initial detection phase and also during the redetection phase (different to sudden onset); this is the only discrimination criterion that applies during redetection.
In theory, the stability criterion can be used to differentiate between atrial fibrillation (irregular) and ventricular tachycardia (regular); however, this parameter cannot be used to distinguish between ventricular tachycardia and sinus tachycardia or atrial tachycardia/flutter, where the ventricular rhythm is usually regular.
The stability criterion may be misleading in certain instances and may lead to an erroneous diagnosis in patients 1) in rapid AF with a rate in excess of 170 beats/minute where the rhythm is generally stable, 2) in certain patients where the onset of VT is accompanied by an irregular ventricular rhythm with regularisation at a later stage.
Discrimination by stability analysis should not be used for tachycardias >180-200bpm to avoid the risk of not treating polymorphic VT and VF.

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