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Discriminators available on a single-chamber Discriminators available on a single-chamber defibrillator - Virtuoso VR

Patient


Male with ischaemic cardiomyopathy implanted with a single-chamber defibrillator (Virtuoso VR) presenting with multiple episodes of VT.

Trace


1- What is the diagnosis made by the defibrillator for this episode?
This episode was classified as VT by the device.

2- How many detection zones are programmed?
Three detection zones have been programmed with a VT zone of 600 to 350 ms.

3- What diagnosis does the interval plot suggest?
The graph shows an initially stable rhythm of around 60 bpm with sudden acceleration and cycles recorded in the VT zone; a burst is delivered with normalisation of the heart rate.

4- How many discrimination parameters are programmed?
The 3 discrimination criteria available in a single-chamber defibrillator are programmed: sudden onset (81%), stability (40 ms) and Wavelet (70% similarity); Wavelet is programmed on Monitor (data are recorded but not used for the decision).

5- How do you rate the start of the episode?
The onset is abrupt, with ventricular cycles initially around 1000 ms, followed by cycles around 400 ms.

6- How do you describe the stability of the tachycardia?
The tachycardia is monomorphic and visually regular; the intervals detected by the device oscillate between 400 and 450 ms; even when the tachycardia appears visually regular, there may be small variations in the duration of the intervals detected, as the time at which the device detects the R wave may vary with respiration.

7- How do you analyse the morphology of QRS complexes during tachycardia?
The shock channel (between the box and the right ventricular coil) is used to record a tracing similar to an ECG lead; on this channel, the change in morphology is obvious between the initial tracing and the tracing in tachycardia.

8- What is the effect of the burst?
The burst is effective and terminates the tachycardia.

Take home message

  • The initial detection of an arrhythmia episode is based on heart rate and duration criteria; heart rate is essential but too simplistic to differentiate between ventricular rhythm disorders and supra-ventricular arrhythmias; as a standalone criterion, heart rate provides a sensitivity of 100% (all VT episodes exceeding this rate are detected) but specificity of less than 60% (all SVT episodes exceeding this rate are also detected). The improvement in specificity is therefore based on discrimination, which is the ability of a defibrillator to identify a supra-ventricular or ventricular origin of an episode of tachycardia, based on the characteristics of the arrhythmia detected.
  • The detection of an arrhythmia episode, whether supra-ventricular or ventricular, triggers the storage of an endocardial EGM that can be accessed when the defibrillator is interrogated, enabling critical analysis of the defibrillator’s diagnostic conclusions and possible reprogramming in the event of a tachycardia classification error.
  • The performance of a discrimination algorithm is assessed based on its sensitivity (ability to correctly diagnose a VT; this should be as close as possible to 100%; a sensitivity of 95% means that 5% of VTs will be wrongly diagnosed as SVT) and its specificity (ability to inhibit therapies during an episode of SVT; this should be as high as possible without altering sensitivity).
  • In single-chamber defibrillators, 3 criteria can be programmed to discriminate the origin of arrhythmias: Sudden Onset, Stability and Wavelet.
  • Each criterion can be programmed to ‘On’, ‘Off’ or ‘Monitor’; Monitor mode allows the performance of a criterion to be assessed without it being taken into account for discrimination; if all the criteria are set to ‘Monitor’ or ‘Off’, discrimination is inactivated and therapy is delivered regardless of the origin of the tachycardia; depending on the programming, each parameter can be made more or less sensitive, or more or less specific.
  • When the 3 parameters are set to ‘On’, the diagnosis of VT requires a sudden onset (validated at the start of the episode), stability of the ventricular cycles (throughout the tachycardia for initial detection and for redetection) and a change in the morphology of the QRS complexes (assessed over the last 8 cycles before the counter is filled); therefore all 3 criteria must indicate a diagnosis of VT.
  • As indicated on the plot, the episode dates back to 2007 and was therefore recorded on an old defibrillator platform (Virtuoso); although the programmable parameters (number of zones, discriminators, etc.) available on defibrillator platforms have changed relatively little since 2007, the way defibrillators are programmed has changed considerably; it is now very rare to program therapy zones for heart rates as low as 100 bpm; similarly, the 3 discriminators available on a single-chamber defibrillator are rarely simultaneously programmed ‘on’ in the first instance.

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