Patient
Male implanted with a dual-chamber defibrillator (Evera XT DR) for dilated cardiomyopathy.
Graph and trace
1, 2. What diagnosis do you suggest from this graph?
The episode was classified as VT-NS by the device (1); the graph shows atrioventricular dissociation with cycles oscillating around the VT zone (2).
- What is your diagnosis?
The EGM shows VT (ventricular rate > atrial rate); some cycles are classified as TS, others as VS; cycles classified as VS reset the VT counter to 0.
Take home message
Messages to remember
- This plot shows one of the possible limits of the TV counter, which requires a programmable number of consecutive cycles classified as TS, and highlights the importance of programming the lower limit of the TV zone.
- In this patient, the diagnosis of VT is indisputable, with obvious atrioventricular dissociation; however, there is a delay in diagnosis due to the specificity of the operation of this counter; the tachycardia oscillates with the lower limit of the VT zone; when a cycle is classified as SV, the VT counter is reset to 0 even if, as in this case, the diagnosis of VT is obvious.
- It is therefore essential to program a sufficient margin in relation to the frequency of the clinical tachycardia to avoid this type of problem; it is usual to program the VT zone 10 to 20 beats/minute slower than the clinical VT, this margin should probably be slightly increased for this brand's devices, as the occurrence of a cycle classified as SV does not have the same consequences as for the defibrillators of competing devices.
- This specificity also means that VT meters are not generally used for tachycardias above 200 beats/minute, as the risk of intermittent under-detection is increased (more frequent polymorphic tachycardias), resulting in delayed or non-diagnosis and therefore treatment; the probabilistic VF meter seems more appropriate for this type of tachycardia.
- In the latest recommendations, there are marked differences in terms of programming the detection zone terminals depending on the manufacturer; the different operation of the meters explains some of these differences (in primary prevention, MedtronicTM recommends a single VF zone from 188-200 beats/minute, compared with a VT zone up to around 230 beats/minute and then a VF zone for the other manufacturers).