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Oversensing and risk of sudden death - Virtuoso DR

Patient


Patient implanted with a dual-chamber defibrillator (Virtuoso DR) for ischaemic cardiomyopathy and hospitalised due to syncope followed by a shock from the device.

Trace

1-What diagnosis is suggested by the interval plot?
The plot shows an initial rhythm of just over 100 bpm with 1 :1 AV association that may correspond to sinus tachycardia ; this is followed by a sudden acceleration of the ventricular rhythm with cycles detected at the border between the FVT zone and the VF zone; a burst is delivered with acceleration of the rhythm into the VF zone; a maximum output shock terminates the arrhythmia.

2- What is your diagnosis?
On the EGM, there is initially sinus tachycardia followed by oversensing of the T wave with cycles classified as either FVT or VF; there is alternation between 2 morphologically different signals, a high-frequency signal (R wave) and a low-frequency signal (T wave); the RT intervals are more or less equal to the TR intervals, which explains the absence of a ‘railroad track’ appearance on the interval plot; the VF counter is full (programmed at 12/16); the episode is classified as VF since at least one cycle out of the last 8 is classified in the VF zone (FS) and a burst during charging is delivered.

3- What do you think of the cycle length of the burst?
The cycle length of the rapid pacing intervals during a burst is calculated from the average of the last 4 tachycardia cycles before the counters were completed; in the presence of T wave oversensing, the heart rate calculation is incorrect and there is a risk that the first cycle of the burst is delivered on the T wave (50% probability); this was the case in this example which explains the rest of the trace.

4- What is the effect of the burst followed by a shock?
The burst degrades the ventricular rhythm into an extremely rapid and polymorphic arrhythmia, threatening the patient’s life; the shock is effective and restores a viable rhythm.

Take home message

  • This patient’s device was implanted for primary prevention, i.e. he had never experienced a sustained, symptomatic ventricular arrhythmia before the defibrillator was fitted. A defibrillator is indicated because it can significantly reduce mortality; however, it should be remembered that in some patients, the defibrillator can paradoxically increase the risk of sudden death by inducing serious ventricular arrhythmias.
  • T wave oversensing should be considered an emergency; even if there are algorithms to circumvent this, the risk of one or more inappropriate therapies being delivered is not zero.
  • This tracing shows the potentially pro-arrhythmogenic effect of an ATP pacing sequence delivered for T wave over-sensing; in this example, the first cycle of the burst is delivered on the T wave, during a vulnerable period, which induces a polymorphic ventricular arrhythmia.
  • As a general rule, ventricular arrhythmias induced by the device are usually very concerning because they are extremely rapid and polymorphic, and the effectiveness of the shock becomes essential to re-establish a viable rhythm. Oversensing of the T wave is enhanced by the presence of a low amplitude R wave, there is an increased risk of of under sensing of induced VF in these patients.
  • The occurrence of an inappropriate shock due to T wave oversensing can also have consequences beyond the pain felt by a patient; in this setting, the shock is synchronised to either the R wave or the T wave; patients with T wave oversensing and a high defibrillation threshold are particularly at risk; in fact, as explained above, in the presence of T wave oversensing, there is a 50% chance that the shock will be delivered on the T wave and therefore in the vulnerable ventricular period; if the defibrillation threshold is high and close to the maximum capacity of the device, the upper vulnerability value is also high; therefore a shock delivered on the T wave has a very high probability of inducing ventricular fibrillation (due to the concept of upper limit of vulnerability) which will then be very difficult to cardiovert even with a maximum output shock (high defibrillation threshold).
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